Alternaria pathogenicity and its strategic controls

Abstract

The Deuteromycetes fungal genus Alternaria comprises of different saprophytic as well as endophytic species and is well known for its notoriously destructive plant pathogen members. It has been found to have a drastic effect on the members belonging to the plant families such as Cucurbitaceae, Brassicaceae, Solanaceae which are having nutritional as well as economical food value. Majority of the members of Alternaria lack sexuality altogether, although few species have been found to have sexual stage in their life cycles. Several types of genes ranging from protein encoding genes to those involved in signal transduction cascades are found to be responsible for the pathogenesis. Production of host-specific toxins (HSTs) is found to be an affirming factor of pathogenesis. Most fungal host-specific toxins are metabolites although toxic substances including despipeptides and fucicoccin-like compounds. Genes encoding the biosynthesis of these HSTs are often contained on mostly conditionally dispensable chromosomes. The necrotrophic nature of Alternaria species typically leads to extensive damage of the plant and harvest product, with seedlings seldom surviving an attack. Apart from the role of toxins in Alternaria pathogenesis, few genes and /or gene products have been found to have a propounding effect as a pre-requisite for pathogenicity. For controlling the diseases, numbers of new chemicals are evaluated along with various biological control agents including bacteria, actinomycetes and fungi. Some plants and plant products are also found to be useful in controlling Alternaria infection.

Keywords

Alternaria, crop pathology, disease management, pathogenicity factors, toxins

Introduction

Mustard (Brassica juncea) forms an important part of the total oilseed production in India. As far as the statistical figures are concerned, out of 75.55 million tonnes of estimated rapeseed (Brassica napus) and mustard production over 30.51 m ha across the Globe, India produces 7.36 m tonnes from 6.18 m ha with 1190 kg/ha productivity (GOI, 2009; Meena et al., 2010). As far as the Indian perspective of the disease is concerned, the losses caused by the disease is estimated to be 47% of the yield loss (Kolte, 1985) with no established source of transferable resistance in any of the hosts. Average yield losses in the range of 32-57% due to Alternaria blight have been reported by several workers (Conn and Tewari, 1990).Therefore, studies on the effective control of diseases caused by Alternaria is of utmost importance.

The focused pathovars belonging to the genus Alternaria affect most Cruciferous crops, including broccoli and cauliflower (Brassica oleracea L. var. botrytis L.), field mustard and turnip (B. rapa L. (synonym: B. campestris L.), Chinese mustard or leaf mustard (B. juncea), Chinese or celery cabbage (B. pekinensis), cabbage (B. oleracea var. capitata), rape (B. campestris) and radish (Raphanus sativus).

The genus Alternaria belongs to the Phylum: Ascomycota, Subdivision: Pezizomycotina, Class: Dothediomycetes, Order: Pleosporales and Family: Pleosporaceae. Alternaria belongs to the division Deuteromycota with several species. Its multicellular pigmented spores are produced in chains or in branching fashions. The spores are broadest near the base and taper gradually to an elongate beak. When Alternaria attacks the host leaf, morphologically it produces a series of concentric rings around the initial site of attack. This gives a "target spot" effect that is associated with early blight. Species of the genus are cosmopolitan and can survive as saprophytes as well as weak parasites. The genus is characterized by the formation of polymorphous conidia either singly or in short or longer chains and provided with cross, longitudinal as well as oblique septa and having longer or short beaks. The spores of these polyphagus fungi occur commonly in the atmosphere and also in the soil. The telomorphs (sexual stage) are known in a very few species and placed in the genus Pleospora of Loculoascomycetes (under Sub-division: Ascomycotina), in which sleeper-shaped, muriform ascospores are produced in bitunicate asci (Verma and Verma, 2010). A great number of species were recorded for the genus Alternaria infecting different crops causing world-wide economic loss (Kirk, 2008). Works pertaining to the collection, isolation and characterization of other Alternaria species are also being carried out for its elaborate studies. Alternaria alternate caused early blight of potato, leaf spot disease in Withania somnifera (Pati et al., 2008) and can infest many other plants. It also causes upper respiratory infections in AIDS patients, asthma in people with sensitivity and has been implicated in chronic rhinosinusitis. Alternaria arborescens (causes stem canker of tomato), Alternaria arbusti (causes leaf lesions on Asian pear), Alternaria blumeae (causes lesions on Blumea aurita), Alternaria brassicae (infests many vegetables and roses), Alternaria brassicicola (grows on cole crops), Alternaria brunsii (causing cumin bloosem blight), Alternaria carotiincultae (causes leaf blight on carrot), Alternaria carthami, Alternaria cinerariae, Alternaria citri, Alternaria conjuncta (grows on parsnip), Alternaria dauci (grows on carrot), Alternaria dianthi, Alternaria dianthicola, Alternaria euphorbiicola (infests cole crops), Alternaria gaisen (causes ringspot disease of pears), Alternaria helianthicola, Alternaria hungarica, Alternaria infectoria (infests wheat), Alternaria japonica (infests cole crops), Alternaria limicola (earliest diverging lineage of Section Porri), Alternaria linicola, Alternaria longipes (infects tobacco), Alternaria molesta (may cause skin lesions on porpoises), Alternaria panax (causes ginseng blight), Alternaria petroselini (causes parsley leaf blight), Alternaria radicina (causes carrot decay), Alternaria raphani, Alternaria saponariae, Alternaria selini (causes parsley crown decay), Alternaria senecionis, Alternaria solani (causes early blight in potatoes and tomatoes), Alternaria smyrnii (infests alexanders and parsleys), Alternaria tenuissima, Alternaria triticina, Alternaria zinniae

Disease symptoms caused by Alternaria

Among the different diseases caused by the genus Alternaria, blight disease is one of the most dominant one that causes average yield loss in the range of 32-57% (Conn and Tewari, 1990). Symptoms of this disease include presence of irregular, often circular brown to dark brown colour leaf spots on the leaves with concentric lines inside the spots. Often the circular spots coalesce to form large patches resulting in the leaf blight. In several cases, small dark coloured spots are also formed on pods and tender twigs (Valkonen and Koponen, 1990). For Alternaria blight management, early sowing (Meena et al., 2002) of properly stored clean certified seeds after deep ploughing along with clean cultivation, timely weeding and maintenance of optimum plant population , avoidance of irrigation at flowering and pod formation stages are some of the steps to be followed for an efficient management of the disease.

