Effect of summer temperatures on survival of Alternaria brassicae in infected Indian mustard (Brassica juncea) debris and thermal death point variations amongst geographical isolates

Abstract

During 2010-11 and 2011-12, experiments were conducted to determine the effect of summer temperature
on survival of Alternaria brassicae (Berk.) Sacc. in Indian mustard (Brassica juncea L.) plant debris, and
seeds in the field, and growth, sporulation and thermal death point variations (40-65oC) in vitro amongst 30
isolates collected from different geographical locations of India. Intact infected plant debris, and leftover
seed after harvest from the surfaces of both unploughed and deep-ploughed fields yielded viable spores of
pathogen on potato dextrose agar (PDA), and moist blotting paper ranging from 13.3 to 84.5%. The highest
per cent viability (84.5%) was obtained on moist blotting paper from infected seeds collected from deepploughed
(30cm depth) mustard fields. In both years, the maximum summer atmospheric temperature varied
between 47-48oC in the experimental area. Survival of A. brassicae in high proportion during the study,
clearly showed that the primary infection in Indian mustard came from inoculum survived in over-summered
(April to June) infected plant debris and seeds. Results of growth and sporulation experiments after exposure
of culture for 10 minutes at temperatures ranging from 40-65oC, showed that mycelial growth as well as
sporulation decreased significantly with increase in temperature. Very sparse to poor sporulation of 16 isolates
at 60oC, and complete absence of growth of 9 isolates at 60oC and all 30 isolates at 65oC, showed that the
thermal death point temperature of A. brassicae is between 61-65oC. Results of survival and thermal death
point could be utilized in eliminating primary inoculum in the farmer’s field through appropriate strategy.