Bean Yellow Mosaic

Beans are easy to grow, they help improve soil structure, and they add nitrogen to the soil. They can also become infected with bean yellow mosaic.

Squash leaf showing yellow mosaic virus (Babahu) Public Domain

There are three different bean mosaic diseases that occur here in California: bean common mosaic, cucumber mosaic, and bean yellow mosaic. These are all viral diseases that cause downward cupping and wrinkling of leaves, especially as leaves get older, along with the telltale mosaic pattern. Bean leaves that develop a bright yellow mosaic pattern may be infected with the bean yellow mosaic virus. There are several strains of bean yellow mosaic (BYM). In addition to beans, bean yellow mosaic can infect peas, peanuts, soybeans, black locust, and fenugreek.


Bean yellow mosaic symptoms

You can differentiate between bean yellow and the other mosaic infections because bean yellow has a yellow mosaic, rather than a light or dark green mosaic. Bean yellow mosaic also exhibits as bright yellow spots on leaves. Plants infected at an early stage of development can become severely stunted and should be removed from the garden and tossed in the trash.


Bean yellow mosaic lifecycle

The bean yellow mosaic pathogen is called, very unimaginatively, bean yellow mosaic virus, or BYMV, for short. This poorly named virus commonly overwinters in legume crops, such as fava beans, alfalfa, clovers, and vetch, as well as in certain weeds and gladiolus. The virus moves from plant to plant in aphids. When an aphid feeds on an infected plant, it becomes a carrier, transporting the disease to every plant it feeds on from that point forward.


Since resistant cultivars are not yet available, these tips may help prevent bean yellow mosaic in your garden:

• buy disease-free seeds from reputable suppliers [Do not use grocery store dried beans!]
• avoid using broad spectrum insecticides that kill aphid predators
• start seeds early enough that they reach flowering stage before aphids become a problem
• heavy aphid populations can be treated with insecticidal soap or neem oil
• plant different legume crops as far away from each other as possible
• remove legume weeds from the area
• reflective mulches may reduce the number of aphids attracted to your plants

Finally, if you see an infected plant, trash it.