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Sporulation of Bremia lactucae Affected by Temperature, Relative Humidity, and Wind in Controlled Conditions

The effects of temperature (5 to 25°C), relative humidity (81 to 100%), wind speed (0 to 1.0 m s^-1), and their interactions on sporulation of Bremia lactucae on lettuce cotyledons were investigated in controlled conditions. Sporulation was affected significantly (P < 0.0001) by temperature, with an optimum at 15°C, and by relative humidity (RH), with sporulation increasing markedly at RH ≥ 90%. There was a significant effect of exposure time in relation to temperature (P = 0.0007) but not to RH. In separate experiments, both RH and wind speed significantly (P < 0.0001) affected the number of cotyledons with sporulation and the number of sporangia produced per cotyledon. No sporulation was observed at wind speeds of >0.5 m s^-1, regardless of RH. In still air, the number of sporangiophores produced per cotyledon increased linearly with RH from 81 to 100% (P = 0.0001, r = 0.98). Histological observations indicated that sporulation may be affected by stomatal aperture in response to RH, as more closed stomata and correspondingly fewer sporangiophores were present at lower RH. These results are important for understanding the mechanism of RH effects on sporulation and for predicting conditions conducive to downy mildew development.