A. H. C.
First and second authors: Department of Plant Pathology, University of California, Davis 95616; third author: Department of Plant Pathology, University of California, Davis, c/o U.S. Agricultural Research Station, 1636 E. Alisal St., Salinas 93905
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Accepted for publication 21 September 1999.
A suction-impaction mini—spore trap was developed to study the effect of light initiation and decreasing relative humidity (RH) on spore release of Bremia lactucae in a controlled environment. Three light periods (from 0400 to 1600, 0600 to 1800, and 0800 to 2000 h, circadian time) at a constant RH of 99 to 100% were used for studying the effect of light initiation on spore release. Few spores were released during the dark periods. Spore release increased sharply after the initiation of the three light periods, reached a maximum 1 to 2 h after light initiation, and then declined until only a few spores could be detected. The effect of reduction in RH on spore release was studied by comparing decreases in RH 2 h before and 2 h after light initiation at 0800 h. When RH decreased from 100 to 94% 2 h before light initiation, spore release increased within 1 h, followed by a second increase after light initiation. When RH decreased 2 h after light initiation, spore release continued to increase after the initial increase after light initiation, reached a maximum 1 h after the reduction in RH, and then declined. The results suggest that both light initiation and reduction in RH can trigger spore release and that these factors have separate effects on spore release of Bremia lactucae.
© 2000 The American Phytopathological Society