Variability and growth response among Alternaria brassicae isolates causing black spot disease in oilseed Brassica

Alternaria brassicae (Berk.) Sacc., the most common and destructive fungus of oilseed Brassica worldwide, generally sporulates poor with slow growth on common media. A comparative analysis among 30 Indian geographical isolates of A. brassicae was undertaken to observe the variability as well as better growth conditions. Among 6 natural liquid media, the profound growth with maximum mycelial biomass of A. brassicae isolates was found of BAB-08, BAB-19, BAB-23, BAB-29, BAB-40, BAB-41, BAB-44, BAB-45, BAB 50, BAB 54, and BAB-56 on tomato broth, Brassica leaf extract, alfa-alfa seed decoction, and potato carrot broth. Tomato agar and Brassica agar medium were found suitable for the growth of A. brassicae isolates, whereas, Brassica seed agar medium does not favour the growth of A. brassicae due to presence of glucosinolate content responsible for antifungal activity. Brassica leaf extract supplemented with peptone as inorganic nitrogen source produced maximum growth among the tested natural media. Brassica leaf extract supplemented with maltose and yeast extract was found to be the most suitable for mycelial growth, and sporulation of A. brassicae. Variation in mycelial growth was also observed due to optimum pH, and light condition. In SDS-PAGE, a high genetic divergence among A. brassicae isolates was observed with 55 bands of 0.029 to 0.84 Rm value. Our results confirmed the existence of variability among isolates indicating that isolate BAB-18, BAB-29, and BAB-50 were more aggressive while tomato followed by Brassica leaf broth were found best medium for their growth, and sporulation of A. brassicae.