This is an extremely destructive and greatly feared disease of beans. Take every precaution to ensure that you don’t use infected seed, especially at times or in areas when the bacteria are most active. Also, be vigilant in checking that your neighbour is not planting infected seed.
Of the two major bacterial diseases in South Africa, this one favours cooler, moist conditions and is most active around 20ºC, although it will still be destructive at higher and lower temperatures. However, it is seldom a problem during the hot part of summer, except in areas that experience prolonged spells of cool, cloudy weather.
Symptoms
Diseased areas on plants are a yellowish colour. The disease starts off as small water-soaked spots with a light yellow halo, which gives the disease its name. The bacteria can become systemic and then the whole plant becomes yellow. Halo blight on the pods manifests as a small white spot surrounded by a darker, water-soaked, greasy area. A creamy ooze sometimes escapes from these lesions, particularly when the beans have been packaged.
The disease is easily spread by mechanical means, particularly by workers and implements moving through the crop. Insects, birds and animals can also easily transfer bacteria from an infected field to a clean one. Transmission is higher when the leaves are moist. If there is a small patch in a field, it can also be spread by driving rain and strong wind. Any wound makes it much easier for the bacteria to enter the plant. Wind bruising and hail can produce vulnerable lesions.
Take precautions
Drip irrigation in dry areas can give rise to lower incidences of disease. Never use overhead irrigation for seed production, and avoid sprinkler irrigation in wind.
The disease can easily be carried over for a year in plant debris. When any bacterial disease develops in a bean crop, it should be ploughed in well as soon as the crop is harvested and beans should not be planted nearby for over a year.
Never work in a bean field when the foliage is wet, whether you see any disease or not. Avoid picking in an old land and then moving to a new land. Rather reverse the process. If any symptoms of halo blight are discovered, go over to a full-cover copper-spraying programme. This is the only product that will slow the disease down.
Green beans can be very profitable, but input costs are high and bacterial diseases can lead to increased spraying costs and even a total loss. – Bill Kerr ([email protected] or call (016) 366 061). |fw
