Crop Diseases in Uzbekistan Caused by the Species of the Genus Fusarium: An Overview

Abstract

In Uzbekistan, plant diseases caused by Fusarium species are common. Some of them are the dominant pathogens of certain crops, causing economically important diseases such as wilts of cotton, melon, tomato and chickpea, root/crown rot of wheat, damping-off, root, crown or foot rot of pepper plants, vegetables and other crops. Reportedly, 26 Fusarium species cause root rot on wheat in Uzbekistan. An annotated list of 22 valid taxa is presented. As well, 26 Fusarium species or forms are reported on cotton. Of many Fusaria found on other crops, only a few credible reports have been analyzed. The composition of Fusarium species infecting crops still needs to be studied since their identification was based on morphology. Molecular identification started to be used recently, and so far, the results of some previous morphological identifications were confirmed, races of Fusarium oxysporum f. sp. vasinfectum and several Fusarium species causing root rot of wheat, bell pepper and hot pepper, chickpea, cowpea and soybean were identified. Undoubtedly, studies of Fusarium species, based on classical morphological methods and DNA analysis of informative genes, will be used more widely in the coming years, ensuring a reliable identification of species of this complex genus in Uzbekistan.

1. Introduction

1.1 Lifestyles and importance of the Fusarium species

Fusarium fungi are ubiquitous worldwide and usually live in the soil as facultative plant parasites (necrotrophic pathogens) or as saprotrophs feeding on plant residues or other organic substrates. Specific populations of some Fusarium species, e.g., Fusarium oxysporum and Fusarium solani, can live in soil as plant pathogens or saprotrophs. Some other Fusarium species may exist also as endophytes of plants [1, 2].

Plant pathogenic representatives of the genus Fusarium can be highly aggressive and severely infect or even kill their host plants. Other Fusarium species may be weak (opportunistic) pathogens that infect weakened plants, aging leaves, or other parts of plants, causing only minor damage. Some species of this genus often colonize plant tissues previously affected by other pathogens (including Fusarium species) as secondary invaders.

Most plant pathogenic representatives of the genus Fusarium are widely specialized and can infect many plant species from various families. However, some species, such as Fusarium oxysporum, have formae speciales (f. spp.) and physiologic races, more or less narrowly specialized to infect, respectively, only the species of their host plants or their varieties [1, 2].

The negative significance of Fusarium fungi in people’s lives is enormous and the main three reasons for this are as follows.

1. Species of this genus are pathogens of such economically important diseases as pre- and post-emergence damping-off, root rot, crown rot, foot (stem) rot, fruit rot of vegetables and other field crops, head scab of cereals.

2. The great importance of these pathogens is also evidenced by the fact that among the 10 economically most important fungi in molecular plant pathology, two species—Fusarium graminearum and Fusarium oxysporum, are placed by scientists in 4th and 5th places, respectively [3].

3. Affecting seeds of plants, especially of cereals, many Fusarium species can produce mycotoxins such as trichothecenes, fumonisins, zearalenones and enniatins that are dangerous to human and animal health ([4] and references therein).

4. Some Fusarium species are opportunistic pathogens of humans (and animals), especially with compromised immunity.

More than 70 pathogenic Fusarium species have been identified, which can affect people’s eyes, nasal cavity, nails of hands and feet, skin, cause difficult-to-treat systemic mycoses of the lungs, intestines, bones, nervous system and refractory diseases that often lead patients to death [5, 6, 7].

1.2 Methods of identification of plant pathogenic Fusarium species

Due to the great importance of fungi of the genus Fusarium, they are objects of research by scientists in many countries. Often, the purpose of research is to determine the composition of diseases of the studied crop, for which a survey of crops, sampling of diseased plants, their mycological analysis, isolation of pure cultures of the causative agent of the disease and its identification to the level of the species is carried out.

To identify fungi, including Fusarium species, three main concepts used are morphological concept, biological concept and phylogenetic concept. Each concept has its advantages and disadvantages [1, 2, 8].

The morphological concept is classical, and traditional, and has been used by researchers for over two centuries [8]. It is based mainly on studying characteristics of colonies, size, shape and other characters of macroconidia, the presence or absence of microconidia and chlamydospores [1, 2]. However, it later became clear that, with rare exceptions, the morphological method does not allow differentiating most species of the genus Fusarium from each other [8].

The biological concept for identifying Fusarium species provides for crossing of the studied isolate of an unknown species with tester isolates whose species are known. Obtaining viable and fertile offspring will indicate that the studied isolate belongs to the same species as the tester isolate, crossing with which gave a positive result [1, 2]. This method is somewhat limited, since it cannot identify Fusarium species, in which the sexual process is unknown, nor for homothallic and some heterothallic representatives of this genus [9].

When using the molecular genetic method (in the phylogenetic concept) to identify Fusarium species, DNA sequences of one or more “preserved” genes in the studied isolate are determined, and with the help of the Basic Local Alignment Search Tool (BLAST) program they are compared with the available sequences in one of the databases, for example, in FUSARIUM-ID v.3 NCBI (http://isolate.fusariumdb.org/blast.php) [4, 5, 6, 10, 11].

Many “candidate” genes have been studied for molecular identification of Fusarium species. Although the internal transcriber spacer (ITS) region of the nuclear ribosome was selected as the universally accepted DNA barcode for fungi [12], it turned out to be less informative for the genus Fusarium than the other three genes, namely, partial sequences of translation elongation factor (TEF-1α), DNA-directed RNA polymerase II largest (RPB 1) and second largest (RPB 2) subunits [5, 13, 14]. Of these, TEF-1α was particularly suitable for this purpose, which alone was able to differentiate all species of the genus Fusarium [4, 10]. Therefore, this housekeeping gene is called the “marker of choice” for single locus identification in Fusarium [4, 9, 10, 13]. Based on many years of research, experienced scientists recommend using only the TEF-1α gene for molecular genetic identification of Fusarium species, and not using such genes as ITS rDNA (ribosomal DNA), 28S rDNA, LSU rDNA (large subunit ribosomal DNA) or TUBB (β-tubulin) [4, 10].

For identifying some other species, particularly members of the Fusarium tricinctum species complex, small portions of the gene encoding phosphate permease (PHO) enzyme were very informative [15].

Using the molecular method, more than 400 phylogenetically distinct species of the genus Fusarium, which belong to 23 monophyletic groups designated as “species complexes,” were identified in 2022. Over 80% of these species have been discovered in the last 25 years. Due to the ongoing discovery of new species at present, this process is expected to continue in the foreseeable future [10].

Information about morphological characteristics is available for many species of the Fusarium genus, about biological features—for a limited number of them; there was also little information about DNA sequences of fungi of this genus; however, they are becoming more and more prevalent every year. In cases where information on all these features is available, “polyphasic” description method can be applied, which makes it possible to identify species of the genus Fusarium very reliably [2, 16].

Here, it is also necessary to mention the classical Koch’s postulates, the fulfillment of which (i.e., the determination of the pathogenicity of the studied isolates) is always necessary in studies on the identification of plant pathogenic species of the genus Fusarium.

2. Diseases of crops in Uzbekistan caused by Fusarium species

Studies conducted in Uzbekistan revealed widespread crop diseases caused by Fusarium species. There are publications on wilt of cotton, root rot of wheat, vegetable crops and mulberry trees caused by species of this genus. In a number of these studies, the species of pathogens were determined tentatively—considering the species of the affected host plant and symptoms of the disease. In other studies, identification was based on the cultural and morphological characteristics of the studied isolates; in some cases, their pathogenicity to host plants was also determined. However, as mentioned above, using only the morphological concept often does not allow for the correct identification of species of this genus.

