Effects of Fusarium Diseases on Forest Nursery and Its Controlling Mechanisms

Abstract

Fusarium spp. cause severe harm to forest plants. These fungi can induce latent infections that lead to planted seedlings rooting, as well as pre- and post-emergence damping. However, a phylogenetic study suggests that the most virulent strains of Fusarium oxysporum Schlechtend, which has been identified as the primary cause of root and root rot in nurseries, are more closely aligned with the recently identified Fusarium genus, Skovgaard, O’Donnell, and Nirenberg. Before planting, soil fumigation was the primary method of treating Fusarium illnesses in nurseries with bare roots. Alternative therapies are being investigated as rules impede the supply of the most effective fumigants. This entails improving sanitation, preserving a healthy microbial population that inhibits pathogens infecting trees through their roots, and refraining against actions that increase the risk of disease in trees, such as overfertilization and inadequate soil drainage. Although Fusarium circinatum, Nirenberg, and O’Donnell can be problematic in nurseries, they can harm ancient trees in native forests, plantations, seed farms, and landscape plantings. It is not advisable to transfer seeds or seedlings from contaminated to uninfected areas because they can spread the virus. To stop F. circinatum from spreading to nations where it has not yet been discovered, quarantine measures must be upheld. F. circinatum infections are linked to harm from weather-related events, insect activity, pruning, and seed harvesting, among other forest management practices. Pruning during the cold, dry season, when conditions are less conducive to infection, can help minimize the risk of illness in managed plantations and control insects that have the potential to be vectors and pests. Ecologically friendly biological strategies, such as using endophytic fungi and bacteria that are antagonistic to F. circinatum, plant essential oils, chitosan, or phosphite, have also been researched as ways to lessen the impact. Additionally, to reduce the number of contaminated seeds introduced into nurseries in disease-free areas, heat treatment is an easy and affordable way to eradicate the pathogen from contaminated seeds. Therefore, to address the problems of Fusarium spp,effects on nurseries, natural forests, and plantations using integrated approaches is required for sustainable managements of the forests.

1. Introduction

Most of the world’s surface area is covered by forests, which are essential ecosystems that play a major role in the global carbon cycle, supply valuable economic products, and provide ecosystem services. Approximately 112.95 million hectares of Europe’s forest area are covered by conifer trees, primarily Pinus species [1].

Pinus is a highly significant tree in terms of both ecology and economics. Furthermore, they are a major supplier of wood, pulp and paper, edible seeds, charcoal, resin, and many other biological activities in forest ecosystems [2]. However, free-market policies and commercial globalization have brought forest infections, particularly dangerous alien species, as invasives into any nation. This causes serious environmental and social impacts and economic losses globally [3, 4, 5].

In this regard, plant protection regulations often fail to prevent biological invasions [6, 7, 8]; and as a result, the number of invasive and damaging forest pathogens has increased dramatically recently [9].

To cope with this global challenge, new pest and disease management approaches are urgently needed to preserve the diverse services of forests under the increasing risks of globalization and climate change [10]. Fusarium species are among the ecologically harmful fungi that can infiltrate the soil and give older trees or young seedlings symptoms of neck and root rot. On the other hand, little is known about the diseases that afflict trees or seedlings in natural and semi-natural forests.

Fusarium species were found in the roots of oak (Quercus robur L.), white oak (Q. petraea (Matt.) Liebl.), and ash (Fraxinus Excelsior L.) in the former Czechoslovakia [11], Austria [12], and Poland [13]. Numerous ecological niches, including those for pathogens, endophytes, and saprotrophs, are occupied by the genus Fusarium in various climatic zones, particularly in tropical and subtropical areas [14]. This species is deemed dangerous primarily because it seriously harms forestry, which restricts seedling output globally [15], as seen with Fusarium circinatum Nirenberg in numerous coniferous O’pine species. Donnell can result in significant drops in the productivity and quality of fruits, vegetables, and other edible plant materials, particularly in South Africa [16, 17, 18, 19]. Fungal pathogen-caused plant illnesses are becoming more and more noticeable to consumers today [16, 17, 18, 20].

