Effects of heat stress on loss of C, germination and pathogenicity from chlamydospores of Fusarium oxysporum f. sp. ciceri

Chlamydospores of Fusarium oxysporum f. sp. ciceri lost different amounts of organic C when exposed to continuous lethal heating (45 or 50°C for 6–72 h), short-duration sublethal heating (40 or 45°C for 30 min) and a daily intermittent heating cycle (2 h at 40°C for 1–15 d). Loss of 14C was detected as 14CO2 evolved from the chlamydospores and by 14C exudation in soil. Chlamydospores showed highest C losses (32.5%) under intermittent heating, followed by the sublethal (15.2%) and continuous lethal (11.9%) heat treatments. Most of the 14CO2 evolution, and 14C exudation occurred, during the first 24 h of exposure to soil after treatment. Loss of ability to germinate was observed for all heat-treated chlamydospores. Pathogenic aggressiveness of the chlamydospores on chickpeas exposed to various heat-stress conditions was less than that of unheated chlamydospores. Chlamydospores which were subjected to various heat-stress conditions and then colonized by bacteria, fungi and actinomycetes in soil, sustained a further loss of viability and pathogenic aggressiveness. The latter results demonstrate an important mechanism involved in biological control of this pathogen.