Hort Update for August 19, 2019

Hort Update for August 19, 2019, Nebraska Extension, http://communityenvironment.unl.edu/update8192019
Serious ConcernsMajor Symptom:
1. Fruit harvest - preharvest intervals Do not harvest crops until the preharvest (PHI) interval has passed
Minor IssuesMajor points:
2. Soybean caterpillars result in higher than normal butterfly occurrence Enjoy the show; control is not needed
3. Leaf spot defoliation Leaf lesions and early leaf loss due to spring fungal infections very evident and common; too late for control
4. Twig girdlers Small terminal twigs turn brown; variety of trees affected
5. Peonies & powdery mildew management Grayish white powder on leaf surfaces
6. Aster lacebugs Yellow to brown scorched leaves caused by insect feeding
Timely TopicsMajor points:
7. Bagworms - treat now or not? Chemical control is not effective if insects are not actively feeding
8. Spider mites on burning bush White to brown stippling on leaves
9. Site preparation for lawn seeding/overseeding Seedbed preparation key to seeding success
10. Perennial warm season weedy grass management Nimblewill and windmill grass
11. Yellowing turfgrass Turf root dysfunction in hot/wet soil can result in yellowing lawn
12. Turf browning from inefficient or broken irrigation systems Scattered browning of turf often results from uneven irrigation or broken irrigation heads
13. Watering turf vs. trees Leaf scorch and early dropping of leaves common symptoms of water stress



1. Fruit harvest & preharvest intervals Do not harvest crops until the preharvest (PHI) interval has passed

Growers have been harvesting vegetables for several weeks, but fruits are now also near or ready for harvest, including plum, peach, grape, apple and pear. With any food crop when pesticides are used to control insect or disease problems, it is essential for growers to wait the required number of days following the last pesticide application before harvesting. This is called the preharvest interval or PHI. Crops must stay in the field or application site until the PHI has passed before they can be harvested. 

Check pesticide labels to determine each product's PHI. Every pesticide product's label has a "Directions for Use" section which states "It is a violation of Federal Law to use this product in a manner inconsistent with its labeling." This makes the label a legal document and PHI a legal requirement.  Crops cannot be harvested and brought into storage, to be held until the PHI has passed. Crops must stay in the field to allow sunlight, heat and microbes to degrade the pesticide. 

Preharvest Interval, National Pesticide Information Center

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2. Soybean caterpillars result in higher than normal butterfly populationsEnjoy the show; control is not needed

Many people are commenting on the high number of painted lady butterflies (pictured above) in Nebraska this summer. The higher numbers are the result of the many thistle caterpillars (the immature stage of these butterflies) seen in soybean fields across Nebraska earlier in the summer. Control of the caterpillar stage in home gardens or landscapes is not necessary. 

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3. Leaf spot defoliationLeaf lesions and early leaf loss due to spring fungal infections very evident and common; too late for control

Leaf infections early this year were favored due to extended cool wet weather. Heavily infected leaves are now evident to homeowners and falling causing tree defoliation. Unfortunately, it’s too late to treat; fungicide applications are not effective at preventing infections this late in the growing season. Encourage homeowners to rake up and discard fallen leaves to reduce disease pressure on trees next year. Keep trees well watered this summer to prevent additional stress and make sure trees are mulched around the base. Otherwise healthy vigorous trees usually tolerate some leaf loss without serious long-term consequences. Fungicides should be applied next spring on trees with a history of serious defoliation.

Apple Scab, Nebraska Extension
Cedar Apple Rust and Related Rusts of Apples and Ornamentals, Nebraska Extension
Diseases of Broadleaf Trees, Nebraska Forest Service
Leaf Spot Diseases of Trees and Shrubs, University of Minnesota Extension

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4. Oak twig girdlerSmall terminal twigs turn brown; variety of trees affected

Twig girdler causes terminal leaves to turn brown; a symptom called 'flagging'. It also causes twig dieback and the girdler can attack oak, elm, linden, hackberry, honeylocust, poplars, hickory, pecan, persimmons and some fruit trees like apple. The girdler is a long-horned beetle that emerges in late summer. As part of egg laying, the female girdles the twig to kill it because the larvae cannot develop in healthy wood. The dead tip may fall to the ground or hang in the tree until wind knocks it out. While damage is obvious, it is rarely severe, and there is usually no need for control. Larvae overwinter inside twigs. Pick up and discard dead twig sections that fall to the ground to reduce this insect. Squirrels clipping tree twigs can be confused with girdler damage.

Twig Girdler, Kansas State University
Twig Girdler and Twig Pruner, University of Missouri

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5. Peonies & powdery mildew managementGrayish white powder on leaf surfaces

Powdery mildew causes grayish white powder-like patches on upper leaf surfaces and can cause heavily infected leaves to die. It is promoted by warm temperatures and high humidity. Many types of plants are susceptible, including peony, Phlox, Monarda and Zinnia. If plants have been heavily infected this summer consider replacing them with resistant varieties.  Remove heavily infected, brown or dying stems now. Moving plants to an area of higher sunlight and better air circulation will help reduce infections in the future. 

