SHADE TREES AND ORNAMENTALS
IPM TRAP COUNTS
A severe case of northern leaf blight was diagnosed last week from a corn field in western Kentucky. This occurred in a hybrid known to be susceptible to this disease. Northern leaf blight usually is a minor component of our corn leaf disease complex. However, the very wet, cloudy conditions of most of May and June were favorable for development of this disease. Periods of below-normal temperatures also have helped to favor the disease. If cool, humid weather continues this summer, we can expect to see more of this disease, although most hybrids grown in Kentucky have adequate resistance.
Symptoms of northern leaf blight include elliptical, grayish-green or tan lesions 1 to 6 inches long with smooth margins. During damp weather, greenish black fungal sporulation is produced in lesions. Older leaves are affected first. Severely affected leaves can be killed when lesions coalesce. On hybrids carrying an Ht2 resistance gene, long, yellow to tan lesions with wavy margins and no sporulation are observed on infected leaves. These resistance-reaction lesions can be easily confused with Stewart's wilt.
The fungus survives in undecomposed corn residue. Spores are spread by air currents. Spores germinate and infect leaves during wet weather with moderate (64- 81oF), temperatures. Severe yield loss can occur when leaves become blighted during early grain fill. The disease is more severe in fields with corn following corn under reduced tillage. The fungus also infects sorghum.
Where the disease is known to occur, use resistant hybrids, especially when grown without rotation under conservation tillage. Hybrids with either single-gene (Ht) or multiple-gene resistance are available. Rotate away from corn and sorghum for 1-2 years. At least 85% of corn fields are past silking, which means they are too late for economic benefit from a fungicide application.
Holcus spot was also diagnosed last week. This is a very common bacterial disease that can easily be confused with chemical spray drift. It produces round to elliptical dead spots on corn leaves. The spots usually have a cream to tan color with a reddish brown to brown margin. Spots usually show up suddenly.
The bacterium that causes Holcus spot thrives on the surface of corn leaves during humid, rainy weather. When its population reaches a large enough size, the bacterium is driven into the natural pores (stomates) on the leaf surface by rain. Once there, it infects small portions of the leaf tissue, resulting in the spots seen on the leaf.
Usually the damage this disease causes has very little to no impact on yield, particularly because it rarely progresses past the initial flush of symptom development observed as silking approaches.
Gray leaf spot should also be beginning to show up. Symptoms of this disease are gray to tan, narrow, rectangular lesions 1/4 to 2 inches long. Lesions on some hybrids exhibit a yellow border. Lesions are restricted by veins. Substantial numbers of leaf lesions usually do not appear until tasseling or later. Older leaves are affected first; severely affected leaves can be killed when lesions coalesce. Weakening and lodging of stalks may occur if a severe outbreak blights leaves during grain fill.
Damage: Yield is reduced through shorter ears and smaller kernels. Yield losses in the range of 10 to 20% are typical in susceptible hybrids grown in Kentucky, although losses of 50% or more may occur under very high disease pressure. Test weight may also be reduced. When leaf blighting is severe, stalks may weaken and lodge as the plant draws nutrients from the stalk to fill ears.
The gray leaf spot fungus survives for 1 to 2 years in undecomposed residue of infected leaf blades and sheaths. Spores are spread by air movement. Leaves become infected during prolonged periods (11 to 14 hours or more) of high relative humidity (>95%) and warm temperatures (72 to 86oF). The disease is most severe in fields with corn following corn under conservation tillage.
As far as management, consider using a hybrid with high levels of resistance in fields where: (1) last years' crop was corn; or (2) corn was grown two years ago and residue cover is at least 30%; or (3) there is untilled corn residue within 150-500 ft. of the field to be planted (the later the planting, the further it should be from untilled corn residue if it is a susceptible variety). Fungicidal control of gray leaf spot may occasionally be economically justified in certain fields of specialty corns. However, fungicide sprays usually do not show justifiable economic returns for commercial dent corn production.
The squash vine borer, a key pest of winter squash, gourds and pumpkins in Kentucky is active throughout the state at this time. Unfortunately, it is usually noticed only after it has done its damage. Symptoms appear in mid-summer when a long runner or an entire plant wilts suddenly.
Infested vines usually die beyond the point of attack with squash vine borer. Sawdust-like frass near the base of the plant is the best evidence of squash vine borer activity. Careful examination will uncover yellow-brown excrement pushed out through holes in the side of the stem at the point of wilting. If the stem is split open, one to several borers are usually present. The caterpillars reach a length of 1 inch and have a brown head and a cream-colored body.
