Extension Communications
3614 Administrative Services Building
Ames, Iowa 50011-3614
(515) 294-9915
11/29/99
Contacts:
Mark Gleason, Extension Plant Pathology, (515) 294-0579,
mgleason@iastate.edu
Elaine Edwards, Extension Communication Systems, (515)
294-5168, eedwards@iastate.edu
Yard and Garden Column for the Week Beginning Dec. 3
Disease In a Dry Season
By Mark Gleason
Extension Plant Pathologist
Iowa State University Extension
When you think fungus, you think wet. Sodden shower curtains, damp socks and decaying fruit in the refrigerator. Most fungi will confess to a fondness for moisture. Fungi that attack plants follow the same formula. Most fungi do everything -- grow, attack and make babies -- better in water.
Gardeners know this fundamental fungal fact, since garden-variety plant diseases tend to be most severe when rainfall is frequent and relative humidity is relatively high. Crabapples defoliate from scab, sycamores succumb to anthracnose, tomato plants wither from the bottom up and peonies break out in purple spots.
In most of the state, every growing season this decade has had above-normal rainfall -- several inches above normal. It's been wet for so long, we've just about forgotten what dry looks like.
Until now, that is. Here in central Iowa, thanks to La Nina, we've had nothing wetter than spit since early September. The fall of 1999 recalls the fall of 1987, when a dry spell followed a near-ideal growing season. If you were living in Iowa then, you remember what happened next &endash; the most punishing drought since the 1930s.
I'm not rash enough to predict a 1988-style drought for 2000. But after a well-watered decade, we're about due for a dry year. What would happen to fungal diseases if the rain stayed away?
As you'd expect, the fungi that have thrived on plant foliage during the 1990s would take a nosedive. All crabapples would have their full complement of leaves, because scab would be a non-factor. Peonies would be perfect, irises spotless and tomatoes blight-free. Even Austrian pines would start to look better, since Diplodia tip blight would give them a break. None of these diseases would be gone forever, but it would be nice to get a breather from them after all the sodden, disease-ridden years.
There are nonconformists in every group, even among fungi. A few leaf-attacking fungi thrive when rainfall is scarce. Powdery mildew fungi grow mostly on the surface of leaves, causing patches of white, webby fungus on lilacs, zinnias, turfgrass and hundreds of other plants. If your lilacs look flocked in July, you can thank powdery mildew. Powdery mildew prefers low-rainfall periods -- especially if the humidity is high. If we find ourselves in a siege of sticky, humid, won't-rain weather next summer, look for an explosion of powdery mildews.
Powdery mildew fungi are called obligate parasites since they're obliged to let their host plants survive. Even though they don't kill plants, they weaken them, turning leaves yellow. And not everyone enjoys flocked lilacs. Management efforts usually take the form of mildew-resistant varieties; the French hybrid lilacs are more resistant than the traditional Persian lilacs, for example. In a few cases, such as muskmelons or cucumbers in the garden, fungicide sprays against powdery mildew may be justified as soon as disease symptoms appear. If you go the chemical-warfare route, make sure the fungicide you use lists powdery mildew on its label, because many fungicides won't impress these fungi at all.
The scavengers, another group of misfit fungi, prey on stressed plants, especially trees and shrubs. Scavengers belong to Mother Nature's cleanup crew, whose mission statement says, "remove the weaklings from all walks of life." A vigorous plant defends itself very effectively against microbial invaders, including fungi, by sealing off and poisoning them. But it takes energy to defend yourself, and energy is exactly what a stressed plant lacks.
Stress can come in many forms -- freeze injury, poor soil aeration, flooding, etc. -- but drought is probably the most common source of stress for woody plants in Iowa. An insufficient water supply can cause wilting, a slowdown in photosynthesis and a gradual depletion of stored energy. When the stored energy is gone, the tree or shrub has nothing left to devote to defense. The next scavenger fungus to invade is in luck, because it's stumbled onto a great big food supply that can't fight back -- an unguarded grocery store.
Canker diseases result when scavenger fungi invade stressed-out woody plants. A "canker" is an area of dead bark -- usually sunken and discolored in the case of fungal cankers. The surface of the canker may even show numerous tiny, pimple-like bumps, which are the spore-making factories of the canker fungus. These invaders usually slip in through branch stubs or cracks in the bark.
In a vigorous tree, these break-ins would be arrested almost before they got started. In a stressed tree, though, the canker can spread over weeks, months, or years, becoming inches or even feet long, and eventually girdle the limb or trunk -- whereupon everything above that point withers and dies. The calling card of canker diseases is a hit-and-miss pattern of scattered dead limbs in an otherwise healthy-appearing tree.
In Iowa landscapes, canker diseases are especially common on blue spruce, Russian olive, honey locust, Scots pine and shrub-type dogwoods. But other trees and shrubs will break out in cankers, too, if sufficiently stressed out.
White pine, for example, developed canker problems on a few sandy sites after the 1988 drought -- the first time this disease had been reported on white pine since the 1930s drought. Cankers can appear during a drought, or as much as several years later -- as long as the tree remains weak and energy-deficient.
The only defense against cankers is stress prevention. The challenge is to keep plants healthy enough to fight off canker fungi on their own.
When it comes to drought periods, watering is the first line of defense. To water a tree adequately, soak the entire drip zone -- the circle of ground under the tree's canopy -- so that the wetting goes at least a foot deep. You can check the wetting depth with a shovel. Once-a-week watering is about right; daily spritzing can actually do more harm than good, since over-watering can smother roots.
Mulching is one of the best ways to drought-protect your trees and shrubs. A 3- or 4-inch-deep layer of wood chips over the entire drip zone will do your woody plants a world of good by encouraging root growth and retaining moisture. Even better, the mulch layer backs the turfgrass away from your trees. Your tree will be grateful to you, because turfgrass is a vicious competitor for water.
Wet or dry, there's a fungus that will thrive and cause problems for your plants. With a drought looming, a good time to get out and start watering your woody plants would be -- yesterday. Wetting the soil now will help trees and shrubs cope with winter, and provide some stress protection as we head into the new millennium.
ml: isugarden
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