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Effects of Temperature, Moisture, and Cucumber Cultivar Resistance on Lesion Size Increase and Conidial Production by Colletotrichum lagenarium. D. C. Thompson, Former graduate research assistant, Department of Plant Pathology, North Carolina State University, Raleigh 27695-7616; S. F. Jenkins, professor, Department of Plant Pathology, North Carolina State University, Raleigh 27695-7616. Phytopathology 75:828-832. Accepted for publication 21 March 1985. Copyright 1985 The American Phytopathological Society. DOI: 10.1094/Phyto-75-828.

Conidial production by Colletotrichum lagenarium on Cucumis sativus 'Calypso' in the greenhouse, increased linearly over 48 hr when free moisture was maintained on leaves, but remained low if free moisture was not maintained. Lesions appeared 7, 6, 6, 5, and 7 days after inoculation at constant 16, 20, 24, 28, and 32 C, respectively. Initially, lesion size increase was more rapid at 20, 24, or 28 C than at 16 or 32 C. Later, however, lesions were significantly larger at 16 C than at other temperatures and that increase was best described with a quadratic function. Lesion size increase from 20 to 32 C was best described with linear functions. Conidial production commenced when lesions became visible and was greatest at 24 C in the early stages of lesion size increase, but total conidial production was greatest at 16 C and became progressively less at higher temperatures. Quadratic functions best described conidial production. Conidial production was greater on plants that had been misted during the night prior to sampling than on nonmisted controls, however, misting nightly did not increase conidial production or lesion size over plants misted once prior to sampling. Lesion size increase and conidial production were greater on older leaves than on younger leaves. Leaf area increased and the proportion of old necrotic leaves decreased as temperature increased, indicating that higher temperatures were more favorable for growth of the cucumber plants. In seven field environments, lesion appearance and conidial production were initiated 5 or 6 days after inoculation. Lesion size increase and conidial production were 2- 4 times greater on the susceptible cultivar Earlipik 14 than on cultivars Calypso or Calico. A third-order polynomial based on time from inoculation explained approximately 50% of the variation in lesion size increase and conidial production; a model composed of environmental variables (degree-days, hours of leaf wetness, hours of RH >85%, daily maximum temperature >32C, irrigation, and rain) explained only about 20%. Combining time with environmental variables in one model did not increase predictive accuracy. In field experiments, more conidia were present in early morning samples (66,176 per lesion) than in late afternoon (38,678). In all studies, the leaves became senescent or necrotic before lesion size increase and conidial production ceased.

Additional keywords: anthracnose, Cucumis sativus, disease cycle components.