In the previous issue of Kentucky Pest News, I wrote about a potentially serious threat of Northern corn leaf blight in 2005. To understand the potential risk, I'll frame the situation in the disease triangle, the most fundamental concept in plant pathology. What the disease triangle tells us is that three components are necessary for plant disease to develop: a causal agent (such as a pathogenic microorganism) must interact with a susceptible host plant in an environment that favors disease development. Like a three-legged stool, it one leg of this triangle is missing, the disease triangle "falls down" and no disease develops. When all three are present, disease develops. So consider the Northern leaf blight (NLB) situation from that standpoint.
The causal agent (=pathogen) The fungus that causes NLB (Setosphaeria turcica, but also known as Exserohilum turcicum and Helminthosporium turcicum), is present in the region in many fields at higher levels now than they have been in several decades. I base this statement on diagnostic records, scattered field observations, discussions with industry representatives, and observations from Ohio, with a long history of study of NLB. The good news is that there is no indication of a new race of this fungus. Race 0 & 1 are known to be widespread in the region, and thus far, tests by colleagues have indicated no races beyond those, although tests are continuing. This suggests that hybrids with genes such as Ht2, Ht3, and HtN will perform as expected, providing very good levels of disease control. (Note that these genes do allow the development of varied types of NLB lesions. However, in hybrids with one of these genes, overall leaf damage is greatly reduced compared to susceptible hybrids.) While the race situation is good news, the overall level of inoculum is still high in many areas, particularly in areas of western Kentucky.
The host plant
Dr. Pat Lipps from The Ohio State University has conducted studies recently to determine susceptibility of a wide range of corn hybrids to NLB. His studies, which he shared with me since my previous article, suggest that the levels of partial resistance among many-perhaps the majority of--hybrids without Ht genes is quite low. This is worrisome. Work continues on the question of how much partial resistance exists in the corn hybrids on the market, but the data from Dr. Lipps certainly raise flags, given the high inoculum levels that are now present on many farms.
The presence of an infectious agent on a susceptible plant does not necessarily lead to disease. The environmental conditions that favor disease development must also be present; otherwise, no disease develops.
As stated above, many farms have high inoculum levels, and there are indications that quite a few hybrids on the market are moderately to highly susceptible to NLB. That is not enough to lead to an epidemic, but producers are at the mercy of the environment. Will we have a repeat of the cool, cloudy, wet weather that predominated in 2004? Of course, the odds are that we will not. However, weird things happen on Earth, and a repeat of that weather pattern is within the realm of possibilities. If we do see a repeat, things could get "ugly" in many corn fields.
While sowing fear is not my usual educational style, I am very concerned about the potential risk for serious damage from NLB next season. The key here is that this is potential risk. High levels of NLB inoculum are present on many farms, and there are indications that many hybrids in use are rather susceptible. Thus, we have two "legs" of the disease triangle. Will we have the third?
We don't know if the weather in the 2005 season will favor disease development. Only time will tell. In the meantime, I encourage producers to pay close attention to the level of NLB resistance in the hybrids they choose for next year, particularly for use in or near fields that had serious damage this past season.
For information about corn pests, visit
"Insect Management Recommendations".
There have been a number of insecticide/miticide recommendation changes to the Small Fruit Spray Guide, ID-94. The 2005 version of this publication will be available hopefully by the first of the year. In this article, I'll summarize those changes for insect and mite control for grapes, blueberries, bramble crops, and strawberries.
Blackberries and Raspberries
Fall colors at the end of the growing season provide inspiration to gardeners and a reminder of the need to maintain health of deciduous tree plantings. When landscape trees failed to perform up to expectations this past growing season, gardeners ask questions in an effort to find remedies for these maladies. This short list of questions and answers may be helpful to County Extension Agents responding to inquiries from their clients.
1. Question: Why do some flowering crabapple trees lose most of their leaves by mid-summer?
Answer: In spring, leaves of flowering crabapples develop scab disease, caused by the fungus Venturia inaequalis. This is a common fungal disease on susceptible varieties of crabapple, and is especially active during wet spring weather. When leaves become infected with scab, they develop olive-green to black spots on the leaf and infected leaves often turn yellow and drop from the tree, resulting in premature defoliation. Although fungicides are available to prevent scab disease, frequent, timely applications are needed. The best defense is to plant a crabapple cultivar resistant to scab. For more information on apple scab management, consult U.K. publication PPA-24, Apple Scab and also U.K. publication ID-68, The Flowering Crabapple, available through Kentucky County Extension Offices. The latter publication provides a list of scab-resistant crabapple varieties.
