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Rate of Lesion Development in Relation to Sporulating potential of Pseudoperonospora cubensis in cucumber. Y. Cohen, The Volcani Institute of Agricultural Research, Bet Dagan, Israel; J. Rotem, The Volcani Institute of Agricultural Research, Bet Dagan, Israel. Phytopathology 61:265-268. Accepted for publication 9 September 1970. DOI: 10.1094/Phyto-61-265.

Increased rate of development of lesions caused by Pseudoperonospora cubensis in cucumbers was associated with a low sporulating potential of the pathogen. In plants incubated at different temperatures shortly after inoculation, the quickest necrotization occurred at 25 C. In chlorotic lesions already formed, however, the conditions enhancing quickest necrotization were 40 C and darkness. Sporulating potential was highest when plants were incubated at 15 continuously or at 20:15 C day :night thermoperiods, i.e., at temperatures favorable for development of chlorotic lesions. Low temperature regimes delayed appearance and necrotization of lesions, but extended the period of sporangium productivity and increased total sporangium production. Increases in light intensities and in the length of daily photo-periods were associated with an increase in the sporulating potential. In relation to epidemiology, a distinction is suggested between direct field damage caused by a pathogen and the danger of its spread to neighboring areas. Potential for spread reaches a peak before maximum crop damage occurs.