How to Manage Pests

UC Pest Management Guidelines

SYMPTOMS

There are two main types of symptoms associated with bean common mosaic disease: common mosaic and common mosaic necrosis. The occurrence of either type of symptom depends on the particular virus present and whether or not the bean cultivar possesses the dominant I resistance gene. If the cultivar has the dominant I gene, it is resistant to strains of the Bean common mosaic virus, but hypersensitive to strains of the Bean common mosaic necrosis virus.

Common mosaic symptoms. In California Bean common mosaic virus is the more prevalent of the two pathogens involved with this disease. When it infects susceptible cultivars, Bean common mosaic virus causes common mosaic symptoms that appear as a light green-yellow and dark green mosaic pattern developing on the trifoliolate leaves. (Often veins are dark green whereas the interveinal areas become light green-yellow.) Leaf discoloration is usually accompanied by puckering, blistering, distortion, and a downward curling and rolling. The intensity and severity of the symptoms depends on the strain of Bean common mosaic virus, the bean cultivar, and the age of the plant when infected. Plants infected at a young age may be stunted and distorted.

Common mosaic necrosis symptoms. Common mosaic necrosis symptoms are rare in California because the Bean common mosaic necrosis virus is not endemic. Necrosis symptoms only develop when the virus infects cultivars that possess the dominant I gene. The symptoms begin as small, red-brown spots that appear on primary or trifoliolate leaves shortly after the virus has been introduced via an aphid vector. The veins around these spots become brown-black, and this vein necrosis then spreads into the phloem tissue of the plant, causing first a wilting, and then death (necrosis) of young leaves and the meristem. The entire plant eventually dies. Cross sections of stems and pods reveals a red-brown streaking in the vascular tissue. These symptoms are often referred to as black root rot (not to be confused with the fungal disease black root rot caused by Thielaviopsis basicola). Common mosaic necrosis symptoms can be confused with those of Fusarium yellows caused by Fusarium oxysporum f. sp. phaseoli. However, necrosis in the vascular tissue of the pods, which is absent in plants having Fusarium yellows, is diagnostic of bean common mosaic necrosis disease.

In bean varieties that lack the I gene, Bean common mosaic necrosis virus induces common mosaic symptoms that are similar to those caused by Bean common mosaic virus. Certain other viruses can also cause necrosis symptoms in common bean. Thus, development of necrosis alone is not sufficient for a positive diagnosis of bean common mosaic necrosis disease, and additional tests must be performed.

COMMENTS ON THE DISEASE

Bean common mosaic virus is the most common and widespread virus of common bean because it is seed- and aphid-transmitted. Bean common mosaic necrosis virus strains were previously referred to as "necrotic strains of Bean common mosaic virus" but it was found that the necrotic strains were actually a distinct virus species. Thus, these strains were given the name Bean common mosaic necrosis virus. As both Bean common mosaic virus and Bean common mosaic necrosis virus cause similar symptoms on varieties without the I gene, they are distinguished by their reaction on I gene-containing varieties or other tests, such as ELISA.

Bean common mosaic necrosis virus is considered to be endemic to Africa. It has been spread throughout the world in infected seeds of non-I gene varieties, and it has been introduced into Idaho, Michigan and New York. In California, Bean common mosaic necrosis virus was detected in a single bean field in 1996, and it has not been detected since.

Bean common mosaic virus and Bean common mosaic necrosis virus are members of the potyvirus family of plant viruses, and both are related to Bean yellow mosaic virus (BYMV) and Clover yellow vein virus (ClYVV), two other potyviruses that infect common bean in California. Bean common mosaic virus and Bean common mosaic necrosis virus are differentiated from Bean yellow mosaic virus and Clover yellow vein virus based on symptoms, host range, seed transmissibility, and antibody tests (e.g., ELISA). However, because the symptoms of these viruses can overlap in certain bean cultivars and mixed infections are not common, antibody tests (ELISA) are the most reliable method for identifying these viruses.

MANAGEMENT
Bean common mosaic disease in California can be effectively controlled through the planting of certified seed (e.g., seed certified by the California Crop Improvement Association, CCIA) and/or resistant varieties that contain the I gene or other Bean common mosaic virus resistance genes. There are now a number of well characterized Bean common mosaic virus resistance genes, and these have been or are presently being incorporated into commercial dry and snap bean varieties. Thus, it is important to know if a variety possesses Bean common mosaic virus resistance and, if so, which resistance gene(s). For varieties that are susceptible to this virus, the disease may be minimized by establishing fields in isolated areas (i.e., not near established bean fields or in areas with extensive bean production).

Because many bean cultivars grown in California possess the I gene for resistance to Bean common mosaic virus, it is imperative to prevent Bean common mosaic necrosis virus from becoming established in California. Bean common mosaic necrosis virus cannot be carried in seed of varieties that possess the I gene because of the necrosis reaction it causes in these varieties; it can only be carried on seed of non-I gene varieties. Therefore, the best way to keep it from becoming established in California is to minimize the planting of non-I gene seed from areas known to have Bean common mosaic necrosis virus.

PUBLICATION

UC IPM Pest Management Guidelines: Dry Beans
UC ANR Publication 3446
Diseases
R. M. Davis, Plant Pathology, UC Davis
Acknowledgment for contributions to Abiotic Disorders:
A. E. Hall, Botany and Plant Sciences, UC Riverside
Acknowledgment for contributions to virus sections in Diseases:
R. L. Gilbertson, Plant Pathology, UC Davis
Acknowledgment for co-authorship of Ascochyta Blight:
C. A. Frate, UC Cooperative Extension, Tulare Co.

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