2. Question: Why do dogwood leaves look off-color, scorched, and unhealthy in summer?
Answer: During recent years, dogwood powdery mildew, caused by the fungi Erysiphe pulchra and Phyllactinia guttata, has become more common on dogwoods in the landscape. Leaves may take on a yellowish or reddish cast in summer or may develop reddish blotches or dead, scorched patches. In some cases, a white powdery growth is visible on the leaves, but this is not always apparent. Yearly infections may weaken trees and cause reduced flowering. Gardeners should avoid cultural practices that stimulate succulent growth and encourage powdery mildew. Rake up the fallen leaves and remove them from the site. Fungicide applications can be used to prevent infections on the most badly affected dogwoods, but must be applied throughout the season. Not all dogwoods in the landscape are equally susceptible to powdery mildew. New powdery mildew-resistant dogwoods are being developed.
3. Question: Why are hawthorn tree shoots and fruits swollen with orange dusty spores on them?
Answer: The shoot and fruit swellings and orange spores which appear on hawthorns in summer and are due to a fungal disease called cedar-quince rust. By fall, only the dark brown, dead, swollen branch tips may remain as evidence of the disease. The causal fungus, Gymnosporangium clavipes, spends part of its life on cedars and junipers and part on hawthorns, apples, crabapples, and quince. Removing nearby cedars or junipers, the alternate hosts of the fungus, will help control this disease. Fungicides can be used in early spring to prevent cedar-quince rust disease. For more information, consult U.K. Cooperative Extension publication PPA-23, Rust Diseases of Apple.
4. Question: Why did some oaks turn brown and drop prematurely this fall instead of waiting until October to turn brown?
Answer: During wet seasons, oak leaves can turn brown prematurely due to fungal leaf diseases such as anthracnose and actinopelte leaf spot, However, many oaks, especially pin oak and red oak in Kentucky are infected with bacterial leaf scorch, caused by Xylella fastidiosa. Leaves of infected branches are green in springtime but in late summer become brown around the leaf margins or at the ends while the part of the leaf nearest the mid-vein or the leaf petiole remains green. Scorched leaves fall from the tree prematurely, but the tree again leafs out normally in the spring. After several years, all the branches of the tree are infected and some small twigs and branches fail to leaf out and so they die. Eventually, the dieback becomes more pronounced and involves larger limbs over the whole tree. Finally, after about 10 or 15 years, the tree has so much dieback that it must be removed. There is no practical cure for bacterial leaf scorch disease. Towns and neighborhoods need to continue replacing lost trees and to emphasize planting diverse species along streets and roads and in home and public landscapes.
5. Question: What are the large, black swellings which have appeared on flowering plum twigs and branches?
Answer: Flowering Prunus species such as purple-leaved plum are susceptible to black knot disease, caused by the fungus Apiosporina morbosa. Plum twigs are infected in early spring and by fall may begin to show evidence of swelling. By the second year, these swellings become rough and black and where they occur, twigs and branches begin to die back. Black knot also affects many backyard fruiting plums. The disease is managed mainly by pruning out and destroying the knots as they appear. More information on black knot and its management is available in U.K. Plant Pathology fact sheet PPFS-FR-T-4.
6. Question: Why are fungus "brackets" or "conks" growing on the trunks of some landscape shade trees?
Answer: Whenever fungal conks (also called fruiting bodies or basidiocarps) are found growing out of the trunk or at the base of a tree, it is an indication that the tree is decaying internally. There are many different kinds of wood decay fungi that can cause heart rot or root and butt rot of living trees. Some wood decay fungi may form hard, leathery perennial conks while others produce soft, ephemeral mushrooms. Infection by these wood decay fungi are almost always associated with stress to the tree and wounding. The decay process is typically slow, and by the time the basidiocarps bodies are visible, internal decay is extensive, and trees are subject to falling or breaking in wind storms. Trees with multiple trunks are often weaker and more susceptible to internal decay. When conks are noticed anywhere on a tree, assume that the tree may have significant internal decay and have it checked by a certified arborist or other tree care professional to make sure it is not posing a safety hazard. Minimize stress and injuries to healthy trees.
Several different species of flies can enter structures in the fall along with the influx of lady beetles and boxelder bugs. The steps outlined in KPN # 1039, October 11, 2004, provide the strategy to follow.
Here are the common culprits:
All three species do not reproduce during the winter so the number in a structure depends on how many get in before cold weather stops their movement.
The vacuum cleaner or shop vac is the most practical way to gather them up for destruction.
Diagnostic lab samples over the past two weeks have included Diplodia ear rot on corn; storage molds on tobacco; common rust on bean; cucumber mosaic virus and Rhizoctonia root rot on pepper.
Ornamental and turf samples included numerous cases of bacterial leaf scorch on oak and a few cases on maple; Actinopelte leaf spot on oak; Phytophthora root rot on rhododendron; Phoma fruit spot on bittersweet; powdery mildew on dogwood; black root rot on ash, boxwood and holly; rust on Panicum sp. (grown as an ornamental grass) and fescue; and Pythium root rot on bentgrass.
NOTE: Trade names are used to simplify the information presented in this newsletter. No endorsement by the Cooperative Extension Service is intended, nor is criticism implied of similar products that are not named.
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