Only in recent years, to identify plant pathogenic species of the genus Fusarium, researchers in Uzbekistan have begun to apply methods based on the DNA analysis of certain marker genes, along with studying morphology and pathogenicity.

Below, we provide information about the Fusarium species, which, according to the reports of local researchers, cause diseases of crops in Uzbekistan, and discuss the status of validity of these species.

2.1 Fusarium species reportedly registered in Uzbekistan as wheat pathogens

As in all countries of the world, soft wheat (Triticum aestivum L.) in Uzbekistan is an important food crop, cultivated mainly on irrigated lands. In 2012–2021, it was sown annually on ∼1.28 M hectares on average, and almost 4.68 tons of grain was harvested from each hectare [17]. Durum wheat (Triticum durum Desf.), previously cultivated in the country on small areas, is currently not sown.

The steady production of high grain yields in the country is hindered by wheat diseases. The most dangerous of them is stripe rust (Puccinia striiformis West. f. sp. tritici Eriks.). In some regions or in some seasons, crops may also be affected by leaf rust (Puccinia triticina Eriks.), common bunt (Tilletia laevis Kühn), loose smut (Ustilago tritici (Pers.) Rostr.), powdery mildew (Blumeria graminis (DC. ex Merat) Speer f. tritici É.J. Marchal), Septoria leaf blotch (Septoria tritici Rob. et Desm.), tan spot (Drechslera tritici-repentis (Died.) Shoem.) and other diseases [18, 19, 20].

In recent years, the attention of researchers has been increasingly attracted to root, crown and stem diseases of wheat plants. Fusarium root rot (FRR), Fusarium crown rot (FCR) and Fusarium head blight (FHB) are sporadically found in wheat fields in the country almost everywhere. However, in recent surveys, many fields were recorded where most of the plants have been infected at the tillering, heading and flowering growth stages. The external symptoms were stunting of plants and a general yellowing. Examination of the affected plants revealed the presence of light brown spots (necroses) on the crown and lower stem internodes. Later, the affected plants usually died prematurely, and white-head and white-stem symptoms usually appeared. In mycological analyses, fungi with morphology similar to those of Fusarium often grew from samples of infected plant parts. Such isolates were selected, and their species’ identity was further determined, mainly by studying their morphological characteristics. In some experiments, their pathogenicity to the host plant was also determined [21, 22].

According to the literature, >60 Fusarium species are registered on wheat worldwide, including >40 species as causes of root and crown rot [23], and about 45 species as pathogens that infect heads [24]. According to local researchers, out of these >60 species, 26 species were registered in Uzbekistan as a cause of wheat root rot, three of which (Fusarium avenaceum, F. graminearum and Fusarium poae), in addition to the roots, also affected the ears of plants (Tables 1 and 2).

Fusarium species		Reference
Currently accepted name	Reported as
F. avenaceum	F. avenaceum	[21, 25]
F. buharicum	F. bucharicum	[25]
F. culmorum	F. culmorum	[21, 25, 26, 27, 28]
F. equiseti	F. gibbosum	[[21, 26, 27]
F. fujikuroi	F. fujikuroi	[28]
F. graminearum	F. graminearum	[21, 25, 28]
F. heterosporum	F. heterosporum	[21, 25]
F. lactis	F. lactis	[21]
	F. moniliforme var. lactis	[25]
F. lateritium	F. lateritium	[21, 25]
F. oxysporum	F. oxysporum	[21, 25, 29, 30]
	F. oxysporum var. orthoceras	[21, 25]
F. poae	F. poae	[21]
	F. sporotrichiella var. poae	[25]
F. pseudograminearum	F. pseudograminearum	[29, 30]
F. redolens	F. redolens	[25]
F. sambucinum	F. sambucinum	[21, 25]
F. solani	F. solani	[21, 25, 29, 30]
	F. javanicum var. radicicola	[21, 25]
F. sporotrichioides	F. sporotrichioides	[28]
F. subglutinans	F. moniliforme var. subglutinans	[26, 27]
F. ventricosum	F. solani var. argillaceum	[21, 25]
F. verticillioides	F. verticillioides	[21]
Incertae sedis	F. javanicum	[21, 25]
Incertae sedis	F. moniliforme	[25]
Incertae sedis	F. sambucinum var. ossicolum	[25]
Not considered Fusarium	F. merismoides	[25]

Table 1.

Fusarium species reported to infect soft and/or durum wheat plants in Uzbekistan in 1972–2019.

Fusarium species and strain nos.	Year and Region recorded	Isolated from	NCBI ID Code No.
F. culmorum R93	2020 Tashkent	Root, crown	OP529856
F. culmorum A78	2021 Kashkadarya	Root	OP529854
F. equiseti AN5	2020 Jizzakh	Crown	OP856716
F. graminearum AN10	2020 Tashkent	Head	OP529853
F. incarnatum AN7	2021 Khorezm	Crown	OP856717
F. oxysporum AN44	2021 Tashkent	Root	OP856720
F. poae AN15	2021 Tashkent	Root, crown	OP893793
F. poae AN42	2021 Tashkent	Root, crown	OP856719
F. proliferatum AN20	2020 Tashkent	Head	OP856718
F. solani AN81	2020 Tashkent	Root	OP856721
F. tricinctum AN18	2021 Andijan	Root, head	OP893792
F. tricinctum S83	2021 Fergana	Root, head	OP529855

Table 2.

Fusarium species reported to infect soft wheat plants in Uzbekistan in 2020–2021 (unpublished data).

All species listed in Table 1 were identified according to their morphological characteristics. Most of them were identified by Prof. A. Sheraliev [25] and his colleagues [21] at the Tashkent State Agrarian University (TSAU) on the basis of morphological features of isolated fungi; for their identification, they used keys compiled by Bilai [31].

The species listed in Table 2 were identified by the researchers of the Institute of Genetics and Plant Experimental Biology of the Academy of Sciences of the Republic of Uzbekistan (IGPEB ASU) by analysis of morphological characteristics [1], pathogenicity (Figure 1) and partial sequences of TEF-1α gene (unpublished data).

Figure 1.

The yellowed, dying soft wheat plants (Krasnodar 99 variety) in three pots, which soils were inoculated with the Fusarium graminearum strain AN10 (the “sickpot” trial). On the left is shown a pot with uninoculated soil and healthy plants (check treatment) (courtesy of AG Sherimbetov).

Twenty-two species names listed in Tables 1 and 2 are considered valid and are part of the following species complexes (SCs): six species belong to Fusarium sambucinum SC (FSAMSC), five species—to Fusarium fujikuroi SC (FFSC), two species each to Fusarium incarnatum-equiseti SC (FIESC), Fusarium solani SC (FSSC) and F. tricinctum SC (FTSC), and one each to Fusarium buharicum SC (FBSC), Fusarium heterosporum SC (FHSC), Fusarium lateritium SC (FLSC), Fusarium oxysporum SC (FOSC) and to Fusarium redolens SC (FRSC) [10].

The taxonomic status of some other binomials is doubtful; these are Fusarium javanicum Koord. [32], F. sambucinum var. ossicolum (does not exist in Internet sources); or rejected (Fusarium moniliforme Sheldon nomen rejiciendum) [33]. Another species, Fusarium merismoides Corda, is currently transferred from the genus Fusarium to genus Fusicolla under the name of Fusicolla merismoides (Corda) Gräfenhan, Seifert & Schroers [34].

