BEAN COMMON MOSAIC DISEASE

Pathogens: Bean common mosaic virus (BCMV) and Bean common mosaic necrotic virus (BCMNV)

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.