Management Strategies and Alternatives for Fungicidal Resistance in Potato

Abstract

Fungicides have been used for over 200 years to protect plants from damage by fungi, but today fungicidal resistance is very common among potato pathogens and it is difficult to control. The best and intensively studied example is metalaxyl resistance in Pytophthora infestans. Causes are many to get pathogen resistance against the fungicides like intensive use or misuse of it, repeated application of same fungicides, etc. Hence, it is today’s need to find out the different strategies like different cultural practices, use of bio-agents, use of green chemicals, elimination of disease source, etc. to manage this fungicidal resistance. There are also alternative ways like increasing host resistance, use of new molecules, etc. that can be adopted to reduce the risk of fungicidal resistance.

1. Introduction

Development of human civilization has been closely associated with the cultivation of crops, and plant diseases have been a concern to mankind probably since plants were cultivated more than 10,000 years ago [1]. The record of severe epidemics, which threatened crops, is reported in early Greek and Roman literature in 500 BC, and in the Roman world, Robigus, the God for cereals, was worshipped to prevent crop failure. Chemical plant protection schemes were developed at the beginning of the twentieth century with copper and sulphur as antifungal agents to control downy and powdery mildews. There was the multi-billion-dollar industry that has modern fungicides belonging to various chemical classes, differing in their mode of action against pathogens and characteristics of uptake and distribution within the plant.

Later, numerous cases of fungicide resistance have occurred worldwide because of release of fungicides with several target sites (so-called multi-side inhibitors) to the market, like organomercurials, regardless of their human toxicity and environmental pollution.

Along with that, systemic fungicides are also released which were taken up by the plant and are subsequently distributed within the entire plant, protecting also newly formed tissues. Thus, these fungicides were curative and allowed to control pathogens after infection had occurred. In the progress highly specific modern fungicides, which block only one target in the pathogen (monospecific fungicides or single-site inhibitors), were developed. Examples of single-site inhibitors are strobilurins, phenylamides and benzimidazoles, which were released to the market in the late 1970s and in the mid-1990s. Surprisingly, after 2 years, the apple scab fungus Venturia inaequalis and the polyphagous grey mould fungus Botrytis cinerea developed resistance against benzimidazoles. Also, in other single-site inhibitors such as the phenylamides and the strobilurins, resistant strains got developed within 2 years after the compounds, were introduced to the market and widely used.

2. Evolution of resistance

The life cycle of fungus is so small that evolution happens frequently. Its interaction with fungicide forces it to modify itself for its survival. Basically, fungicides disrupt metabolism and threaten the fungal survival, and as a result, pathogenic fungi can initiate mechanisms to resist lethal effects. Fungal genomes are very unstable and may contain thousands of polymorphisms [2]. Fungicides target a specific biochemical step, and a single point mutation causing one amino acid change can rapidly and effectively block fungicide binding within the target site (single-site inhibitors) and generally cause high levels of resistance. Fungicides of multisite inhibitors that target many biochemical steps require a combination of many mutations and so resistance evolves slowly.

The basic process of development of fungicidal resistance is depicted in Figure 1. The fungal spore populations are having the genetic potential to resist the disease and here the resistance can be developed initially (represented by the filled circles in the figures). When a newly formed fungicide is applied, maximum of the fungal spores are killed but very few get survived and that became the resistant spores. These spores are extremely low in numbers and that will be the start of the process. Along with the resistance spores, some sensitive spores also survived, because they ‘escaped’ the fungicide treatment and were not got exposed to the applied fungicides. The survived spores get developed and started the disease activity in favourable environmental conditions and produce a new crop of spores. This new crop of spores has a higher percentage of resistant spores because of its survival in the previous crop of spores.

Figure 1.

Process of fungicidal resistance development. (A) Population of spores before fungicide use. Most spores are sensitive (open circles), but sometimes a very low number are genetically resistant to the fungicide (filled circles). (B) After a fungicide application, the number of surviving spores is greatly reduced and only the resistant spores survived the treatment. Also, some sensitive spores (open circles) escaped the treatment. (C) If environmental conditions favour a new cycle of disease activity, the next generation of spores will have a higher percentage of resistant spores. Continued use of the fungicide selects for these resistant spores. (D) Multiplication of resistant spores in the next generation and spread.