Below, we provide additional information about the species of the genus Fusarium registered in Uzbekistan on wheat. The abbreviated names of the species complexes that these species belong to are given after their names in square brackets.

Fusarium avenaceum (Fries) Sacc. [FTSC]. Many plant pathologists consider this species to be a saprotroph or weak (opportunistic) pathogen of cereals [1, 35, 36], while others [37, 38] place it among quite strong pathogens that cause wheat FRR. The high pathogenicity of F. avenaceum to the wheat root system is shown in experiments with artificial infection [39, 40]. Others reported that this species did not cause wheat infection in experiments [41].

In Uzbekistan, F. avenaceum is reported to occur sporadically in six regions of the country; its isolates from infected roots of soft and durum wheat [21, 25], as well as barley, corn, rice, etc., in total from 14 plant species, have been identified by morphological characteristics [25].

Fusarium buharicum Jacz. ex Babajan & Teteterevn.-Babajan [FBSC]. In Uzbekistan, this species was isolated from the affected roots of cotton [31] and wheat [25], and identified by morphological features. However, the pathogenicity of F. buharicum to wheat has not been experimentally proven, so there is no reason yet to include it among the pathogens of wheat FRR.

Fusarium culmorum (W.G. Smith) Sacc. [FSAMSC] is a strong pathogen causing FRR and FHB in soft and durum wheat, as well as cultivated and wild species of the genera Triticum and Hordeum (e.g., [1, 39, 42, 43, 44, 45, 46, 47, 48, 49, 50]). This species is also a pathogen of garlic, sugar beet and carnation [51].

In Uzbekistan, F. culmorum isolates recovered from samples infected with root rot samples of soft wheat, corn and rice collected in three regions of the country, as well as from the soil of rainfed fields, were identified by morphological features; furthermore, pathogenicity of the isolates to wheat has been confirmed experimentally [25, 26, 27].

Later, in 2020 and 2021, F. culmorum was isolated from the roots of soft wheat in the Tashkent and Kashkadarya regions, and two representative isolates were identified by morphological and molecular characteristics (Table 2). In experiments with artificial infection, these isolates showed pathogenicity to the roots of wheat plants (unpublished).

Fusarium equiseti (Corda) Sacc. [FIESC]. This species is cosmopolitan, and in mycological analysis it is very often isolated from various organs of many plants [1, 36], including infected wheat roots (e.g., [37, 42]). Recognized experts on the genus Fusarium (e.g., [1, 36, 42]) believe that F. equiseti is a common saprotroph that colonizes aging or already affected by other pathogen tissues of various plants as a secondary invader. However, in the experiments of other researchers in the USA, F. equiseti showed pathogenicity to seedlings of two wheat varieties [40]. It is also suggested that F. equiseti may be a minor pathogen of wheat FHB [43, 44].

In Uzbekistan in the 1970s, some fungi were isolated from wheat and rhizosphere soil, which were identified by morphological features as Fusarium gibbosum Appel & Wollenw. [26, 27]. This name is known to be synonymous with F. equiseti. Then, in the 2000s, F. gibbosum was isolated from Upland cotton, corn, etc., in total from 12 plant species in seven regions of the country [25]. However, whether all these isolates are pathogens of wheat (and other crops) has not been proven.

Later, in 2020, F. equiseti was isolated from the crowns of soft wheat in the Jizzakh region and was identified by morphological and molecular features (Table 2). In experiments with artificial infection, one isolate of the fungus showed some pathogenicity to the roots of wheat plants (unpublished).

Fusarium fujikuroi Nirenberg [FFSC]. This species, which causes the “foolish seedling” (bakanae) disease of rice, is not considered as wheat pathogen [1]. It has been, obviously, isolated by accident from wheat with root rot in the Syrdarya region of the country and was identified by the morphology of spores and molecular features [28, 52].

Fusarium graminearum Schwabe [FSAMSC]. F. graminearum s. str. is a cosmopolitan species, and, together with F. culmorum and Fusarium pseudograminearum, is found everywhere in the world where wheat is cultivated, but more often in regions with a temperate or warmer climate [1, 35]. It is the causal agent of very serious and economically important wheat diseases—FHB, FRR, FCR and damping-off [3]. For example, in the USA, ≥30% of the wheat grain yield is lost annually due to these diseases (e.g., [44]).

The researchers’ analyses show that the causal agent of wheat FHB in the world in ∼80 to 90% of cases is F. graminearum (and its phylogenetic “relatives,” e.g., Fusarium asiaticum O’Donnell, T. Aoki, Kistler & Geiser). The total “share” of all other Fusarium species in causing FHB is estimated to be at about 10–15% [1, 43, 44, 48].

In addition to wheat species, F. graminearum affects other crops, and corn is the main alternative host. In susceptible varieties of this culture, the fungus strongly affects stalks and cobs (e.g., [1, 35]).

Recently, legumes have also been included among the alternative hosts of F. graminearum, F. culmorum and some other Fusarium species. Thus, isolates of these species received in the USA from chickpea and lentil infected with FRR in experiments with artificial inoculation, infected, in addition to their original host plants, also wheat and barley [53].

The registration of F. graminearum in eight regions of Uzbekistan on 32 crop species, including the roots and heads of soft and durum wheat, was reported back in the 2000s; identification was based on the morphology and pathogenicity of isolated strains [25]. Later, this species was also isolated by other researchers from wheat infected with FHB and FCR [21, 28].

In 2020, isolates of F. graminearum recovered from FHB-infected wheat heads were identified for the first time in the country on the basis of molecular genetic studies (Table 2; unpublished).

Fusarium heterosporum Nees ex Fries [FHSC]. Cosmopolitan, especially common in Australia and Africa on millet panicles affected by ergot [36]. It is believed that this species is not a wheat pathogen [1]. Therefore, its isolation from infected with FRR wheat in mycological analyses in Uzbekistan and elsewhere ([25, 42], a. o.) is probably associated with the invasion of the fungus into tissues as a saprotroph and secondary invader. In addition to soft and durum wheat, F. heterosporum was also reported in nine regions of the country on 32 crop species, including Upland and Pima-type cotton, and corn [25].

Fusarium incarnatum (Desm.) Sacc. [FIESC]. This species was isolated from a sample of the infected wheat crowns collected in the Khorezm region in 2021. A representative isolate was identified by morphological and molecular characteristics (Table 2).

There is no information on the Internet that Fusarium incarnatum can infect the roots or stems of wheat. It was only reported that this fungus is often isolated from barley and oat seeds in the north-west of Russia [54], and from wheat heads in Kenya [55]. It is believed that the ability of F. incarnatum to infect the roots or heads of wheat has not been experimentally proven, so it should not yet be included among the pathogens of this crop.

Fusarium lactis Pirotta & Riboni [FFSC]. This species, identified morphologically, is reported to cause FRR on wheat [21, 25] but it is a well-studied pathogen of fig trees, and it causes a serious fig disease called “endosepsis” in California, USA [1, 36]. Considering the limited area (only the USA) and the absence of pathogenicity in F. lactis to wheat, cotton and other plant species, one can doubt its correct identification; however, we have no reason to consider this species as a pathogen of wheat.

