EFFECTS OF HUMIDITY, LEAF WETNESS, TEMPERATURE AND LIGHT ON CONIDIAL PRODUCTION BY PHAEOISARIOPSIS-PERSONATA ON GROUNDNUT

Controlled-environment studies of conidial production by Phaeoisariopsis personata on groundnut are described. With constant relative humidity (RH), conidia were only produced above a threshold (94 . 5% RH) and there was a linear increase between 94 . 5% RH and 100% RH. Conidial production was less with continuous leaf wetness (resembling heavy dew) than with continuous 98-99% RH, but it was similar with intermittent leaf wetness and intermittent 98-99% RH (8 h at 70% RH each day). With alternate high (greater than or equal to 97% RH) and low humidity, daily conidial production depended both on the duration of high RH and on the low RH value. With 99% RH at night (12 h), night-time conidial production decreased with the previous daytime RH. After conidial production had started, small numbers of conidia were produced even when the RH was well below the threshold (94 . 5%). Conidia were produced in continuous light when the photon flux density was 2 mu mol/m(2)/s., but production was completely inhibited with 60 mu mol/m(2)/s. With constant RH, more conidia were produced with a 12 h photoperiod than in continuous darkness. However, more than 75% of the conidia were produced in the dark. With continuous darkness, more conidia were produced during the night (18.00-06.00 h) than during the day, but this biological rhythm was overcome with a (light-night)/(dark-day) regime. With constant 98-99% RH there was a linear increase in conidial production with temperature between 10 and 28 degrees C, and virtually no conidia were produced at 33 degrees C. The daily production of conidia increased with time for 2 to 6 days, depending on the treatment.