The fungicide with the same mode of action is the core feature of fungicide resistance and hence the same specific resistance mechanism, show cross-resistance, but not resistance to other modes of action. This resistance may generate resistance between products with different modes of action, known as multidrug resistance (MDR), especially in laboratory assays.

Sometimes the interaction of fungicides of the same mode of action group (FRAC) may be different to a particular change in the target site, which differs in different resistance levels of fungi resulting in the evolution of resistance within pathogen populations [3]. For example, the interaction of Prothioconazole, with the haem component of the target-site sterol 14α-demethylase (CYP51), was differently from other azoles [4], showing lower resistance but still effective control of some cereal diseases. In Mycosphaerella graminicola, the cause of wheat leaf blotch, azole resistance was shown to different target-site mutations alone, or in combination, generate different cross-resistance patterns [5] and indeed improved the performance of prochloraz [6].

3. Genetics of fungicidal resistance

Development of fungicidal resistance in fungal plant pathogens is a challenge in modern crop protection. Because of the short life cycle, fungi are indeed very able to adapt to changing environmental conditions, like the introduction of a new fungicide in the agricultural practice. These changed environmental conditions forced the genetic material to change and several genetic mechanisms happen in fungus and influence the chance and time of its appearance and spreading in fungal populations.

Acquired resistance develops in fungi in their wild-type form are sensitive and may develop resistance after their exposure to new fungicides. This resistance is due to genetic modifications transmissible to the progeny so that a chemical, which was once effective against the organism, is no longer effective. Fortunately, till the late 1960s, fungicides used in crop protection were sulphur, copper derivatives, dithiocarbamates and these were multisite inhibitors, affecting multiple target sites and hence interfering with many metabolic processes of the pathogen. Afterwards, the single-site fungicides were introduced and as a consequence of their frequent and repeated use, fungicidal resistance has become a major concern in modern crop protection seriously threatening effectiveness of several fungicides [7]. Hence, Fungicidal resistance is a result of adaptation of a fungus to a fungicide due to a stable and inheritable genetic change, leading to the appearance and spread of mutants with reduced fungicide sensitivity [8].

3.1 Genetic bases of fungicide resistance

Borck and Braymer [9] have analysed the genetics of fungicide resistance and found four important factors responsible for the resistance, these are (1) involvement of number of loci, (2) the number of allelic variants at each locus, (3) the existence and relevance of dominant or recessive relationship between resistant and wild-type alleles and (4) the additive or synergistic interactions between resistance genes.

These resistance genes may be located either in the nucleus on chromosome or in the cytoplasm on extrachromosomal genetic determinants and that can be differentiated by their inheritance patterns. Chromosomal genes typically show disomic inheritance in sexual crosses, in which one allele from each parent is received by the zygote and in cytoplasmic extrachromosome a uniparental (usually maternal) transmission [10]. In cytoplasmic genes, resistance stability gets affected by vegetative segregation and intracellular selection [11, 12].

In pathogenic fungus, mutation occurs, as a result of fungicidal resistance, in single major genes or from additive or synergistic interactions between several mutant genes [13].

Mostly this resistance is the result of mutations in major genes and these genes conferring resistance to fungicides having different modes of action may also occur in the same isolate, causing multiple resistance. Laterally, these genes have an appreciable influence on the phenotype which results in qualitative change in response to a fungicide, with the appearance in the field of new fungicide-resistant sub-populations well distinguishable from the wild-type sensitive ones. In the case of oligogenic resistance, many different major genes are involved, any one of which can mutate to cause an increase in resistance to the same fungicide. Like, in Pyricularia oryzae the resistance against kasugamycin as well as resistance to the two fungicides ethirimol and triadimenol in Blumeria graminis f. sp. hordei may be controlled by three different loci where a resistance allele at any one locus confers resistance [14].

3.2 Ploidy level

Cellular ploidy is the number of complete sets of chromosomes in a cell. Many eukaryotic species have two (diploid) or more than two (polyploid) sets of chromosomes [15]. The difference in ploidy level affects the number of alleles at each locus, which constitutes an Fmajor genomic trait that results in the evolution of fungicidal resistance. This ploidy level directly affects the frequency of mutations which may arise in single individuals as a result of the different numbers of mutational targets [16]. Most of the plant pathogenic fungi are in haploid state for a major part of their life cycle. On the contrary, Oomycetes typically show a diploid life cycle, and the haploid phase is restricted to the gametes [17]. Moreover, polyploids have been frequently identified among Oomycetes, such as Plasmopara viticola and Phytophthora spp. [18, 19].

