Effects of humidity and air speed on sporulation of Phytophthora infestans on potato leaves

An apparatus is described for the control of humidity, air speed and temperature around detached potato leaflets and whole plants. Leaflets and plants inoculated with Phytophthora infestans were incubated in air at 15°C. Abundant sporangia were formed in an air speed of 0.3 × 10 −3 m/s when the ambient humidity was 90–100% RH, but not at 85 or 80% RH. At air speeds of 5.5 × 10 −3 and 13.7 × 10 −3 m/s there were numerous sporangia at 100% RH, but not at 95–80% RH. The number of sporangia formed on leaflets kept in air flowing at 5.5 × 10 −3 m/s with alternating humidities of 80 and 100% RH each for 12 h per day was intermediate between numbers formed on leaflets incubated at each humidity continuously, and higher than numbers formed at a constant 90% RH. Removing leaflets from plants before inoculation did not affect the number of sporangia produced.