Rodrigo A. Valverde

Soybean viruses have been shown to cause significant yield losses throughout the soybean-producing areas of the world. In the United States, many different viruses infect soybeans. Viruses, alone or in combination, can cause foliar mosaic, leaf malformations, stunting, seed discolorations and, more important, lower yields. Because insects are the main vectors of plant viruses, the occurrence of viral diseases is often associated with an increase in insect populations.

During 2008 and 2009, soybean leaf samples showing virus-like symptoms were collected from 11 different locations across Louisiana. Samples were tested for the presence of viruses by an enzyme-linked test using blood serum containing antibodies against antigens specific for several known soybean viruses.  Results indicated that the two most common viruses in Louisiana are the aphid-transmitted soybean mosaic virus and the beetle-transmitted bean pod mottle virus. These viruses have been shown to cause significant yield losses in other states.

Some plants showing virus-like symptoms did not give positive reactions with the test. Therefore, it is likely that these plants were infected with other viruses for which the test was not used or by soybean viruses that have yet to be described. The effects of bean pod mottle and soybean mosaic on yield depend upon the time of virus infection and the occurrence of mixed infections. In spring 2010, a greenhouse experiment was conducted to determine the effect of bean pod mottle and soybean mosaic, alone and in combination, on soybean yield. The experiment was conducted in the greenhouse rather than in the field to avoid natural spread of the viruses by insect vectors.

Maturity Group IV soybeans were planted in 1-gallon clay pots, and two weeks after emergence, plants were mechanically inoculated with soybean mosaic, bean pod mottle or a mixture of both viruses. Some plants were used as healthy control. Plants were kept in the greenhouse until maturity. Symptoms evaluated throughout plant growth con sisted of foliar mottle, mosaic, malformation and reduced plant growth.

Bean pod mottle alone induced more severe symptoms than soybean mosaic; however, the most severe symptoms were caused by mixed infections of both bean pod mottle and soybean mosaic. The severity of the symptoms was associated with reduced yields. When compared with healthy plants, the average reductions on seed weight ranged between 26-33 percent depending on the virus (Table 2). Simultaneous infections by both viruses resulted in an 84 percent loss in seed weight. Stained seed was common among plants with mixed virus infections. In general, when viral infections occur late in the plant development, the effect on yields is not as severe as when the infections occur earlier in plant development.

Some plants infected with bean pod mottle alone or in combination with soybean mosaic exhibited green stem at maturity; however, not all bean pod mottle infected plants developed the green stem disorder.

Increased insect activity, lower yields, mottled seed and green stem plants at harvest are indicators of a potential virus disease problem. Foliar symptoms such as mottling and leaf malformation are good indicators of virus infection. Using soybean varieties resistant to these viruses and controlling insect populations are the best approaches to avoid virus problems.

This information will help soybean producers identify management options that reduce losses caused by plant viruses. Moreover, this type information is of great value to personnel conducting evaluations and selections of new soybean varieties for Louisiana. 

Rodrigo A. Valverde, Professor, Department of Plant Pathology & Crop Physiology, LSU AgCenter, Baton Rouge, La.

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