Aeciospore release in Gymnosporangium

Abstract

Aeciospore release was studied in Gymnosporangium clavipes Cke. & Pk., G. globosum Farl., and G. juniperi-virginianae Schw. under natural and controlled conditions. In nature diurnal periodicity occurred in all species with spores released in the morning as humidity dropped. Precipitation caused immediate release in G. clavipes and G. globosum but prevented release in G. juniperi-virginianae because of the hygroscopic peridia. Under constant conditions of 70 °F, 90% R.H., photoperiod 12 hours beginning at 8 A.M., spore release in G. clavipes and G. globosum appeared to be continuous. In G. juniperi-virginianae under the same conditions there were twin peaks, a major one between 8 and 10 A.M. and a smaller one at 8–10 P.M. With a dew cycle inserted for 4–12 hours, G. clavipes and G. globosum released spores during the dew period but the hygroscopic peridia of G. juniperi-virginianae closed the opening, preventing aeciospore release. Upon return to 90% R.H., spore release in G. clavipes and G. globosum declined for 3–9 hours, then returned to a steady level but G. juniperi-virginianae released great numbers of spores 2–4 hours after the dew period, because of the action of the peridial strands. In all species, fresh dry spores are shrunken and irregular, expanding immediately in contact with free water, shrinking as water is lost. Spore release is correlated with free moisture in G. clavipes and G. globosum and in all species with rapidly declining humidities.