Downy Mildew

Alan Henn

Extension Plant Pathologist

V.05.2003scp

In the last several years Downy mildew on roses has only be found on plants shipped into the state from either California or Texas. Conditions in Mississippi are not very conducive to the disease so I suspect that lack of infection in native roses is normal.

The Downy mildew fungus, Peronospora sparsa, is active only under cool, damp conditions. The fungus overwinters without the special resting structures of its relatives. It survives as dormant mycelia in stems and may survive as long as a month in dried, fallen leaves.

When humidity rises to 85% or more, the mycelia resumes growth and produces tree-like sporangiophores that grow out of the lower leaf surfaces. These produce small “seed-like” spores called sporangia.

The spores are captured by slight wind currents and may eventually settle on new host tissue. As long as the humidity stays high and the temperature moderate, spores will be produced.

The most favorable temperature for spore germination is about 65 F, but they can germinate as low as 41 F and as high as about 80 F. It takes about four hours for the spores to germinate once on a new host. Under favorable conditions the new infection may produce sporaniophores in three days. Infected leaves most often have a chevron-like appearance.

Management is best achieved by pruning diseased/dead canes, removing fallen leaves and a rotating series of fungicides. Resistance to fungicides by this fungus is known, therefore be sure to rotate active ingredients. Two of the best fungicides are Subdue and Compass. Coppers can help suppress the disease.

When receiving new plants from out-of-state, carefully prune diseased canes. Watch the plants carefully during periods of cool, damp weather. This years cool spring and cool period the week of April 7 was perfect for Downy mildew. Many imported plants all over the state showed infections.

Early Downy mildew lesions, above and moderate lesions below.

Advanced Downy mildew lesions.