CAA (carboxylic acid amide) fungicides, which inhibit cellulose biosynthesis in Oomycete phytopathogens, are considered at low to medium resistance risk depending on the fungal species. Classic genetic analysis showed that resistance to all CAA fungicides co-segregates and has the same genetic basis [20, 21]. The intrinsic risk of resistance is estimated to be significantly higher than CAA due to their genetic differences; however, no cross-resistance exists between CAA and other fungicides currently available against Oomycetes, such as phenylamides and QoI fungicides. In phenylamides, the resistance is a monogenic trait conferred by a semi-dominant chromosomal gene [22, 23], while QoI resistance is due to mutations in the mitochondrial cytb gene [24].

The probability of resistance is significantly increased by the occurrence of gene recombination, although, to express it phenotypically and for making resistance fixed several sexual reproduction cycles are required.

3.3 Heterokaryosis and nuclear number

Heterokaryosis is the association of genetically distinct nuclei coexisting in a common hyphal compartment and is a process involved in the generation of fungal variation occurs frequently in some fungal taxa and is a potential source of genetic variation. Heterokaryosis permits changes in the proportions of different nuclei in Ascomycetes, for selection and is essential for parasexual recombination [25]. In Basidiomycetes, the stable dikaryotic state is established in which two distinct parental haploid nuclei coexist (heterothallic) without fusion in each cell, which is genetically equivalent to a diploid. They both, heterokaryons and dikaryons, provide the chances for genes to complement each other (genetic complementation). In heterokaryons, fungicidal resistant and fungicide-sensitive genes may be able to develop in the presence or absence of fungicides [25].

It is very rare that the resistant mutants gain competitiveness under the selection force of fungicide sprays and are selected to frequencies at which disease control becomes unsatisfactory [26]. Mutation for resistance occurs at different rates depending on the number of genes conferring resistance. A rapid shift towards resistance may occur in monogenic resistance, leading to a discrete resistant sub-population. In polygenic resistance, mutation for resistance occurs slowly, leading to a reduced sensitivity of the entire population. Two types of selection pressures are able to keep resistant and wild-type sub-populations in a dynamic equilibrium, (1) the disruptive selection (directional selection), which develops because of repeated sprays of fungicides having the same mode of action and favours resistant sub-population(s), and (2) stabilizing selection, is developed because of a negative pleiotropic effect of resistance mutations leading to reduced fitness and favours the wild-type sensitive populations.

4. Management strategies and alternatives

Potato is considered a poor man's food and carries many diseases. The healthy potato will have a direct impact on people's food security and increase the income in potato growing countries. Worldwide, efficient use of land, water and nutrients can be improved by achieving healthy potato tubers but practically it is not possible. To reduce the disease loss, lots of fungicides get spread on the crop, which creates the cause of resistance development. This is one of the reasons for fungicidal resistance in potato pathogens and it can be reduced with the adaptation of good management practices and strategies, which are discussed below.

To delay the development of fungicidal resistance is the primary goal of resistance management rather than managing resistant fungal strains and a management strategy should be implemented before resistance becomes a problem. In this way, resistance can be prevented from becoming economically important. Also, minimizing the use of at-risk fungicides helps to avoid the development of fungicidal resistance without sacrificing disease control. This can be accomplished by using the at-risk fungicide with other fungicides and with non-chemical control measures, such as an integrated disease management program and the use of disease-resistant varieties. Also, the use of resistant cultivars, growing the crop in pathogen-free areas, lengthening of crop rotation, disease forecasting tools, proper use of fungicides (repeated use of same fungicide, dose, time, place of fungicide), creating unfavourable environment helps to avoid the development of fungicidal resistance without sacrificing disease control. Elimination of disease source will be the alternative with which development of fungicidal resistance can be avoided. Anyhow, it is critical to use an effective disease management program to delay the build-up of resistant strains. The larger the pathogen population exposed to an at-risk fungicide, the greater the chance a resistant strain will develop. However, in the broad aspect the management strategies and alternatives for fungicidal resistance in potatoes can be exploiting host resistance; exploiting host resistance; exploitation of race-nonspecific resistance; enhancement of natural disease resistance in potatoes; biotechnology approach; biological approach; use of botanical and many more.

