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 degrees C for 6-72 h), short-duration sublethal heating (40 or 45 degrees C for 30 min) and a daily intermittent heating cycle (2 h at 40 degrees C for 1-15 d). Loss of C-14 was detected as (CO2)-C-14 evolved from the chlamydospores and by C-14 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 (CO2)-C-14 evolution, and C-14 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.