Studies on seed borne aspects of Alternaria porri (Ellis) Cifferi, the causal agent of purple blotch of onion, its molecular characterization and disease management
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Date
2013-11
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G.B. Pant University of Agriculture and Technology, Pantnagar - 263145 (Uttarakhand)
Abstract
Purple blotch (A. porri) in conjuction with Stemphylium blight (S. vesicarium) is the most destructive foliar and seed stalk pathogens of onion which cause significant qualitative and quantitative losses both in bulb and seed crop. Thrips are found to aggravate the incidence ofpurple blotch. Keeping in view, the significance of these diseases, the present study was carried out in foot hills of Tarai and Bhabar regions of Uttarakhand to investigate the severity of diseases, morphological, molecular and culture characterization of pathogens, seed qualitylosses due to these plant pathogens, detection and location of infection in seed, transmission of pathogen, effect of pathogen on germination and longevity of pathogens in seed during storage and their effective management. For effective management of both the diseases, seed treatment and foliar spray of bioagents and fungicides were carried out under field conditions. To analyze the molecular characterization of 14 isolates of A. porri were collected and studied from different regions of Uttarakhand. The symptoms of purple blotch appeared as small, elliptical, water-soaked, whitish and orange coloured lesions surrounded by yellow hallow in the initial stage. As, the disease progressed the lesions enlarged and became eye shaped having purplish concentric rings which resulted in the development of necrotic patches. Initially, Stemphylium blight showed small, yellow to tan water-soaked lesions with elongated spots turnning dark olive brown to black. The leaves and flower stalks of onion showed completely blighted appearance as the lesions coalesce. The infestations of the thrips observed on the onion leaves reflected characteristic longitudinal streaking which gave the appearance of rasping damage. The best growth of the fungus was reported in Malt Agar medium at 27±10C. The deep freeze method was found to be the best for maximum recovery of pathogen from onion seed. The histopathological studies of infected onion seed revealed the presence of mycelium in or on the seed coat and pericarp but not in embryonic tissues. The maximum percent germination was maintained in seeds stored in cold and dry conditions. The longevity of pathogens (A. porri and S. vesicarium) in seed was higher in cold and dry condition as compared to ambient storage conditions. The seed treatment with CMC + T. harzianum @ 10g/kg of seed was found effective for enhancing percent ermination and reducing seed rot. Amongst chemicals used for seed treatment, thiram @4g/kg is found the most effective followed by bio control agents, T.harzianum against A. porri. In vitro, propiconazole at all the concentrations 25ppm, 50ppm, 100ppm, 250ppm) followed by tebuconazole at 250 ppm concentration completely inhibited the growth of pathogen (A. porri) however, out of eleven treatments tried in foliar spray for management of both the diseases mancozeb (@ 0.3%) + monochrotophos (@ 0.05%) followed by neembicidine (@ 0.03%) at every 15 days interval was found to be very effective in reducingthe diseases and increasing the yield including the management of thrips population.To assess the degree of genetic diversity and relatedness, 14 isolates of A. porri collected
from different locations of Uttarakhand were subjected to molecular characterization using 9 SSR markers (selected on the basis of polymorphism pattern). A total of 36 loci were obtained. Allelic diversity was found ranging from 0.0531-0.3707. The isolates A.p.B4 and A.p.B1 showed maximum similarity coefficient (92%). Based on UPGMA cluster analysis, genetic similarity coefficients was observed between pair wise isolates ranging from 0.61-0.91. The PIC value (0.2978) was recorded to be maximum for D. N. alt-14 while minimum (0.0495) for D. N. alt-5.
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Thesis-PhD
Keywords
alternaria porri, fungal diseases, onions, crop losses, blight, bulbs, thrips