Our Private War With Downy Mildew

By Gaird Hamilton
National Award of Merit Winner

If you read in the average book about the care and raising of roses you will find that it will probably say that there are three main fungus diseases of roses. These are blackspot, powdery mildew, and rust. Since these diseases are very prevalent wherever roses are grown, there will be instructions on the various ways to rid your garden of these problems. This is usually a regular spray program either of an advanced spray which can be very effective, or a more environmentally friendly spray which can keep these diseases down to an acceptable level. Only a very few up to date books mention downy mildew at all.

Since most of us live in the cool coastal fog belt which exists along the Pacific Ocean, this lack of information is very crucial. Downy mildew, while being the scourge of many other plants, has not really been recognized as a serious problem in rose gardens until just recently. It was a problem in florist roses raised in greenhouses but that was it. Our problem is that our moderate climate which is so close to greenhouse temperatures and very humid, is just right for downy mildew too.

If you are raising roses here and are not having a problem with downy mildew, count your blessings. If however you are having a problem with something and you are not sure just what it is, then this may be the answer. We have had a regular spray program for a number of years, and we are frequently told that our leaves look “good enough to eat”. That is because we have the other fungus diseases well in check. Some parts of our yard have been troubled from time to time with downy mildew though. When the conditions are right for it, you can get a bad outbreak in a matter of hours. Unlike blackspot which starts at the bottom of the bush usually by spores bouncing up from the ground, downy mildew start in the upper part of the bush. It attacks the fastest growing nice new canes and their flowers. Obviously it spreads through moisture plus wind since we get it in the windiest part of our garden usually after an untimely rain or extreme fog, ie. drizzle.

The symptoms begin with some of the healthy big young leaves turning their bottom side up and the bottom side has a redder look. Then the newest leaves developing right below the bud will seem less vigorous and the bud will be smaller than usual. At this stage to a visitor your roses will still look very good and heathy, but if you have had it before you know what is coming next. The next thing is the appearance of irregular patches on the leaves which are a purplish red color with some of the leaves having a jagged patch right up the main vein of the leaf. The health of the cane begins to get worse as it loses color and the leaves and flowers on the affected canes look like all of the nutrients have been cut off. The flower if it does bloom is very small and deformed and is the wrong color looking washed out. If allowed to continue, the canes might develop areas of a deep purple color. Leaves start falling. By this time the bush is looking very sick if it is a variety that is very susceptible, and it looks in danger of dying.

In most parts of the country, summer would take care of the problem because downy mildew can’t function at temperatures over 80degrees, for that matter 40 degrees is the low side boundary which ends its cycle. Here, we must remain vigilant and if you have ever had these kind of problems, get a regular spray program going. The first spray which can be effective on downy mildew would be your dormant spray. A copper based spray such as Kocide mixed with horticultural oil (for the aphid eggs and scale, etc.), is a good combination, but must be put on before the new growth starts to grow. When you begin your spray cycle, sprays which can be added to your spray for blackspot, powdery mildew, and rust include Aliette, an aluminum based product, Subdue 2E, a systemic containing metalaxyl, and Pace which contains Subdue along with Mancozeb. Subdue 2E can also be used as a drench where you would mix the drench and pour about 2 gallons per bush. This goes to the roots and as a systemic on into the bush. We have heard that Daconil is useful against downy mildew and since it is also a good contact spray against other funguses too, it could be mixed with a systemic to make a great overall spray. I must admit though that we have not tried Daconil specifically against downy, so we can’t be sure.

If you keep close watch on weather conditions and have your sprays and drenches up to date, hopefully you will avoid the problem, but no controls are really 100 per cent effective against downy mildew so you might have some show up anyway. If so, the only solution is to treat the plants some more and to cut away the diseased parts of the bushes. After the downy has been in the canes for awhile, you can look at the affected part and if you look on down the cane the rose will put out new growth below the diseased part. Cut back to right above the new growth and burn the material which you removed. Sterilize your pruners with alcohol or Lysol after you make your cut. If no new growth has started below, either cut off the entire cane or cut it back to where you think that it is healthy. Cleanup around the bushes and keep it clean.

Like I said earlier, if you don’t have downy mildew, count your blessings. We will be expanding our roses in the healthy part of our yard where the roses are out of the wind and doing good. We will be retreating from the bad areas of our yard and probably putting in easy care perennials. I’m sure that we will still be trying to win the war against downy as best we can, and I’ll keep you posted if they come out with a great new cure. Good luck!