ARS | Publication request: The Effect of Solar Irradiance on the Mortality of Phakopsora Pachyrhizi Urediniospores

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 15, 2005
Publication Date: January 1, 2006
Citation: Isard, S.A., Miles, M.R., Hartman, G.L., Russo, J.M., De Wolf, E.D., Morel, W. 2006. The effect of solar irradiance on the mortality of Phakopsora pachyrhizi urediniospores. Plant Disease. 90:941-945.

Interpretive Summary: Phakopsora pachyrhizi, the causal agent of soybean rust, may be the most serious foliar disease of soybean worldwide. Substantial economic losses attributed to this plant pathogen have occurred in Asia, Australia, Africa, and South America. The spores of P. pachyrhizi have demonstrated swift and frequent aerial movement at local, regional, and global scales. The objective of this study was to determine the relationship between spore viability and exposure to solar radiation. Urediniospores of P. pachyrhizi were exposed in the field to determine the deleterious effects of sunlight. The major finding of these studies was that solar radiation and especially its ultraviolet (UV) component was the primary cause of mortality during exposure. The proportions of the spores that germinated were a linear function of solar irradiance. The relationship between spore viability and exposure to solar radiation has been incorporated into the US Department of Agriculture soybean rust aerobiological model that provides North American soybean growers with decision support for managing soybean rust. This information is useful to epidemiologist, disease forecasters and other scientists that are interested in soybean rust.

Technical Abstract: Soybean rust, caused by Phakopsora pachyrhizi, may be the most important foliar disease of soybean. Within the last 10 years, the fungus has moved to many new geographical locations via spread of airborne urediniospores. The objective of this study was to determine the relationship between spore viability and exposure to solar radiation. Urediniospores of P. pachyrhizi were exposed in the field to determine the deleterious effects of sunlight. Concomitant total solar and ultraviolet irradiance measurements were used to predict spore germination. Urediniospores exposed to doses of solar and UV radiation greater than or equal to 28 MJ m-2 and greater than or equal to 1 MJ m-2 ,respectively, did not germinate. The proportions of the spores that germinated, normalized with respect to the germination proportion for unexposed spores from the same collections, were a linear function of solar irradiance (R2 = 0.83). UV measurements predicted normalized germination proportions equally well. Results of inoculation experiments with exposed P. pachyrhizi urediniospores supported the results of the germination trials. The relationship between spore viability and exposure to solar radiation has been incorporated into the US Department of Agriculture soybean rust aerobiological model that provides North American soybean growers with decision support for managing soybean rust.