Horticulture Pest Update - Summer Pests (June and July)
submitted by Mary Jane Frogge, Extension Associate
Anthracnose Diseases on Trees (June):Anthracnose diseases on trees: Anthracnose is a common problem on trees in Southeast Nebraska during cool, wet spring weather. Symptoms first appear as water-soaked spots on the expanding leaves. The spots enlarge and develop brown lesions. The lesions are often associated with leaf veins and margins. Growth of the infected tissue slows or stops as the rest of the leaf continues to expand. As a result, the leaf tissue around the lesion becomes twisted and wrinkled. Defoliation also may occur. See NebGuide: Anthracnose Diseases of Shade Trees G1200
Herbicide Damage on Tomato
Broadleaf Herbicide Damage (June):
Numerous plants affected, but tomato, potato, grape, and redbud very sensitive. Leaves on affected plants are cupped, thickened, distorted or leathery, and develop an uncharacteristic fan shape. To determine whether herbicide damage is to blame, look at surrounding herbicide-sensitive plants such as potato, pepper, grape and redbud to see if they also show twisting or distortion. Plant growth-regulating herbicides are commonly used in home lawns early in the season to control dandelions and other broadleaf weeds. If misapplied and accidentally sprayed on sensitive plants, they cause severe injury or even plant death. Unfortunately, these plants don't have to be sprayed directly with herbicides to cause damage. Some broadleaf herbicides such as 2,4-D are volatile, especially during hot weather, and may drift across the yard or even from adjacent yards in concentrations sufficient to cause injury. Therefore, you don't necessarily have to be using broadleaf herbicides in your yard to suffer damage.
Brown Patch - Turf (July):
Brown patch of turfgrass is caused by the fungus Rhizoctonia solani Kuehn. All commonly cultivated turfgrasses in Nebraska are affected by this disease, but differences in susceptibility exist within cultivars of the various turfgrass species. A primary host is tall fescue. Certain species of Rhizoctonia are capable of attacking turfgrass plants from seedling stage to mature plants and are pathogenic over a wide range of environmental conditions.
Brown patch symptoms vary, depending primarily on turfgrass species and mowing height. The degree of turfgrass injury depends largely on susceptibility of the cultivar, management practices and weather conditions.
On home lawns, the disease is the presence of roughly circular patches of dead and dying grass. Diseased areas may encompass large portions of the turf. Turf with these patches appears somewhat "sunken." The grass in the center of the diseased patches may be less affected, giving the appearance of the "frog-eye" symptom commonly associated with summer patch. However, the presence of the characteristic brown patch leaf spot on individual blades should distinguish it from this disease. Also, the affected turf appears less matted than that affected by summer patch.
Green plants within the affected turf have grayish-colored leaf spots that are long, irregularly shaped and surrounded by a dark brown margin.
Brown patch occurs on dense, heavily fertilized and watered turf in hot (above 85 degrees F), humid weather when night temperatures remain above 60 degrees F. Poorly drained soils, thick thatch and night irrigation lengthen the period of leaf wetness and promote greater infection. High levels of nitrogen and low levels of phosphorous or potassium may contribute to increased disease severity. Mowing with a dull mower blade frays leaf blade tips and causes excessive wounding that enhances infection through those frayed blade tips.
Integrated Disease Management Tips for Brown Patch:
- Avoid heavy, early spring and summer fertilization, particularly with soluble nitrogen.
- Avoid over fertilization of turfgrasses growing in shaded areas.
- Use slow-release nitrogen fertilizers. Fertilize to maintain adequate but not lush growth during the growing season. Properly fertilized turf will recover quicker from disease injury than will under-fertilized turf.
- Remove and dispose of clippings from infected areas or when conditions are conducive to disease development. Mulching mowers that chop clippings to 1/4 inch or less do not contribute to brown patch development.
- Prune woody landscape plantings and trees to allow better air movement and light penetration to reach the turfgrass canopy.
- Water infrequently, but deeply, in the early morning, and avoid late evening and night watering.
- When available, plant turfgrass cultivars with a known level of resistance to brown patch.
- Use a preventive fungicide program on high-value turfs with a history of brown patch. On other turfs, apply a registered fungicide at first evidence of the disease. Integrate fungicide treatments with other management practices to maximize their effectiveness.
- Provide good surface and subsurface water drainage to reduce humidity in the turf canopy.
- Manually remove dew in early morning by dragging a garden hose or rope laterally across the grass.
- Annually aerify to reduce thatch buildup.
- Control Options: benomyl, mancozeb, maneb, propiconazole, thiophanate-methyl, triadimefon
Powdery Mildew on Turf (June):
Description: Individual blades of Kentucky bluegrass are grey in color. Closer inspection reveals a white, powdery growth primarily on the upper surface of the leaves. As the disease progresses, turfgrass blades wither and die. Kentucky bluegrass grown in the shade is the most likely to be affected. Wet weather, high relative humidity, poor air movement, and air temperatures around 65°F favor disease development.
Recommendations: Disappears after area dries out and temperature warms up. Pruning of trees and shrubs to increase light and air movement can also be helpful. Chemical control is effective but often unnecessary.
