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

Abstract

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.