SHORT-TERM VARIATION IN ASCOSPORE RELEASE BY PHACIDIUM INFESTANS ON THE NEEDLES OF PINUS-SYLVESTRIS

Ascospore release by Phacidium infestans was monitored with volumetric -recording spore traps in Finland. Detached branches of Scots pine inf ected naturally during the previous winter were used as a spore source . Spores were trapped during two autumns with simultaneous recording o f weather factors. Free water from rain or thawing of rime or snow on the needles was required to initiate the spore release. After the onse t of rain, the number of spores reached its maximum in 4-6 h, while he avy rain decreased the liberation of spores into the air. The number o f spores had a higher correlation both with relative humidity and with precipitation delayed 3-7 h than with current precipitation. Temperat ure did not directly affect the ascospore release. Only a few spores w ere trapped at temperatures below 0 degrees C. The effect of wind on t he release of spores was mainly negative, and indirect through decreas ing humidity. No diurnal periodicity was detected in the spore release .