Fusarium lateritium Nees & Link [FLSC]. This cosmopolitan species is a parasite of trees (e.g., mulberry) on which it causes wilt, cankers and die-back, and it does not infect wheat or other field crops [1, 36]. Therefore, we assume that isolates of this species recovered from infected wheat roots in Uzbekistan [25], roots [56] and heads [57] elsewhere are probably random species that have colonized already previously infected plant tissues.

Fusarium oxysporum Schlecht. emend. Sn. & Hans. [FOSC]. This cosmopolitan fungus has both plant pathogenic and saprotrophic populations in the soil.

Wheat and other cereals are not infected by F. oxysporum, but this species, especially its saprobic strains, easily penetrates roots already infected by other pathogens as a secondary invader, and are easily isolated from their tissues during mycological analyses ([1].

Consequently, cultures of F. oxysporum recovered from wheat tissues infected with FRR/FCR in different countries, including Uzbekistan [21, 25, 29, 30] and elsewhere (e.g., [54]), are also not the true causal agents of these diseases. This has also been proved by researchers in special experiments. Thus, isolates of F. oxysporum and F. solani recovered from infected wheat roots and crowns in the USA did not show pathogenicity in tests with artificial inoculation of wheat seedlings [40].

Fusarium poae (Peck) Wollenw. [FSAMSC]. The ability of F. poae to cause FRR/FCR in wheat has been proven many times, but it is considered less virulent in comparison with F. pseudograminearum, F. graminearum and F. culmorum [1, 35, 38, 50].

In Uzbekistan, strains of this species isolated from wheat roots were morphologically identified under their own name, while those isolated from roots and heads—under the name Fusarium sporotrichiella var. poae [21, 25].

In 2021, F. poae cultures isolated from infected with FRR/FCR wheat tissues were identified for the first time in the country on the basis of molecular studies (Table 2; unpublished).

Fusarium proliferatum (Matsush.) Nirenberg ex Gerlach & Nirenberg [FFSC]. This cosmopolitan species is one of the aggressive pathogens causing corn stalk rot, and it also affects drooping sorghum, grasses and asparagus. Wheat is not affected by F. proliferatum, but it is found in its tissues as an endophyte [1, 36].

One isolate of Fusarium proliferatum recovered in 2020 from wheat infected with FHB was identified for the first time in Uzbekistan on the basis of molecular studies (Table 2; unpublished).

This species in Russia [54], China [38], Turkey [41, 42] and Iran [56] was isolated from wheat plants infected with FRR/FCR, and in some other countries—from wheat heads infected with FHB (e.g., [43, 58]). This was probably due to the transfer of F. proliferatum spores to wheat fields from corn or rice fields. There have been no reports of experimental establishment of the pathogenicity of F. proliferatum to wheat.

There was only one report to suggest that this species infected wheat leaves very weakly during artificial infection [59]. Therefore, it can be assumed that F. proliferatum is probably a secondary invader on the roots and heads of wheat.

Fusarium pseudograminearum Aoki & O’Donnell [FSAMSC]. This species was created in 1966 in Australia on the basis of one of the two populations of F. graminearum s. l., which more often cause FCR in wheat and barley, and do not form perithecia in cultures derived from a single spore. Another population of the fungus, which more often infects heads of wheat, barley, oats and corn stalks, and regularly forms perithecia in monosporic cultures, was actually F. graminearum s. str.

Subsequently, F. pseudograminearum spread from Australia to other continents [36, 60]. In recent years, it has been identified mainly as the causal agent of wheat FCR in many regions of different countries with dry and warm climates and often occupied first or second position on its incidence. It is not found in regions with cooler climates (e.g., [38, 39, 61]).

It was reported in Turdieva et al. [29] that F. pseudograminearum in Uzbekistan was isolated from wheat plants infected with root rot, but the identification was preliminary, not confirmed by molecular studies. For this reason, the identification of this species on wheat in Uzbekistan must be considered as yet unproven.

Fusarium redolens Wollenw. [FRSC]. This species occurs in temperate regions of the world as a saprotroph in the soil, on the rotting residue of various plants, and is the causal agent of root rot of asparagus, beans, peas, carnation, roses and spinach [1, 36]. Sometimes it is isolated from the seeds of cereals [31]. In studies conducted in the USA, F. redolens has shown pathogenicity to wheat seedlings [40].

It was reported that F. redolens has been registered in nine regions of Uzbekistan on 24 crop species, and has been isolated from soft wheat, rice, corn, Upland and Pima cottons infected with root rot. The identification of F. redolens was based on its morphological characteristics [25].

Fusarium sambucinum Fuck. s.str. [FSAMSC]. This species is widespread in regions of the world with a temperate, cool climate on various substrates, is often isolated from cereal seeds and causes dry rot of potato tubers [1, 36].

There are reports of isolation of F. sambucinum from diseased wheat roots in Iran [56] and Uzbekistan [21, 25]. The identification of the species by all these researchers was carried out on the basis of its morphology. F. sambucinum was included in the lists of wheat FHB pathogens [43], and isolated from the affected heads of this crop [57]; however, we have not encountered reports of experimental establishment of its pathogenicity to wheat.

Fusarium solani (Mart.) App. & Wollenw. emend Sn. et Hans. s. str., syn. Neocosmospora solani (Mart.) Lombard & Crous [FSSC]. This cosmopolitan fungus has plant pathogenic and saprotrophic populations in the soil. F. solani is the causal agent of dry and jelly-end rot of potatoes, a strong pathogen of many species of the families Fabaceae, Solanaceae, Cucurbitaceae, cotton and some trees, but it does not affect cereals, in particular, it is not the causal agent of wheat root rot [1, 32]. In experiments with artificial infection, F. solani showed no pathogenicity to wheat [40, 41].

Hence, it can be concluded that strains of F. solani isolated from the infected with FRR/FCR wheat tissues in different countries (e.g., [41, 42, 54, 56]), including Uzbekistan [21, 25], obviously are not the true causal agents of the primary root infection, but are secondary invaders.

Fusarium sporotrichioides Sherbakoff [FSAMSC]. This species occurs in regions of the world with a temperate or cold climate (even under snow). It is isolated from seeds of cereals and other plant species. In general, it is considered an opportunistic (weak, minor) pathogen of cereals (e.g., [1, 31, 36, 54]).

Fusarium sporotrichioides was isolated from infected root samples in studies conducted in Turkey [41] and Uzbekistan [28, 52]. It was identified by morphological features. The Turkish isolate did not cause wheat infection in the experiment [41]. One isolate (no. 404) in Uzbekistan was identified by molecular characteristics [52], but its pathogenicity had not been tested.

Fusarium subglutinans (Wollenw. & Reinking) Nelson, Toussoun & Marasas [FFSC]. This species causes corn stalk and cob rot in regions with a cold climate and in cooler regions of countries with a temperate climate [36]. In different countries, it was isolated from grasses, millet, drooping sorghum, wild rice and soybean [1], and in Iran [56] and Uzbekistan [26, 27]—from infected wheat roots.

Although some researchers consider F. subglutinans to be one of the pathogens of wheat FHB [43], there have been no reports of experimental establishment of its pathogenicity to wheat.

Fusarium tricinctum (Corda) Sacc. s. str. [FTSC] This species is more common in temperate regions of the world [1, 36]. Researchers considered it a weak pathogen, saprotroph or secondary invader, colonizing roots or heads of cereals, and sometimes wheat, previously affected by other pathogens [1, 41, 43, 44, 54, 58].