Biology of Alternaria

Conidiophores of majority of the species of Alternaria produce asexual spores (conidia) measuring between 160- 200 μm long. Under in vitro conditions, sporulation occurs at a temperature range of 8-24 ºC, where mature spores occur after 14-24 h. Optimum temperatures are between 16 and 24 ºC where sporulation time ranges from 12 to 14 h. Moisture in the presence of rain, dew or high humidity are essential for infection and a minimum of 9-18 h are required for majority of the species (Humperson-Jones and Phelps, 1989). Continuous moisture of 24 h or longer practically guarantees infection (Rangel, 1945; Chupp and Sherf, 1960). Relative humidity of 91.5% (at 20 ºC) or higher will result in the production of large numbers of mature spores in 24 h (Humpherson-Jones and Phelps, 1989).

Epidemiology

The major sources of transport of these pathogens are the infected seeds with spores on the seed coat or the presence of mycelium under the seed coat. The dissemination of spores occurs by wind, water, tools and animals. The fungus can survive in susceptible weeds or perennial crops (Rangel, 1945; Chupp and Sherf, 1960; Maude and Humpherson-Jones, 1980a, b). Presence of infected crops left on the ground after harvest also serves as a source of infection for majority of the Alternaria species. In one study, infected leaves of oilseed rape and cabbage placed outdoors on soil produced viable spores for as long as leaf tissues remained intact. For oilseed rape, this was up to 8 weeks and for cabbage up to 12 weeks (Humpherson-Jones, 1989). This type of spread is likely to occur in seedling beds as well, and seedlings from infected seed beds can carry the inoculum to the field (Rangel, 1945).

Alternaria toxins

A number of plant pathogenic fungi including Alternaria produce toxins that can damage plant tissues. Toxins are often classified as host selective (host specific) or nonspecific. Host-selective toxins (HSTs) are toxic only to host plants of the fungus that produces the toxin. In contrast, nonspecific toxins can affect many plants regardless of whether they are a host or non-host of the producing pathogen. Yoder (1980) classified toxins of plant pathogens as a pathogenicity factor or a virulence factor by considering the possible involvement of toxins in pathogenesis: pathogenicity is the ability to cause disease (a qualitative term), whereas virulence refers to the extent or severity of the disease caused (a quantitative term). Most HSTs are considered to be pathogenicity factors, which the fungi producing them require to invade tissue and induce disease (Wolpert et al., 2002; Howlett, 2006). All isolates of the pathogen that produce an HST are pathogenic to the specific host; all isolates that fail to produce HSTs lose pathogenicity to the host plants. Plants that are susceptible to the pathogen are sensitive to the toxin. Such correlations between HST production and pathogenicity in the pathogens and between toxin sensitivity and disease susceptibility in plants provide persuasive evidence that HSTs can be responsible for host-selective infection and disease development. On the other hand, the exact roles of nonspecific toxins in pathogenesis are largely unknown, but some are thought to contribute to features of virulence, such as symptom development and in plant-pathogen propagation.

The Alternaria HSTs involve a diverse group of low-molecular-weight substances, and most were found in culture filtrates as families of closely related compounds. The Alternaria HSTs cause necrosis on leaves of susceptible cultivars at concentrations as low as 10-8 to 10-9 M and no necrosis on leaves of resistant cultivars even at higher concentrations (Otani et al., 1995). Several different types of genes have been found to be responsible for the pathogenicity of the fungus. Genes encoding for different physiological parameters such as cell wall degrading enzymes, toxins and transporter proteins involved in signal transduction cascades such as mitogen activated proteins (MAP) kinases are some of the different types of genes responsible for the pathogenicity. The toxins produced by the different pathotypes of Alternaria are mainly low molecular weight secondary metabolites. Some of the types of toxins are reported to have a sphingolipid like molecular structure (Wang et al., 1996; Gilchrist, 1997). Other types of toxins include some desipeptide- based molecules (Johnson et al., 2000). Most fungal toxins are metabolites but in some cases a toxic peptide has been found to be a major virulence factor such as in the case of wheat pathogen Pyrenophora triticirepentis (Ballance et al., 1989; Tomas et al., 1990; Tuori et al., 1995). Likewise, proteinaceous toxin (AB- toxin) is produced by A. brassicola and is produced only on host plants (Otani et al., 1998).

Alternaria species also produce types of toxins that are non-host specific. In addition to AB- toxin, other toxic substances including desipeptides and fucicoccin- like compounds are also being produced by different pathotypes of Alternaria (McKenzie et al., 1988; Cooke et al., 1997; MacKinnon et al., 1999). Although different structurally diverse suits of toxic substances are being produced by Alternaria species, some pathotypes of the species share common toxin biosynthetic building blocks (Nakashima et al., 1985; Nakatsuka et al., 1986, 1990; Feng et al., 1990; Kohmoto et al., 1993). With the DNA sequences corresponding to the toxin biosynthetic genes becoming available, two characteristics became evident: 1) these genes were part of larger gene clusters responsible for toxin production; 2) these toxin biosynthetic clusters were localized to the small chromosomes noted previously (Akamatsu et al., 1997). Studies on different Alternaria pathotypes reveal that the fungi bearing the additional chromosomes could be cured of them or lose them through repeated sub-culturing, suggesting that they might be not required for normal saprophytic growth implying that genes located on these elements might confer selective advantages in certain situations or ecological niches (Johnson et al., 2000). In light of this fact, it has been found by Masunaka et al. (2005) that there is a strong possibility of an occurrence of a genetic hybrid.

