Heat shock induces resistance to Cladosporium cucumerinum and enhances peroxidase activity in cucumbers

A brief heat shock induced resistance to the scab pathogen, Cladosporium cucumerinum, in cucumber plants normally susceptible to the fungus. Immersion of seedlings in a 50 °C water bath for 40 or 50 sec was found to be the optimal treatment for the induction of resistance. Plants inoculated with C. cucumerinum as soon as 3 h after the heat shock exhibited increased resistance to the fungus; a 12 h interval from heat shock to inoculation allowed for development of maximum resistance. The resistance was still fully effective when plants were inoculated 48 h after heat shock. All scab susceptible cultivars that were tested became more resistant to C. cucumerinum after heat shock. There was a direct correlation between the activity of soluble peroxidase induced by heat shock and the resistance induced by the same treatment. Heat shocked cucumbers had an increase in activity of the same isoperoxidases seen to increase in cucumbers with systemic resistance induced by prior Colletotrichum lagenarium inoculation. The relationship of heat shock induced resistance to other stress responses and the role of peroxidases in induced resistance is discussed.