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