LSU AgCenter plant pathologist Dr. Zhi-Yuan Chen is teaming up with AgCenter soybean breeder Dr. Blair Buckley to work on a new project that should take them closer to finding a soybean variety that might repel Cercospora.
There are no known soybean varieties that are resistant to Cercospora kikuchii, which infects leaves, stems and pods of soybeans, causing purple seed stain and Cercospora leaf blight, Chen said. The first symptoms include leaves with reddish-purple, angular-to-irregular lesions, which become leathery and dark purplish-red with bronze highlights at the late R5 and early R6 soybean growth stages, Chen explained.
The disease has been reported in most soybean-growing regions. Heavily infected leaves rapidly turn yellow and fall and cause substantial yield loss – ranging from 15 to 30 percent in Brazil.
This disease has emerged in the past few years in the southern United States and is spreading steadily to as far north as Ohio and Iowa, Chen said.
In 2006, the yield loss caused by C. kikuchii in 16 southern states in the United States was estimated to be 2.14 million bushels, with 43 million bushels of that loss in Louisiana. It is now considered the No. 1 cause of soybean yield losses in our state, Chen said.
Chen conducted field evaluations of different varieties and found that soybean lines with resistance to purple seed stain were susceptible to Cercospora leaf blight although both diseases are caused by the same pathogen
Identification of soybean lines with resistance to Cercospora leaf blight is critical to the effective control of this disease since there are no effective fungicides against it, Chen explained.
The disease appears to be more prevalent in the southern United States than in the northern states, indicating a possible geographical or environmental effect on disease development. "Understanding how environment affects the disease development can help us to avoid culture practices that favor the disease development," Chen said.
Buckley has been crossing soybean lines with soybean varieties with enhanced resistance to Cercospora leaf blight. This project is evaluating 20 of the most promising soybean varieties from Buckley at both the LSU AgCenter’s Ben Hur and Red River research stations.
Objectives are to determine the responses of different soybean varieties to Cercospora leaf blight disease, determine the effects of environmental factors on disease development and determine the source of Cercospora kikuchii inoculums, Chen said.
The soybean leaves have been collected on a weekly basis starting two weeks after planting to determine the differences among the 20 lines of the C. kikuchii infection and growth via a molecular technique called real time polymerase chain reaction. These lines also will be evaluated for differences in disease symptom development and Cer-cosporin toxin production among the lines to help better evaluate their level of resistance to C. kikuchii infection.
Special attention will be given to determine whether there is a connection between color of pubescent pods and disease severity, Buckley said. Any differences in Cercospora leaf blight disease severity among varieties differing in color of young pods may be more directly related to pod thickness as measured by the ratio of the weight of the pod and beans to the weight of the beans. Many gray pubescent pods appear to have thinner pods than varieties with brown pubescent pods, according to Buckley.
Pod samples of each variety will be collected at the R6 growth stage and weighed. The pods then will be shelled and the beans weighed.
"We expect to identify soybean varieties with different levels of resistance to Cercospora leaf blight disease and determine whether pod color or thickness is related to disease severity," Chen said.
This field study is only the first step, however. Promising lines from this study will be further evaluated
under greenhouse conditions to determine their differences in resistance under artificial inoculation conditions and the effects of environmental conditions on Cercospora leaf blight development, Chen explained. –Mary Ann Van Osdell
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