At the same time, there are reports that F. tricinctum is becoming a dangerous pathogen of cereals, causing FRR and FHB on wheat, which leads to severe yield losses, reduces the quality of grain, polluting it with dangerous mycotoxins such as enniatins, moniliformins, bovericin, T-2 toxin, etc. The ability of F. tricinctum to cause FHB in soft wheat and barley has been proven by artificial infection of plants with the isolates received from them [62, 63, 64].

In general, not only F. tricinctum became the emerging predominant pathogen of cereals, legumes and other economically important crops, but also other species of the F. tricinctum species complex [15]. F. tricinctum turned out to be a polyphagous that also affects worldwide oats, rice, oat, potatoes, corn, soybean, lily, onion, pumpkin and other crops [64].

In Uzbekistan, F. tricinctum was isolated in 2021 in the Andijan and Fergana regions from the infected FRR roots and FHB ears of wheat. Fungal isolates were identified by morphological and molecular characteristics (Table 2). The pathogenicity of isolates to wheat was proved in tests with artificial infection of the host plant (unpublished).

Fusarium ventricosum Appel & Wollenw., syn. Rectifusarium ventricosum (Appel & Wollenw.) Lombard & Crous [supposedly FSSC]. This little-known fungus was originally isolated from potato tubers and soil in a wheat field in Germany [65].

According to the report, this species (under the synonymic name F. solani var. argillaceum) was registered in eight regions of Uzbekistan on 21 plant species, including soft wheat, corn, rice and cotton. It caused root rot on wheat plants [21, 25]. Identification of this species was based on its morphological characteristics. No information is provided on testing the pathogenicity of recovered isolates to host plants. Therefore, there is no reason to consider this species a pathogen of wheat yet.

Fusarium verticillioides (Sacc.) Nirenberg, syn. F. moniliforme Sheldon [FFSC]. This fungus is found everywhere in the world where corn is cultivated. It is considered the most dangerous pathogen of corn, affecting its stalks and cobs. F. verticillioides is a polyphage, and its hosts also include drooping sorghum, sugarcane, rice, asparagus, etc., in total 11,000 plant species. However, it is believed that the pathogenicity of the fungus to these plants must be confirmed anew to make sure that no other species from the Fusarium fujikuroi SC is accepted for F. verticillioides [1, 36].

There are reports that F. verticillioides (sometimes given under the invalid synonymous name F. moniliforme) has been isolated from wheat tissues with root rot in Turkey [42, 56], Algeria [66] and Uzbekistan [21, 25]. In all these studies, identification was based on morphological features of isolates, and their pathogenicity to wheat was not tested. Therefore, it can be assumed that this species is not among the pathogens of wheat.

2.2 Fusarium species reportedly registered in Uzbekistan as cotton pathogens

Cotton has been cultivated in Uzbekistan for more than 2.5 thousand years and is important for the country’s economy as the main cash crop. According to official sources, the total area under cotton in the country in 2019–2022 was approximately 1.1 to 1.3 million hectares annually, and the harvest ranged from 3.1 to 3.8 million tons of raw cotton.

In Uzbekistan, Upland cotton (Gossypium hirsutum L.) is mainly cultivated. Pima cotton (Gossypium barbadense L.) is grown only in the south of the country, in the Surkhandarya and Kashkadarya regions. In recent years, G. barbadense has occupied 4 to 5000 hectares, and in 2023 it reached 7 to 8000 hectares.

A number of abiotic and biotic factors threaten the sustainable production of high cotton yields. Of the latter, the most common and dangerous are such diseases of cotton as damping-off (Figure 2), root rot, Verticillium wilt, Fusarium wilt and bacterial blight. Seedling diseases and root rot diseases in the country have remained poorly studied until recent years, but wilt diseases have been known for many years and studied intensively.

Figure 2.

The dying seedling of the upland cotton (Bukhara 6 variety) infected with Fusarium sp., in the field of the Jondor district of the Bukhara region. Part of a healthy seedling is visible on the left (courtesy of BA Khasanov).

Fusarium species isolated from infected cotton roots and stems during mycological analyses in 1940 and 2001, and identified by morphological characteristics are listed in Table 3.

Fusarium species		Reference
Currently accepted name	Reported as
F. acuminatum	F. scirpi var. acuminatum (Ell. &. Ev.) Wollenw.	[67]
F. anthophilum	F. anthophilum	[67]
F. buharicum	F. bucharicum	[25]
F. compactum	F. scirpi. Var. compactum (Ell. &. Ev.) Wollenw.	[67]
F. culmorum	F. culmorum	[67]
F. equiseti	F. equiseti	[25, 67]
F. filiferum	F. scirpi var. filiferum (Preuss) Wollenw.	[67]
F. heterosporum	F. heterosporum	[25, 67]
F. lactis	F. moniliforme var. lactis	[25]
F. lateritium	F. lateritium	[25]
F. longipes	F. scirpi var. longipes (Wollenw. & Reinking) Wollenw.	[67]
F. oxysporum	F. orthoceras App. & Wollenw.	[67]
	F. oxysporum	[25]
F. oxysporum f. sp. vasinfectum	F. vasinfectum G.F. Atk.	[67]
F. sambucinum	F. sambucinum	[67]
F. scirpi	F. scirpi	[67]
F. solani	F. martii Appel & Wollenw.	[67]
	F. solani	[25, 67]
	F. javanicum var. radicicola	[25]
F. sporotrichioides	F. sporotrichioides	[67]
F. subglutinans	F. moniliforme var. subglutinans	[25]
F. ventricosum	F. ventricosum	[25, 67]
F. verticillioides	F. moniliforme	[25, 67]
Incertae sedis	F. javanicum	[25, 67]
Incertae sedis	F. moniliforme var. minus Wollenw. & Reinking	[67]
Not considered Fusarium	F. nivale	[25]

Table 3.

Fusarium species reported to infect cotton plants in Uzbekistan in 1940 and 2001.

Thorough studies of the Verticillium and Fusarium wilt diseases of cotton in Uzbekistan (and in general, in Middle Asia) were conducted by A. I. Solovyova with colleagues at the Plant Protection Station of the All-Union Scientific Research Institute of Cotton Growing (PPS CRI), Tashkent region, in 1938–1945 [67, 68, 69]. During mycological analyses, the PPS employees in the 1930s recovered 25 isolates from cotton stems with wilt symptoms, which they identified by morphological features and assigned to the following 15 species: Fusarium acuminatum, Fusarium anthophilum (A. Braun) Wollenw., Fusarium compactum (Wollenw.) Raillo, F. culmorum, F. equiseti, Fusarium filiferum (Preuss) Wollenw., F. heterosporum, F. longipes Wollenw. & Reinking, F. oxysporum, F. oxysporum f. sp. vasinfectum (FOV), F. sambucinum, Fusarium scirpi Lamb. et Fautr., F. solani, F. sporotrichioides, F. ventricosum, F. verticillioides and incertae sedis taxa F. javanicum and F. moniliforme var. minus. But in the experiments of these researchers with artificial infection, none of these isolates, except FOV, infected cotton plants [67].

Subsequently, the widespread and severe infections of cotton wilt caused by FOV were also registered throughout the Andijan region [70].

