Bean Mosaic Virus (BCMV)

The virus that causes Bean Common Mosaic (often abbreviated as BCMV) is a common problem for growers of older varieties worldwide. Most modern commercially grown bean varieties are resistant however.

Bean Mosaic causes a mottled light and dark green mosaic-like pattern on leaves (they often look like they are variegated) and downward cupping along each leaflet. Depending upon the age, variety of plant and strain of virus they may not have a significant effect on the plant (maybe slowing growth slightly) or they may cause stunting and severely reduce the harvest. In some varieties there is a hypersensitive reaction that quickly kills the plant. There is also another, less common strain of BCMV known as Bean Common Mosaic Necrosis virus that is much more virulent and is commonly fatal.

Bean Mosaic is most often the result of planting infected seed (especially home saved seed, so be careful where you get it). Even a single infected seed can be a problem, as the virus can then be transmitted to other plants by aphids and can quickly spread through a planting. Whereas plants grown from infected seed are usually stunted and yield poorly, plants infected by aphids may still produce well (though the seed will be infected and shouldn’t be saved for replanting).

Mosaic is sometimes introduced into the garden on vegetable seedlings, so it’s best to grow your own, using disease-free seeds (beans are so easy to grow from seed that buying seedlings is a waste of money anyway). If the disease is prevalent in your area you might want to use row covers to keep aphids and other sucking insects off of your plants (though it’s usually easier to plant resistant varieties). If any plants start to exhibit symptoms, remove them immediately to reduce source of infection. Viruses are also frequently spread by gardeners, so don’t touch wet plants and wash your hands frequently with soap and water (especially after touching infected plants). You can minimize the effect of viruses by giving the plants plenty of nutrients and water to keep them in good health.

Image: Howard F. Schwartz, Colorado State University, Bugwood.org