Photo 1. Long bean infected by Bean common mosaic virus showing distorted leaves with light and dark green patches.
Photo 3. Old leaf of long bean showing yellow and green patches (called a mosaic) from infection by Bean common mosaic virus.
Bean common mosaic virus, common mosaic of beans.
Bean common mosaic virus
Worldwide. Asia, Africa, North, South and Central America, the Caribbean, Europe, Oceania. It has been recorded from Australia, Fiji, New Zealand, and Solomon Islands.
The virus has been recorded in the following legumes in Fiji and Solomon Islands: Calopogonium mucunoides, Centrosema pubescens, Clitorea ternatea, Desmodium heterophylum, Macroptilium atropurpureum, Macroptilium lathyroides, Phaseolus vulgaris (French beans), Phaseolus coccineus, Pisum sativum, Pueraria phaseoloides, Stylosanthes guianensis, Vigna marina, Vigna unguiculata ssp. sesquipedalis (long bean or yard long bean). Cowpea and soybean are also hosts. In a 2007 survey in Solomon Islands the virus was recorded in wild passion fruit, Passiflora foetida.
Patches of light and dark green on the leaves (mosaics), with distortions. If the first leaves are discoloured, and misshapen, and the plants are stunted (Photo 1), then it is likely that the seed was infected by Bean common mosaic virus.
Aphids (green fly) spread the virus in a 'non-persistent way'; this means that after feeding on a plant infected with virus it can straight away move to a healthy plant and transmit (spread) the virus as it feeds again. The virus is also spread in seed. Seed infection can be high, over 30%. It can then be spread from these plants by aphids, and it is not uncommon to see all long beans or French beans in a plot with symptoms of the virus.
The virus can remain alive in bean seeds for many years. The virus is also spread in pollen. However, seed and aphid transmission are the most important methods of spread.
The virus is important on long bean, especially when infection comes from planting diseased seed. In this case, symptoms appear early: plants are dwarfed, pods are shorter and fewer than normal and, consequently, yields are low.
Look for patches of light and dark green on the leaves (this symptom is called a mosaic - Photos 1-4), and for leaves that are misshapen. The leaves may be puckered, with bumps on the surface of the leaves (Photo 1).
Cultural control is important. The following applies to food legumes, yard long and French beans, in particular:
Although there are known differences between Phaseolus vulgaris varieties in other parts of the world, as yet, there have been no reports from Pacific island countries. Currently, trials with long bean (Vigna unguiculata ssp. sesquipedalis) are being done in Pacific island countries with varieties of long beans from AVRDC (now known as The World Vegetable Center).
The use of insecticides for the control of aphids that spread the virus is not recommended. The time is short between an aphid sucking up the virus when it feeds on a diseased plant and spreading the virus as it feeds again on a healthy plant. By the time the insecticide has killed the aphid it has spread the virus.
Should growers wish to use an insecticide to kill aphids, use those given under Aphids (see Fact Sheet no. 38):
AUTHORS Helen Tsatsia & Grahame Jackson
Produced with support from the Australian Centre for International Agricultural Research under project PC/2010/090: Strengthening integrated crop management research in the Pacific Islands in support of sustainable intensification of high-value crop production, implemented by the University of Queensland and the Secretariat of the Pacific Community.
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