Anthracnose of Stylosanthes scabra: Effect of Leaf Surface Wetness on Disease Severity. S. Chakraborty, Division of Tropical Crops and Pastures, CSIRO, St. Lucia, Queensland 4067, Australia. D. Ratcliff, and F. J. McKay. Biometrics Unit, Institute of Plant Products and Processing, and Division of Tropical Crops and Pastures, CSIRO, St. Lucia, Queensland 4067, Australia. Plant Dis. 74:379-384. Accepted for publication 31 October 1989. Copyright 1990 The American Phytopathological Society. DOI: 10.1094/PD-74-0379.

Leaf surface wetness periods (SWPs) most commonly lasted less than 2 hr during the day or night at Samford, Queensland, Australia. In controlled environment and glasshouse studies, the severity of anthracnose, caused by Colletotrichum gloeosporioides, on two accessions of the tropical pasture legume Stylosanthes scabra increased with increasing duration of SWP. An SWP of 12 hr or longer favored disease development in the susceptible cultivar Fitzroy, and maximum severity was reached after 36 hr. Increases in severity were generally associated with improved infection efficiency, although this association was modified by defoliation and coalescence of lesions. Anthracnose was not significantly less severe when SWP was preceded by 6 hr of low or 12 hr of high relative humidity. However, longer periods of low relative humidity before the onset of wetting resulted in lower disease severity and infection efficiency. Severity was relatively unaffected by brief interruptions in SWP during the initial 12 hr after inoculation, provided a continuous SWP or relative humidity above 85% was maintained for the next 24 hr. We concluded that anthracnose can develop over a range of durations of leaf surface wetness.