Common blight and halo blight (7 September) are the two most destructive bacterial bean diseases. Whereas halo blight thrives in cool conditions, common blight is active in warm, moist conditions. With common blight, the leaves appear scorched, although the scorched areas also have a pale perimeter. Symptoms start off as water-soaked spots on the underside of the leaf which become brown and coalesce, so the leaves look scorched.
The pale yellow perimeters of these scorch marks are not as widespread and pronounced as with halo blight. disease may become systemic and even cause the plant to wilt, and then the developing seeds will also be systemically infected. Pod symptoms are similar to those of halo blight – a spot with a water-soaked area surrounding it and sometimes yellow ooze in the centre. Some varieties are less inclined to develop pod symptoms, but their yields will be reduced proportionate to the damage on the leaves.
By far the most common means of introduction is by infected seed. The disease can also carry over from a previous infected crop or infected land in the vicinity. Areas that use disease-free seed should not get common blight or halo blight. he disease is most active when temperatures approach 30ºC and when the air is humid with occasional thundershowers. H ail or other mechanical damage will increase infection. The same precautions should be taken as outlined in last week’s article on halo blight. A s for all bacterial diseases of beans, use a copper fungicide spray programme as soon as the first symptoms appear. This will not cure the disease, but does lessen the spread and economic damage. Bacterial brown spot his is a less destructive disease than the other bacterial diseases.
Although seed-borne, it can also be introduced by weeds in the vicinity. I have only had one outbreak in 15 years and it was not very severe. Overcast, cloudy, warm, humid conditions are favourable for its development, and it spreads like common blight. an-coloured, circular lesions with a dark outer ring and a light halo develop on the leaves. spots are 3mm to 8mm in diameter but form larger lesions when they coalesce. centres may fall out, leaving holes. Pods develop small, water-soaked, dark green spots which become sunken and tan-coloured with a darker outer rim. These sunken spots can cause the pods to become distorted, especially if the onset of symptoms occurs when the pods are young. he bacteria can survive on plant debris for a year, so infected material should be well dug in after harvest. – Bill Kerr ([email protected] or (016) 366 0616).
