From March 6, 2006 Kentucky Pest Newsletter
HOW WILL THE DROUGHT OF 2005
AFFECT LANDSCAPE PLANT DISEASES?
By John Hartman
Much of the bluegrass and
eastern parts of Kentucky
suffered dry weather during 2005. Rainfall in most Kentucky locations was below normal every
month except January and August. August would have been more deficient except
for the rainy aftermath of Hurricanes Dennis and Katrina. Indeed, the Bluegrass
region suffered moderate to severe drought for most of the summer and eastern Kentucky was in a state
of severe drought by summer's end. Even now, in late winter, Bluegrass
and eastern regions are in a state of mild drought. Woody plants. Wilt and leaf scorch symptoms are often associated with dry weather. In addition, drought-stressed plants close their stomata which reduces their rate of photosynthesis. Reduction in photosynthesis may not kill a tree or shrub, but it means fewer carbohydrates are made and stored for future use. In the landscape, seedlings and recently transplanted trees and shrubs were at greatest risk because they lacked extensive root systems.
With drought, there are some fungal diseases of landscape trees and shrubs that often do not show symptoms until the following season, after the drought has passed. The role of water stress in encouraging opportunistic plant pathogens is unclear. It is possible that the stress condition interferes with the plant's defense against such pathogens, or possibly, the reduced carbohydrate reserve allows the plant little energy to fight invasion by pathogens.
Expect certain fungi such as Hypoxylon, primarily an oak pathogen, and Armillaria, which attacks many woody plants, to appear in 2006 because of the 2005 drought stress. In addition expect symptoms of diseases caused by other fungi such as Thyronectria, (honey locust canker); Cytospora or Valsa, (cankers on prunus, poplar, willow, maple, spruce and other conifers); Diplodia, (pine tip blight); and Botryosphaeria and Nectria (cankers of many woody plants such as rhododendrons, crabapples, dogwoods, maples, and others) to appear the season following the dry weather.
In searching for water, some woody plants could have sacrificed surface roots to the drought while relying more heavily on roots that were deeper in the soil. If excessive rains return, partial flooding could render these deeper roots more prone to root rot diseases, thus leaving the woody plants with few functional roots. Thus, expect additional woody plant death when the drought breaks.
One possible benefit of the drought could be the reduction in foliar diseases this year. There could be less carry-over inoculum from shade tree anthracnose diseases, crabapple scab or rose black spot, for example. The benefit could be short-lived, however if spring weather is wet and rapidly repeating cycles of these diseases occur. Looking ahead even farther, the rust infections of cedar that should have occurred, but didn't, during the dry 2005 summer might result in fewer cedar galls in the spring of 2007 and less rust on crabapples and hawthorns that same summer.
Herbaceous ornamentals. Perennial flowers and ground covers, like their woody counterparts could have reduced energy reserves due to the drought. This could make them more susceptible to cankers and to root, corm, or bulb rot diseases. There is not much research on the role of stress on diseases of herbaceous ornamentals, so it is difficult to know how the drought will affect these plants. A few diseases such as Volutella blight of Pachysandra, are known to be more severe on stressed plants, but most likely the disease would have appeared during the drought. For foliar diseases, the situation is similar to that of woody plants - reduced primary inoculum might result in less disease, at first.
Tree fruits. Tree fruits in the landscape and orchard are subject to many of the same diseases as shade trees. Fungi such as Nectria, Cytospora and Botryosphaeria cause cankers of tree fruits suffering from drought stress. The effects are likely to be the same as for landscape trees. As for reduced inoculum for foliar diseases such as apple scab or cherry leaf spot, again the response should be about the same as for landscape trees.
Small fruits. Blueberries and brambles in the garden are especially susceptible to fungal cankers, and grapes also can become cankered. They are likely to react to drought in a similar way as woody landscape plants. Reduced foliar diseases could also be expected for these crops, at first. Strawberries that were not watered probably died last summer from lack of water or from the black root rot complex which is usually more severe on drought-stressed crops. On the other hand, if they did survive, this season could bring a reduced threat from leaf spot and anthracnose diseases, at least at first.
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