INDUCTION OF HEAT-RESISTANCE IN FUSARIUM-OXYSPORUM AND VERTICILLIUM-DAHLIAE CAUSED BY EXPOSURE TO SUBLETHAL HEAT-TREATMENTS

The effects of sublethal heat treatments on heat resistance were studi ed for Fusarium oxysporum f.sp. dianthi (Fod) and Verticillium dahliae (Vd), one isolate of each pathogen. Treatments of propagule suspensio ns of Fod at 55-degrees-C and of Vd at 45-degrees-C for 30 min were su rvived by less than 0.001% and 0.01% of the propagules, respectively. Pretreatment of suspensions of Fod at 45-degrees-C increased survival of the 55-degrees-C treatment up to 0.73% of the propagules and pretre atment of suspensions of Vd at 40-degrees-C increased survival of the 45-degrees-C treatment up to 0.40%. Induction of heat resistance was d ependent on duration of the exposure to the sublethal temperature. Wit h Fod, this duration was shorter for propagules from old cultures than for those from young cultures. Pretreatment at 45-degrees-C of a susp ension of an 1-week-old culture of Fod induced resistance when lasting 30 min or longer, but not when 20 min or shorter. With Vd, the durati on of the pretreatment inducing heat resistance depended on type of cu lture - white or black - due to differences in microsclerotia formatio n. Implications of induced heat resistance for control of plant diseas es by thermotherapy are discussed.