ABSTRACT
Sesame (Sesamum indicum L.), is an important oilseed crop in the tropics and subtropics, referred as “Queen of Oilseeds” owing to its high cooking quality and medicinal value. Sesame production, particularly in India, has been declining since last decade and ‘Leaf blight’ caused by Alternaria spp. is reported to cause yield loss up to 30-40%. Here, we investigated the fungal toxin produced by Alternaria and its pathogenicity. A total of 164 Alternaria strainswere isolated on potato dextrose agar media from the infected sesame leaves showing circular concentric rings with dark brown spots symptoms. All the isolates were screened for cultural and morphological characters. Colour of the fungus was grey to dark brown, formed smooth, raised, fluffy, and regular to irregular margins. Among 164 isolates, 43 isolates were moderately growing and 121 were fast in growth. The DNA of the isolate was amplified with ITS primers and sequence of BLAST results confirmed seven different species of Alternaria of NCBI database. Further, toxigenic potentiality of the isolates was tested with dilutions of culture filtrate (1:1 to 1:5) on sesame leaves. Among 164 isolates, 23 showed toxigenicity, varied from highly toxigenic to least toxigenic. Pathogenicity of the isolates showed that they were highly virulent to less virulent when tested by the detached leaf method. Based on the toxigenicity, the toxin was partially purified and brown coloured paste was recovered. Chemistry of the toxin was confirmed based on the IR, UV, NMR and mass spectra analyses, and it resembled the structure of alternariol mono methyl ether and altenuene which are mycotoxins in nature. Further, bioassay of toxin was carried out at different concentrations (50 to 2000 ppm) on seeds and seedlings of sesame. Maximum inhibition of seed germination of 81.1% was observed at 2000 ppm and the least was 6.67% at 50 ppm. With the increase in the concentration of toxin, the manifestation of the symptom was conspicuous and quick such as marginal, veinal necrosis, drooping and yellowing with lesion formation. From the present study, it is found that the species of Alternaria are responsible for the cause of blight disease symptoms and the toxicity of toxin produced by the pathogen was very high. The Alternaria toxin could inhibit the growth of the plant as well as seed germination rate.
ABSTRACT
When electrophoretic profiles of native proteins from vegetative mycelia of Phytophthora palmivora, Phytophthora capsici and Phytophthora citrophthora causing black pod disease of cocoa in India were compared on a single Polyacrylamide gel, the isolates of same species were readily distinguished both qualitatively by visual similarity in banding patterns and quantitatively by calculating similarity coefficients. Similarity coefficients were generally much higher between isolates within a species than between isolates of different species. The dendrograms obtained after unweighted pair grouping with arithmetic averaging cluster analysis, revealed that all the isolates of Phytophthora capsici were highly homogenous and formed a single cluster. The isolates of Phytophthora citrophthora were resolved into two electrophoretic types which were clustered into two distinct sub groups. Phytophthora palmivora formed a separate group. Thus, the results reveal that polyacrylamide gel electrophoresis can be used successfully in distinguishing species and sub groups within a species of Phytophthora encountered on cocoa.