4.1 Exploiting host resistance

To avoid the development of fungicidal resistance the cultivar resistance could be exploited to reduce fungicide input while achieving an acceptable control of potato disease, especially in late blight, in both foliage and tubers. Nærstad et al. [27], in 2007, confirmed that the host resistance against the pathogen is responsible to avoid the development of fungicidal resistance (i) by spraying at the right time with the recommended dose in cultivars with low field resistance to blight; (ii) the fluazinam dose can be reduced to 80% of the recommended dose by exploiting medium foliar resistance at high disease pressure and to approximately 40% at low disease pressure, by applying fungicides at right time, when field resistance to tuber blight is high; (iii) exploiting a high level of foliar resistance carries a high risk when the level of field resistance to tuber blight is low because a light foliar infection can provide enough spores to cause a high frequency of infected tubers; (iv) The application intervals may also be extended at high levels of field resistance to blight.

4.2 Enhancement of natural disease resistance in potatoes by chemicals

It is possible to enhance the existing host resistance against potato pathogen by exogenous application of some chemicals like acetylsalicylic acid (ASA), acibenzolar-S-methyl (BTH), 2,6-dichloroisonicotinic acid (INA), DL-3-aminobutyric acid (BABA), etc. The expression of the pathogenesis-related (PR) gene was observed by the spray of INA to tomato [28, 29], ASA and BTH to tobacco [30], and benzothiazole to potato plants and results in disease resistance, but the level of resistance and the set of PR-proteins induced are highly plant-specific [28, 31].

BABA induces the accumulation of high levels of three PR-protein families, PR-1, PR-2 and PR-5 in potatoes, and protects against late blight caused by Pytophthora infestans [32]. BABA has also been reported in the partial protection of potato plants against P. infestans in field experiments [32]. The fosetyl aluminium (aluminium tri(ethyl hydrogen phosphonate)) is a systemic fungicide, which has acropetal and basipetal mobility and is active against Oomycetes. Its mode of action showed that it can act directly on the fungus and indirectly by activating disease resistance mechanisms, such as phytoalexin production in tomato, tobacco, capsicum and grapevine plants [33].

4.3 Bio-technological approach

This approach is one of the most promising approaches for avoiding the development of fungicidal resistance and getting disease-free potato tubers by making the host more compatible to fight with pathogens.

This approach develops the host resistance by understanding the knowledge of molecular biology and genetics of inter-action between plant and Oomycetes which helps in discovering many resistance genes, numerous effector proteins and their mode of action [34]. Mainly two approaches are there in biotechnology aspect i.e., Cis-genic and Trans-genic.

4.3.1 Cis-genic approach

In this, resistant genes naturally occurring in the plant itself or from other related species are used and it is mainly based on the availability of resistant genes in potato crop. This approach is ethically and socially more acceptable to the public [35].

The start and end product during this programme is potato varieties which consist of potato genes (resistant) only. In this programme, no new varieties were developed and only point is that in the old variety resistant genes of wild potato species were incorporated. This cis-genic modification approach with potato's own gene is societally acceptable and also results in simplification in the legislation on the use of cis-genic modification approach [34]. To develop the durable resistance in potato crop, the DuRPh (Durable Resistance against Pytophthora infestans) programme was made in which cloning, transformation and selection of desired resistance were involved. Surprisingly, no markers are used in this approach, so the variety obtained will be made free, and to confirm the presence of resistance gene for P. infestans, PCR (polymerase chain reaction) technique is used.

4.3.2 Transgenic approach

This includes detection, isolation, cloning and transformation of gene from wild species or any other species into existing varieties through a bacterial vector (Agrobacterium tumefaciens). The mutate plantlets regenerated through callus culture and are screened to assess for resistance. Importantly, the mutant should have the same phenotype as the wild variety into which resistance genes are introduced. In this, there are two sub approaches, genetic engineering and RNAi technology. Plant genetic engineering is the act of inserting one or more agriculturally important genes into the genome of a plant by in vitro techniques. The genes inserted by genetic engineering are called transgenes that may (partly) originate from other organisms (such as bacteria or fungi) or non-crossable plant species. The first transgenic potato was developed about 20 years ago, and many of the transgenic potato plant products with enhanced characteristics are to be commercialized in the present decade [36]. Another important class of transgenes is based on RNAi for silencing existing traits coding for starch composition, processing traits or other quality traits.