Even later, A. Sheraliev from the TSAU [25] has reported that in surveys conducted on the territory of Uzbekistan, 13 Fusarium species were registered and identified on cotton by morphological characteristics (Table 3). Of these, four species were isolated from Upland cotton and nine species—from both Upland and Pima cotton species. Twelve of these 13 species were indicated by this author [25] also on wheat (see Table 1). In this work, it was claimed that six species, namely F. oxysporum, F. moniliforme, F. lateritium, F. heterosporum, F. solani and F. equiseti, were causal agents of cotton wilt. All these species, except F. equiseti, as well as five more species—F. buharicum, F. javanicum (incertae sedis), F. lactis, F. subglutinans (under the name of F. moniliforme var. subglutinans) and F. ventricosum were also indicated as pathogens causing FRR and/or FCR on cotton [25].

When analyzing the information given here, some questions arise about their validity. Thus, it is doubtful that as many as five Fusarium species can be true causal agents of the cotton wilt, and 11 Fusarium species can be true causal agents of FRR/FCR on cotton. Currently, only Fusarium oxysporum f. sp. vasinfectum is recognized as the only causative agent of cotton wilt [1, 71].

In addition, another species, Fusarium nivale Ces. ex Berl. & Voglino, was indicated as a pathogen of Upland cotton, as well as corn and five other plant species [25]. This species is excluded from the genus Fusarium, and its current name is Microdochium nivale (Fr.) Samuels & I.C. Hallett [1]. Due to the fact that M. nivale causes pink snow rot of cereals usually in regions with a cold climate [72], the report of its presence on cotton in hot and arid conditions of Uzbekistan does not seem convincing.

Obviously, all of the above information indicates once more that the morphological identification of species of this genus is often unreliable and can lead to confusion. Therefore, in order to obtain reliable results in the differentiation of fungi, in particular plant pathogenic Fusarium species, along with traditional methods of morphological identification and determination of their pathogenicity, it is necessary to use molecular genetic methods [10, 73].

After the statement by A. Sheraliev [25] that Fusarium moniliforme is one of the causal agents of the cotton wilt, another publication claimed that this fungus allegedly causes wilting on almost all varieties of Gossypium hirsutum deployed in various zones, and this disease is widespread in 10 regions of Uzbekistan; the identification of F. moniliforme was based on its morphological characteristics [74]. However, considering the fact that the name Fusarium moniliforme has long become illegitimate [33], researchers [75, 76] have questioned the reliability of this message. Later, these authors [77] started to use a name F. verticillioides instead of F. moniliforme, and its teleomorph stage they called Gibberella fujikuroi (Sawada) Ito. In fact, the teleomorph of F. verticillioides is Gibberella moniliformis Wineland, and not G. fujikuroi (Sawada) Ito in Ito & K. Kimura [1]. In that paper, the authors also claimed that they conducted molecular studies, and “genes from NIR and rDNA were sequenced and turned out to be similar to the genes of Gibberella fujikuroi (Sawada) Ito, which is the sexual stage of Fusarium moniliforme Sheld.” [77]. The details of the study of these genes, the results of comparing their sequences with those of F. verticillioides in this work were not presented, and we also did not find them in the National Center for Biotechnology Information, USA (NCBI) GenBank database. For this reason, it is necessary to consider the statement that F. verticillioides is the causative agent of cotton wilt as unproven.

Further studies with the causal agents of Fusarium wilt and root rot of cotton were carried out by employees of the Center for Genomics and Bioinformatics of the Academy of Sciences of Uzbekistan (the CGB ASU) [52, 78, 79]. In their experiments, they used more than 45 isolates from the collection of the Institute of Genetics and Plant Experimental Biology of the Academy of Sciences of Uzbekistan (IGPEB ASU), 34 of which were isolated in 2002–2013 from symptomatic cotton plants in different regions of Uzbekistan and identified by morphological characteristics according to Booth [80]. The nucleotide sequences of the gene translation elongation factor 1α (TEF-1α), β-tubulin (TUBB), nitrate reductase (NIR), ribosomal DNA (rDNA) and phosphate permease (PHO) were determined in these isolates and compared with the information of the NCBI GenBank using the BLAST method. The studies also included 13 FOV isolates with known races, which they received from the USA [52, 78].

In these studies, of the 34 isolates studied, 14 were identified as F. oxysporum f. sp. vasinfectum (FOV), 3—as F. solani, 1 each—as F. oxysporum, F. oxysporum f. sp. phaseoli and F. verticillioides, 5—as F. proliferatum and 3—as F. fujikuroi (Table 4). Isolates 319 and 552 could not be accurately identified. The results of morphological and molecular identification coincided for eight FOV isolates (nos. 30, 34, 316, 328, 347, 489, 526 and 534), three isolates of F. solani (nos. 319.1, 519 and 520) and one isolate of F. verticillioides (no. 602). It was also found that FOV populations in Uzbekistan are represented by complex race 1, 2 & 6, race 3 and race 4 or 7 [52, 78, 79].

Isolate No. and region collected	Morphological Identification	Molecular Identification	Reference
30 Andijan	FOV	FOV	[79]
34 Kashkadarya	FOV	FOV	[79]
316 Tashkent	FOV	FOV, race 3	[52, 78]
328 Bukhara	FOV	FOV, race 7	[52]
		FOV, race 4 and 7	[78]
347 Syrdarya	FOV	FOV, race 3	[52, 79]
375 Tashkent	F. oxysporum	F. oxysporum	[52]
		FOV, race 1, 2 and 6	[78]
444 Tashkent	F. oxysporum	FOV	[52]
		Fusarium sp.	[78]
460, 491 Andijan	F. oxysporum	FOV	[52]
489 Bukhara	FOV	FOV, race 1	[52]
496 Kashkadarya	F. oxysporum	FOV, race 4	[52]
526 Bukhara	FOV	FOV, race 4	[52]
527 Tashkent	Fusarium sp.	FOV, race 3	[79]
534 Andijan	FOV	FOV	[52]
489 Bukhara	FOV / Fusarium sp.	F. oxysporum	[52]
139 Surkhandarya	F. oxysporum	Fusarium equiseti	[52]
315, 327 Unknown	F. oxysporum	Fusarium equiseti	[52]
498, 525 Kashkadarya	F. sambucinum	Fusarium equiseti	[52]
528 Tashkent	F. verticillioides	Fusarium equiseti	[52]
319 Namangan	F. oxysporum	F. solani	[52, 78]
		Fusarium sp.	[78]
319.1 Namangan	F. solani	F. solani	[79]
519, 520 Bukhara	F. solani	F. solani	[52]
490 Syrdarya	F. oxysporum / F. verticillioides	F. proliferatum	[52]
502 Tashkent	F. solani	F. proliferatum	[52]
509 Tashkent	F. solani	F. proliferatum	[52]
	F. oxysporum	F. oxysporum f. sp. phaseoli	[52]
518 Tashkent	F. oxysporum	F. proliferatum	[52]
524 Tashkent	F. oxysporum / F. verticillioides	F. proliferatum	[52]
552 Tashkent	Fusarium sp.	F. proliferatum or F. fujikuroi	[52]
487 Tashkent	F. verticillioides	F. fujikuroi	[52]
559 Kashkadarya	F. lateritium	F. fujikuroi	[52]
561 Tashkent	Fusarium sp.	F. fujikuroi	[52]
602 Bukhara	F. verticillioides	F. verticillioides	[52]

Table 4.

Identification of Fusarium isolates recovered from upland cotton and some other crops in Uzbekistan in 2013–2016.