Fusarium species cause inhibition of germination, root rot, seedling wilt, stem rot, and seed rot in orchards. Many species of Fusarium, including F. oxysporum sensu lato., F. parish K. Skovg., F. equiseti (Corda) Sacc., F. chlamydosporum Wolverine., F. circinatum Nirenberg ja O’Donnell, F. proliferatum (Matsush.) Nirenberg., F. acuminatum Ellis kaj Everh. Vaca Reinking., F. moniliforme J. Shield, and F. tricinctum (Corda) Sacc. Young seedlings in nurseries and germination-prone seeds are susceptible to infection by Necosmospora solani (Mart.) L. Lombard and Crous (formerly known as Fusarium solani (Mart.) Sacc.). The oxysporum sensu lato species complex poses the greatest threat among Fusarium members [16, 17, 18, 21, 22, 23, 24].

It is unknown if Fusarium species are associated with the spontaneous regrowth of forest trees, despite the fact that members of this type of phytopathogenic fungus have been identified in forest litter and soil. For example, F. avenaceum and F. lateritium were discovered in oak and sycamore (Acer pseudoplatanus L.) in Great Britain [25], whereas F. proliferatum was discovered in the soil of pine forests (Pinus sylvestris L.) in Scotland [26] and Poland [27]. No representatives of the genus Fusarium are more frequently discovered in soil samples taken from open fields than in samples taken from regenerating and mature forests. Outside Europe, Fusarium spp. was found in the roots of naturally regenerating Douglas fir (Pseudotsuga menziesii (Mirb.) Franco) seedlings in Canada [28, 29] and in Japanese hardwood seedlings with declining symptoms [30]. Pathogenic Fusarium spp. were reported to be associated with tree roots and rhizomes in western Iran [31], whereas F. avenaceum and Freziera longipes Wollenw and Reinking were found as inhibitory pathogens in naturally regenerating Eucalyptus obliqua L’Hértier and E. radiata in Sieber ex DC. in Australia [32]. Recently, F. solani, F. oxysporum, F. verticillioides (Sacc.) Nirenberg, F. equiseti, F. fujikuroi Nirenberg, F. pseudocircinatum O’Donnell Ning Nirenberg kaj F. subglutinans (Wollenw. and Reinking) P.E. Nelson, Toussoun, and Marasas having the potential to cause leaf branch darkening were confirmed in pathogenicity tests [33].

Members of the genus Fusarium are commonly found in soil and forests in different parts of the world. For instance, populations of different Fusarium species have been reported in forests of Canada [34], Australia [35], Sri Lanka [36], India [37], Malaysia [38], Argentina [39], and Indonesia [40]. Fungal pathogens living in the litter layer can reduce the number of budding beech trees. According to a recent study, Neonectria and Ilyonectria species, which inhabit beech beds, are pathogenic and can harm F. sylvatica’s normal regeneration [41]. Numerous members of the genus Fusarium are more common, including unexplored species linked to the early stages of forest trees, according to a 2011 investigation of beech soil fungi in Central Europe [42]. Hence, this book chapter focuses more on Fusarium sp. effects on forest nurseries and its controlling mechanisms.

2. Causative agents of Fusarium fungi and others

Phytophthora, Pythium, Fusarium, and Rhizoctonia are the most frequent causes of plant growth inhibition [43]. However, Table 1 below includes fungi from a number of other genera, such as Colletotrichum, Alternaria, and Cylindrocladium. Commonly, Rhizoctonia spp. have been found to be significant contributors to damping-off in ornamental orchards [60, 61] and to be a source of disease in tree seedlings. It is not isolated more often in the US; it may be that its presence is masked by faster-growing fungi such as Fusarium spp [62]. Recent literature [63] lists the most common damping-off pathogens.