Transgenics for late blight resistance: The disease caused by the Oomycetous fungus, has a history of causing catastrophic famine in Ireland where people depended heavily on this crop. In recent years, India and China emerged as the global leaders in potato production together contributing about 27% of world production. Occurrence of both A1 and A2 mating types of P. infestans resulting in sexual reproduction and survival through resilient oospores have been reported that may give rise to immense variability in the pathogen population, thereby endangering durability of a cultivar. Moreover, this population is gradually becoming tolerant to higher doses of prophylactic fungicides. As a consequence of this, a hidden but serious population shift in P. infestans has succumbed to this disease, in Kufri Jyoti, the most popular Indian cultivar, after a sustained performance for about 30 years. The other popular cultivar Kufri Bahar does not have any resistance to P. infestans. Together, these two cultivars occupy >60% of the potato area in India creating an imminent danger under our nose.

Race-specific, major genes from the wild potato species Solanum demissum have been extensively used in resistance breeding programmes throughout the world including India. However, the efficacy of such major genes had been too short-lived to justify their deployment. Because of this stress, late blight breeding has now moved to deployment of multi-gene, horizontal resistance. Although, identifying the genes responsible for horizontal resistance and their pyramiding is a difficult task. Recently, a new gene has emerged, i.e., the RB gene, which behaves like non-host resistance and is effective against all known races of P. infestans. This gene has been mapped and cloned by two independent groups in the USA and The Netherlands. The potato cultivar Katahdin, Transgenic clones of RB gene, showed late blight resistance at Toluca valley, the centre of origin of P. inefstans. The Agricultural Biotechnology Support Project-II operating from The Cornell University, USA has initiated a programme to popularize the use of RB gene in South and South-East Asia.

Bacterial wilt resistance: Bacterial wilt is another chronic disease problem that does not have any reliable resistance source. Therefore, an antimicrobial peptide gene, bovine enteric beta defensin (EBD) is being used for conferring bacterial wilt resistance in potatoes. Transgenic lines of Kufri Badshah showed a very high level of resistance to bacterial wilt in glass house screening. In India, the gene has now been transferred to two commercial potato cultivars Kufri Giriraj and Kufri Jyoti. Kufri Giriraj was selected because of its popularity in Shimla and Nilgiri hills where bacterial wilt is prevalent. Kufri Jyoti is a popular variety in eastern plains where bacterial wilt is endemic. Twenty-seven putative transgenic lines of Kufri Giriraj and 12 lines of Kufri Jyoti have been developed that are being characterized at present.

Viruses are also important pathogens which are ubiquitous and cause 80% losses in potato yield. Potato has been infected by more than 40 viruses and 2 viroids [37]. So far, only 9 viruses and 2 viroids are of economic significance for the growing potato industry. These potato viruses include potato viruses A, M, S, V, X, and Y (PVA, PVM, PVS, PVV, PVX, and PVY), potato leafroll virus (PLRV), tobacco rattle virus (TRV) and potato mop-top virus (PMTV). PLRV and PVY are currently considered the most dangerous viruses [38]. There are numerous variable factors i.e., plant genetic diversity, biology, lifecycle of the host plant/pathogen, vector species, biotype and environmental conditions that affect the incidence and severity of viral diseases. Potato is clonally propagated by planting tuber, which enhances the risk of accumulation of viruses in the next crop and tuber generations. Viral infection on potato (either individual or mixed infection) results in varied tuber infections i.e., spraying (TRV); necrotic ringspots (PVY NTN), net necrosis (PLRV) and deformed tubers (potato spindle tuber viroid) that render the tubers unsaleable [39]. PLRV is among the most prevalent viral diseases of potato in India, which almost causes 50–80% loss in potato yield and produces only a few, small to medium tubers [40]. Mineral oil and pesticide spray (chemical spray) are partial protection techniques that are not effective and efficient means to control viral diseases. The generation of resistant cultivars is considered the most economic and environmentally acceptable way of controlling viral diseases in potatoes [38]. Transgenic development by using pathogen-derived resistance is, therefore, being pursued for their management. The molecular technique involves two sets: (i) cellular technique which involves transformation and regeneration and (ii) includes identification, isolation and specific genes coding for interesting traits.