It is puzzling here that at every mention of the F. verticillioides (isolate nos. 487, 490, 524, 528 and 602), the name Gibberella fujikuroi is mistakenly given as its teleomorph stage. In addition, without questioning the reliability and significance of the results obtained in these studies, it should also be noted that we could not find in the NCBI GenBank database at least some information about the species of the genus Fusarium identified by the authors using molecular method [52, 78, 79].

Pathogenicity of the individual representative isolates of Fusarium species was studied in laboratory, greenhouse and field experiments. The ability to cause wilting of cotton plants has been confirmed for a number of FOV isolates. F. solani caused root rot of cotton seedlings, but its separate isolates differed greatly in the degree of pathogenicity: some of them were avirulent or showed moderate pathogenicity, while others severely affected cotton plants, often killing them. Fusarium proliferatum and F. equiseti isolates did not show pathogenicity to cotton plants in tests, and the pathogenicity of F. fujikuroi and F. verticillioides isolates was not studied [52, 78].

2.3 Fusarium species reportedly registered in Uzbekistan as pathogens of other crops

According to the results of many years of research, A. Sheraliev [25] has reported the isolation and morphological identification using keys of Bilai [31] of 17 species and 10 varieties of Fusarium fungi from 57 species of agricultural crops and weeds. However, due to the fact that major changes have taken place in the taxonomy and nomenclature of the genus Fusarium in recent years, the status of many taxa identified by this researcher requires clarification. In addition, some of his statements raise doubts about their reliability; for example, it was stated that as many as 10 Fusarium species cause wilt, and as many as 14 Fusarium species cause root rot on mulberry trees (Morus spp.) [25, 81].

Studies conducted in Uzbekistan have shown that Fusarium species cause economically important diseases not only on cotton and wheat, but also on a number of other agricultural crops (Table 5). It was found that most of these diseases are caused by F. oxysporum or its formae speciales. Such diseases included wilts of melon and tomato, root and crown rots of pepper and some other crops. Below is a summary of these and other diseases registered in the country.

Isolate No. and region collected	Host	Morphological Identification	Molecular Identification and NCBI ID No.	Reference
– All regions	Misc. crops	Misc. species	—	[25]
	Melon	F. oxysporum f. sp. melonis	—	[82]
	Tomato	F. oxysporum f. sp. lycopersici	—	[83, 84]
	Misc. crops	F. oxysporum	—	[85]
	Potato, onion	F. solani	—	[85]
– Kashkadarya	Bell pepper	F. solani	—	[86]
– Tashkent	Bell pepper	F. solani	—	[86, 87, 88]
– Kashkadarya	Bell pepper	F. oxysporum	—	[86, 87, 88]
– Fergana	Chili pepper	F. oxysporum	—	[86]
– Tashkent	Chili pepper	F. oxysporum	—	[86]
– Surkhandarya	Rice	Fusarium sp.	—	[89]
– Tashkent	Radishes	F. oxysporum f. sp. raphani	—	[90]
28 Tashkent	Melon	F. oxysporum	F. equiseti	[52]
378 Jizzakh	Bindweed	F. oxysporum / F. longipes	F. equiseti	[52]
546 Tashkent	Tomato	F. oxysporum / F. verticillioides	F. proliferatum	[52]
547 Tashkent	Tomato	F. oxysporum / F. verticillioides	F. proliferatum	[52]
		F. oxysporum	F. oxysporum f. sp. phaseoli	[79]
560 Tashkent	Tomato	Fusarium sp.	F. fujikuroi	[52]
UZF-21 Andijan	Bell pepper	F. oxysporum	F. oxysporum OQ466308	IGPEB*
UZF-23 Andijan	Bell pepper	F. oxysporum	F. oxysporum OQ466309	IGPEB
R-22 Karakalpakstan	Cowpea	F. oxysporum	F. oxysporum OP716769	IGPEB
R-23 Tashkent	Soybean	F. acuminatum	F. acuminatum OP716770	IGPEB
R-24 Syrdarya	Soybean	F. acuminatum	F. acuminatum OP716771	IGPEB
R-25 Jizzakh	Chickpea	F. equiseti	F. equiseti OP716774	IGPEB
R-29 Tashkent	Soybean	F. culmorum	F. brachygibbosum OP716772	IGPEB
R-37 Tashkent	Chickpea	F. camptoceras	F. persicinum OP716773	IGPEB

Table 5.

Identification of Fusarium isolates recovered from some other crops in Uzbekistan.

*

IGPEB – group of scientists of the IGPEB ASU (unpublished).

Melon wilt caused by F. oxysporum f. sp. melonis Snyder & Hansen is widespread almost everywhere in Uzbekistan and is the main disease of this crop. The disease is severe and causes large crop losses, especially when there is a lack of soil moisture. Often in such cases, most of the plants of susceptible melon varieties are killed in the late season, usually 15–25 days before the full ripening of the fruit [82, 83].

It was reported that tomato wilt, caused by F. oxysporum f. sp. lycopersici (Sacc.) Snyder & Hansen, in Uzbekistan is among the dominant diseases of this crop both in the open fields and in greenhouses [83, 84]. However, there are no exact data on the incidence of the disease in different regions of the country.

Recently, a frequent occurrence in the Tashkent region of wilt disease on green and red globe radishes caused by F. oxysporum f. sp. raphani Kendrick & Snyder was reported in Akbarov and Khakimova [90].

It is also indicated that in Uzbekistan F. oxysporum (the form is not specified) causes wilt diseases on cucumbers, pumpkins and watermelons, and rotting of carrot fruits, and F. solani causes rotting of potato tubers and onion heads [85].

In 2010, in the paddies of “Sholi Invest” LLC in the Surkhandarya region, spots on panicles and spots and lesions on rice seeds were encountered. On the panicles just emerging from the sheathes of the flag leaf, 5–10% to 60–80% of the seeds already had symptoms of the disease. At the request of farmers, samples of the affected plant parts were collected, which were examined visually and subjected to mycological analysis. Fusarium sp. was consistently isolated from ∼60% and Curvularia lunata (Wakk.) Boed. from ∼15% of the samples. The symptoms of the disease on the panicles and seeds were typical of the disease called “pecky rice” [91]. It is caused by various fungi in different countries of the world, among which C. lunata [92] and several species of the genus Fusarium [93] are often found.

In recent years, cases of sudden withering and death of sweet and hot pepper plants (Capsicum annuum L.) have been registered in six districts of four regions of Uzbekistan. The number of dead plants in these areas ranged from 1 to 3 to 5% in the greenhouses, and from 5 to 21.1% in the open fields [86, 87, 88]. Examinations showed the presence of two groups of symptoms on plants. The signs of the disease (Figure 3) on the plants of the first group were very similar to those on the crown rot of pepper, found earlier and described as a new pathogen in Spain, named F. oxysporum f. sp. radicis-capsici Lomas-Cano et al. [94, 95].

Figure 3.

The stem rot of hot pepper plant (Shakira F₁ variety), caused by Fusarium oxysporum in a field of the Kibray district of Tashkent region: A general view of the affected, withered, and healthy plants (at center), and their lower parts, and roots, and the cross-sections through the areas of their crowns (courtesy of AA Khakimov).

This form of the disease was found on seedlings and young plants in all surveyed regions of Uzbekistan. From the symptomatic crown samples of plants collected in Uzbekistan, colonies of the same morphology were constantly isolated, representative isolates from which were identified as F. oxysporum [86, 87, 88].

