Postharvest Diseases of Vegetable Crops and Their Management

6.1 Sclerotinia: rot

White mold (Sclerotinia sclerotiorum) appears in warm and moist weather (>95% relative humidity) and favors fungal growth on infected pods which develops as a white, fluffy mycelial mat often with large, irregular, black-colored sclerotia, typical of S. sclerotiorum [11, 12, 13]. Within the superficial mycelium, initially white but later hard dark black sclerotia are formed. Infected pods show brown discoloration and soft rot. The isolated fungus was identified as S. sclerotiorum based on morphological and cultural characteristics of the mycelia and sclerotia (Figure 1) [14].

6.2 Ascochyta: blight

Ascochyta blight (Ascochyta pisi) black spot symptoms on pods result in the production of round tan-colored sunken spots bearing dark margins. Pycnidia develop in the centers of such spots on pods (Table 1).

6.3 Phytophthora: late blight

Tomato (Solanum lycopersicum, solanaceae) is one of the most important vegetable crops. In the last couple of the years, the disease has become one of the most devastating threats to the cultivation of tomatoes in eastern Uttar Pradesh [15, 16]. Initial disease symptoms appeared in the form of irregular; water-soaked and light brown lesions on leaves which are normally covered with white cottony mycelial growth on the lower side of leaves. Water-soaked brown lesions expanded rapidly on stem and green fruits. Infected green fruits of tomato usually developed olivaceous, brown-colored leathery, and hard structures. All infected fruits eventually fall of from the plants and they were neither fit for marketing nor human consumption. Microscopic studies of the colonized pathogen on potato slices revealed hyaline, coenocytic, branched hyphae, and aseptate sporangiophores with lemon-shaped, papillate sporangia. Sporangia dimensions were 32 ± 6.3 × 20 ± 4.9 μm, with a length to width ratio of 1.6. On the basis of morphological characteristics and sporangia size, the pathogen was confirmed as P. infestans (Figure 2) [17].

6.4 Colletotrichum: fruit rot

Chili (Capsicum annum, solanaceae) is an economically important spice crop, widely grown in India. Colletotrichum sp. is an anamorphic fungal genera ranked in 8th position among top 10 fungal plant pathogens in the world. Infected fruits showing typical anthracnose symptoms of sunken necrotic lesions with a black dot like acervuli in concentric rings collected and collected fruit samples were examined under a light transmission microscope. Anthracnose (Colletotrichum lindemuthianum, C. orbiculare) symptoms appear on immature pods. Sunken cankers with lighter or gray central areas of about 5–7 mm size are seen. The spots on vegetable pods are enlarged and produce tiny black acervuli in the centers which in humid conditions ooze viscous droplets consisting of a mass of pinkish spores. Pure culture of the pathogen isolate was established on PDA by the hyphal tip method. Under the light microscope, one-celled, smooth-walled hyaline falcate, tapered ended conidia (16–26 × 3–4 μm) and acervuli with numerous setae (15–27 × 2–5 μm), were recorded. In this respect, this documentation will play an important role for better understanding of the pathogen and formulation of disease management strategies for the prevention of pre and postharvest crop losses under changing climatic scenarios (Figure 3).

6.5 Didymella: blight/rot

Gummy stem blight (GSB) is caused by Stagonosporopsis cucurbitacearum (syn. Didymella bryoniae). S. cucurbitacearum is an airborne, seed-borne and soil-borne facultative necrotrophic plant pathogen. A black dot like pycnidia is observed on the infected fruits. Its incidence was recorded on cucurbits such as cucumber, bottle gourd, ash gourd, watermelon, etc. in the field and polyhouse. Inoculated PDA plates were produced white mycelium after 4 weeks of incubation at 24°C. Conidia were cylindrical non-septate to monoseptate and 60 × 40 μm in size. Based on the morphological and microscopic characteristics, the pathogen was identified as Stagonosporopsis cucurbitacearum (syn. Didymella bryoniae) (Figure 4 and Table 2).

6.6 Phomopsis: blight

Brinjal (Solanum melongena, Solanacae) is one of the most important vegetables worldwide. Phomopsis vexans is one of the notorious seed-borne fungal pathogens that causes destructive Phomopsis blight which ranked second topmost disease of brinjal in India (Table 3). Brinjal fruit rot due to the incidence of this disease has been estimated up to 60%. The pathogen was identified on the basis of colony morphology and size of conidia (20–40 × 40 μ) (Figures 5 and 6).

7.1 Xanthomonas: blight

X. campestris pv. vesicatoria now reclassified as X. euvesicatoria, the causal agent of bacterial spot of tomato. The disease is prevalent in warm, humid, and temperate regions of the world. The genus Xanthomonas comprises 20 different species [21] with many pathovars causing economically important diseases on different vegetable crops [22]. Xanthomonas is a rod-shaped, gram-negative bacterium. It produces a yellow soluble pigment, called xanthomonadin, and extracellular polysaccharide (EPS). EPS is an important pathogenicity factor of bacteria that protect from desiccation and for the attenuation of wind- and rain-borne dispersal (Figure 7).

