Symptoms and Signs
There are two main types of symptoms associated with bean common mosaic disease: common mosaic symptoms and common mosaic necrosis symptoms. The occurrence of either type of symptoms depends on the particular virus present and whether or not the bean variety possesses the dominant I resistance gene. If the variety 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 because it spreads both by seed and aphid. When it infects susceptible cultivars, Bean common mosaic virus causes mosaic patterns of light green-yellow leaf tissue, dark green tissue, or both light and dark mosaics together on the trifoliolate leaves. 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 variety, 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 widespread. Necrosis symptoms only develop when the virus infects varieties 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 reveal 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 wilt caused by Fusarium oxysporum f. sp. phaseoli. However, necrosis in the vascular tissue of the pods, which is absent in plants having Fusarium wilt, is diagnostic of bean common mosaic necrosis disease (see photo comparison).
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.
|Dominant I resistance gene||Bean common mosaic virus||Bean common mosaic necrosis virus.|
|yes||Resistant = no symptoms||Hyper sensitive = BCMNV symptoms|
|no||BCMV symptoms||Induces common mosaic symptoms similar to those caused by BCMV|
Comments on the Disease
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. Since both Bean common mosaic virus and Bean common mosaic necrosis virus cause similar symptoms on bean varieties without the I gene, they are distinguished by their reaction on I gene-containing varieties or other tests, such as ELISA or PCR-DNA detection methods.
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, antibody tests (e.g., ELISA) or PCR-DNA detection methods. However, because the symptoms of these viruses can overlap in certain bean varieties and mixed infections are not common, antibody tests (ELISA) are the most reliable method for identifying these viruses.
Bean common mosaic disease in California can be effectively controlled through the planting of certified, disease-free seed (i.e., seed certified by the California Crop Improvement Association, CCIA) 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 susceptible varieties, the risk of 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 varieties 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.
The following beans are resistant to one or more strains of Bean common mosaic virus:
- Light red kidney: CA Early Light Red, Red Kidney M, Sacramento
- Dark red kidney: CA Dark Red Kidney, UC Nichols
- Cranberry: Etna, Hooter
- Pink: Yolano
- Black: Black Turtle T-39, Midnight
- Large white: Cyrrus
- Yellow: Canario 707
See Common Dry Bean Production in California , ANR Publication 8402 for more information on resistance.