The adult squash vine borer is a stout dark gray moth with 'hairy' red hind legs, opaque front wings, and clear hind wings with dark veins. Unlike most moths, they fly about the plants during the daytime, appearing more like a paper wasp than a moth. Adult moths begin to emerge about the time the plants begin to run, and moth flight continues through mid August. The small brown eggs, laid individually on leaf stalks and vines, hatch in seven to 10 days. The newly hatched larva immediately bores into the stem. A larva feeds for 14 to 30 days before exiting the stem to pupate in the soil.
The key to squash vine borer management is controlling the borers before they enter the stem. Once inside the vine, insecticidal control is ineffective. Poor timing of sprays is the usual cause of inadequate control. Monitor plants weekly from mid-June through August for initial signs of the borer's frass at entrance holes in the stems. Very early signs of larval feeding indicate that other eggs will be hatching soon. Use two insecticide applications 7 days apart to control newly hatching larvae and continue to monitor for additional activity. Sprays need to penetrate the canopy to cover the vines to be effective.
Other Causes of Wilting. There can be several different causes of wilting of individual vines or the entire plant. It is important to determine what is the causing the problem so the proper preventive controls can be used next season. Often more than one factor may be causing a mixture of problems in a single field. Bacterial wilt transmitted by cucumber beetles will also begin to appear at this time. Single vines followed by the collapse of the entire plant is common. Prior to collapse, the plant usually retains its green color. Yellow vine decline, transmitted by squash bug, can also cause various types of wilts and decline. With some types of cucurbits, there will be yellowing of the plants before wilting.
Walnuts, hickories, and pecans growing in Kentucky are prone to a variety of leaf spot diseases caused by parasitic fungi. These diseases commonly result in premature defoliation especially during wet seasons such as this. Walnut, hickory, and pecan leaf spotting and leaf drop are being observed commonly in landscapes and forests now.
Walnut anthracnose. Anthracnose, caused by the fungus Gnomonia leptostyla, causes premature defoliation of affected black walnut and butternut trees. Symptoms appear on newly expanded leaves in spring as brown spots about 1/4 inch in diameter typically surrounded by a yellow halo. Tiny brown blister-like acervuli of the imperfect stage of the fungus (Marsoniella) develop in the spots. When spots are numerous, they cause leaflet yellowing, marginal browning, curling and premature dropping of leaflets. Lesions may also develop on leaf petioles and rachises, fruit husks and sometimes on twigs. During wet seasons, the disease can cause susceptible trees to be nearly leafless by July or August. Premature defoliation may yield nuts with dark, shriveled kernels, thus reducing nut quality and possibly tree vigor.
Hickory leaf spot. This leaf spot characteristically appears in Kentucky on several species of hickory in the latter part of the season. Although infections are thought to occur in spring, these apparently latent infections do not normally begin to appear as leaf spot symptoms until late in the summer. Large, irregular spots on shagbark, bitternut, mockernut and pignut hickories are the result of infection by the fungus Gnomonia caryae. A related fungus causes leaf spots on pecan as well. Typically, the 1/4 to 3/4 inch diameter spots are reddish brown on upper leaf surfaces and brown on the lower. Small brown specks, the fruiting bodies (acervuli) of the fungus (Gloeosporium, the imperfect stage), are sometimes visible on lower leaf surfaces. Where spots are numerous, they cause browning of parts of the leaflet, yellowing, and premature leaf cast. Damage from the disease is usually most noticed late in the season when it may hasten yellowing and senescence of the foliage. Normally, this leaf spot is inconsequential and for the most part, spotting and defoliation should not damage vigorous trees.
Pecan anthracnose. The fungus Glomerella cingulata, cause of anthracnose of many hosts, also causes leaf spot and blight of pecan. The disease is favored by cool, wet spring weather and susceptible trees are often defoliated by mid-summer. Anthracnose usually begins as a small brown area, often along the leaf vein, and spreads so that large sections of the leaf may die. The imperfect (Colletotrichum) stage of the fungus produces tiny dark acervuli in the dead tissues. Anthracnose may also cause twig cankers.
Downy leaf spot. The causal pathogen, Microstroma juglandis, causes white mold or white leaf spot of walnuts, hickories, and pecans. The disease develops on leaflets in early spring as pale yellow leaf spots with downy or powdery fungal growth on the underside. Lesions often coalesce and become brown and dry and by midsummer leaflets begin to shrivel and drop. On several hosts, the fungus induces formation of brooms allowing the fungus to grow perennially within twigs and buds. Although noticeable, this disease is not considered serious.
Disease Management. The leaf spot disease fungi of walnuts, hickories, and pecans generally overwinter in the fallen leaves although in some cases, brooms and twigs provide shelter for the fungus. After surviving the winter in fallen leaves, fungal spores are released and infect trees early in spring. Infections are favored by wet weather and repeating cycles of the diseases occur throughout the spring. To reduce fungal inoculum, rake up and destroy fallen leaves. These leaf spot diseases rarely require fungicide treatments.
