J. Blair Buckley

Soybean producers annually face diseases, insects and environmental stresses that affect the yield and quality of their crop. The use of varieties with resistance to specific pests and stresses is generally considered the most cost-effective solution to these problems. The focus of the LSU AgCenter’s soybean program is to develop high-yielding varieties with resistance to major Louisiana diseases.

Currently, the disease of most concern to Louisiana soybean producers is Cercospora leaf blight caused by the fungus Cercospora kikuchii. The first symptoms of the disease typically occur late in the season as the seeds within the pods approach full size. The leaves in the upper portion of the plants exposed to the sun develop a purplish-bronze appearance. Dark lesions may occur on the leaf stems and on the upper and lower leaf surfaces. The fungal toxin cercosporin is translocated into the leaf and is converted into its toxic form by light. Cercosporin damages the leaf tissue and causes defoliation of upper leaves and the presence of green leaves below the defoliated area. Cercospora leaf blight severity is influenced by temperatures between 82-90 degrees Fahrenheit and high relative humidity. Severe infections can cause substantial yield loss and reduce seed quality. Fungicides have not been highly effective in controlling Cercospora leaf blight.

LSU AgCenter researchers evaluate Cercospora leaf blight resistance in soybean varieties developed in the United States and many other countries. These varieties are grown in the field and rated for Cercospora leaf blight symptoms. Varieties identified as having the least amount of symptoms are crossed with varieties possessing other desirable traits, such as high yield, strong stems and pods that don’t open prematurely. The goal is to combine Cercospora leaf blight resistance with these desirable traits into new varieties that possess both.

Soybean flowers contain both male and female parts. Varieties used in crosses are designated as either male or female parents. Crosses are made by the tedious process of taking pollen from a male parent plant and placing it on the female flower part of the designated female parent plant. Seeds from the crosses then are planted to produce a second generation of seed.

The second generation is evaluated for Cercospora leaf blight resistance at the Red River Research Station in Bossier City. Promising plants are selected and advanced for further evaluation. Testing for yield begins in the fourth generation. Advanced-generation breeding lines that continue to perform well for Cercospora leaf blight resistance, yield and other desirable traits are evaluated at additional state and regional locations. Soybean lines that perform well in state and regional trials will be considered for release as varieties.

Soybean breeding lines currently with the greatest Cercospora leaf blight resistance exhibit a few leaf-stem lesions and slight bronzing of upper leaves, but they do not have any apparent yield loss, decrease in seed quality or premature defoliation. Disease symptoms in these lines consistently develop at a later growth stage than in highly susceptible lines.

Research includes studies to determine when the fungus infects the plants. A better understanding of when infection occurs and how symptoms develop in moderately resistant lines compared with highly susceptible soybean varieties will aid the effort to develop Cercospora leaf blight-resistant varieties. Previous research found considerable genetic diversity within the Cercospora leaf blight fungus and that the Louisiana Cercospora kikuchii population is significantly different from samples of the fungus collected elsewhere. Knowledge of this variability amount the fungi can be incorporated into the breeding strategy for Cercospora leaf blight resistance.

In addition to breeding for resistance to Cercospora leaf blight, the breeding program is placing some effort on breeding for frogeye leaf spot resistance, drought tolerance and salt tolerance. The general approach for breeding for these traits is somewhat similar to that for Cercospora leaf blight. However, unlike Cercospora leaf blight, there are numerous sources of resistance for frogeye leaf spot and several for salt tolerance. Soybean breeders, however, have made less progress in breeding for drought tolerance.

The continued development of soybean varieties with resistance to diseases and stresses common to Louisiana will provide soybean producers with an economical method of control for these limits to production.

J. Blair Buckley, Associate Professor, Red River Research Station, Bossier City, La.

(This article was published in the spring 2011 issue of Louisiana Agriculture Magazine.)