7.2 Pseudomonas: blight

The disease is caused by pathogen, Pseudomonas syringae pv. pisi. Another bacterium, P. syringae pv. syringae has also been reported to infect vegetable pea in a temperate region. Tender pods are chocolate brown, thin, twisted, and shriveled. Lesions are large on older pods and they become thin, twisted, and dry. Seeds become discolored and shriveled. Dried bacterial ooze makes the pod surface glossy.

7.3 Pectobacterium: soft rot

Pectobacterium carotovorum and Pectobacterium atrosepticum (formerly Erwinia carotovora subspecies carotovora and subspecies atroseptica) causes huge losses of economically important fleshy vegetables worldwide. P. atrosepticum was the first genomically sequenced plant bacterial pathogen that is taxonomically related to animal pathogens. Genomic information is now available for P. carotovorum strains and other “former Erwinia” species now reclassified in the genus Dickeya. Geographically, P. carotovorum is widely distributed and causes soft rot diseases of several vegetable crops. However, P. atrosepticum is an economically important pathogen of blackleg disease of potato and restricted into the temperate region of the world (Table 4) [23].

8.1 Chemical control

Chemical fungicides are commonly used for the management of postharvest disease in vegetables. For postharvest pathogens which infect produce before harvest, the fungicides should be applied at field level during the crop season, and/or strategically applied as systemic fungicides. At the postharvest level, the fungicides are often applied to reduce infections already established in the surface tissues of produce or they may protect against infections occurring during storage and handling. Fungicides used during postharvest are actually fungistatic rather than fungicidal under normal usage. The fungicides are applied on the produce as dips, sprays, fumigants, treated wraps, and box liners or in waxes and coatings. Dip and spray methods are very common in postharvest treatments. The fungicides generally applied as a dip or spray method are benzimidazoles (e.g. benomyl and thiabendazole) against anthracnose, and triazoles (e.g. prochloraz and imazalil) and fumigants, such as sulfur dioxide, for the control of gray mold used for postharvest disease control [24, 25]. Dipping in hot water (at 50°C for 5–10 min, depending on the size of produce in combination with the fungicide) is also used for effective control of the disease. Sodium hypochlorite as a disinfectant is used to kill spores of pathogens present on the surface of the vegetable produce.

8.2 Biological control

International markets reject produce containing unauthorized pesticides, with pesticide residues exceeding permissible limits, and with inadequate labeling and packaging. Hence, biological control of postharvest diseases has great potential because postharvest environmental conditions like temperature and humidity can be strictly controlled to suit the needs of the biocontrol agent. Much information has been provided in relation to postharvest biocontrol and the problems faced by the development of commercial products [26, 27]. Biological control is used through microbes such as fungi, bacteria, actinomycetes, and viruses (bacteriophages) to control the postharvest disease of vegetables [1, 28, 29, 30, 31]. The degree of disease control or disease suppression achieved with these bioagents can be comparable to that achieved with chemicals. As per estimates, the market of Indian bioagents is equivalent to 2.89% of the overall pesticide market in India with the worth of rupees 690 crores. It is expected to show an annual growth rate of about 2.3% in the coming years [32, 33]. In India, so far only 18 types of bio-pesticides have been registered under the Insecticide Act of 1968. Among agriculturally important microbes, Trichoderma viride, T. harzianum, Pseudomonas fluorescens, and Bacillus subtilis are the most potential bio-agents which as act as producers of biologically active metabolites like antibiotics and bacteriocin, elicitors and inducers of systemic resistance in plants. Biocontrol mediated pathogen inhibition is found to be more effective when the antagonist is applied prior to infection taking place. Antagonists which act against postharvest pathogens of vegetables by competitive inhibition at wound sites include the yeasts Pichia and Debaryomces species. Chitosan, for example, is not only an elicitor of host defense responses but also has direct fungicidal action against a range of postharvest pathogens. Trichoderma has potent antifungal activity against Botrytis cinerea, S. sclerotiorum, Cortictum rolfsi, and other important biotic stresses. Microbial pesticide active ingredients of Streptomyces griseoviridis K61 against bacterial soft rot, gray mold, Phytophthora; Gliocladium catenulatum against gray mold; Candida oleophila strain against postharvest diseases; Coniothyrium minitans against Sclerotinia sclerotiorum, Sclerotinia minor; Trichoderma aspellerum (formerly T. harzianum) against Pythium, Phytophthera, Botrytis, Rhizoctonia; Trichoderma atroviridae against B. cinerea and B. subtilis against Botrytis spp. is the most commonly used biocontrol agents for postharvest diseases.