Coat protein mediated virus resistance: In 1986, Abel et al. reported the first example of resistance derived from coat protein. Transgenic potatoes expressing the PVY CP gene were found to be highly resistant to PVY and PLRV [41]. The PVYo strain was collected from field infected samples. The CP gene has been amplified by RT-PCR, cloned and sequenced. Sense, antisense and hairpin constructs have been designed and cloned in pDrive. The constructs were sub-cloned in the binary vector pBinAR. The vectors were then mobilized into A. tumefaciens strain EHA 105. Co-cultivation was done with sense construct and 11 putative transgenic lines of Kufri Bahar regenerated so far. Further screening of these 2 lines for PLRV resistance is to be undertaken in the glass house at CPRI, Shimla.

Generation of virus resistance through RNA silencing: Resistance to PVY in potatoes was done by induction of RNA silencing an ectopically expressed dsRNA, conserved between different PVY strains. PVY strains express Hc-Pro suppressor protein that interferes with the plant host defence. The expression of virus-derived dsRNA from transgenes can fully suppress viral infection through RNA silencing, thus overcoming viral suppressors [42, 43, 44]. Coat protein of PVY was cloned in pT3T7 vector followed by subcloning in binary vector pART7/27 and transferred into A. tumefaciens strain LBA4404. Transgenic lines generated were found resistant to viral infection as confirmed by ELISA measurements, northern hybridizations and RT-PCR. The transgenic lines generated have not been yet tested under field conditions, which would be necessary for further use of these lines [45].

Engineering virus resistance using a modified potato gene: Virus resistance genes have recessively inherited that function in a passive manner, whereby host factors evolve to avoid an interaction, which is essential for an invading virus to complete its lifecycle [46]. Using this theory, a study was carried out by [47] where natural mutations in translation initiation factor eIF4E confer resistance to potyviruses in potato plants. elf4E from potato cultivar Russet Burbank strain ‘Ida’ was cloned in TOPO cloning vector and further subcloned in a plant cloning vector pBI121 further, Agrobacterium-mediated transformation was performed on potato stem internode segments. All control plants (wild-type ‘Russet Burbank’ and transgenic lines over-expressing the1+ or GUS genes) developed typical PVY symptoms and tested positive for DAS-ELISA. To determine eIF4E expression, Northern blot and cDNA sequencing analysis were conducted on transgenic and non-transgenic plants.

Movement protein-mediated resistance: Resistance to virus movement in plants reduces the initial infection site [48]. Potato transformed with sequence from PLRV open reading frame (ORF) 4, encode a protein (pr17) i.e., phloem specific movement protein [49]. The mutant pr17 binds to the plasmodesmata and inhibiting cell to cell movement of unrelated viruses. Transgenic plants showed reduced accumulation of PLRV on secondary infection (operating at RNA level) and were also found resistant to PVY and PVX virus infection (protein-mediated resistance).

Fungicide Resistance Action Committee (FRAC) were developed to facilitate managing resistance by categorising fungicidal group and coding them, which designate chemical group of that fungicide and mentioned on the front of label. The fungicides with the same mode of action have categorised into one group. For managing resistance, it is critically important to know the group code for the fungicides being used to avoid alternating among chemically similar fungicides. Currently, there are 48 numbered FRAC Group Codes plus NC (not classified), 7 numbered with a ‘P’ (for host plant defence), 7 numbered with a ‘U’ (for unknown mode of action), and 12 numbered with an ‘M’ (for multi-site contact activity).

The codes for all fungicide active ingredients are based on a common name that can be downloaded from the FRAC website and can be used to spray the effective fungicides against a particular disease.

4.4 Use of bio-agents

In biological control, living micro-organisms provide disease protection through the production of antibiotics, competition for food and space, induced plant resistance, etc. This helps to avoid the use of fungicides which directly reduce the chances of development of fungicidal resistance. Various fungi and bacteria were tested against P. infestans in potato crop [50] and results in suppressed blight infection in leaflets [51]. Daayf et al. [52] also studied on biological control of potato late blight by detached leave method, whole planting testing system and in vitro.

Trichoderma (formulation) @ 10 g/l and Pseudomonas (formulation) @ 10 g/l found antagonistic behaviour and best results against late blight disease [53]. Bio-agent Xenorhabdus spp. gave most consistent results of biological control against late blight disease. Application of Steinernema feltiae was also studied against late blight both in vivo and in vitro [54].

In search of antagonistic against P. infestans, lot of work has been done and Burkholderia spp., Streptomyces spp., Pseudomonas spp., and Trichoderma spp. were obtained from leaves, stems, tubers and rhizoplane of potato plants were tested. The efficacy of these bio-agents to A1 and A2 mating type of P. infestans was assessed in greenhouse; field and on potato leaves in moist chamber and all three found to reduce the P. infestans infection applied individually or in combination [55].

