Abstract

Sudden wilt is the major disease problem confronting the capsicum industry in tropical and subtropical Australia. Plants are healthy until fruit set, when they suddenly wilt and defoliate, producing small, shriveled and unmarketable fruit. Observations of apparently healthy and sudden wilt-affected plants in the field showed that root rotting was consistently associated with the disease. Isolations from rotted roots yielded over 200 fungal isolates, predominantly Fusarium spp., Pythium spp., Macrophomina phaseolina and Rhizoctonia solani. However, only P. aphanidermatum and P. myriotylum were able to destroy root systems within a few days of inoculation. Root rotting caused by these pathogens was much more severe at soil temperatures of 35°C and 40°C than at 30°C. High temperatures were also sub-optimal for capsicum, as root health was poor and root and shoot growth was markedly reduced at 35°C and 40°C in the absence of pathogens. Since soil temperatures greater than 35°C may occur at certain times of the year in beds used for capsicum production, these observations suggest that sudden wilt is the result of a pathogen × environment interaction in which heat-stressed plants are attacked by P. myriotylum or P. aphanidermatum.