Saturday, July 24, 2010

Plant pathology:Leaf spot diseases on Rhubarb

Rhubarb plants (Rheum rhabarbarum) with Downy Mildew

Damp cold weather is favourable for the development of fungal leaf diseases on Rhubarb. The two main ones are Downy mildew, Peronospora jaapiana and Leaf spot, Ascochyta rhei. In Downy mildew, the symptoms are first seen on the upper leaf surfaces. Light brown patches develop first and then this spreads until large patches of the leaf turn brown and fall out giving the leaf a tattered appearance. In Leaf Spot, small brown circular patches appear on the leaf first. As the spots get bigger, they develop reddish brown borders and small black dots appear on their surfaces.These are the fungal spores which can be spread around by splashing water. The pieces of affected leaf tissue eventually die out. In both cases, it recommended that affected leaves are removed and put in a bin, never in compost.

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