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Downy Mildew

GARDEN BAD GUYS - DOWNY MILDEW
by Nanette Londeree, Consulting Rosarian

I’ve been growing roses for a long time, and I can’t recall a Spring when I had so many “bare” plants. The roses are flowering like crazy, but many don’t have any leaves. If you have similar symptoms, it’s a good idea to figure out if it is a typical bout of blackspot or if the spots look like something else entirely – the dreaded downy mildew. This disease is the most insidious of the fungi that attack roses, and can defoliate a plant in just a day or two. If you have had nice healthy foliage on all your roses during spring this year, you can stop reading now. If you have some “bottomless” roses like me, read on to find out what this disease is, how to distinguish it from blackspot, and most importantly, how to control it.

Downy mildew is caused by the fungus Peronospora sparsa. Contrary to its name, it really doesn’t look “downy” on roses. Symptoms on leaves vary from yellow, purple to brown irregularly shaped interveinal blotches surrounded by a yellowing of the leaves to a scorch-like burn.

Downy Mildew

The spores are produced on the undersides of the plant’s leaves, but the effects are often first observed on the upper sides. Downy mildew also affects rose canes and stems where it appears as purplish, brown or black spots. Within three days of infection, damage can be observed on the plants, but in some cases, plants have been known to drop all of their leaves before any sign of the disease is obvious.

The fungus is active only under cool, damp conditions, and in our area is usually a springtime problem. Downy mildew spores begin active growth when they are wet for only four hours. Perfect conditions for the growth and spread of this disease are wet weather, with humidity above 85% and temperatures between 65 – 75F. Spores can remain dormant on plants for long periods of time until weather conditions favor their growth and reproduction.

How do you tell blackspot from downy mildew? Blackspot lesions are usually more or less round and mostly on the upper surface. Downy lesions are usually more purple in color and are often less regular in shape and are often bounded by veins of the leaf, at least on one side. Both will cause leaf drop, but downy does it much faster; an infection can defoliate a rose garden very rapidly.

Downy Mildew

Clearly, the bad news is that if you have this disease, it can devastate your roses in short order. If you do see downy mildew on your roses, it is too late to prevent severe leaf drop. Prune your plants back to encourage healthy new growth, and destroy all cut material. Control with fungicides generally requires using some very potent and costly chemicals. Protect foliage with Fore or Dithane; these products contain mancozeb, a dithiocarbamate compound. You can also try products with fosetyl-al, an organic phosphate found in Aliette, Start and Signature, or mefenoxam, the active ingredient in Subdue, and probably the easiest of the chemicals to obtain in our area.

Spores can live on fallen leaves for up to one month - the good news is that they become inactive when humidity drops below 85% and are killed when temperatures stay at more than 80F for several days. Preventing this fungal disease relies on the same basic cultural requirements as blackspot and rust – provide good garden sanitation, good air circulation around your plants, and ovoid overhead watering. And, keep your fingers crossed that Mother Nature doesn’t produce another long, cool, wet spring next year!


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Last Modified: 08/06/2013