Bean pod mottle is a viral disease of soybean, snap bean and other legumes caused by Bean pod mottle virus (BPMV). Like many plant viruses, BPMV is spread by an insect. In the North Central region, the most important insect vector is the bean leaf beetle, Cerotoma trifurcata, which feeds on infected plants, then transmits the virus particles to the nest plant on which it feeds. Foliar symptoms include yellow and green mottled areas. Young leaves show symptoms more severley than older leaves, sometimes with a raised, blistered or distorted appearance. Symptoms con be transient and most obvious during periods of rapid plant growth and cool temperatures, but they may disappear during hot weather and during the reproductive stages of the crop. Symptoms may resemble injury from herbicide drift and are similar to those caused by other viruses. This makes it difficultu to diagnose bean pod mottle and most other viral diseases based on symptoms alone. Laboratory tests can be done at diagnostic clinics to distinguish among suspected viruses.
BPMV is associated with green stem syndrome, a delayed maturity of the stems and petioles, which can make harvesting more difficult. Infection also decreases pod formation, reduces seed size, weight and number and my cause seed mottling. There are three potential sources of BPMV: overwintered bean leaf beetles, perennial host species (e.g., Desmodium species) and infected seed (usually less than 0.1 percent). Although the level of virus transmission by overwintered beetles is low, beetles acquire BPMV from infected perennial host species and soybean seedlings infected via seed transmission. The presence of bean leaf beetles is an indicator for increased risk of BPMV infection. The first- generation peak in beetle numbers ccurs during late V or early R stages - around early July. the second- generation peak occurs during pod-fill stages (R3 through R6) in August.
Anytime during the growing season is fine to scout; however, the most practical way to check for virus infection is to look at the seed. Check plants on the edge of fields. Early planted fields may be at higher risk for infection.
Variety selection: Although tolerance to BPMV infection has been identified in soybeans, commercial varieties are not clearly characterized for this trait. Currently, varieties different in tolerance to BPMV, although the differences are not clearly studied.
Insecticide seed treatments: Consider planting treated seed if overwintering survival of bean leaf beetles is predicted. Also consider insecticide seed treatments if bean pod mottle has been confirmed in fields in previous years and bean leaf beetles have been present.
Foliar applied insecticides: Foliar applied insecticides can manage bean leaf beetle populations and may reduce incidence of bean pod mottle.
Photo by Alison Robertson