Antagonistic yeast forms a biofilm to stick pathogen and parasitize on the hyphae of the pathogen. Bar-Shimon et al. [34] reported that biocontrol efficacy of yeast correlates with the production of lytic enzymes and their ability to tolerate high concentrations of salts. Further, molecular approaches were used to examine the role of glucanases in the biocontrol activity of the yeast C. oleophila and biocontrol activity was enhanced by overexpression of antimicrobial peptides. By early 2000, three postharvest biological products, Aspire™ (the USA and Israel), Bio-Save™ (the USA), and Yield Plus™ (South Africa) were available in the market. However, Aspire was initially involved to combine the product with a low concentration of postharvest fungicide [35] or salt solutions (1–2%) of calcium chloride or sodium bicarbonate and also with other additives which are commonly used in the food industry [36]. These products were also combined with physical treatments like hot air, curing, hot-water brushing, and combinations of the above with pressure infiltration of calcium for improvement of efficacy [37]. To increase bio-efficacy, the antagonists can also be combined with a sugar analogue (2-deoxy-D-glucose).

An effort has been made to develop two new products based on yeast antagonist Candida saitoana and a derivative of either chitosan (Biocoat) or lysozyme (Biocure). These products had been evaluated worldwide. They showed strong eradicative activity. The two commercial products based on the use of a heat-tolerant strain of Metschnikowia fructicola also contain other additives such as sodium bicarbonate. The additives are found highly effective to increase biocontrol efficacy to levels equivalent to those found with available postharvest fungicides. The product is marketed under the name ProYeast-ST and ProYeast-ORG in Israel by the company AgroGreen and found effective against rots incited by Botrytis, Penicillium, Rhizopus, and Aspergillus.

8.3 Plant essential oils

Botanical pesticides cause no adverse effects on non-target biota with biodegradability. It should be noted that most of the crops sprayed with botanical pesticides are quite safe for consumption after a short period after spraying. A large number of defensive of rich chemicals such as terpenoids, alkaloids, phenols, tannins, coumarins, flavonoids, etc. are present in plants which cause physiological effects on pathogens. These compounds have already been identified in the extracts/exudates of many plants. They have antimicrobial activities and are used for postharvest disease control.

The use of natural botanical products would be a supplement or an alternative to synthetic fungicide. Examples include 1,8-cineole, the major constituent of oils from rosemary (Rosmarinus officinale) and eucalyptus (Eucalyptus globus), eugenol from clove oil (Syzygium aromaticum), thymol from garden thyme (Thymus vulgaris), and menthol from various species of mint (Mentha species). The majority of research is progressing in this regard to develop plant oil-based pesticides. Therefore, essential oil-based formulations have great scope in the future to use as green pesticides as plant protectants in the integrated pest and disease management of value-added agriculture and horticulture crops.

Many exhaustive studies have been carried out on the utility of neem oil against various fungal pathogens. Its efficacy has been evaluated against fungal pathogens and found to be on par with the fungicide hymexazole in the control of the soil pathogens Fusarium oxysporum, Fusarium ciceri, R. solani, S. rolfsii and S. sclerotiorum. Researchers have reported in-vitro inhibition of 16 aromatic compounds against five major seed-borne fungal pathogens in the concentration range of 100–8000 ppm and the minimum inhibitory concentration (MIC) value for all the test fungi was 270–1704 ppm. Essential oils under commerce used as biopesticides have many problems, such as non-tariff barrier, scarcity of natural resources, need of quality control, and difficulties of registration. Some plant products have been commercialized. SPIC Science Foundation has developed a fungistatic product “Wanis” which has a single monoterpene as an ingredient and it is reportedly very effective in controlling more than 30 different types of phytopathogenic fungi. It is non-toxic to human beings and livestock. Recently, an antifungal agent by the name “TALENT”, containing carvone as the active ingredient, derived from the essential oil of Carumcarvii, was commercialized. Mycotech Corporation product Cinnamite™, based on cinnamon oil, has been developed as a fungicide/miticide for glasshouse and horticultural crops. World-leading essential oil-based pesticide producing EcoSMART technologies developed EcoPCORR under the name Bioganic™ as insecticide and miticide for nursery crops, horticultural crops, and value-added crops under glasshouse conditions. The EcoSMART formulation is based on rosemary oil, viz. EcoTrol™ (insecticide/miticide), Sporan™ (fungicide) and eugenol oil formulation Mataran™ (weedicides) were classify as generally recognized as safe (GRAS). The search for antifungal agents of plant origin is important, which can further broaden the arsenal for disease management and can be used as alternatives or complementary to synthetic fungicides. These chemicals of biological origin are safe to use, and in a few cases can even be produced by farmers and rural communities. Thus plant essential oils are safe to the user and the environment and have a good potential as crop protectants and integrated pest management under organic farming and value-added agricultural and horticultural crops [38].