Pathogen	Host sp	County	Source
Fusarium spp.	Pinus nigra	Spain	[44]
etotrichum acutatum, Fusarium oxysporum	Cornus florida	USA: Georgi	[45]
Alternaria tenuis, Fusarium spp., Pythium spp., Rhizoctonia solani	Eucalyptus spp.	China	[46]
Rhizoctonia solani	Caragana arborescens	Canada	[47]
Cylindrocladium scoparium, Rhizoctonia solani	Eucalyptus spp.	Brazil	[48]
Rhizoctonia spp, Pythium spp	Picea smithiana	India	[49]
Rhizoctonia spp.	Pinus palustris	USA: Florida	[50]
Fusarium spp.	Pinus sylvestris	Finland	[51]
Fusarium spp., Pythium spp., Thanatephorus spp	Eucalyptus spp., Pinus caribaea, Acacia spp.	Zimbabwe	[52]
Phoma herbarum, Phomopsis occulta	Larix decidua	France	[53]
Fusarium spp., Phytophthora spp., Rhizopus spp.	Santalum album	India	[54]
Colletotrichum dematium	Fagus crenata	Japan	[55]
Fusarium spp.	Pseudotsuga	USA: Idaho	[56]
Phytophthora spp.	Fagus sylvatica	Poland	[57]
Cylindrocarpon destructans	Pinus sylvestris	Sweden	[58]
Cylindrocladium scoparium	Pinus resinosa	Canada	[41]
Fusarium spp., Pythium spp., Rhizoctonia spp	Acacia mangium	Phillipines	[59]
Fusarium spp., Alternaria spp., Pythium spp.	Pinus sylvestris, Larix silbirica	Russia	[48]

Table 1.

Damping-off is a cosmopolitan disease affecting plants from around the world.

3. Fusarium fungi disease control

Fusaria pathogens can be carried out by the soil and are extremely difficult to eliminate once they have taken over a field. Currently, fumigants such as chloropicrin, hot water, sunlight, or cultivars that are resistant to the pathogen are commonly employed to disinfect the soil and fight diseases caused by soil-borne pathogens.

Unfortunately, the results of these treatments are not as strong as anticipated, will harm the environment, and necessitate reducing their use.

Biological control refers to the use of microorganisms in the management of plant diseases. To combat tobacco Rhizoctonia, for instance, Trichoderma lignorum was registered as a fungicide in 1954 under Japan’s Agricultural Chemicals Regulation Act. This was the world’s first instance of biofungicides being documented. Many investigations were carried out to accomplish biocontrol of Fusarium illnesses throughout the last 3 to 4 decades, including the rice-causing Bakanae disease, which is brought on by soil-borne F. oxysporum and F. fujikuroi. As a biofungicide against F. oxysporum sp on sweet potatoes, a nonpathogenic strain of F. oxysporum was registered in 2002. However, Talaromyces flavus was registered in 2007 after Trichoderma atroviride was registered in 2003 as a biofungicide to control Bakanae rice through seed or nursery treatment. They are both available in the market currently. Furthermore, for plant health certification and effective disease management, soil cleaning, the use of disease-resistant cultivars, the application of fungicides, and the early detection of plant pathogens in the field are crucial. Nevertheless, there was no practical procedure for Fusarium spp. except for the isolation and inoculation test with test plants.

Seeds and seedlings should not be transferred from contaminated to uninfected areas because they can spread the disease. To stop F. circinatum from spreading to nations where it has not yet been discovered, quarantine measures must be upheld. F. circinatum infections are linked to harm from weather-related events, insect activity, pruning, and seed harvesting, among other forest management practices. Pruning should only take place during the cold, dry season when conditions are less conducive to infection. Additionally, insects that can serve as pests and vectors should be carefully controlled to lower the risk of diseases in managed plantations. Furthermore, heat treatment is an easy and affordable way to get rid of the pathogen from contaminated seeds to reduce the entry of the infected seeds into nurseries of disease-free areas.

4. Type of pest and method of propagation

The most common culprits are several genera of fungi (Fusarium, Rhizoctonia) and Oomycetes (Pythium, Phytophthora), but other fungi can also be involved (Table 1). If general controls are not effective, the presence of the pathogen must be confirmed by culturing it in an artificial medium [64]. However, the way of propagation is very different for each pest. Dispersal in contaminated soil or growing medium is typical for all species. Fusarium spp. spores are spread through contaminated seeds, soil, or growing medium and used containers. The role of seed dispersal is readily apparent in cotyledon blight. Although spores are airborne, they are primarily responsible for secondary spread. Thick-walled chlamydos spores help the fungus overwinter in plant litter, and sclerotia also can form [65]. Rhizoctonia sp. can be spread by seeds or airborne spores, but transmission is most common through contaminated soil, as the fungus overwinters in the soil as sclerotia [66]. Oocyst producers of Pythium spp. and Phytophthora spp. have unique zoospores that can swim in water and overwinter in soil or plant debris such as thick-walled oospores or chlamydospores. None of these pathogens produce airborne spores, although spores can be spread by water spray [67].

