Temperature, leaf wetness, and isolate effects on infection of Minneola tangelo leaves by Alternaria sp.

Alternaria brown spot causes necrotic lesions on immature leaves, twigs, an d fruit of tangerines and their hybrids, reducing yield and fruit quality. The effect of temperature, leaf wetness, and isolate was evaluated in an in vitro system using immature detached leaves of Minneola tangelo. Infection was greatest at 27 degrees C, decreased gradually as the temperature decli ned to 24, 20, and 17 degrees C, and dropped sharply at 32 degrees C. Level s of infection were low at 4 and 8 h of leaf wetness and continued to incre ase with longer wetting periods up to 36 h. A polynomial equation was devel oped that provided a good fit for the data (adjusted R-2 = 0.93). Isolates differed in aggressiveness, but there was no significant difference among i solates in their response to temperature and leaf wetness duration.