Why Are All the Birches Dying?
Marianne C. Ophardt
Washington State University Cooperative Extension
Area Extension Agent
Chances are that if you=re a gardener you=ve already heard of a plant disease called powdery mildew. It=s a fungus disease that creates a white powdery coating on the surface of plant leaves, shoots, and sometimes even buds, flowers and fruit. What you might not know is that “powdery mildew@ isn’t just one fungus, it=s a group of similar but distinctly different species of fungi. Each species of powdery mildew fungus infects a narrow range of related hosts. This means that the powdery mildew fungus that attacks roses isn’t the same as the one that attacks sycamore... or the same one that attacks grape vines... or the same one that attacks crabapples.
While there are numerous different powdery mildew fungi, they all look and behave very similarly. A powdery mildew infection on plant tissues creates white to gray-white spots or areas on the surface of leaves, shoots, buds, or fruit. Under magnification, one can see the cottony looking strands of fungus mycelium and chains of spores.
Tender, younger plant tissues are more prone to attack by mildew fungi than mature leaves or shoots. As these younger tissues develop, the fungus takes its nourishment from the plant. This usually leads to distorted and stunted growth. When infections are severe, tissues may become severely distorted or even killed.
Before we talk about how to control powdery mildew, let=s talk about the conditions that favor the disease. Unlike many other fungi that attack plants, powdery mildew doesn’t need free water on the surface of plant tissues to enable it to germinate and infect the tissues. All powder mildew needs is high humidity and moderate temperatures (between 60 and 80 degrees Fahrenheit). Garden conditions that favor the disease include:
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damp shady areas |
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poor air circulation due to the crowding of plants or the location in the landscape |
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frequent sprinkler irrigation keeping the humidity high around plants |
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both excessive nitrogen fertilization and watering that favor succulent plant growth |
While powdery mildew can be severe on certain types of plants under certain conditions, it=s seldom a death knell. It can seriously weaken or disfigure some plants or destroy the fruit of some crops, such as highly susceptible rose and grape cultivars. However, powdery mildew is not typically a serious problem in most area gardens and landscapes. That=s because of our hot summer days and usual low humidity.
Perhaps its our spring weather or higher humidity, but this season I have been seeing more powdery mildew than I usually do at this time of year. Now that hot weather has arrived, this shouldn’t cause undue concern because the mildew fungus doesn’t grow much when temperatures are above 85 degrees.
For most of us area gardeners, powdery mildew is only a nuisance that can usually be managed with some simple changes in our growing practices and gardening approaches. These include:
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Remember that old joke.. “Doctor, it hurts when I do this@. The doctor replies, “Don=t do that.@ You can solve a mildew problem by not growing mildew prone plants or highly susceptible cultivars. The most mildew prone plants in our region seem to be modern roses, apple, Kentucky bluegrass, grape, euonymus, crabapple, sycamore, peach, and cherry. When possible, select mildew resistant cultivars. For some types of plants, such as roses, these are readily available.
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Don=t encourage succulent growth with excessive amounts of nitrogen fertilizer and heavy irrigation.
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If air circulation is poor due to crowding, remedy the situation by thinning plants or pruning to facilitate air movement.
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Where heavy shade contributes to the problem, consider thinning the crowns of trees and shrubs to allow for more light... making sure the pruning is done correctly and doesn’t compromise tree and shrub health.
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You can reduce mildew problem by hosing down and thoroughly wetting the surfaces of leaves and stems of infected plants. Do this two or three times a week. This washes some spores off the plant and it also destroys other spores by causing them to burst. Hose the plants down early enough in the day so that they dry off quickly.
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During the growing season, where practical, prune out and remove severely infected leaves or branch tips. In the fall, rake up fallen leaves and prune out badly infested stems to remove sources of reinfection. Remove infected annual plants when killed by frost.
Fungicide sprays aren’t warranted for most mildew problems in area garden and landscapes. Cultural management can usually keep powdery mildew in check, but sprays may be necessary to control it on certain troublesome plants. The key to adequate control is catching the problem early.
The best time to apply a fungicide spray is early in the season as soon as the disease is noticed. The fungicide needs to be reapplied as the plant grows, because these new plant tissues are not protected with fungicide. Good coverage of the entire plant is important for good control of the disease. Be sure to cover stems and both sides of leaves with the spray.
There are a number of fungicides available for control of powdery mildew on plants. Before you use them, make sure the plant to which you=ll be applying the spray is listed on the label. This is especially important for food crops to make sure that it will be safe to eat the produce later. Some chemicals may also damage certain plants and you=ll want to make sure it=s safe to treat your infected plant.
One new product on the home garden market is >Remedy= produced by Bonide. It contains potassium bicarbonate (closely related chemically to sodium bicarbonate, known as baking soda) and has very low toxicity. It=s a “least toxic@ fungicide to use for mildew control, but should be used as soon as mildew first appears.
CAUTIONS:
When using oil or sulfur sprays to control powdery mildew, you need to be aware that plants can be injured if you apply these materials when the temperature is above 90 degrees. It also doesn’t make much sense to apply the fungicides during the hot parts of summer, because hot weather inhibits mildew growth.
With any pesticide product you should read and follow all label directions, especially regarding the safe use of the material. An example of this is triforine (Funginex), which contains sulfur and is often used for mildew control. The label will advise you to wear protective goggles and gloves because of its corrosive nature.