5. Critical control points for nursery damping

The best management method is to keep pathogens out of the nursery; however, this isnot always feasible, particularly in nurseries with bare roots where diseases can still survive in the soil. Because benches, containers, and other surfaces are easily sanitized, seedling preservation is made easier from the perspective of disease prevention [68], mostly diseases that inhibit seeds, such as Fusarium species and Rhizoctonia species. Table 1 also lists additional, less aggressive fungi that can be problematic for certain species. To remove this possible source of inoculum, seeds are rinsed under running water or sterilized with a diluted (1:10) solution of hydrogen peroxide or chlorine before being sown [69]. Because all suppressive pathogens are frequent soil dwellers, they can readily get caught in-between crops or develop into dormant spores that might linger for months or even years in plant debris. Therefore, sterilizing the soil before sowing would be a wise management practice for bare-root nurseries.

One of the main ways that Rhizoctonia spreads in bare-root at nurseries is through soil splashing, which can be reduced by plant cushions [70]. The majority of media used in container gardens, such as perlite and vermiculite, are practically sterile, and pathogen suppression is hindered by the low pH of sphagnum peat [71]. The most typical way that Pythium and Phytophthora spread is through water because of their motile zoospores. Reducing surface water contact and polluted runoff by storing containers on raised benches is advised to utilize a raised bed in bare-root nurseries to prevent water from standing around the seedlings and to select coarser-textured, well-drained soil for seed sowing [72].

6. The role of the environment

Because most suppressive organisms are opportunistic pathogens, using the right culture techniques can lessen or even eliminate illness. For more than a century, water logging can be avoided by simply maintaining low pH levels in the soil or growing medium. Another way to avoid wetness is to keep the soil or growing medium “moist but not wet” [73].

6.1 Impact of climate change on Fusarium circinatum and other plant pathogens

A virulent infection, a susceptible host plant, and ideal environmental circumstances work together to generate plant disease. Damage caused by diseases has a high correlation with environmental changes [74]. The intensity of the illness and the coevolution of pathogens and plants can be impacted by changes in temperature, precipitation, drought, and other factors [75, 76]. In addition to having a direct impact on pathogen biology, environmental variables have a significant impact on plant germination and reproduction. For foliar pathogens, these parameters include temperature, relative humidity, and leaf wetness [77, 78]. Besides these, temperature and relative humidity can affect pathogen overwintering in the absence of a host.

Because pathogens have evolved to withstand a wide range of temperatures and have evolved at high latitudes, it is anticipated that climate change will boost their fitness and raise the danger of disease outbreaks [79]. Reduced rainfall can lessen grape downing, but higher temperatures offset shorter leaf wetness durations because infections begin earlier in the growth season, giving epidemics more time to spread [80].

Pathogens such as Fusarium circinatum (pine canker), which is more common in Europe, are favored by a drop in the average minimum temperature accompanied by more frost [81].

Conversely, variables such as increased CO₂ concentrations, elevated temperatures, or dry spells impact host physiology, which modifies the way biotrophic infections colonize host tissues [82]. According to reports from central Italy, the invasive exotic species Heterobasidion irregular—which is more suited to spread in the Mediterranean climate than the local species Heterobasidion annosum—is directly impacted by dry circumstances [83].

Numerous diseases differ depending on the temperature and humidity. Because they can spread swiftly by wind and have shorter generations than plants, most diseases have an advantage over plants that allows them to move and adapt to changing climate conditions [84].

