But if you recently planted a rose and notice all the leaves are scorching and falling off, it may be a result of a disease called downy mildew. This fungal disease is almost always introduced on infected, newly purchased roses grown out of state. It does not appear that the fungus survives our winters, or maybe we don't have the right weather conditions to initiate the disease. Although we don't worry much about this disease in older plantings, it can be a serious problem on recently transplanted roses, especially during rainy, relatively cool springs. It can also cause significant damage in holding areas in the nursery or garden center.
Downy mildew starts out as light yellow blotches on the upper surface of the leaf.
The fungus produces a sparse tuft of growth on the bottom surface of the leaf just beneath the yellow discoloration. But this fungal growth is difficult to see and often overlooked. Leaves eventually develop light tan, angular scorched areas that correspond to infected areas and will eventually drop off. Extensive defoliation may weaken the plant, especially during the transplant period. The best way to control this disease is to avoid accepting plants, either in the nursery or home, that show symptoms of downy mildew.