Tell Powdery Mildew to take a Powder!

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Q. Dear Mike: Our garden was hit with late blight on tomatoes and powdery mildew on squash, gourd, and melons. We ripped out the tomato vines and used neem oil and milk spray on the powdery mildew. My question is about next year's garden. We won't plant tomatoes in the same place, of course. But is there anything else you would recommend to stop the blight and mildew before they get a foothold? Thanks!

---Jason in southwest Virginia

A. It's always good to grow your tomatoes in a different spot every year, but it has nothing to do with the dreaded late blight that struck so many gardens last season. Rotating tomatoes is primarily done to avoid soil-borne wilts. But rotation of all susceptible plants is recommended for powdery mildew prevention, especially on the vining plants you name. Even more important are sanitation and clean-up. Promptly remove affected plants at the end of the season and remove and destroy any mulches that were near them—those mulches are harboring fungal spores that could well get things going again.

But the spores that cause powdery mildew can still blow in from other gardens, and often do—they're prevalent in the environment. So, provide lots of airflow around those vines during the growing season. Leave extra room between the plants, and trellis or otherwise support them—it can be impossible to avoid disease on sprawling plants in a crowded garden. (And 'growing up' allows you to give the plants more breathing room in the same or even smaller footprint.)

Again, most mulches are notorious for harboring disease spores. Wood and bark are the worst. And even though they're one of my favorites, shredded fall leaves should also be avoided when you're trying to get through at least one symptom-free season to break the cycle of disease. Compost is the ideal mulch. A one to two inch layer of compost on the surface of the soil feeds your plants, prevents weeds, and provides a hostile home for spores looking to breed; high-quality compost contains living organisms that eat disease spores as soon as they land!

Unlike other diseases—like anthracnose, downy mildew and almost all the others—mildew can occur in dry conditions. And although it seems counter-intuitive, British researchers reporting in the journal Plant Pathology found that rinsing the whitish-grey powder off your plants with sharp sprays of water can be a surprisingly effective cure. Just be aware that wetting leaves will make other diseases worse; so make sure you have powdery mildew and do the wetting in the AM only.

(Powdery Mildew Symptoms: small 'powdery' round spots, white or grey in color, that start on oldest leaves and quickly get bigger and more numerous, eventually covering the plant in what appears to be white powder as the leaves start to turn brown and die. The unrelated downy mildew makes leaves turn yellow first, the mildew appears more purple in color, the spots don't start out round, it looks more like down than powder, and tends to appear only on one type of plant in each garden, not affecting different plants nearby.)

I like Jason's use of neem oil, as the oil form of this plant seed is a very effective anti-fungal. (In another, harder-to-find form, neem acts as an anti-feedant; and if pest insects eat anyway, they die. You go, neem tree seed!)

And although you may be scratching your heads at Jason's mention of milk, he's on the money with that one as well. I've always wondered who figured this out, but a 10 percent milk solution sprayed on affected leaves beat chemical fungicides in controlled studies of powdery mildew remedies. To try this at home, mix nine parts water with one part skim milk and spray affected leaves well in the morning, trying to coat both sides. If the disease progresses despite your use of milk, neem or the other remedies we'll mention in a moment, cut your losses by destroying the affected plants before they can make the rest of your garden sick.

Q. I've been plagued by powdery mildew on my floribunda and hedge roses for several years. I've also had it on my lilacs, which are nowhere near the roses. I was close to giving up last year, though I love my roses and have tended them for 15 years. Please help! I don't have the ability to make my own compost.

---Distressed in Salisbury, MD

A. Well then, buy some compost, "Distressed", as it is essential in helping those two disease-prone plants fight infection. (You seem to know this by your mention of it at the end of your email.) And you live in Maryland, home to the great "Leaf Gro" program that takes all the state's leaves and turns them into widely available premium compost. You Marylanders have easier access to high-quality compost than listeners in virtually any other state!

If you're pruning your roses in the fall, stop. Wait until they begin growing anew in the Spring, prune them then, get rid of any old mulch along with the prunings and lay an inch of compost on the surface of the soil as your new, disease-preventing mulch.

Remove any mulch under the lilacs as well, and replace it with compost as soon as possible in the Spring. Obviously you want to wait until after the lilacs have bloomed to prune them. When you do, try to open up the centers of the plants to increase sunlight penetration and air circulation. And if they're crowded by other plants, prune or remove some of those airflow stoppers. (Do this over the winter if you can.)

If disease appears anyway, compost tea or the Cornell Formula might work better here than the cures we've already mentioned. (They also work well on other kinds of plants.) You'll find all the details on compost tea, the Cornell Formula and many other helpful things in our previous Question of the Week on those topics; just click on the link above.

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