Common Mosaic Virus (BCMV)
BCMV can cause a systemic mosaic or darkening (necrosis) in infected plants, depending on the variety and its genetic background. The systemic mosaic symptom occurs in susceptible plants infected by the virus. Symptoms appear as green to bluish-green mottled or mosaic pattern on leaves which usually exhibit a downward curling or cupping. This mosaic symptom is often associated with and follows the network of leaf veins which become slightly darker than the areas between the veins. This symptom can sometimes be more easily observed by shading the leaf from direct sunlight. Severe, early infection can greatly reduce plant vigor and productivity, and increase seed transmission. Plants infected early in the growing season or grown from infected seed may have fewer pods and fewer seeds per pod than healthy plants. Plants infected through secondary means later in the season usually produce near normal yields, but a high percentage of the seed may be infected.

The systemic necrosis symptom (black root reaction) occurs in varieties with a specific gene which is being used frequently in the development of new bean varieties with more stable forms of resistance to BCMV. These varieties are resistant to all strains of BCMV under most conditions. However, when plants growing at high temperatures are inoculated with BCMV, the black root reaction may develop. The systemic necrosis begins with a slight wilting of young leaflets at any stage of plant growth, followed by a browning or blackening of the vascular tissue and various plant parts, wilting of leaves, and eventually plant death. Discoloration of the water conducting tissue of stem and pods can be observed by making a cross sectional cut through them. The rapid wilting and death of plants prevents them from serving as sources of infection for healthy plants. However, when large numbers of plants are affected by black root, yield losses are substantial.

BCMV may be severe when susceptible varieties are grown near a virus source (i.e. infected plants in the field or nearby field) and are subjected to aphid infestations. BCMV generally does not cause serious problems east of the Rockies, unless seed stocks of susceptible varieties are badly contaminated and unusually high populations of aphids exist. Yield losses from BCMV are generally more severe when infection occurs earlier in the growing season.

Certified seed of BCMV resistant varieties is available in areas where this disease is a problem. Even a low percentage of infected seed (1-5%) can result in the introduction and buildup of new strains of BCMV in Michigan that could induce the black root reaction in resistant cultivators. Crop rotation, early planting, and deep plowing of infected fields are NOT effective in controlling BCMV, but are recommend practices for other diseases of dry beans. Adjustment of cultivation equipment and timing is important to minimize mechanical rubbing and injury of plants, especially if BCMV or any bacterial is present in the field.