Changes in the flora of some fungus appear to be linked to climate change, which poses a threat to plant health and, in certain situations, may also increase the prevalence of pests and diseases. Infections with Phytophthora infestans have increased in Finland as a result of climate change [85]. In Canada and the United States, high temperatures have been linked to significant beetle outbreaks [20, 86, 87]. Models indicate that the number of canola diseases in Scotland will decline due to Pyrenopeziza brassicas and Leptosphaeria maculans [88]. Nonetheless, in the long run (2071–2100), increased average temperatures in northern Germany are anticipated to benefit canola pathogens such Alternaria brassicae, Sclerotinia sclerotiorum, and Verticillium longisporum [89]. Although there is currently little knowledge on how climate change may affect plant disease biological control, high temperatures may, in certain situations, make the introduction of biological control easier [90, 91]. Crops, fruits, and other edible plant materials can suffer significant yield and quality losses due to plant diseases. In contemporary times, fungal pathogen-caused plant diseases have garnered the interest of consumers as a significant global health concern [92].

7. Fusarium wilt, root rot, and disease

It has been documented that suppressing pathogenic fungi such as Fusarium oxysporum, R. Solanum, and Pythium aphanidermatum can lead to damping-off illnesses, root rot, and Fusarium wilt in a variety of crops, which can seriously reduce growth and yield [93].

Endophytic bacteria that colonize plant interiors have even been demonstrated in recent research to enhance plant growth and health [94]. For this reason, they seem to be great candidates as biocontrol agents against fungal phytopathogens. Three endophytic bacterial strains were assessed for their antifungal qualities: B.B. Simbolis H18 (Bacillales: Bacillaphy), Pseudomonas aeruginosa H40 (Pseudomonadales: Pseudomonadaceae), and Stenotrophomonas maltophila H8 (Xanthomonadales: Xanthomonadaceae). By treating plants with endophytic bacteria, they were able to lessen the severity of their diseases and boost seed production and seedling survival.

Furthermore, treated seedlings exhibit an apparent increase in the antioxidant enzymes peroxidase, PPO, and catalase (CAT). The capacity of bacterial endophytes to generate a broad range of antifungal chemicals, in addition to bioactive substances, which can cause systemic resistance in infected cotton seeds, has been linked to their antagonistic action against R. solani [95].

Maximum chitinase production was seen in B. thuringiensis GS1, an endophytic bacterium that was isolated from Pteridium aquilinum, a rough fern. It was also investigated if B. thuringiensis GS1 may make cucumber plants resistant to R. solani. R. solani wilt in cucumber plants may be prevented by B. thuringiensis GS1 induction of PRPs and defense-related enzymes [96], as well as the use of endophytic actinomycetes to biocontrol R. solani, which caused tomato wilt disease [97]. The endophytic P. fluorescent strain ENPF1 inhibited the growth of the stem blight pathogen Corynespora casiicola (Berk and Curt) Wei in vitro. P. fluorescents increased defense-related enzymes such as peroxidase, PPO, chitinase, and β-1,3-glucanase in Phyllanthus amarus after exposure to Corynespora cassiicola [98].

8. Biological control of emerging forest diseases

8.1 Biological control of nonnative pathogens

Severe disease outbreaks can result from novel pathogen-host interactions. In actuality, nonnative viruses may have detrimental effects on nearby nonnative tree species as well as native tree species. For instance, Fusarium circinatum on Pinus species (mostly P. radiata) in the Northern and Southern Hemispheres, P. pini on Pinus radiata in Chile, and Phytophthora ramorum on Japanese larch (Larix kaempferi) in the British Isles [98]. The native location of the virus is more likely to host coevolved hyperparasites.

It is true that a freshly distributed pathogen epidemic that intensifies can alter the afflicted ecology significantly. Here, polyphagous species coexisting in the same ecosystem might eventually switch to a different carbon source and modify their biology to take advantage of the newcomer [99]. However, attempts have been made to biologically control several of the most devastating nonnative pathogens using a bottom-up strategy; many potential biological control agents (BCAs), although promising in vitro, have almost completely failed in the field (e. g., Fusarium circinatum; failure in situ, and this may be climatic constraints, lack of an alternative host, development of resistance to BCA, and biotic disturbance by native organisms).

