Infection Periods ofGymnosporangium juniperi-virginianaeon Apple

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Abstract
Duration of leaf wetness period (LWP) required for light and severe infection of apple (M. pumila ''Rome Beauty'') seedlings by basidiospores of G. juniperi-virginianae were determined experimentally at 2-32.degree. C. Infection occurred from 2.degree. C (light infection, LWP 24 h) to 24.degree. C (light infection, LWP 2 h). The optimum temperatures for infection were 10-24.degree. C (light infection, LWP 2-5 h; severe infection, LWP 4-6 h). Leaves 4, 6 and 8 days old generally bore more lesions than did 10 and 12 day old leaves, which bore very few lesions at low temperatures. Models developed to predict light and severe infection accounted for 98 and 97%, respectively, of the observed variation. The models were tested with data from LWP monitored in the field for 2 yr and successfuly predicted infection or no infection in 23 of 24 LWP. The models, together with available information on requirements for teliospore germination and basidiospore formation, enable prediction of infection periods in the field, thus allowing efficient use of eradicant fungicides for the control of cedar apple rust.