Bean common mosaic virus is the most likely virus to be found in your bean crop. The usual way it starts off is through infected seed, although infection is not as common as it used to be.

Years ago, most bean varieties were not controlled by plant breeders’ rights, so less attention was paid to ensuring the seed was disease-free, as doing so would incur an additional cost.

Each seed company now has exclusive protected varieties and makes an extra effort to ensure that the seed is clean and of good quality, which makes business sense. It does come at a higher price but provides a higher success rate for the farmer.

When I started farming, many bean farmers would allow a portion of their crop to go to seed when the market price hardly justified selling the beans. They would thus keep this seed for their next crop.

However, it takes a barely noticeable number of diseased plants to cause big crop losses as a result of planting this seed, which is a very risky practice. Rather buy new seed and consider it as insurance against virus infection.

Bean common mosaic virus, of which there are eight known variants, can be transmitted through infected seed of Phaseolus (bean, wild bean) cultivars only.

Spread via aphids

The usual way the virus spreads on the land is through a number of aphid species. It takes less than one minute of feeding for the aphid to become infected, and the same brief period is enough for it to transmit the virus to another plant.

You may also find that the virus is spreading without the presence of aphids. This is because aphids ‘probe’ the crop to determine whether it is to their liking, and thus can spread the virus simply by inspecting plants.

The species Myzus persicae (green peach aphid) does, however, readily colonise a bean crop.

The virus can also be spread mechanically from plant to plant.

Apart from countering aphids if they start to colonise your crop, the disease can be also controlled by resistant genes. Often, bean seed adverts claim a variety is resistant to bean common mosaic virus.

The virus can also be spread by pollen. Although Phaseolus vulgaris is strictly self-pollinating, carpenter bees, which we incorrectly call bumblebees in South Africa, are able to spread the virus, as they cling to the flower and with their hind legs push open the curled pollen-containing section to collect pollen.

Bean yellow mosaic virus

Bean yellow mosaic virus is not seedborne in Phaseolus varieties but has many different hosts among leguminous crops like peas, fava beans, vetch and clovers, as well as a number of ornamental plants.

As many of these crops go through winter, they can become reservoirs for the virus, which is then spread to bean fields by aphids.

While symptoms of bean common mosaic virus may vary greatly, those of yellow mosaic virus look the same on all infected bean plants, with all leaves developing a yellow mottle.

Bill Kerr is a vegetable specialist and breeder.