Studies of the epidemiology and control of tomato stem rot caused by Didymella lycopersici

Pycnidiospores of Didymella lycopersici were capable of inducing stem lesions when used at concentrations as low as two spores per 15-μl droplet, provided the inoculum was placed on wounded tissue and the relative humidity maintained at more than 90%. At c. 75% RH, lesions were not produced even at high spore concentrations and with pre-inoculation stem wounding. Pycnidiospores remained viable when stored for 17 weeks in sterile water and 14 weeks in nutrient solution. Detailed examination of tomato stems with a single lesion indicated that, in some cases, the pathogen may be widely distributed as it was isolated at distances of up to 1000 mm above the lesion and 750 mm below. Experiments on the transmission of D. lycopersici using sciarid flies demonstrated that, although this may be possible, it is probably infrequent in occurrence. None of the fungicides tested for the control of stem lesions were better than the standard benomyl in Actipron, but benomyl in medical-grade liquid paraffin was as good.