Ants are the most frequent and persistent pests encountered around homes and buildings. Besides being a nuisance, they contaminate food, build unsightly mounds on clients' property, and cause structural damage by hollowing out wood for nesting.
To most householders, all ants look pretty much the same. In truth, dozens of different species occur around homes and buildings, each having unique characteristics, which may influence the method of control. In Kentucky, the most common household- invading ants include pavement ants, carpenter ants, acrobat ants, and odorous house ants. The latter ant species has become a huge problem in recent years, causing fits to householders and pest control professionals alike. Knowing which ant(s) you have often requires the help of an entomologist or knowledgeable pest control firm. Collecting a few of the non-winged worker ants in a plastic bag or vial will help with subsequent identification.
Dealing with ants can be very frustrating. The following recommendations pertain to all common structure- invading ants in Kentucky except carpenter ants, which were discussed in a previous (5/10/04) newsletter. For additional information, see the newly revised entomology publication, Ant Control for Homeowners (Entfact- 619).
THE BATTLE PLAN
The mistake most people make when trying to control ants is only spraying the ones they see. This approach usually fails because the ants seen foraging over exposed surfaces is only a small portion of the colony. Typically, there will be thousands of additional ants, including one or more egg-laying queens hidden somewhere in a nest. Eliminating queens and other colony members within nests is often the key to effective control.
Ants Nesting Indoors- Buildings contain many favorable hiding and nesting sites for ants. Preferred sites include spaces behind walls, cabinets, or appliances; behind window and door frames; or beneath floors and concrete slabs. Most of these areas are hidden, making it difficult to determine their precise location. When the location of the nest cannot be determined or are inaccessible, insecticide baits often are a good option, especially for homeowners. The advantage in using baits is that foraging ants take the insecticide back to the nest and feed it to the queen(s) and other colony members. As a result, the entire colony often is destroyed.
Most baits sold to homeowners come pre-packaged with the insecticide and food attractant confined within a plastic, child-resistant station. Some of the more effective ant baits sold in grocery, hardware and retail stores are Combat Quick Kill® Formula bait stations and Combat Ant Killing Gel; Raid Ant Bait II, and Terro® Ant Killer II.
Place the baits next to wherever ants are seen, preferably beside ant "trails" - invisible odor trails that worker ants follow between food and the nest. Do not spray other insecticides or cleaning agents around the baited locations as this will deter ants from feeding on the bait. Initially you should see an increase in the number of ants around the bait station. Do not spray them. This indicates that the ants are feeding on the bait and transporting the insecticide back to the nest. Ant activity often will subside in a matter of days as the number of ants in the colony declines. Continue to place additional baits wherever ants are seen.
Ants are rather finicky in their food preferences and may alter them throughout the year. If one bait product isn't attractive or doesn't seem to be working, try another. Optimal results usually require a sustained period of feeding, not just a brief visitation by a few ants. Professional pest control firms have a wider selection of products to choose from, and can usually provide relief when homeowner efforts are unsuccessful. Retail baits usually will not control carpenter ants, although the Combat® Ant Killing Gel or Terro® baits may be worth a try.
Ants Nesting Outdoors- Ants noticed inside the home may actually be nesting outdoors in the yard. Try to trace the ants back to the point where they are entering from outside; this may be along a windowsill, beneath an entrance door, or where the exterior siding meets the foundation wall. Ants usually prefer to trail along lines and edges. When tracing ant trails indoors or outdoors, pay particular attention to seams and edges created by baseboards, the tack strip beneath perimeter edges of carpeting, mortar joints, foundation/siding interface, etc. Nests often will be located in the ground, marked by a mound or anthill. Other times, the nest will be concealed under stones, mulch, landscaping timbers, pavement, or beneath grass adjoining the foundation wall. Some kinds of ants prefer to nest behind exterior siding or wood trim that has been damaged by moisture. While it takes patience to locate an ant colony outdoors, results will be more permanent than if you spray only where ants are seen trailing. One way to entice ants to reveal the location of their nest(s) is to place small dabs of honey or jelly next to where ants are observed. After the ants have fed, they will head back to the nest.
When a belowground nest is discovered, the colony can often be eliminated by spraying or drenching the nest location with a liquid insecticide such as carbaryl (Sevin), or a pyrethroid insecticide such as Spectracide Triazicide®, Ortho Home Defense System®, or Bayer Advanced® Lawn & Garden Multi-Insect Killer. Large colonies will require greater amounts of liquid to move the insecticide throughout the network of underground galleries within the nest (using a bucket to apply the diluted insecticide is an effective method). Follow label directions for treating ant mounds, paying attention to precautions for mixing and application. Another effective and convenient way to control some species of outdoor and indoor-nesting ants is with a granular bait product, such as Combat® Ant Killing Granules. Sprinkle the bait in small amounts beside outdoor ant mounds, along pavement cracks, and other areas where ants are nesting or trailing.
