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Downy Mildew

Downy mildew is a bad actor. A very bad actor.  Most rose diseases we struggle with here in the Northwest only weaken and disfigure plants, but downy mildew kills them.  And it kills very rapidly.  I have seen reports of rose bushes being wiped out within a few days following attack by downy mildew.  It is also contagious  rapidly spreading to adjacent bushes under proper conditions.  So, while downy mildew is a disease you wont see often, the prudent gardener will nevertheless learn to recognize it and to take immediate action when it appears.

Downy mildew affects many types of plants, being a major pathogen of onions, cucurbits, alfalfa, soybeans, and grapes.  Virtually all varieties of roses are also susceptible to the disease.  Some are more susceptible than others.  Several years ago I was unlucky enough to see downy mildew in my own rose garden, but it seriously affected only one variety  Liverpool Echo.  Other surrounding varieties appeared unaffected.

Downy mildew is a disease of early spring.  It is particularly virulent in those unusually cool springs when the weather remains moist, cold, and overcast for a prolonged period.  The fungal organism that causes downy mildew, Peronospora sparsa Berk., cannot survive when the humidity drops below about 85% and/or the temperature exceeds about 810F.  Therefore, you will almost never see it in summer.  Spore germination, hence infection, is most effective when the temperature is around 650F.

The most striking symptom of downy mildew is spectacular leaf drop.  An infected bush can almost completely defoliate within days.  Upon closer inspection of the plant, you will note that the younger leaves exhibit dark, purplish angular blotches on their upper, but not lower, surfaces.  These blotches tend to follow the leaf veins.  The surrounding leaf tissues are often yellowish or light green, and the leaf petioles may be red.  In severe cases, purplish lesions may also develop on the canes.  White powdery conidia and conidiophores sometimes, but not always, form on the undersides of the leaves.

The keys to identifying downy mildew, then, are: (1) rapid and alarming early spring leaf drop, (2) dark, purplish, angular (not rounded) blotches on only the upper surface of the leaves, and (3) occasionally, a white powdery material on only the lower leaf surfaces.

Okay, if you see downy mildew on your roses (pray that you dont) what do you do about it?  The most important thing is to act quickly  a few days hesitation on your part can mean life or death for your rose plants.  Fortunately, we have a product that is effective against downy mildew.  Aliette,. by Bayer, is an inorganic aluminum phosphate (fosetyol-AL) that not only kills the fungus on contact, but is also taken up by the plant and translocated through its vascular system to provide a level of systemic protection.  When using this product be aware that you cannot mix it with other chemicals and you should not use a spreader-sticker with it.

When using any garden chemical, be certain to read and follow all label instructions.  In the longer term, good sanitation (removal of leaves in autumn) can prevent carry-over of the organisms from year to year.  All infected plant parts should be removed from the garden and destroyed.  Once the disease has been eradicated, the rescued plants should recover fully and do not need to be removed.

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Updated July 02, 2004
Copyright 2004 ARS Pacific Northwest District