The Alternaria pathosystem

Brassicaceae, the crucifer plant family, consists of approximately 3,500 species in 350 distinct genera. The important crop species keeping in view the economic perspective falling in the genus Brassica include B. oleracea (vegetables), B. rapa (vegetables, oilseeds, and forages), B. juncea (vegetables and seed mustard), and B. napus (oilseeds) (Westman et al., 1999). Black spot disease caused by Alternaria brassicola is of worldwide economic importance (Humpherson-Jones and Maude, 1982a, b; Humpherson-Jones, 1983, 1985, 1989; Humpherson-Jones and Phelps, 1989; Rotem, 1994; Sigareva and Earle, 1999a). The black spot can be a devastating disease resulting in 20-50% yield reductions in crops such as canola or rape (Rotem, 1994). A. brassicicola, however, is not limited to infection of leaves, and can infect all parts of the plant including pods, seeds, and stems, and is of particular importance as a post-harvest disease (Rimmer, 1995). The necrotrophic nature of the Alternaria species leads to extensive damage of the plant and harvest product (Humpherson-Jones, 1985; Rimmer, 1995). Spread of the disease is mainly by the rain and wind dislodged spores. The optimum conditions for sporulation and infection include a minimum wet period of 13 h and ambient temperatures of 20-30 ºC (Humpherson-Jones and Phelps, 1989; Rotem, 1994). Some weedy cruciferous plants such as A. thaliana, C. sativa and C. bursa-pastoris have been found to have immunity against the pathogen but no satisfactory source of resistance has been identified among cultivated Brassica species (Conn et al., 1988; Sigareva and Earle, 1999a, b; Westman et al., 1999).The genetic basis for the resistance have been found to involve additive and dominant gene action (King, 1994).

Identification of pathogenicity factors

The work done by Yao and Koller (1994, 1995), Berto et al. (1999) and Cho et al. (2006) reveal the functional redundancy of lipases in regards to pathogenicity. Interestingly, one of the factors responsible for the pathogenicity has been predicted to be secondary metabolite production. Recently a non-ribosomal peptide synthase gene (NPS6) in Cochliobolus heterostrophus and A. brassicicola was found to direct the biosynthesis of a siderophore metabolite important for oxidative stress tolerance and pathogenicity (Oide et al., 2006). The secondary metabolite corresponding to or synthesized via AbNPS2 has yet to be characterized. Clearly more research is needed to further characterize secondary metabolite biosynthetic genes and their role in pathogenicity and fungal development. Another important area of investigation in the Alternaria- Brassicaceae pathosystem is the fungal signal transduction. For example, disruption of the Fus3/Kss1 MAP kinase homolog (Amk1) in A. brassicicola resulted in a complete loss of pathogenicity as observed in other fungi (Cho et al., 2006, 2007). Interestingly, in the latter study it was shown that addition of long polypeptide nutrients partially restored pathogenicity to the mutants. In addition, two novel virulence factors by Cho et al. (2008) were predicted to encode a transcription factor (AbPro1) and a two-component histidine kinase gene (AbNIK1). Both of these kinases are pathogenicity factors in phytopathogenic fungi. Slt2 was found to be associated with cell wall integrity and HOG with oxidative stress tolerance (Xu, 2000). Another major work pertaining to the studies related to the identification of virulence factors was the disruption of Aso-1, a gene required for hyphal fusion (anastomosis) which was also found to be required for pathogenicity in Alternaria species (Craven et al., 2008). Eventually, over a hundred genes have been functionally analyzed through various techniques like gene knockout and overexpression experiments making A. brassicicola the species of choice for functional genomics research to define conserved virulence mechanisms for this important genus of fungi (Oide et al., 2006; Cho et al., 2006, 2007; Kim et al., 2007; Cho, 2008).

With the objective of identification of A. brassicola, an attempt was made to examine the role of cutinase genes in A. brassicicola pathogenesis (Yao and Koller, 1994, 1995). In these studies, biolistic transformation was used to disrupt the CUTAB1 gene. Disruption of CUTAB1 affected saprophytic growth since cutin was no longer able to be utilized as a sole carbon source, but this disruption had no significant effect on A. brassicicola pathogenicity. An extracellular lipase was found to be produced by A. brassicicola in vitro (Berto et al., 1999). In this study anti-lipase antibodies were found to significantly decrease of the ability of A. brassicicola to cause disease on cauliflower leaves. However, disruption of four predicted A. brassicicola lipase genes expressed during plant infection did not result in reduced virulence on cabbage (Cho et al., 2006).

One area of interest regarding A. brassicicola pathogenicity lies in the area of secondary metabolite biosynthesis. Recently a non-ribosomal peptide synthase gene (NPS6) in Cochliobolus heterostrophus and A. brassicicola was found to direct the biosynthesis of a siderophore metabolite important for oxidative stress tolerance and pathogenicity (Oide et al., 2006). In another study, a non-ribosomal peptide synthase gene (AbNPS2) was found to be important for cell wall integrity, conidial viability, and virulence of aged spores of A. brassicicola (Kim et al., 2007). The secondary metabolite corresponding to or synthesized via AbNPS2 has yet to be characterized. Clearly more research is needed to further characterize secondary metabolite biosynthetic genes and their role in pathogenicity and fungal development.