The bio-control agents in combination with products such as neem oil could be effective to manage late blight severity [56] and it could be another option to reduce crop losses caused by the pathogen. Among the seven potato phylloplane fungi, only three fungi viz., Fusarium spp., Trichoderma spp., Aspergillus spp. showed antagonistic potential against P. infestans [53].

Systemic acquired resistance: Induction of SA (salicylic acid) is elicited by both bacilli and pseudomonad PGPR strains but ethylene and jasmonic acid dependent [57].

Bio-fungicide: Chaetomium mycofungicide found to reduce incidence of late blight and reduce its population in the soil with significant reduction the potato late blight [58]. A significantly reduction in P. infestans sporangial germination was observed with the spray of T. viride and P. viridicatum formulation and has potential to control potato late blight under control condition [59].

Rhizobacteria: Kim and Jeun [60] reported the drenching with plant growth promoting rhizobacteria isolates increased the total weight of tubers per potato plants, in addition to effectively controlling late blight. Yang et al. [61] also reported the Bacillus pumilus and Pseudo-monas fluorescens induced resistance to P. infestans and there was reduction in zoospore formation and germination.

4.5 Green chemicals

Biological origin pesticides, especially extract and natural substances originating from plants, microorganisms, algae and animals, are called green pesticides, like botanicals, essential oil, etc., also called ecological pesticides, which are considered environmentally friendly and are causing less harm to human and animal health and to habitats and the ecosystem are gaining a lot of interest for the integrated management of fungal diseases. Botanicals are one of them which is a substance obtained or derived from a plant such as a plant part or extract used for many purposes and can be used against the pathogens. Green plants are a huge reservoir of various effective chemotherapeutics and could serve as an environmentally friendly natural alternative to fungicides and directly the risk of fungicidal resistance development can be avoided.

Eleven extracts from different plant species were tested for antibacterial activity against potato soft rot bacteria, E. carotovora subsp. carotovora (Ecc) P-138, under in vitro and storage conditions and found effective. These are bael (Aegle marmelos L.), mander (Erythrina variegata), chatim (Alstonia scholaris L.), marigold (Tagetes erecta), garlic (Allium sativum L.), onion (Allium cepa), lime (Citrus aurantifolia), turmeric (Curcuma longa L.), jute (Corchorus capsularis L.), cheerota (Swertia chirata Ham.) and neem (Azadirachta indica) [62]. The work on botanicals with anti-oomycetes activity has increased over the years and the efficacy of botanicals against pathogens has also been demonstrated. Several preliminary in vitro studies have been conducted in China and India [63, 64]. Few plant extracts from different plant materials were tested for controlling effects against the infection of P. infestans on potato tuber slices, seedlings and detached leaves and Galla chinensis showed the best inhibiting effect among Terminalia chebula, Sophora flavescens, G. chinensis, Rheum rhabarbarum, Potentilla erecta and Salvia officinalis [65]. Cao and Van Bruggen, (2001) observed the treatments of garlic cloves extract at 1 or 2 percent completely inhibit the zoospore formation and colony growth of pathogen [63].

Essential oils are used to manage potato diseases which are obtained from plants through fermentation, enfleurage, extraction and steam distillation. Essential oils are used because of two prominent features, i.e., low toxicity for people and the environment due to their natural properties and low risk for resistance development by pathogenic micro-organisms [66]. The antifungal activity of essential oil obtained from three medicinal plants, i.e., Zataria multiflora, Thymus vulgaris and Thymus kotschyanus against phytopathogenic fungi were reported [67].

5. Conclusion

Today, food security is dependent on crop protection and fungicide-based plant protection is indispensable for efficient and large-scale crop production. As it takes a long time to develop new fungicides, it is necessary to take proper care to avoid the development of fungicidal resistance. In single-target fungicidal resistance, development may occur within a few years and fungicides with novel modes of action are found rarely. The earlier strategy was to apply single-target fungicides, which impose a strong selection pressure for the development of resistant mutants. The fungicides with more than one target are not easily overcome by mutations. Hence, there is a need to explore and enhance the existing host resistance against the potato pathogen. Among all, transgenic approach is the most promising to avoid the risk of development of fungicidal resistance; use of alternatives like green chemicals, bio-agents and explore and enhance the host resistance.