Sporadic bean diseases

This week I look at bean diseases that you will not encounter frequently.
Issue Date: 28 September 2007

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A plant severely infected with angular leaf spot.

This week I look at bean diseases that you will not encounter frequently.

Angular leaf spot
This fungal disease generally occurs in prolonged wet weather in summer, particularly at temperatures approaching 25ºC. It can be seed-transmitted, although this does not usually happen these days, and survives in undecomposed crop residue. When conditions are very favourable for this disease, one does not generally get rust. The first symptoms are spots on the leaves that can have zonate rings. these spots reach leaf veins, they don’t cross over them but turn aside, giving the marks a distinctive angular appearance. Dark patches on the pods make them unmarketable. Many systemic fungicides that control rust will also control this disease. It can spread very rapidly and control should begin at first signs of the disease.

I have not seen this fungal disease for some time because improved seed is now available. In the past it was quite frequently introduced through infected seed. The disease, which can be very destructive, occurs in cool to moderate temperatures in wet conditions. The first symptoms are reddish-brown streaks along the veins under the leaves and on the stems. These marks grow until brown, discoloured lesions appear on the upper surface of the leaves in line with the veins. Pod symptoms, which render the crop unmarketable, are sunken spots on the pods with ash-grey centres surrounded by a dark ring. Minute black spots can be seen in the grey area. Implement a fungicidal spray programme as soon as the disease appears, and avoid using the land for two years.

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Viral diseases
These are not the problem they were in the past, again mainly thanks to better seed production and the fact that many varieties are resistant. Occasionally the odd plant becomes infected but it needs aphids to spread further, so don’t allow aphid build-up. Different cultivars may show slightly different symptoms. Some plants may turn yellow and die, and others will have light and darker patches on the leaves.

Yellow mosaic virus
This occurs occasionally but have never seen it do much damage. This virus is not seed-borne but generally arrives via infected winged aphids. The diseased plant develops a yellow mottling on the leaves. A lthough an infected plant looks alarming, do not panic – the virus will not spread without the aphids that introduce the disease, and these do not usually colonise beans. – Bill Kerr ([email protected] or (016) 366 0616). |fw