Another area ripe for exploration in the A. brassicicola-Brassicaceae pathosystem is fungal signal transduction mechanisms. Disruption of the Fus3/Kss1 MAP kinase homolog (Amk1) in A. brassicicola resulted in a complete loss of pathogenicity as observed in other fungi. Interestingly, in the latter study it was shown that addition of long polypeptide nutrients partially restored pathogenicity to the mutants (Cho et al., 2006, 2007).

Disease management

Since a number of Alternaria species infect crops of economic importance, there is a strong need to effectively control for this pathogen. There are different methods which are therefore needed for its control.

By Planning

The planting of susceptible varieties in field should be avoided with infected residues from a previous crop retained on the surface.

By Ground Preparation

The residues from the previous crop should be incorporated. Apart from this, balanced crop nutrition especially of potassium should be provided.

By Fungicides

One of the most effective measures to control the disease caused by Alternaria is the effective application of fungicides. Thiram (75%) proved as the most effective fungicide at 5000 ppm while complete inhibition of Alternaria was noticed at 10,000 ppm in the case of Thiram (TMTD 80%) and Arasan 50% (Sahni and Singh, 1967). Apart from this, work done by Fugro et al. revealed that Dithane M-45 was significantly superior to others against A. cucumerina causing leaf blight of watermelon. It was followed by Bavistin, Dithane Z-78, Difoltan, Blitox and Bordeaux mixture. Similarly for control of Alternaria blight of cauliflower, Captafol was found to be the best followed by Dithane M-45 to provide maximum yield (Sinha and Prasad, 1989) where as for Alternaria blight of radish seed crop, Dithane M-45 (0.25%) proved most effective, followed by 0.4% Bordeaux mixture (Hussaini and Singh, 1989). Mancozeb (0.2%) was found most effective for inhibiting the mycelial growth of A. solani (Choulwar et al., 1989). The effectiveness of Mancozeb in controlling early blight of tomato was confirmed by Singh et al. (2001). Different hormones such as Indole-3-Butyric Acid or Naphthalic acid at 200 μg/lit concentrations for 30 min have been found to delay the fruit rot caused by A. alternata (Datar, 1996). In controlling Alternaria blight of potato, the combination of Emisan-6 with Indofil M-45 was found to be most effective followed by the combination of Emisan-6 with Indofil Z-78 (Singh et al., 1997). Mancozeb followed by Thiram, Bavistin and Iprodione also proved effective as seed dresser. Among non-systemic fungicides Iprodione and Mancozeb and among systemic fungicides thiophanate methyl was found to be effective under in vitro conditions by Prasad and Naik (2003). Singh and Singh (2006) tested efficacy of seven fungicides viz., Chlorothalonil, Copper oxychloride, Azoxystrobin, Propineb, Copper hydroxide, Mancozeb at 2500, 2000, 1000, 500 and 250 ppm and Hexaconazole at 1000, 500, 200, 100 and 50 ppm against A. alternata causing blight of tomato. Their observations revealed that all the fungicides significantly reduced the radial growth of the fungus. However, hexaconazole was very effective as it caused 100% growth inhibition (Verma and Verma, 2010). The best control of Alternaria leaf spot disease of bottle gourd was obtained by spraying recommended @ 0.2% Indofil M- 45 followed by Chlorothalonil, Cuman L, Ridomil, Indofil Z-78, Copper oxychloride, Jkstein and Topsin-M (Katiyar et al., 2001). Indofil M-45, Indofil Z-78, Vitavax and Kavach were found to be most effective in reducing the mycelial growth of A. alternata infecting brinjal in vitro followed by Bavistin, Benlate and Thiram (Singh and Rai, 2003). Sidlauskiene et al. (2003) found that Amistar was very effective in controlling Alternaria leaf spot in cucumber, cabbage and tomato as it reduces the disease incidence by 88-93%; whereas Euparen plus Bion were found to increase biological efficiency (Verma and Verma, 2010). Singh and Singh (2002) reported that three sprays of 0.25% Dithane M-45 proved superior to other fungicides e. g., Kavach, Foltaf, Bayleton, Baycor and Contaf 5 EC, in terms of additional yield. They advocated three sprayings of Dithane M-45 (0.25%), Kavach (0.1%) or Foltof (0.25%) at 10 days interval for adoption by the farmers for controlling A. brassicicola on cabbage (Verma and Verma, 2010). The sulfanilamide derivatives of chitosan prepared by Mei et al. (2007) showed significant inhibiting effect on A. solani at 50 to 500 μg/ml concentrations. The potassium and sodium bicarbonate and Nerol (a commercial product of the citrus essential oil fractions) had great inhibitory effect against A. solani causing early blight of potato. Complete inhibition of fungus was obtained with potassium or sodium bicarbonate at 2% and Nerol at 0.5% (Abdel Kareem, 2007).

By seed treatment

This method is an effective measure in controlling Alternaria diseases as it helps in reducing primary inoculums. The hot water treatment of seeds at 50°C for 30 min to control Alternaria diseases in cabbage was recommended by Walker (1952) while Ellis (1968) recommended same temperature for 25 min to eliminate Alternaria infection from Brassicaceae seeds. Seed treatment with Thiram plus Captan (1:1) 0.3% and four sprays of Zineb (0.25%) were found quite effective to control this disease in chilli (Jharia et al., 1977).

