Documenting variation in Alternaria brassicae isolates based on conidial morphology, fungicidal sensitivity and molecular profile

摘要

Alternaria black spot (Alternaria brassicae) is an economically devastating disease of rapeseed-mustard in India. Losses in yield can exceed 50 percent depending on Brassica species, cultivar planted and prevailing weather. There is no known source of resistance to alternaria black spot in crop Brassica germplasm. In the absence of appropriate documentation on pathotype variation in the causal organism the past attempts at germplasm screening had at the best been tentative and had poor repeatability, possibly due to yearly variation in prevalent pathogenic profile. This communication documents the results of our attempt to collect and catalogue pathogenic variability in Alternaria brassicae. 322 isolations were made from alternaria black spot infested Brassica leaves collected from a wide geographic spread of north-west India. Detached leaf technique was used to test pathogencity of the isolates. 114 proved pathogenic. Among the pathogenic isolates 31, 24, 48 and 11 each were collected from B. rapa, B. napus, B.juncea and B. carinata respectively. These were purified using single spore culture and characterized on the basis of conidial morphology, colony diameter and sporulation patterns. Variation was recorded with range of 51.4-481.2 μm, 6.9-36.0 μm and 16.3 - 266.9 μm for conidia length, breadth and beak length respectively. Average number of horizontal, vertical septa in conidia and beak were 9.7, 0.8 and 3.7 respectively. Colony diameter and sporulation were in range of 13.0-77.5 mm and 1.0-4.0×106 spores / ml respectively. Colonies were brown, white, and olevaceous green with smooth or wavy margins and thick velvety to sparse growth. Morphological data was subjected to diversity analysis. Seven broad groups could be inferred, representing a very wide range of diversity. A representative isolate from each such morphological grouping was tested for relative sensitivity to six fungicides; dithiocarbamate, chlorothalonil, copper oxychloride, tebuconazole, difenoconazole and propiconazole by poison food technique at five concentrations each. Seven groups characterized on the basis of fiungicidal reaction were concordant with those obtained on the basis of conidial morphology. Data on molecular diversity of pathogen isolates will be presented.