CHYTRIDS AND ALGAE: II. FACTORS INFLUENCING PARASITISM OF RHIZOPHYDIUM SPHAEROCARPUM ON SPIROGYRA

Host–parasite interrelationship studies showed that temperature was the most important environmental factor associated with epidemics of Rhizophydium sphaerocarpum (Zopf) Fischer on Spirogyra. Certain highly susceptible spring species of Spirogyra grew poorly or not at all at 30 C, the optimum temperature for R. sphaerocarpum, while resistant summer species grew well at 30 C, and even at 35 C. Both pure-culture growth of R. sphaerocarpum and infection of Spirogyra were optimal at approximately pH 7.0–7.5, falling off markedly below pH 6.0 and above pH 8.5. Spirogyra itself grew over a wider pH range, especially on the alkaline side. Infection of Spirogyra was negligible in darkness but a light intensity of 40 ft-c permitted some infection. No difference in infection occurred over a range of light intensities from 65 to 980 ft-c or in day lengths varying from 8 to 16 hours at a light intensity of 600 ft-c. Light saturation for Spirogyra was 400 ft-c. In contrast to the effect of light upon infection, pure culture growth of R. sphaerocarpum was considerably greater in total darkness than at light intensities of 20 ft-c or higher.