By disease resistant varieties

With the release of various disease resistant varieties, the in-built resistance is increased and it becomes economical for the farmers making it effective throughout the life. For example, Cucumis melo line MR-1 is resistant to A. cucumerina (Thomas et al., 1990), whereas Mathur and Shekhawat (1992) found watermelon varieties Sel-1 and Sugarbaby to be resistant and Meetha, Durgapura, AY, WHY & WHY-4 to be highly susceptible and RW-177-3, RW-1, RW-187-2 and Milan as moderately susceptible against Alternaria leaf spot. Katiyar et al. (2001) found three varieties of bottle gourd namely, Azad Harit, 7002 and 7003 to be resistant against A. cucumerina. Two highly resistant chilli genotypes, CA 87-4 and CA 748 were identified against fruit rot caused by Alternaria (Sujatha Bai et al., 1993), whereas tomato genotypes viz. Arka Alok, Arka Abha, Arka meghali, Arka Saurabh, IIHR-305, IIHR-308, IIHR-2266, IIHR-2285 and IIHR-2288 were found to be resistant against early blight (Matharu et al., 2006). Similarly, workers across the world are working on the expression of various genes encoding for proteins vital for inducing resistance in various crops.

By bio-control agents

Keeping in view the antagonistic properties of various bacteria and actinomycetes, the use of various bio-control agents is being encouraged. Another important reason of their increased application is the fact that they are eco-friendly too. The antagonists like Chaetomium globosum, Trichoderma harzianum, T. koningii and Fusarium spp. effectively controlled seedborne A. raphani and A. brassicicola in radish (Vananacci and Harman, 1987) Effective inhibition of mycelial growth of A. solani causing leaf blight of tomato by Bacillus subtilis and Trichoderma viridae has also been reported (Babu et al., 2000). It was also found that Bacillus and Pantoea had strong antifungal activity both in in vitro as well as in vivo conditions, but Curtobacterium and Sphingomonas showed antifungal activities only in in vitro against A. solani isolated from tomato (Zhao et al., 2008).

By herbal extracts and natural products

The use of various herbal extracts and natural products is being encouraged because these cause no health hazard or pollution. The extracts of Canna indica, Convolvulus arvensis, Ipomoea palmata, Cenchrus catharticus, Mentha piperita, Prosopsis spicigera, Allium cepa, A. sativum, Lawsonia inermis, Argemone mexicana, Datura stramonium and Clerodendron inerme completely inhibited the spore germination of A. brassicae isolated from leaves of cauliflower (Sheikh and Agnihotri, 1972). The inhibitory effect of garlic bulb extract on the mycelial growth of A. tenuis –causal organism of brinjal leaf spot was reported by Datar (1996). The strong inhibitory action of ethanol or methanol extract of speed weed (Polygonum perfoliatum) against conidial germination of A. brassicicola causing leaf spot of spoon cabbage was reported from Ching (2007). The neem leaf extract showed high efficacy to inhibit the radial growth of A. solani (43.3 and 26.7% at 0.1% and 0.01%, respectively) (Sharma et al., 2007). Hence there are a number of herbal extracts and herbal products which are found effective in controlling diseases caused by Alternaria with no health hazards or pollution.

By other methods

Apart from the various methods mentioned above, several other methods can also be employed which would help in combating devastating effects caused by Alternaria species. Gomez-Rodriguez et al. (2003) found that intercropping of tomato with marigold (Tagetes erecta L.) induced a significant reduction in early blight caused by A. solani. This was achieved by means of three different mechanisms like:

(i) the allelopathic effect of marigold on A. solani conidial germination,

(ii) by altering the microclimatic conditions around the canopy, particularly by reducing the number of hours/day with relative humidity ≥ 92%, thus diminishing conidial development and

(iii) by providing a physical barrier against spreading the conidia.

In addition to this, incorporation of residues as soon as possible after harvest is another measure to reduce the harmful effects of Alternaria. Control of alternative weed hosts also help in the same.

Conclusions

From the above studies, it is concluded that Alternaria is a very destructive pathogen causing a widespread destruction in vegetables and other economically important crops. But with the utilization of advanced techniques, it becomes easier to control this cosmopolitan fungus. Substantial progress has been made in studying the molecular basis for the biosynthesis of phytotoxic secondary metabolites and their role in plant disease development. Utilization of various techniques like gene disruption will allow for an elaborate understanding of its various virulence factors and its physiology. As far as the control of Alternaria is concerned, application of fungicides is a common method for the same. But keeping in view, the various health hazards these cause to the human beings, emphasis is being laid on the other method of disease control like growing disease resistant varieties, use of plant and natural products, bio-control agents and alterations in agronomic practices etc. because they are more economical, eco-friendly and safe.

