Avoid bacterial diseases in green beans

Bacterial diseases are a menace to all crops, because unlike the case with fungal diseases, we have few means of controlling them.

Ragged brown patches on the leaves – symptoms of common blight.
Photo: Bill Kerr

With green beans, the main source of infection is through the seed itself. This is why you should never buy bean seeds of questionable provenance.

Price should not be an issue. It is far more economical to pay a higher price for clean seed than to get it cheaply and then spend money on repeated sprayings – only to end up with a lower yield or an unmarketable product.

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Three bacterial diseases are of concern: bacterial brown spot, halo blight and common blight.

Of these, bacterial brown spot is the least harmful. Halo blight is more of a problem in cooler, wet conditions and common blight appears during the warmer months.

Halo blight
Caused by Pseudomonas syringae pv. phaseolicola, halo blight is a highly destructive disease that you should prevent at all costs.

If it does strike, do not use the land for beans for two, or preferably three years. Implements used to handle the infected seed and crops should be disinfected.

The disease is active at temperatures of 16°C to 23°C. At 28°C, the bacteria may be present even though no symptoms are evident. In such cases, the disease will again become active when the temperature drops.

The first symptoms are small brown spots on the underside of leaves, which then develop a greenish-yellow halo caused by toxins formed by the bacteria. Seen from above, these light patches appear rather greasy, hence the Afrikaans name vetvlek.

The bacteria can become systemic and infect the entire plant, leading to a loss in production. The pods are usually also infected and unmarketable. Halo blight symptoms appear six to 10 days after infection.

It often happens that the farmer knows there is a problem with the crop, but is not too concerned as the pods seem to be in good condition. He then receives a report from the market a couple of days later saying the consignment has been rejected due to diseased pods.

Two resistant genes provide protection against halo blight, one for the leaves and another for the pods. Some varieties have the resistant gene only for the pods so that the pods can be marketed even if the leaves are infected.

Common blight
Caused by Xanthamonas phaseoli, which thrive in warm, wet weather, especially light hail.

The hail causes tears and bruising, allowing the bacteria to enter the plant more easily.

Symptoms include irregular brown patches on the leaves, especially along the edges, where there is a thinner border of yellow. As with halo blight, lesions appear on the pods, rendering them unmarketable.

Both diseases are spread by the same means – splashing (caused by irrigation or rain), workers and implements going through the land, especially when wet, and even animals such as antelope, hares, birds and insects (locusts are a particular problem).

The infection may be carried from a neighbour’s land by these means.

The only way of reducing damage once the pathogen is present is by spraying thoroughly with a copper compound.

Make sure the sprayer covers the underside of the leaves; this reduces secondary infection. Be warned, though: once common blight takes hold, you will never be able to clear the infection completely.

Bacterial brown spot
This is caused by Pseudomonas syringae pv. syringae, and it is not as serious as the above two diseases. Inclined to be sporadic, bacterial brown spot favours warm weather and is spread in the same way as the other diseases discussed.

It struck my crops only once, although the infection lasted many years; I suspect it came in with weeds.

It has many host plants and weeds. Small brown spots, each with a thin yellow border, appear on the leaves and pods. Bacteria brown spot, like common blight, responds to treatment with a copper compound.