Effects of post-dew period temperature on development of phakopsora pachyrhizi on soybean

Authors: Bonde, Morris; Nester, Susan; Berner, Dana

Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/26/2012
Publication Date: 7/11/2012
Citation: Bonde, M.R., Nester, S.E., Berner, D.K. 2012. Effects of post-dew period temperature on development of phakopsora pachyrhizi on soybean. Phytopathology. 102:761-768.

Interpretive Summary: Soybean rust is a very important disease of soybean, caused by a fungus. In 2004, soybean rust was discovered for the first time in the United States, apparently having spread from Asia, to Africa, South America, and finally to the U.S. We conducted a study inside the ARS Plant Pathogen Containment Facility at Ft. Detrick, MD, to determine how weather affected the progression of disease. In environmental chambers simulating weather conditions in nature, it was determined that the pathogen was very susceptible to high day time temperatures characteristic of hot summers in southern United States. When day time temperatures peaked at approximately 95 degrees F, the pathogen was not able to reproduce. These results suggest that soybean rust will not be a serious threat to reproduce and spread when summer temperatures are high. However, if summers are cool, the disease could present a very serious problem to growers. This research provides important information to crop consultants and growers to predict rust outbreaks, develop recommendations for disease management, and prevent disease.

Technical Abstract: Soybean plants were inoculated with urediniospores of Phakopsora pachyrhizi, incubated overnight in a 20 degrees C-dew chamber, and then transferred to temperature-controlled growth chambers simulating day/night diurnal temperature profiles representative of the U.S. soybean production areas during spring, summer, and fall. At three-day intervals, beginning 12 days after inoculation (DAI), urediniospores were collected from each plant and counted. At the end of each experiment, the numbers of lesions and leaf areas were determined for each inoculated leaf, and numbers of lesions per cm2 and urediniospores per plant and per lesion calculated for each temperature profile. Representative leaflets from each plant were fixed and stained, and average numbers of uredinia per lesion and uredinium diameters determined for each diurnal temperature profile. From the results, it was determined that the highest numbers of urediniospores per cm2 leaf area were produced when the day temperature peaked at 21 or 25 degrees C and night temperature dipped to 8 or 12 degrees C, respectively. When day temperatures peaked at 29, 33, or 37 degrees C, urediniospore production per plant was reduced to 40, 17, and 0 percent, respectively, of that at the optimum. The maximum numbers of lesions per cm2 leaf area were not significantly different for peak temperatures from 17 to 33 degrees C; however with peak temperatures of 37 and 41 degrees C lesions were nearly absent. The data indicate that when day temperatures repeatedly peak at 33 degrees C or above, urediniospore production by P. pachyrhizi is low to absent. Daily temperature data obtained from the (U.S.) National Climatic Data Center for a four-year period indicated that while temperatures of mid-western states (e.g. Illinois and Iowa) usually were conducive for urediniospore production during May through September, temperatures of southeastern states frequently peaked above 33 degrees C, too high for significant sporulation during summer months.