Bean Mosaic

If you planted beans in fall as a cover crop, or in spring as a food crop, you may notice a problem with leaves looking pale, splotchy, and generally unhealthy. Blotchy, puckered bean leaves may indicate a bean mosaic virus.

Bean leaf mosaic (Scot Nelson) Public Domain

​There are several mosaic viruses that attack beans. Here are the three most common ones:

• bean common mosaic - cost farmers 80% of their crop when it first arrived in the US in 1917; is seed borne, meaning it can arrive on your property through seeds from the grocery store
• bean yellow mosaic - overwinters in clover, gladiolus, and wild legumes; not seed borne
• cucumber mosaic - seed borne; overwinters in weeds

Symptoms of bean mosaic

Bean mosaic symptoms vary between types of beans, life stage, and specific virus, but you will want to keep a lookout for these symptoms in your bean patch:

• infected leaves may be mottled green or yellow
• bands may be visible in the larger plant veins
• leaf chlorosis between the veins may occur
• distorted, cupped, wrinkled or downward curling leaves

Vectors of infection

Bean mosaic is spread by aphids, mealybugs, and leafhoppers. While it sounds convenient, research has shown that using insecticides is not practical because it kills the beneficial insects who would normally feed on the pests. Once a plant is infected, it becomes a vector for disease, as well, and should be completely removed from the site. Infected plants should not be added to the compost pile.


Controlling bean mosaic

Prevention is the best protection when it comes to bean mosaic. Use these handy tips to reduce the likelihood of future generations of beans becoming infected:

• Remove nearby broadleaf weeds. The weeds may provide habitat for the virus and for the vector insects that carry it to your plants.
• Installing reflective mulch may repel insects. You can buy rolls of silver polyethylene from a garden store, or paint some cardboard with silver paint, or lay aluminum foil around your plants for the same results. Just be sure to remove reflective mulch before temperatures get hot or it can cook your plants!
• Buy disease-free seeds from reputable suppliers
• Buy resistant varieties (heirlooms are less likely to be resistant)
• Do not plant beans near clover, alfalfa, rye, or other legumes
• Rotate bean crops each year
• Start plants early enough that they reach flowering stage before aphids become a problem
• Treat heavy aphid infestations with neem oil or insecticidal soap

Bean mosaic is one of the most powerful arguments against using dried beans from the grocery store to plant in your garden. While they are perfectly safe to eat, there is no guarantee that they are safe to sow. Invest in high quality, guaranteed clean seeds from a reputable seller and then save the best seeds from your crop for replanting.

Beans are easy to grow. They improve soil structure, provide excellent food crops, and can look lovely in a landscape, as long as they are kept healthy.