Slime molds are primitive organisms that are common on turf and mulch. Slime molds are not fungi and are no longer classified as such. They belong to the Kingdom Protista rather than Kingdom Fungi. On turf, you might often see large numbers of small gray, white or purple fruiting structures, called sporangia on leaf blades during cool and humid weather throughout spring, summer and fall. Affected areas are often several inches to 1 foot in diameter. During wet weather, the fruiting structures may appear slimy. As the structures dry out in hot weather, they become ash gray, and break up easily when touched. Homeowners often are concerned that this is a disease organism that will kill the grass, but slime mold feeds on bacteria, other fungi and dead organic matter. It simply uses the turf as a structure on which to grow. Chemical control of slime molds is not necessary. Use a broom or a heavy spray of water to dislodge the mold.
Slime molds on mulch often attract attention because of their bright colors and disgusting appearance. Common names are often quite descriptive. For example, the "dog vomit" slime mold is a bright, whitish color that resembles its namesake. It eventually turns brown and then into a hard, white mass. There is also the "scrambled egg" slime mold, "the yellow blob" slime mold and the "regurgitated cat breakfast" slime mold. Slime molds do not hurt anything, but most people do not find them attractive and want to get rid of them. Simply use a shovel to discard the offensive organism and then stir up the mulch for aeration.
Source: Kansas State University Extension, Sedgwick County
Blossom end rot of tomatoes is a common problem. It occurs under conditions of high plant water stress and heavy fruit load. The first sign is a small, water-soaked area around the blossom end of the fruit that rapidly darkens and enlarges. As this lesion enlarges, it shrinks and the area becomes sunken and leather-like. This depressed area may become infected with secondary pathogens. Early fruits on the plant may have blossom end rot, while those that develop later are normal.
Blossom end rot is a symptom of calcium deficiency in the fruit. It can occur even when there is abundant calcium in the soil and tissue tests show high levels of calcium in the plant. Poor calcium distribution in the plant results in low calcium levels where the lesion occurs.
Calcium, dissolved in water, moves through the plant in the vascular system from the roots to the leaves. Leaves are the primary sink for movement of water because of water loss through transpiration. Under high moisture stress, the water containing calcium and other minerals moves rapidly to the leaves. Fruit does not transpire as much as leaves and thus tends to be bypassed. This results in a localized calcium deficiency. This calcium deficiency in an area of rapid growth, the end of the fruit, causes cells to collapse and the sunken-lesion symptom of blossom end rot.
Blossom end rot may appear on some of the first fruit clusters on a plant. This is attributed to the combination of rapid plant growth with a large leaf area for water transpiration, water stress, and fruit enlargement. Even a temporary water stress during early fruit enlargement can cause blossom end rot because the fruits are the last to receive adequate calcium.
Calcium, unlike potassium or phosphorus, is not remobilized from the leaves to the fruits. Thus, foliar sprays of calcium won't correct blossom end rot. Tomato fruits do not have openings in the epidermis (skin) where moisture can be lost or where calcium can enter the fruit from surface application. Thus, direct application of calcium to fruit is ineffective.
Another cause of blossom end rot is over-fertilization, especially of nitrogen, which stimulates vegetative growth. Excessive vegetative growth increases the transpiration surface and further prevents calcium accumulation in the fruit. Tomato varieties with large amounts of foliage tend to be more susceptible to blossom end rot. Adjust the nitrogen rate for each cultivar to reduce blossom end rot. Avoid ammoniacal forms of nitrogen that compete with calcium during uptake from the soil.
Hot, windy conditions with low relative humidity can cause high transpiration rates ideal for inducing blossom end rot. Fluctuations in soil moisture during periods of rapid plant growth create moisture stress and limits calcium distribution to the fruit.
Preventing moisture stress is important to control blossom end rot, especially during fruit set and fruit enlargement. Plants require 1 acre-inch of water per week or more in sandy soil and during hot, windy weather.
Mulch to conserve moisture and adjust the nitrogen rate to the type of tomato being grown to avoid excessive vegetation. Use nitrogen in the form of potassium or calcium nitrate and avoid ammonium nitrate. Check soil pH and soil nutrient levels annually and adjust the pH to between 6.5 to 6.7 if necessary. Apply potassium, phosphorus, and magnesium as recommended because balancing these nutrients with calcium is also important in preventing blossom end rot. The primary factor, however, is maintaining uniformly adequate soil moisture throughout the season.
Tomato Early Blight and Septoria Leaf Spot (July):
Two of the most common diseases of tomato are early blight and Septoria leaf spot. Both diseases may occur anytime during the growing season. Both diseases result in the formation of leaf spots. These spots typically develop first on the older leaves nearest the ground. Under favorable conditions for development these diseases can cause extensive defoliation
Early blight and Septoria leaf spot are fairly easy to distinguish from one another in the field. Early blight results in the formation of irregular, brown leaf lesions or spots that range in size up to ½ inch diameter. The most important diagnostic feature of early blight is the formation of dark, concentric rings within the lesion, giving the spots a target-like appearance. The leaves turn yellow, dry up, and fall off the plant.
Symptoms of Septoria leaf spot first appear as small, water-soaked spots on the lower leaves. The leaf spots generally are smaller and more numerous than those resulting from early blight. Eventually the center portion of the Septoria lesion turns light tan or gray while the margin remains dark.
Both fungi overwinter in plant debris, on seed, or on weeds. Spores of these fungi may be splashed or blown to tomato leaves. Disease development is favored by relatively warm temperatures, abundant rainfall and high relative humidity.
University of Nebraska-Lincoln Extension in Lancaster County is your on-line yard and garden educational resource. The information on this Web site is valid for residents of southeastern Nebraska. It may or may not apply in your area. If you live outside southeastern Nebraska, visit your local Extension office