Figure 1. Chlorotic mottling and rugosity observed on the soybean leaves. Credit: C. Strunk
This year is certainly keeping us busy! It doesn’t seem to matter which way we turn or what crop we are scouting pests are being observed. Bean leaf beetles have been found in both soybean and alfalfa fields. With the amount of bean leaf beetles being observed out in the fields, producers need to also watch for Bean pod mottle virus (BPMV) development. BPMV was first identified in South Dakota in 1998 and is considered an economically important soybean disease.
What does BPMV look like?
Bean pod mottle virus symptoms are commonly confused with herbicide injury and can resemble symptoms of other viruses. Symptoms associated with BPMV include mild to severe chlorotic mottling or mosaic and rugosity (distortion or wrinkling) on foliage, stunting, and delayed maturity (Figure 1). Symptom severity may lessen during hot weather or with maturity; however, the plant still remains infected with the virus. One effect of delayed maturity is the green stem disorder. This is where the stem remains green after the soybean pods have matured. Infection by BPMV decreases pod formation and reduces seed size, weight, and number. Seed coat mottling (the discoloration of the seed due to a black or brown pigmentation bleeding from the hilum) is another symptom caused by this virus. Grain with discolored seeds may be docked at the time of sale. BPMV is also associated with increases in seed infection by Phomopsis spp.
How is BPMV spread?
Bean pod mottle virus is primarily vectored by the bean leaf beetle, Cerotomoa trifurcate (Forster) in the United States. When bean leaf beetles feed on BPMV-infected soybean plants they also ingest the virus and become a carrier (viruliferous). The virus can be obtained with a single bite of an infected plant. Virus transmission occurs rapidly with the next feeding. As the beetle moves throughout the field, it spreads the virus to the healthy plants. Not only is the beetle an efficient vector, but it also feeds on the soybean foliage resulting in defoliation. Later in the season the bean leaf beetles will feed not only on the leaves but also on the soybean pods and are capable of causing yield loss by clipping the pods from the plant prior to harvest.
Bean Leaf Beetle Identification
Adult bean leaf beetles are approximately ¼ inch long and can vary greatly in color from white, yellow, brown, red and several other intermediate shades (Figure 2). Adult beetles have a black triangle directly behind their thorax and varying numbers of black spots (can have 0, 2, or 4) on their elytra (backs). Bean leaf beetles can be difficult to scout for due to their defensive behavior of falling off of plants when disturbed. If fields are exhibiting large amounts of defoliation a sweep net can be used to determine if the culprit is bean leaf beetles.
Figure 2. Bean leaf beetle adult. Credit: A. Varenhorst
Bean leaf beetles can be managed through insecticide applications which will inhibit the spread of BPMV. Bean leaf beetles should be managed when scouted plants have approximately 30% defoliation. Insecticide seed treatments are effective at managing the overwintering population of bean leaf beetles. Transmission of BPMV by the overwintering generation of bean leaf beetles causes severe yield loss. There is no chemical control available for BPMV infected plants found in the field. To date, no soybean cultivars have been found to be resistant to BPMV.