On plants of the second group, the disease was represented by stem rot, which led to sudden withering and rapid death of abundantly fruiting plants (Figure 4). This disease was recorded in one field in Kashkadarya, and one in Tashkent regions, where the number of dead plants averaged 11.9 and 10.5%, respectively [87, 88]. The symptoms of the disease were similar to those of the stem rot of sweet pepper caused by F. solani in Canada [96], but we did not see perithecia of the pathogen on stems in Uzbekistan. The pathogen from symptomatic plants was isolated and tentatively identified by morphological features as F. solani [87, 88].

Figure 4.

The stem rot of hot pepper plants (Lastochka variety), a general view of the affected and withered plants in the field of the Yakkabag district of Kashkadarya region; arrows show infection sites (courtesy of BA Khasanov).

Currently, marker gene sequences are being analyzed with Uzbek isolates of F. oxysporum and F. solani from pepper plants to confirm the results of morphological identification [86, 87, 88].

Reportedly, in recent years, several Fusarium species (Table 5) have been isolated and identified from samples of diseased plants collected in Uzbekistan, by morphological and molecular characteristics, including F. equiseti from melon and bindweed, F. proliferatum and F. fujikuroi from tomato [52] and F. oxysporum f. sp. phaseoli Kendrick & Snyder from tomato plants [79]. Experiments to confirm the pathogenicity of these species to host plants have not been conducted.

Another group of researchers at the Institute of Genetics and Plant Experimental Biology of the Academy of Sciences of the Republic of Uzbekistan (IGPEB ASU) isolated and identified eight strains belonging to five species of the genus Fusarium from samples of root rot-infected crops collected in 2022 in five regions of Uzbekistan by morphological and molecular characters (analysis of DNA sequences of ITS region rDNA) (Table 5) (unpublished). F. oxysporum was isolated from samples of dead or dying seedlings taken from two greenhouses in the Andijan region producing sweet pepper seedlings (Figure 5). The same species was isolated from symptomatic young cowpea plants, samples of which were collected in Karakalpakstan. Isolates of F. acuminatum were isolated from samples of soybean plants affected by root rot in the Tashkent and Syrdarya regions, and another species, Fusarium brachygibbosum Padwick [FSAMSC], was also isolated from Tashkent samples. The latter is a polyphage and an emerging new root/crown pathogen of wheat, legumes, corn and other crops and trees; it has been registered in Algeria, Iraq and some other countries of the world ([97], and references therein). From the samples of symptomatic chickpea plants collected in the Jizzakh and Tashkent regions, F. equiseti and Fusarium persicinum J. W. Xia et al. (FIESC) were isolated, respectively.

Figure 5.

A focus of the seedling damping-off of the sweet pepper plants (Ferrari F₁-Sakata variety), caused by Fusarium oxysporum and Rhizoctonia solani complex in the greenhouse’s nursery of the Shakhrikhon district of the Andijan region (courtesy of DT Aznabakieva).

Fusarium acuminatum and F. equiseti in North America and elsewhere are included in the complex of fungi that cause root rot of legumes [53, 98]. The pathogenicity of Uzbek isolates of F. oxysporum to pepper and cowpea plants, F. equiseti to chickpeas, and F. acuminatum and F. brachygibbosum to soybean plants has been confirmed in experiments with artificial infection. There is no information about the pathogenicity of F. persicinum to plants. It is known to be a mycoparasite on other fungi, more often on basidiomycetes [99].

3. Conclusions

Diseases of crops caused by fungi from the genus Fusarium are common in Uzbekistan and are of great economic importance. Lists of species of this genus have been compiled that were reportedly registered in Uzbekistan on soft and/or durum wheat (26 species, including 22 valid names), Upland and/or Pima-type cotton (26 species or varieties, including 21 proper names) and other crops (> 11 species or varieties). Based on a critical analysis of these reports, the following conclusions can be drawn. The species recorded on wheat in Uzbekistan include well-known and highly aggressive pathogens Fusarium graminearum and F. culmorum, an emerging pathogen F. tricinctum, and less potent pathogens F. avenaceum and F. poae. The finding in Uzbekistan of F. pseudograminearum, the strong wheat crown rot pathogen, is considered not yet confirmed. Possibly, some strains of F. sporotrichioides, F. equiseti and Fusarium redolens could be weak pathogens of wheat, but confirmation of their pathogenicity in experiments with artificial infection of the host plant is required. Several other Fusarium species, namely F. buharicum, F. fujikuroi, F. incarnatum, F. lactis, F. oxysporum, F. sambucinum, F. solani, F. subglutinans, F. verticillioides, and, probably, F. ventricosum, reportedly isolated from wheat plants in the country, are obviously not wheat pathogens, but are secondary invaders.

Among the Fusarium species isolated by various researchers from cotton in Uzbekistan, the pathogenicity to the host plant has been confirmed for many strains of FOV (cause of wilt), and several isolates of F. solani (cause of damping-off and FRR). By studying the nucleotide sequences of the TEF-1α, TUBB and rDNA genes, it was found that the FOV populations in Uzbekistan are represented by complex race 1, 2 & 6, race 3 and race 4 or 7.

Reports that the causal agents of cotton wilt are, in addition to F. oxysporum, five more species, namely, F. equiseti, F. heterosporum, F. lateritium, F. solani and F. verticillioides (reported as F. moniliforme nom. Invalid.), have no justification. So, in the experiments with artificial infection, cotton isolates of F. equiseti and F. proliferatum did not show pathogenicity to this culture. The report that Fusarium verticillioides (the sexual stage of which is erroneously listed as Gibberella fujikuroi) causes cotton wilt in many regions of Uzbekistan is also not confirmed by convincing results of molecular studies. Also unproven are the claims that the same five species mentioned above, as well as F. buharicum, F. javanicum (incertae sedis), F. lactis and F. ventricosum, are among the pathogens of cotton root rot. Besides, the indication of Microdochium nivale (reported as Fusarium nivale) as a pathogen of cotton and other crops is also not credible.

In addition to wheat and cotton, Fusarium spp. in Uzbekistan cause serious diseases on other crops. Surveys conducted in recent years have shown that such diseases caused by F. oxysporum and other Fusarium species include wilt of melons and other cucurbits, tomatoes, and damping-off, root rot, crown rot, and stem rot of sweet and hot peppers, and other vegetables. An unidentified Fusarium species was the main cause of the “pecky rice,” a serious diseases of rice panicles and seeds in some fields in the Surkhandarya region of Uzbekistan. However, the number of surveys conducted in the country still was few, and the species identifications were based mainly on morphological characteristics of their colonies and conidia, and, in some cases, also on the pathogenicity to host plants. Undoubtedly, in further studies, the number of detected diseases on various crops caused by Fusarium species will grow significantly. This process will be particularly facilitated by the fact that in recent years, methods based on the DNA analysis started to be used to identify plant pathogenic fungi, especially Fusarium species, along with the classical methods.

Conflict of interest

The authors declare no conflict of interest. These authors contributed equally.

Abbreviations

BLAST	Basic Local Alignment Search Tool
CGB ASU	Centre for Genomics and Bioinformatics of the Academy of Sciences of the Republic of Uzbekistan
FCR/FRR/FHB	Fusarium crown rot/Fusarium root rot/Fusarium head blight
IGPEB ASU	Institute of Genetics and Plant Experimental Biology of the Academy of Sciences of the Republic of Uzbekistan
NCBI	National Center for Biotechnology Information, USA
PPS CRI	Plant Protection Station of the Cotton Research Institute, Uzbekistan
TSAU	Tashkent State Agrarian University, Uzbekistan