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Plant Diseases

The cause of late blight is a fungus known to scientists as Phytophthora - though I called it much worse than that. I didn't know at the time that the same fungus also caused the Irish potato famine, and a related fungus causes root rot in rhododendrons. Phytophthora is a nasty one.

If I'd been paying attention, I might have noticed the signs of late blight before the infection devastated my entire crop - some 70 plants. According to my reference books, late blight starts with small pale green to dark water-soaked spots on leaves. The upper halves of tomato fruit display grayish green spots that enlarge and darken. I didn't notice any of that. Perhaps if I'd noticed those signs and removed the plants, I might have saved part of the crop. But I didn't.

To prevent a recurrence I brought lots of brush into the garden, threw all the infected plant material onto it, and burned it. Afterwards I went up and down the rows with a propane-fueled flame weeder (available from Fedco Seeds or 207-873-7333) and scorched the earth. The following spring I turned that part of the vegetable garden into perennial flower beds. I was taking no chances on another fiasco.

Even if I had been a conventional gardener, one who uses chemical pesticides, I would not have been able to save my tomatoes that year. According to Dr. Smith Cheryl, plant pathologist at UNH Cooperative Extension, "Once a plant is showing symptoms of disease it's usually too late to cure it or manage the disease. You can't get rid of most infections that are already there. We can't cure the leaves that are already spotted. Either we live with the imperfection or we 'amputate', removing the infected parts - or the whole plant."

The best approach is to prevent disease from occurring. According to Dr. Smith, this is done by growing vigorous, healthy plants that will naturally resist diseases. Her recommendations include:

• Select resistant cultivars where possible.
• Don't over-fertilize. Too much nitrogen produces plants that are often susceptible to diseases.
• Practice good sanitation in the garden: "Start clean, keep it clean, and end clean." Many diseases overwinter in foliage or stems of plants, so clean up the garden in the fall.
• Remove -amputate- any diseased leaves and stems as soon as they show signs of disease. Obviously, that rule must be used with common sense. You won't want to remove all the leaves of a lilac with powdery mildew, for example.
• Don't overreact. Many plant diseases are ugly, but not lethal. Neither apple scab nor powdery will kill your plant. Put up with some imperfections instead of grabbing the sprayer full of chemicals.

I grow some tomatoes inside a plastic hoop house and have never gotten any of the fungal diseases that cause the lower leaves to blacken and dry up. That, I postulate, is because there is no splash-up from the soil, which happens in a hard rain. I water carefully with a watering wand. Outdoors, mulches can help minimize splashing.

In recent years, scientists have been working hard at developing natural bio-fungicides safe for organic gardeners. One such product goes by the trade name "Serenade." It comes as a bacterium-containing powder that you dilute in water and spray on susceptible plants. The bacterium, Bacillus subtilis, penetrates and destroys the disease spores, but does not harm any beneficial insects or wildlife. You can use it against powdery mildew, gray mold, early blight, bacterial leaf blight, botrytis neck rot, downy mildew, fire blight, scab, bacterial spot and pin rot. It is said to stimulate a plant's own disease-fighting mechanisms, as well. It is approved for use by organic gardeners, and has no restrictions about when to use, or on what.

Usually by this time of year I have powdery mildew on the lilac by our front door. But this year I sprayed Serenade twice, and so far there is no powdery mildew. I sprayed some on a peony that looked to be developing botrytis in early June and I cut off the infected portion. The plant bloomed and looks great now. These experiences, of course, do not PROVE that it works. Still, I believe it helps, and will use it next year. I sprayed all my tomato plants, too. I bought mine from Gardens Alive (or 513-354-1482), though some garden centers may sell it, too.

There are no guarantees in the garden. I like the idea of using biology instead of chemistry to solve problems. Maybe one day we'll be able to have safe solutions for all the garden's problems.

Henry Homeyer is the VT/NH associate editor of "People, Places and Plants" magazine. Write him at gardening.guy@valley.net or P.O. Box 364, Cornish Flat, N.H. 03746.