EFFECTS OF RADIATION, ESPECIALLY ULTRAVIOLET-B, ON CONIDIAL GERMINATION AND MYCELIAL GROWTH OF GRAPE POWDERY MILDEW

Conidia of Uncinula necator inoculated on vine leaf disks were exposed to different irradiation conditions during various combinations of ir radiation periods. In controlled experiments at constant leaf temperat ure spore germination and mycelial growth were negatively affected by the UV B doses, irrespective of the exposition duration. In semi-contr olled condition experiments, conidia were exposed to shaded, sunny and sunny without UV B radiation conditions. Shaded conditions were alway s more favourable to spore germination and mycelial growth than sunny conditions. Under two different ranges of temperature (20-24 and 26-31 degrees C for shaded conditions), the effect of radiation on germinat ion and mycelial growth differed. Thus, the effect of radiation on spo re germination and mycelial growth seems to be affected by temperature . In general, radiation effects increased as the number of exposition periods increased, indicating that both spore germination and mycelial growth were reduced, but not totally stopped by the different exposur es. Germination was most affected by exposures applied just after inoc ulation, whereas mycelial growth was most affected by exposures applie d one day after inoculation. These results indicate that radiation is an important factor to consider for a better understanding of the rela tionships between climate and grape powdery mildew epidemics.