Downy mildew of Rhubarb
The symptoms of this disease are first seen on the upper leaf surfaces. The light-brown patches that develop are angular at first because they are bounded by veins. If the weather continues cool and wet, however, the patches may continue to spread until large areas of the leaf are brown. Eventually these areas will tear, and pieces will fall out so that the leaves have tattered appearance. Severe defoliation can occur. Underneath the leaf the furry growth characteristic of down mildew is produced, and the spores on this furry growth are spread about by the wind to other leaves and to other plants.