Ant entry into homes can be reduced by caulking around door throesholds, windows, and openings where utility pipes and wires enter buildings. Ant entry can be further reduced by spraying one of the above-mentioned liquid insecticides around the outside perimeter of the building. Consider applying a 2-to 6-foot swath along the ground adjacent to the foundation, and a few feet up the foundation wall.
Also treat ant trails and points of entry into the home, such as around doors and where utility pipes and wires enter from outside.
In Kentucky, spraying or applying granular insecticides to the entire yard is not recommended, and will seldom if ever, solve an ant infestation indoors. Whole-yard treatments also eliminate beneficial ants, which help to keep other damaging pests of lawns and gardens in check.
BATTLING ODOROUS HOUSE ANTS
The odorous house ant has become the most common and difficult ant species to control in Kentucky and throughout much of the United States. The ant is small (1/8-inch), darkish, and forms distinct trails along outdoor and indoor surfaces. It is often mistaken for the pavement ant, which can readily be controlled with most baits. The most accurate diagnostic difference, visible under magnification, is the absence of a noticeable "bump" (node) along the constricted area between thorax and abdomen of the odorous house ant. Pavement ants have two obvious nodes, and fine grooves or striations along the head and thorax. Pavement ants also are more likely to displace bits of soil from their typical nesting location under sidewalks, driveways and other paved areas. Odorous house ants emit what's been described as a rotten coconut or pine scent when crushed with the finger and sniffed.
Odorous house ants will nest in almost every imaginable location. They commonly nest outdoors under pavement, stones, mulch, woodpiles, flowerpots, and house siding, foraging indoors for food and moisture. Nests also occur indoors within wall cavities, appliances, potted plants, etc., especially near sources of moisture. The nests tend to be mobile; colonies relocate fast and often in response to changes in weather and disturbance. Odorous house ant colonies usually have numerous, egg-laying queens and the primary colonies may split into smaller ones for no apparent reason. Ants foraging indoors feed on all manner of foods, ranging from the trash can to the cereal bowl.
This particular ant is VERY DIFFICULT to control, especially by householders. The better baits to try are often syrupy ones, such as Combat® Ant Killing Gel or Terro® Ant Killer II. As with all ants, activity indoors can sometimes be reduced by removing ready access to food and moisture (water leaks, spillage, trashcans, pet food dishes, etc). Temporary relief can sometimes be had by wiping away the invisible odor trails with a kitchen cleanser or mild detergent. Do not disturb foraging trails, however, if you are using bait. Caulking obvious ant entry points also may be helpful, along with trimming back shrubs and limbs touching the building. In nature, this ant feeds extensively on plant nectar and honeydew excreted by plant-sucking insects such as aphids.
When odorous house ants are the problem, homeowners may be better off calling a professional, although they, too, are challenged by this ant. Some products used by professionals (e.g. Termidor®/Phantom® sprays, certain baits) can be effective, but are not available to the public.
Recent samples in the Diagnostic lab have included sudden death syndrome and potash deficiency on soybean; black shank, blue mold, frogeye leaf spot, angular leaf spot, Rhizoctonia stem canker, Fusarium wilt, Pythium root rot, alfalfa mosaic virus, tobacco ringspot virus and manganese toxicity on tobacco.
On fruits and vegetables, we have diagnosed double blossom on blackberry; Cercospora leaf spot on cherry; bitter pit, and cedar-apple rust on apple; gummy stem blight, Alternaria leaf blight and Pythium root rot on cantaloupe; anthracnose on cucumber and muskmelon; bacterial canker on pepper; bacterial canker, Botrytis canker, early blight and walnut toxicity on tomato; Phytophthora stem rot on squash; and gummy stem blight and magnesium deficiency on watermelon.
On ornamentals and turf, we have seen Pythium blackleg on geranium; rosette disease on rose; powdery mildew and Septoria leaf spot on dogwood; leaf blister on oak; Sphaeropsis canker on filbert; Fusarium canker on golden chaintree, pine and walnut; Phomopsis canker on willow; anthracnose on bentgrass; brown patch on fescue; and summer patch on bluegrass.
|UKREC-Princeton, KY, July 9 - 16, 2004||Black Cutworm 1
||Fall Armyworm 1
||Corn Earworm 1
||European Corn Borer 1
|Southwestern Corn Borer 98|
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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