References

1. Abd-el-Kareem F. 2007. Potassium or sodium bicarbonate in combination with Nerol for controlling early blight disease of potato plants under laboratory, greenhouse and field conditions. Egyptian J. Phytopath., 35(1): 73-86.
2. Akamatsu H, Itoh Y, Kodama M, Otani H and Kohmoto K. 1997. AAL-toxin-deficient mutants of Alternaria alternate tomato pathotype by restriction enzyme-mediated integration. Phytopathol.,87: 967-972.
3. Babu S, Seetharaman K, Nandakumar R and Johanson I. 2000.Efficacy of fungal antagonists against leaf blight of tomato caused by Alternariasolani(Ell. and Mart.)Jones and Grout. J. Biol. Cont., 14(2): 79-81.
4. Ballance GM, Lamari L and Bernier CC. 1989.Purification and characterization of a host-selective necrosis toxin from Pyrenophoratritici-repentis. Physiol. Mol. Plant Pathol.,35: 203-213.
5. Berto P, Commenil P, Belingheri L and Dehorter B. 1999. Occurrence of a lipase in spores of Alternariabrassicicolawith a crucial role in the infection of cauliflower leaves. FEMS Microbiol.Lett.,180: 183-189.
6. Ching HW, Yu WT, Jen SH and Hsiung KW. 2007. Effect of oriental medicinal plant extracts on spore germination of Alternariabrassicicolaand nature of inhibitory substances from speed weed. Plant Dis., 91(12): 1621-1624.
7. Cho Y, Davis JW, Kim KH, Wang J, Sun QH, Cramer RA and Lawrence CB. 2006. A high throughput targeted gene disruption method for Alternariabrassicicolafunctional genomics using linear minimal element (LME) constructs. Mol. Plant-Microbe Interac.,19: 7-15.
8. Cho Y, Cramer RA, Kim KH, Davis J, Mitchell TK, Figuli P, Pryor BM, Lemasters E and Lawrence CB. 2007. The Fus3/Kss1 MAP kinase homolog Amk1 regulates the expression of genes encoding hydrolytic enzymes in Alternariabrassicicola. Fungal Genet. Biol., 44: 543-553.
9. Cho Y, Kim KH, La Rota CM, Scott D, Santopietro G, Callihan M, Mitchell TK and Lawrence CB. 2008. Identification of novel virulence factors associated with signal transduction pathways in Alternariabrassicicola. Mol. Microbiol. (In Press)
10. Choulwar AB, Datar VV, Kurundkar BD. 1989.Efficacy of fungitoxicants on the mycelial growth of A. solani.Pestology, 13: 17-19.
11. ChuppC, and Sherf AF. 1960. Vegetable diseases and their control. The Ronald Press Company, New York, USA.p. 267- 269.
12. Conn KL, Tewari JP and Dahiya JS. 1988. Resistance to Alternariabrassicaeand phytoalexin elicitation in rapeseed and other crucifers. Plant Sci., 56: 21-25.
13. Conn KL and Tewari JP. 1990. Survey of Alternariablackspot and Sclerotinia stem rot in central Alberta in 1989. Can. Plant Dis. Survey, 70: 66-67.
14. Cooke DEL, Jenkins PD and Lewis DM. 1997.Production of phytotoxic spore germination liquids by Alternariabrassicaeand A. brassicicolaand their effect on species of the family Brassicaceae. Ann. Appl. Biol., 131: 413-426.
15. Craven KD, Velez H, Cho Y, Lawrence CB and Mitchell TK. 2008. Anastomosis is required for virulence of the fungal necrotroph, Alternariabrassicicola. Eukaryot. Cell, 7: 675-683.
16. Datar VV. 1996. Efficacy of growth regulators and fungitoxicants on fruit rot of chilli. Indian J. Mycol. Plant Pathol.,26: 239-242.
17. Ellis MB. 1968a. Alternariabrassicae.In: Descriptions of pathogenic fungi and bacteria (No. 162), Commonwealth Mycological Institute (CMI), Kew, Surrey, England.
18. Ellis MB. 1968b. Alternariabrassicicola.In: Descriptions of pathogenic fungi and bacteria (No. 163), Commonwealth Mycological Institute (CMI), Kew, Surrey, England.
19. Feng BN, Nakatsuka S, Goto T, Tsuge T and Nishimura S. 1990. Biosynthesis of host-selective toxins produced by Alternariaalternatapathogens, I: (8r,9s)-9,10-epoxy-8-hydroxy-9-methyl-deca-(2e,4z,6e)-trienoic acid as a biological precursor of AK-toxins. Agric. Biol. Chem., 54: 845-848.
20. Gilchrist DG. 1997. Mycotoxins reveal connections between plants and animals in apoptosis and ceramide signaling. Cell Death Differ.,4: 689-698.
21. Gomez-Rodriguez O, Zavaleta-Mejia E, Livera-Munoz M and Cardenas-Soriano E. 2003.Allelopathy and microclimatic modification of intercropping with marigold on tomato early blight disease development. Field Crops Res., 83(1): 27-34.
22. Howlett BJ. 2006. Secondary metabolite toxins and nutrition of plant pathogenic fungi. Curr.Opin. Plant Biol., 9: 371–375.
23. Humpherson-Jones FM. 1983. The occurrence of Alternariabrassicicola, Alternariabrassicaeand Leptosphaeriamaculansin brassica seed crops in south-east England between 1976 and 1980. Plant Pathol.,32: 33-39.
24. Humpherson-Jones FM. 1985. The incidence of Alternariaspp. and Leptosphaeriamaculansin commercial brassica seed in the United Kingdom. Plant Pathol.,34: 385-390.
25. Humpherson-Jones FM and Phelps K. 1989.Climatic factors influencing spore production in Alternariabrassicaeand Alternariabrassicicola. Ann. Appl. Biol., 114: 449-458.
26. Humpherson-Jones FM and Maude RB. 1982a. Control of dark leaf spot (Alternariabrassicicola) of Brassica oleraceaseed production crops with foliar sprays of iprodione. Ann. Appl. Biol., 100: 99-104.
27. Humpherson-Jones FM and Maude RB. 1982b. Studies on the epidemiology of Alternariabrassicicolain Brassica oleraceaseed production crops. Ann. Appl. Biol., 100: 61-71.