Reinforcement of top-down control forces through the recognition of native forest allies is particularly attractive in a foreign species pathogen × (native or alien) host. In relation to this, interspecific variation in host resistance is usually limited, and coevolution does not occur. The effects of symbionts on host defenses must be general enough to protect against pathogens that have not evolved, according to Halecker et al. [100] who identified an endophyte (Hypoxylon rubiginosum) living in European ash (Fraxinus excelsior) leaves that produces metabolites toxic to the nonsymbiotic pathogen Hymenoscyphus fraxineus. Thus, this endophyte may have the potential to evolve into an effective BCA. In the inoculation experiment, it was found that the presence of a local (Heterobasidion annosum) or a foreign root pathogen (H. irregulare) on Pinus pinea seedlings also affected the mycorrhizal density in the same way. However, the ectomycorrhizal symbiont (Tuber borchii) appeared to be able to discriminate between native and nonnative pathogens, possibly through host plant-mediated signaling [100].

9. Treatment of Fusarium diseases on conifers

Numerous Fusarium species are thought to be responsible for conifer diseases, some of which have detrimental effects on the environment and economy. Among these are fungi that result in poor growth in transplanted seedlings, stunting, and root rot in nurseries [101]. In addition to harming seedlings, Fusarium circinatum Nirenberg and O’Donnell are pathogens that affect adult trees, resulting in pitch canker disease. Along with stem and root rot brought on by Fusarium oxysporum and other Fusarium communities, tonal canker also affects planted pines. Fusarium torreyae T. by Aoki, J.A. Smith, L. Mountain, Geiser, and O’Donnell, a recently reported pathogen of Torreya taxifolia, are among the Fusarium species that have been linked to plant illnesses but have not yet been well explored [102].

Tonal canker, which is caused by F. circinatum, was originally discovered to be a disease in North Carolina, U.S.A. in 1945 [103]. The disease was given the moniker “pitch cancer” because the pathogen was found in malignant tumors that produced a lot of resin. Later, pitch canker made its way to the U.S. Southeast, where it still poses a threat to pine plantations, seeds, and seedlings.

10. Conclusions

The majority of forest diseases, particularly dangerous alien species such as Fusarium, are brought into nonnative areas through free market policies and worldwide commerce. This has detrimental effects on the environment, society, and the forest industries’ bottom line. Therefore, laws pertaining to plant protection sometimes fall short of preventing biological invasions, which has led to a sharp rise in the quantity of invasive and harmful forest pathogens in recent years. To cope with this worldwide challenge and protect the various services provided by forests from the growing threats of globalization and climate change, new methods of managing pests and diseases are desperately needed.

In various climatic zones of tropical and subtropical regions, the fungus Fusarium occupies multiple ecological niches, including those of pathogens, endophytes, and saprotrophs. In particular, Fusarium circinatum Nirenbergi in many coniferous species of the O’pine genus is seen to be a problem primarily because it severely harms globally by restricting the generation of seedlings. In addition, Fusarium circinatum Nirenbergi has the potential to significantly lower fruit, crop, and other edible plant material yields as well as quality.

Hence, to prevent this fungus from damaging nurseries, before sowing, the seeds are cleaned under running water or sterilized with a diluted (1:10) solution of chlorine or hydrogen peroxide that can eliminate this potential source of inoculums.

High latitudes are where pathogens evolve. Because of the pathogen’s ability to withstand a broad range of temperatures, it is anticipated that climate change may boost their fitness and raise the possibility of disease outbreaks, especially those involving Fusarium species. The severity of the disease and the coevolution of pathogens and plants can be influenced by variations in temperature, precipitation, drought, and other environmental factors. These changes are closely linked to disease-induced damage. Several species of Fusarium cause diseases of conifers, some of which have serious economic and ecological impacts. These include fungi that cause stunting and root rot in nurseries and poor growth of transplanted seedlings.

Furthermore, reducing the chance of importing contaminated seeds into nurseries in disease-free areas can be achieved easily and affordably by using thermotherapy to eradicate the virus from infected seeds. However, Fusarium spp. will remain an unsolved problem for the foreseeable future due to the absence of practical field intervention strategies and the challenges of preventing its spread to the forest because of asymptomatic nursery plants. Thus, in the near future, significant efforts will be required to address the integrated management of this disease through the application of eco-friendly technologies.