Modeling the Effects of Cloudy Weather on Regional Epidemics of Soybean Rust.

This study simulated daily development of soybean rust (SBR), caused by Phakopsora pachyrhizi, based on cloud cover conditions. Results from a previous study that determined the relationship between shading and apparent infection rates were applied in this study to simulate SBR progress on a regional scale using a semi-empirical logistic model parameterized according to the observed cloud cover conditions. Depending on local weather data availability, cloudy days were assumed to be either (i) the days with less than 2 h of full sun or (ii) the days with complete cloud cover as measured by three daily observations. Estimated disease progress and final estimates of epidemic intensity were verified by 30 reports of seasonal disease progress in 11 regions of Brazil and South Africa from 2002 to 2007. The model predicted final disease severity and the observed final severity fall into a linear relationship with correlation coefficient r = 0.96 and a slope close to 1. Severe SBR epidemics occurred when 19.5 or more cloudy days were recorded during the period from initial disease detection to the date of final disease assessment near the end of a growing season in Brazil and South Africa. Mild epidemics were observed with less than eight cloudy days in a season.

Effects of Shade Intensity and Duration on Asian Soybean Rust Caused by Phakopsora pachyrhizi.

Field studies to quantify the effects of shade intensity and duration on soybean rust caused by Phakopsora pachyrhizi were carried out in Florida in 2006 and 2007. Soybean plants at the V4 stage were inoculated with urediniospores at 2100, 0000, and 0200 h. Inoculated plants were either placed in cages that were covered with shade cloths of different mesh sizes allowing 70, 50, or 20% transmission of sunlight or were not covered so that the plants received 100% of sunlight. Plants kept under 20 and 100% sunlight were sampled 12, 18, and 36 h after inoculation to determine the in vivo germination percentage of urediniospores and the percentage of germ tubes that formed appressoria. In separate experiments, inoculated plants were placed under the shade (20% sunlight) and moved to unshaded conditions after 1, 2, and 7 days. For all experiments, soybean rust incidence and severity were rated 12 days after inoculation. Higher levels of disease incidence and severity were detected in plants under shade compared with those under full sunlight. Shade duration greater than 2 days favored disease development. Within 36 h, in vivo germination of urediniospores and formation of appressoria were not significantly affected by the treatments. These results may explain why soybean rust is more severe in the lower canopy and shaded areas in the field.