28. Hussaini SH and Singh H. 1989.Efficacy of different fungicides against Alternariablight of radish seed crop. Plant Disease Res., 4: 105-107.
29. Jharia HK, Khare MN and Chand A. 1977.Efficacy of fungicides in the control of fungal diseases of chillies. Indian Phytopathol.,39: 341-343.
30. Johnson RD, Johnson L, Itoh Y, Kodama M, Otani H and Kahmoto K. 2000.Cloning and characterization of a cyclic peptide synthetase gene from Alternariaalternataapple pathotype whose product is involved in AM-toxin synthesis and pathogenicity. Mol. Plant-Microbe Interac.,13: 742-753.
31. Katiyar A, Kant S, Chauhan SS and Alka S. 2001.Chemical control of Alternarialeaf spot of bottle gourd. Ann. Plant Protec. Sci., 9(2): 339-341.
32. Katiyar A, Kant S, Chauhan SS and Alka S. 2001.Sources of resistance in bottle gourd to Alternarialeaf spot. Ann. Plant Protec. Sci., 9(1): 155-157.
33. Kim KH, Cho Y, La Rota M, Cramer RA and Lawrence CB. 2007. Functional analysis of the Alternariabrassicicolanon-ribosomal peptide synthetase gene AbNPS2 reveals a role in conidial cell wall construction. Mol. Plant Pathol.,8: 23-39.
34. King SR. 1994.Screening, selection, and genetics of resistance to Alternariadiseases in Brassica oleracea.Ph.D. thesis, Cornell University, Ithaca, USA.
35. Kirk PM, Cannon PF, Minter DW, Stalpers JA. 2008. Dictionary of the Fungi. 10th ed., Wallingford, CABI.p. 22.
36. Kohmoto K, Itoh Y, Shimomura N, Kondoh Y, Otani H, Kodama M, et al. 1993. Isolation and biological activities of 2 host-specific toxins from the tangerine pathotype of Alternariaalternata.Phytopathol.,83: 495-502.
37. Kolte SJ. 1985. Diseases of annual edible oilseed crops. Vol. 2-3. CRC Press, Boca Raton, Florida, USA.p. 135.
38. MacKinnon SL, Keifer P and Ayer WA. 1999. Components from the phytotoxic extract of Alternariabrassicicola, a black spot pathogen of canola. Phytochem.,51: 215-221.
39. Maude RB and Humpherson-Jones FM. 1980a.Studies on the seed-borne phases of dark leaf spot (Alternariabrassicicola) and grey leaf spot (Alternariabrassicae) of brassicas.Ann. Appl. Biol., 95: 331-319.
40. Maude RB and Humpherson-Jones FM. 1980b.The effect of iprodione on the seed-borne phase of Alternariabrassicicola.Ann. Appl. Biol., 95: 321-327.
41. Masunaka A, Ohtani K, Peever TL, Timmer LW, Tsuge T, Yamamoto M, et al. 2005. An isolate of Alternaria alternate that is pathogenic to both tangerines and rough lemon and produces two host selective toxins, ACT and ACR toxins. Phytopathol.95: 241-247.
42. Mathur K and Shekhawat KS. 1992. Fruit rot of watermelon. Ind. J. Mycol. Plant Pathol.,22: 80.
43. Matharu BK, Sharma JR and Manrao MR. 2006.Synthesis and antifungal potential of 2-chlorobenzal derivatives. Pesticide Res. J., 18(2): 113-115.
44. Mckenzie KJ, Robb J and Lennard JH. 1988. Toxin production by Alternariapathogens of oilseed rape (Brassica napus). Crop Res., 28: 67-81.
45. Meena PD, Chattopadhyay C, Singh F, Singh B and Gupta A. 2002. Yield loss in Indian mustard due to white rust and effect of some cultural practices on Alternariablight and white rust severity. Brassica, 4: 18-24.
46. Meena PD, Awasthi RP, Chattopadhyay C, Kolte SJ and Kumar A. 2010.Alternariablight: a chronic disease in rapeseed-mustard. J. Oilseed Brassica, 1(1): 1-11.
47. Mei ZZ, Rong C, Ronge X, Lin CX, Song L, Yong GZ, Xia J, Lin W and Cheng LP. 2007. Synthesis and antifungal properties of sulfonilamide derivatives of chitosan. Carbohydrate Res., 342(16): 2390-2395.
48. Nakashima T, Ueno T, Fukami H, Taga T, Masuda H, Osaki K, et al. 1985. Isolation and structures of AK-Toxin I and II, host-specific phytotoxic metabolites produced by AlternariaalternataJapanese pear pathotype. Agric. Biol. Chem., 49: 807- 815.
49. Nakatsuka S, Feng BN, Goto T, Tsuge T and Nishimura S. 1990. Biosynthesis of host-selective toxins produced by Alternariaalternatapathogens, 2: Biosynthetic origin of (8r,9s)-9,10-epoxy-8-hydroxy-9-methyl-deca-(2e,4z,6e)-trienoic acid, a precursor of AK-toxins produced by Alternariaalternata. Phytochem.,29: 1529-1531.
50. Nakatsuka S, Ueda K, Goto T, Yamamoto M, Nishimura S and Kohmoto K. 1986.Structure of AF-toxin II, one of the host-specific toxins produced by Alternariaalternatastrawberry pathotype. Tetrahedron Lett.,27: 2753-2756.
51. Oide S, Moeder W, Krasnoff S, Gibson D, Haas H, Yoshioka K and Turgeon BG. 2006. NPS6, encoding a nonribosomal peptide synthetase involved in siderophore-mediated iron metabolism, is a conserved virulence determinant of plant pathogenic ascomycetes. Plant Cell, 18: 2836-2853.
52. Otani H, Kohmoto K and Kodama M. 1995.Alternariatoxins and their effects on host plants. Canadian J. Bot., 73: S453– S458.
53. Otani H, Kohnobe A, Kodama M and Kohmoto K. 1998. Production of a host-specific toxin by germinating spores of Alternariabrassicicola. Physiol. Mol. Plant Pathol.,52: 285-295.
54. Pati PK, Sharma M, Salar RK, Sharma A, Gupta AP and Singh B. 2008.Studies on leaf spot disease of Withaniasomniferaand its impact on secondary metabolites. Indian J. Microbiol., 48: 432-437.
