Frogeye leaf spot can be a severe problem in U.S. soybean fields.

In years with adequate rainfall, fungal diseases will be among the top 10 yield robbers. A research project funded by the North Central Soybean Research Program enabled researchers to determine that very few soybean varieties sold in the Midwest are truly resistant to the pathogen. There are three known resistance genes, but only the Rcs3 resistance gene gives protection against all known races of the pathogen.

Originally found in the hot and humid climate of the southern United States, frogeye leaf spot has spread into the northern soybean-growing regions. Since 2003, frogeye leaf spot has ranked 10th on the list of 21 major diseases that suppress United States soybean yield. In 2003, frogeye leaf spot reduced soybean yield in the United States by 9.8 million bushels with a value of $59.3 million. By 2006, losses from frogeye leaf spot totaled $82.4 million.

Since it is a foliar pathogen, as opposed to living in the soil, frogeye leaf spot needs adequate rainfall or extended periods of dew for infection and reproduction. During drought years, producers are generally safe from frogeye leaf spot, but in years of normal moisture the soybean crop is at risk unless a fungicide is used.

Not every farm or field will have frogeye leaf spot. Host genetics, field location and topography greatly influence disease severity. In most fields a few frogeye leaf spot pathogens are present, however not all fields will have a raging epidemic. Frequently, frogeye leaf spot problems occur in river bottom fields and fields that remain shaded early or late in the day. During the past seven years, the disease appears to be on the upswing in incidence and severity in the U.S. north central region.

Finding soybean varieties resistant to frogeye leaf spot is proving challenging. Many differences exist among pathogen isolates, says Jason Bond, an agronomist at Southern Illinois University.. This means there are isolates with increased aggressiveness toward soybean varieties.

“The good news is that none of the pathogen isolates collected in the NCSRP project caused disease on varieties with the Rcs3 gene. Varieties containing the other two genes however, provide protection against some of the isolates, but not all,” says Bond.

Results from the early phases of the study are prompting NCSRP researchers to screen the plant introduction collection for new sources of resistance to the frogeye leaf spot pathogen.

The scientists are beginning with those soybean lines with known resistance to other pathogens like soybean cyst nematode or those that have other beneficial traits like drought tolerance or high yield potential. While researchers work to discover a broader base of genetic resistance, the best management technique for growers is the use of fungicides. Check with your chemical provider to see if their fungicide is labeled for control of frogeye leaf spot.

Not all soybean producers will see an economic return from the use of fungicides. Trial use on each soybean variety is the most prudent course of action since all varieties vary in their level of resistance to frogeye leaf spot.

SOURCE: NCSRP newsletter January 2008.

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