Effects of Temperature, Humidity, and Wounding on Development of Phytophthora Rot of Cucumber Fruit

The effects of temperature (10, 15, 20, 25, 30, and 35 degrees C) and relative humidity (similar to 35, 60, 70, 80, and 100%) on development of Phytophthora fruit rot, caused by Phytophthora capsici, of pickling cucumber (Cucumis sativus) were investigated in controlled growth chamber studies. The effect of wounding on disease development was characterized for small (2.0 to 2.5 cm diameter x 8 to 9 cm long), medium (3.0 to 4.0 cm diameter x 12.0 to 13.0 cm long), and large (>4.5 cm diameter x >14 cm long) pickling cucumbers. Lesions developed on cucumbers incubated at all temperatures tested except 10 and 35 degrees C. Disease severity was greatest on cucumbers incubated at 25 degrees C at 4 days postinoculation (dpi). Lesions formed on cucumbers incubated at all relative humidities tested. The diameter of water-soaking and pathogen growth increased as the incubation relative humidity increased. Wounding lessened age-related resistance in pickling cucumbers. The diameter of water-soaking was similar for all wounded cucumbers at 4 dpi regardless of fruit size. Sporangial production was greater on small and medium fruits than on large fruits. These results indicate that P capsici is capable of infecting cucumbers over a wide range of temperature and relative humidity conditions. Wounding increases the susceptibility of pickling cucumbers to P. capsici.