55. Prasad Y and Naik MK. 2003. Evaluation of genotypes, fungicides and plant extracts against early blight of tomato caused by Alternariasolani. Indian J. Plant Protec., 31(2): 49-53.
56. Rangel JF. 1945. Two Alternariadiseases of cruciferous plants. Phytopathol.,35: 1002-1007.
57. Rimmer SR and Buchwaldt H. 1995.Diseases.In: Brassica oilseeds-production and utilization [Kimber D and McGregor DI (eds.)], CAB International, Allingford, UK. p. 111-140.
58. Rotem J. 1994. The genus Alternaria: Biology, epidemiology, and pathogenicity. APS Press, St. Paul.
59. Sahni ML and Singh RP. 1967. Bioassay of tetramethylthiuramdisulphide fungicide. Indian Phytopathol.,20: 71-73.
60. Sharma A, Dass A and Pau MS. 2007. Antifungal effect of neem extract on some common phytopathogenic fungi. Adv. Plant Sci., 20(2): 357-358.
61. Sigareva MA and Earle ED. 1999a.Camalexin induction in intertribal somatic hybrids between Camelina sativa and rapid cycling Brassica oleracea.Theor. Appl. Genet., 98: 164-170.
62. Sigareva MA and Earle ED. 1999b.Regeneration of plants from protoplasts of Capsellabursapastorisand somatic hybridization with rapid cycling Brassica oleracea. Plant Cell Rep., 18: 412-417.
63. Singh NK, Saxena RP, Pathak SP and Chauhan SKS. 2001. Management of Alternarialeaf spot disease of tomato. Ind. Phytopathol.,54: 508.
64. Singh M, Singh RR and Narain U. 1997.Efficacy of different fungicides for control of early blight of potato caused by Alternariasolani. Ann. Plant Protec. Sci., 5(1): 114-115.
65. Singh PC and Singh D. 2006.In vitro evaluation of fungicides against Alternariaalternata. Ann. Plant Protec. Sci., 14(2): 500-502.
66. Singh K and Rai M. 2003.Evaluation of chemicals against Alternarialeaf spot of brinjal. Ann. Plant Protec. Sci., 11(2): 394-395.
67. Singh A and Singh D. 2002.Efficacy of fungicides against Alternarialeaf spot of cabbage. Crop Res., 23(1): 192-193.
68. Sidlauskiene A, Rasinskiene A and Surviliene E. 2003. Effect of various protection means on Alternariadiseases of tomato, cucumber and cabbage seed plants. Sodininkyste-ir- Darzininkyste.,22(3): 388-394.
69. Sheikh RA and Agnihotri JP. 1972. Antifungal properties of some plant extracts. Ind. J. Mycol. Plant Pathol.,2: 143-146.
70. Sinha PP and Prasad RK. 1989. Chemical management of Alternariablight of cauliflower seed crop. Ind. J. Mycol. Plant Pathol.,19: 204-205.
71. SujathaBai E, Seetharaman K and Sivaprakasam K. 1993.Alternariafruit rot disease of chilli- a serious malady in Tamilnadu. Ind. Phytopathol.,46: 338.
72. Thomas CE, McCreight JD and Jourdain EL. 1990.Inheritance of resistance to Alternariacucumerinain Cucumismeloline MR-1. Plant Dis., 74: 868-870.
73. Tomas A, Feng GH, Reeck GR, Bockus WW and Leach JE. 1990. Purification of a cultivar-specific toxin from Pyrenophoratritici-repentis, causal agent of tan spot of wheat. Mol. Plant-Microbe Interact.,3: 221-224.
74. Tuori RP, Wolpert TJ and Ciuffetti LM. 1995. Purification and immunological characterization of toxic components from cultures of Pyrenophoratritici-repentis. Mol. Plant-Microbe Interact.,8: 41-48.
75. Verma N and Verma S. 2010.Alternariadiseases of vegetable crops and new approaches for its control. Asian J. Exp. Biol. Sci., 1(3): 681-692.
76. Vananacci G and Harman GE. 1987. Bicontrol of seed-borne Alternariaraphaniand Alternariabrassicicola. Canadian J. Microbiol., 33: 850-856.
77. Valkonen JPT and Koponen H. 1990.The seed-borne fungi of Chinese cabbage (Brassica pekinensis), their pathogenicity and control. Plant Pathol.,39: 510-516.
78. Wang H, Jones C, CiacciZanella J, Holt T, Gilchrist DG and Dickman MB. 1996. Fumonisins and Alternaria alternate lycopersicitoxins: sphinganine analog mycotoxins induce apoptosis in monkey kidney cells. Proc. Natl. Acad. Sci. USA, 93: 3461-3465.
79. Walker JC. 1952. Diseases of crucifers. In: Diseases of vegetable crops, McGraw Hill Book Co., New York, USA. p. 150- 152.
80. Westman AL, Kresovich S and Dickson MH. 1999. Regional variation in Brassica nigraand other weedy crucifers for disease reaction to Alternariabrassicicolaand Xanthomonascampestrispv. campestris. Euphytica, 106: 253-259.
81. Wolpert TJ, Dunkle LD and Ciuffetti LM. 2002. Host-selective toxins and avirulence determinants: what’s in a name? Annual Review of Phytopathology, 40: 251–285
82. Xu JR. 2000. MAP kinases in fungal pathogens. Fungal Genet. Biol., 31: 137-152.
83. Yao C and Koller W. 1994. Diversity of cutinases from plant pathogenic fungi: Cloning and characterization of a cutinase gene from Alternariabrassicicola. Physiol. Mol. Plant Pathol.,44: 81-92.
84. Yao CL and Koller W. 1995. Diversity of cutinases from plant pathogenic fungi: Different cutinases are expressed during saprophytic and pathogenic stages of Alternariabrassicicola. Mol. Plant-Microbe Interact.,8: 122-130.
85. Yoder OC. 1980.Toxins in pathogenesis. Annual Review of Phytopathology, 18: 103–129.
86. Zhao Y, Tu K, Shao XF, Jing W, Yang W and Su ZP. 2008. Biological control of the post-harvest pathogens Alternariasolani, Rhizopusstoloniferand Botrytis cinereaon tomato fruit by Pichiaguilliermondii. J. Hort. Sci. Biotech., 83(1): 132- 136.