Survival of Pyrenochaeta lycopersici and the Influence of Temperature and Cultivar Resistance on the Development of Corky Root of Tomato. Nina Shishkoff, Postgraduate Research Plant Pathologist, Department of Plant Pathology, University of California, Davis 95616. R. N. Campbell, Professor, Department of Plant Pathology, University of California, Davis 95616. Plant Dis. 74:889-894. Accepted for publication 22 May 1990. Copyright 1990 The American Phytopathological Society. DOI: 10.1094/PD-74-0889.

Behavior of Pyrenochaeta lycopersici in soil, root debris, and living roots was studied with the use of a semiselective medium and extraction of propagules by blending and wet-sieving. Abundant propagules (up to 54,000 per gram of tissue) were found in the cortex of tomato root debris 2–8 mo after a tomato crop was harvested. Propagules were recovered from pieces of root buried for 33 mo, the longest period tested. The fungus was unable to colonize tomato root tissue buried in naturally infested soil or in vermiculite infested with microsclerotia produced in vitro. Symptomless infection, which occurred on tomato and Solanum spp., may result in extensive invasion and formation of propagules by harvest. Thus, P. lycopersici is an ecologically obligate parasite that survives in root tissue as dormant propagules (microsclerotia). Effects of environment on infection and symptom expression were studied at constant temperatures in controlled environment chambers and in the field. The disease was more severe at constant temperatures of 16 or 21 C than at 27 C. Cool temperatures stimulated lesion expansion and symptom development, rather than the initial infection process, when plants were incubated under different combinations of temperatures. When resistant and susceptible tomatoes were planted in naturally infested soil in microplots at monthly intervals, disease progress was linear for all planting dates and both cultivars, but the rate of disease increase was greater for the susceptible than for the resistant cultivar and for the early planting (February) than for the late planting (May). Cool temperature, particularly during the first few weeks of growth of seedlings, probably has a significant effect on increasing disease severity, and this effect is not overcome as seasonal warming occurs later.