When I speak of this disease to gardeners, they usually think I am referring to powdery mildew and have changed the name slightly. Downy mildew is not related to powdery mildew. Powdery mildew is a disease that we see every year on rose and many other hosts. Downy mildew is usually a greenhouse or production problem that rarely makes its way to home gardens in Illinois. This year may be an exception.
We have had several reports and two confirmations of downy mildew on rose in Illinois. Both were from commercial growers who spotted the problem, had it identified, and removed infected plants from the sales area.
Downy mildew is a fungal disease that causes brown blotches on rose leaves, often with a yellow area around the blotches or a reddish tint to affected leaves. The first time I saw the disease, I thought that the plant roots had pulled up some herbicide into the leaves. The blotches tend to be limited by veins, resulting in a somewhat blocky pattern. Reddish streaks or blotches may occur on stems and calyxes. The fungus sporulates on the undersides of leaves, but this can go unnoticed, especially in dry weather. Use a hand lens to look at the undersides of the blotches. A white, downy fungus should be present in humid conditions. This disease becomes serious in extended cool, wet weather. Leaves turn yellow and drop, and the entire plant may become defoliated except for the youngest leaves.
Greenhouse and nursery production areas concentrate control practices on maintaining relative humidity below 85 percent and using chemical controls. Homegrowers should watch for this disease on newly purchased plants and destroy infected leaves, stems, and flowers. The fungus overwinters in stems, but it is not clear whether it will overwinter outdoors in Illinois. Fungicides registered as preventives include Aliette
T & 0, Chipco Aliette, and mancozeb. The fungus is an oomycete, so effective fungicide choices are limited.
We do not have an RPD that discusses rose downy mildew. A very good source of information is the Compendium of Rose Diseases by APS Press. Junius Forsberg also discusses this disease in Diseases of Ornamental Plants.