Effects of light on urediniospore germination, appressorium formation and infection efficiency of Phakopsora pachyrhizi, causal agent of soybean rust

Effects of light on infection efficiency, germination, and appressorium formation of urediniospores of Phakopsora pachyrhizi Sydow, the causal agent of Asian soybean rust, were investigated in three experiments separately conducted in China and the USA during 2006-2008. To examine infection efficiency, in Experiments 1 and 2, detached soybean leaves at the V3-V4 stages were inoculated with urediniospores of P. pachyrhizi, and then exposed to different periods of darkness (0, 4, 8 or 12 h ). General linear model analysis indicated that infection efficiency of the urediniospores increased significantly with a longer period of darkness. In Experiment 3, urediniospores treated with different light/darkness periods were examined at 2, 4, 6 or 8 h after incubation for germination and appressorium formation. Multiple comparison and orthogonal contrast analyses indicated significant reduction in spore germination and appressorium formation in the treatments of light contrasted to the treatments of darkness. Within 8 h, proportions of urediniospores appressorium formation were up to 0.35 in darkness and 0.1 under light, which was a more significant relative difference, compared with that of the germination proportions (as high as 0.7 under light and 0.8 in darkness). These results suggest that light was detrimental to germination and infection of P. pachyrhizi and may suppress the development of soybean rust. They may provide an explanation for late season occurrences of this disease in high latitude regions in the northern and southern hemispheres.