Bacterial diseases of dry beans: Every producer's nightmare

HANGWANI MUEDI AND DEIDRE FOURIE, ARC-GRAIN CROPS INSTITUTE

Dry beans are one of the most affordable high protein sources in the world, but its production is not without hassles. Some very small microscopic organisms called bacteria, among other organisms, cause diseases and brings about a negative impact on yield and profitability.

The most important bacterial diseases of dry beans in South Africa are common bacterial blight, caused by Xanthomonas axonopodis pv. phaseoli (Xap) (Smith), halo bacterial blight, caused by Pseudomonas savastanoi pv. phaseolicola (Psp), and bacterial brown spot, caused by Pseudomonas syringae pv. syringae (Pss).

The three diseases mainly affect the foliage and pods (including the seeds) and are all seed-borne. They also penetrate a host plant through wounds and natural openings on a plant.

Symptoms

Once inside the plant tissue, the bacteria start feeding and cause visible damage (symptoms) to the plant. It must be said that these symptoms deny the plant maximum generation and usage of the all-important nutrients that plants require. In addition seeds look unattractive with less chance of germinating or growing properly when planted.

Common bacterial blight
The symptoms associated with common bacterial blight first appear as small water soaked spots on the underside of the leaves. These watersoaked spots become dry (necrotic lesion) and are often surrounded by a small zone of lemon yellow tissue (Photo 1).

The lesions enlarge and join together, making the plant appear as if it has been burnt. When the bacteria have severely infected the plant, the leaves die out and the dead leaves remain attached to the plants even at maturity. Bacteria from infested material act as a source for new infections on healthy plants and healthy plant tissues.

Pod symptoms consist of lesions that are generally circular, slightly sunken, and dark red-brown. The sizes and shapes of the lesions differ. After some rainfall, or heavy mist or lengthy overhead irrigation, the lesions on pods become covered in bacteria. On white dry bean seeds, butter yellow or brown spots can be seen throughout the seed coat. When seeds are severely infected, they shrink and when planted, they do not germinate properly, and those that germinate will appear weak.

Halo bacterial blight
Symptoms first appear as watery spots on the underside of the leaves. These spots quickly dry up, becoming visible also on the upper side of the leaves (Photo 2). The dry areas on the leaves generally remain small in size (1 mm to 2 mm in diameter) and a green halo (from there the name of the disease) surrounds the dry area.

When the disease is severe, the whole plant may generally appear yellowish and eventually die. The general yellowish appearance is more visible when the weather is cooler than when it is warmer. Bacteria may need seven to ten days after first infection in order to cause new infections.

Some red or brown lesions, that also look as if they contain water, may appear on the pods. When the dry bean growing season approaches the end and the pods mature, loosing their green colour, the diseased areas on the pods remain green and some shining substance (bacteria) can be seen on the area. Severe pod infections on the seeds cause the seeds to shrink and loose their original colour.

Bacterial brown spot
Symptoms first appear as small watery spots on the underside of the leaves, which become reddish brown and circular in shape, with different sizes (Photo 3).

Even when these spots become larger than their original sizes, they still remain smaller compared to the other two diseases in general.

A narrow yellow or light green zone surrounds the lesions and they later dry up, with the dry area falling down, thereby leaving holes on the leaves. The watery spots also appear on the pods with a circular shape, later becoming darker green and sunken brown, and causing the infected area on young pods to bend at point of infection.

When the seeds are infected, they first develop a watery appearance at the point of infection and later become brown and shrunken when the disease becomes severe. When bacterial brown spot develops all over the plant, damage also appears on the stems. Sources of these bacteria include infected seeds and volunteer beans, weed hosts and plant debris.

Management of bacterial diseases

The very first thing should be every dry bean producer’s aim, and that is to plant disease-free certified seed as a primary step in managing these seed-borne diseases. However, bacteria may still find their way to get to your crop through other means.

For this reason, planting dry bean cultivars that are resistant (if available) to these bacteria is recommended as this serves as an effective method of controlling bacteria. Since previously bacteria-infested beanstalks may still be present in the soil, dry beans should not be planted on the same piece of land for at least two years.

Other crops, such as oats, barley and wheat may be planted during this two-year period due to the fact that these crops are non-hosts to these bacteria, thereby eliminating bacteria. Producers must remember that bacteria are capable of surviving on dry bean leaves without causing diseases on the crop until such time as conditions are favourable for infection.

Bactericides or chemicals that contain copper can accordingly be applied preventatively during the late vegetative or early flowering stages, especially in areas prone to bacterial diseases. Once the bacterial symptoms are already visible, it is already too late for the chemicals to be effective and it will only control the spreading of the disease.

Debris of infected fields can be burnt or the debris buried as to remove inoculum sources from problem fields. The weeds and volunteer beans may also harbour the bacteria, and should be controlled accordingly. Dry bean bacteria require moisture in order to grow and multiply quickly. It is for this reason that working in the fields should be avoided during wet weather and human, animal or machinery movement should be minimised or completely avoided. These movements can easily spread the bacteria through the field, especially if plants are wet.