Plain as Dirt: It is most likely mildew to you
I don’t know about you, but I’m sick of the term “green.” When I spot that term in an article or headline, it’s a surefire buzz kill for me – which is why I’m not going to use that term here to describe some concoctions appropriate to controlling a couple of fungal infections we are sure to see: powdery mildew, which is a catch-all term for a group of fungi; and downy mildew, which is, specifically, the fungus plasmopara viticola.
I’m certain some of you are seeing some downy mildew damage already. I’ve seen it. It requires wet foliage and spreads even in cool temperatures lower than 45 degrees. (I’ve been out of town. Has it rained here?)
I suspect the worst may be over in the way of downy mildew, but as things warm up higher than 70 degrees, its doppelganger, powdery mildew, is sure to flourish here because of its ability to spread when it’s humid and not wet. Someone wisely named these mildews similarly as the two mildews look alike. That foresight only makes it easier to distinguish the two – particularly in conversation.
Downy mildew shows up commonly as scattered yellowish – sometimes, but infrequently, white – spots a little smaller than a dime on the top side of foliage. Its distinguishing
characteristic occurs on corresponding infected areas on the underside of the foliage which sprout a white, cottony-looking growth. As it progresses, the infected areas will become reddish-brown dead spots on the leaves. I’m certain some of you are beginning to see some downy mildew damage.
With cool temperatures behind us, though, I’m expecting powdery mildew to be the 2009 fungi of the year. Under our normal dry conditions we typically have a pretty good infestation of this group of fungi on roses, lilacs, monarda (bee balm), and phlox paniculata (garden phlox). Increase the humidity and I expect some of those plants to appear in July as being drifted over with snow.
Powdery mildew is so common, I’m sure you’ve seen it. It begins as hazy white blotches on the top sides of leaves, but there is no cottony growth on the underside. The blotches turn yellow as the disease progresses. With the current dampness you can count on an infestation.
Reformed as I am, and believing that this fungus is going to be a problem, I decided not to reach into my old bag of tricks to gain control of these diseases. My old bag includes a once favorite triazole fungicide having a half life in soil and stream sediments of approximately two thirds of a year.
Bugs and birds are not fond of triazoles when attempting to further their breeds. I’ve decided this is not acceptable in my gardens today, although the EPA is still comfy with that concept as it relates to the greater good. I can see their point when the greater good comes into question in controlling soybean rust. But, the average gardener is not going to starve if the lilacs by the front steps are mildewed.
All that aside, my current dilemma is, having led a chemically engineered professional life, I’m lacking a new bag of tricks filled with magic bullets for firing at fungi. So, I had to do some research which led me to a way cool government funded organization whose website I invite you to visit. You can find The National Sustainable Agriculture Information Service website at http://attra.ncat.org. With such an easily remembered web address, I figure it doesn’t hurt to point out that there’s some good, current research worth searching out on this site.
One of the fungicides recommended there for controlling both diseases was Bordeaux Mix, a mixture of copper sulfate and hydrated lime which is designated an organic pesticide by the USDA. It is a fungicide that has been around since grapes were discovered to be better served as wine – well almost – late nineteenth century. I have to admit, I have never used Bordeaux mix. I’ve got an excuse, though. It leaves a residue on leaves, which is not suitable for retail ornamental plant production. Furthermore, Bordeaux mix can burn foliage and fruit if applied when plants have not been adequately watered or are stressed. Bordeaux mix can be purchased relatively inexpensively at almost every garden center or home improvement store.
The other fungicide I found is a concoction using baking soda which doesn’t control downy mildew, but wreaks havoc with powdery mildews. Very nice. My own mother bakes cakes with it. There’s probably a box in your fridge.
There is some question as to the correct dosage, but most recommendations call for 4 teaspoons of baking soda per gallon of water. To beef up the efficacy, add to the mix one _ teaspoon of liquid dish soap. To get the absolute most out of your treated gallon of water throw in a tablespoon of Dormant oil, which is another certified organic pesticide. This recipe may also control black spot and botrytis. Baking soda, soap, and oil may also burn foliage and fruit if your plants are water stressed, but, at the rates recommended here, they won’t persist in your yard for the better part of year.
As the spread of downy and powdery mildews has already begun, I wouldn’t wait to spray your plants – particularly the aforementioned plant varieties. Prevention is better than the cure.
Vail Valley ranch takes a European approach to promoting welfare of this keystone species