Powdery Mildew on Landscape Plants, Nebraska Extension

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6. Aster lace bugsYellow to brown scorched leaves caused by insect feeding

Lacebugs are very small, flattened insects with ornate, lacy membranous wings.  Approximately 160 species of lacebugs occur in North America.  They are highly host specific, meaning each species of insect feeds only on one type of plant or a few closely related plants.  They have piercing-sucking mouthparts, and can often be found on the undersides of leaves of trees and shrubs.  Lacebugs damage plants by sucking plant sap from the leaves, resulting in yellow or white stippling of the leaves. Lacebug excrement is seen as small black tar-like droppings on the undersides of leaves.

Severe infestations can occur on asters, causing leaves to turn completely turn brown and decrease plant vigor. Monitor plants for signs of damage, but insects are usually are kept in check through natural predators.  If control is needed, try washing the insects of plant leaves with a strong jet of water several times per week. If chemical control is needed, heavily infested plants may be treated with carbaryl, bifenthrin or insecticidal soap.

Lace Bug on Deciduous Woody Ornamental Plants, PennState

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7. Bagworms - treat now or not? Chemical control is not effective if insects are not actively feeding

Once August arrives, inspect bagworms closely before applying an insecticide. Effectiveness of insecticides at this time of year depends on if bagworms are still active or if they’ve sealed up their bags to pupate. If bags are sealed, insecticides will no longer control caterpillars. Once bags are sealed, bagworms are no longer feeding and so no additional damage will occur. At that point, insecticide applications should be held off until June of next year. To inspect bagworms, pick a number of them off of the tree and hold them in your hand. If a caterpillar sticks its head out of the bags, some are still active and insecticides may help reduce the population. If the majority of bags inspected are sealed so there is no opening at one end; the caterpillars have stopped feeding for the year and are beginning to pupate. Insecticides are no longer effective at this stage and should not be applied. Mark calendars for June of next year to apply an insecticide. Bagworms can be handpicked and destroyed from now through winter to help reduce overwintering numbers.

Bagworms, Nebraska Extension   

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8. Spider mites on burning bushWhite to brown stippling on leaves 

Spider mites have been an issue in the Omaha area causing discoloration of foliage. To identify spider mite damage, look for a brown to whitish stippling on the upper surface of leaves and very fine webbing on leaf underside. A hand lens may be needed to see webbing. If mites are suspected, a small branch could be cut and brought indoors to tap over a white sheet of paper. Mites will be dislodged and can be seen as tiny specks moving about on the paper; or that smear green or red when you rub your hand across the paper. (Dust specks will not smear green or red). Spider mite control includes a strong jet of water aimed at the underside of leaves and done twice a week. Horticultural oils and insecticidal soaps are helpful. These also need to be aimed at leaf undersides to contact mites and are best applied in the morning when temperatures are cooler. More than one application is needed. Carbaryl (Sevin) is not recommended for spider mites as this insecticide can increase mite populations. 

Spider Mites on Burning Bush, Kansas State Research and Extension

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9. Site preparation for lawn seeding/overseedingSeedbed preparation key to seeding success

Late August into early September is the ideal time to seed the cool season turfgrasses tall fescue and Kentucky bluegrass. For success, seedbed preparation is important to assure seed to soil contact.

New construction: 1). Remove all construction debris, branches, etc. 2). Control perennials weeds with glyphosate (Roundup). Two to three applications at the recommended timing may be needed. 3). Establish grade for proper surface drainage. 4). Use a rotary tiller or other cultivation equipment to work the soil to a depth of six inches, incorporate compost while tilling. Avoid tillage of wet soil as this creates compaction. Do not try to improve clay soil by tilling in sand as this can increase compaction. For clay soils, spread a one inch layer of compost over the site and till it in. Then spread another one inch layer and till perpendicular to the first tillage. 5). Allow soil to settle after tilling and prior to seeding. 6). Keep the soil moist after seeding.

Overseeding/Renovating an Established Lawn: 1). Mow the area 1 to 1.5 inches tall. 2). If there is excess thatch, one-half inch thick or more, power rake aggressively and removed debris. 3). Aerify the area, punching 20 to 40 holes per sq. ft. with the largest tines available. Make at least two to three passes over the area to be seeded. 4). Apply a starter fertilizer. 5). Seed using a drop spreader or power overseeder (slit or slicer seeder).  6). Keep the soil moist.

Establishing Lawns from Seed, Nebraska Extension Turf iNfo
Improving Lawns in Fall, Nebraska Extension Turf iNfo

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10. Perennial warm season weedy grass managementNimblewill and windmill grass

Nimblewill and windmill grasses are best controlled when young and actively growing. Control should begin shortly after green up in summer and stop once they go dormant. These warm season grasses will go dormant in late summer sooner than cool season grassy weeds. They can be killed non-selectively with glyphosate. Two to three applications may be needed. Follow label directions for frequency of application and the number of days to wait prior to reseeding bare areas. For selective control, the herbicide mesotrione (Tenacity) has been show to control nimblewill and windmill grass. Three applications made 7 to 10 days apart are needed. Know that control of perennial grassy weeds is difficult and time-consuming and clients may choose to tolerate these weeds.

Perennial Grassy Weed Control, Nebraska Extension Turf iNfo

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11. Yellowing turfgrassTurf root dysfunction in hot/wet soil can result in yellowing lawn

While summer yellowing of Kentucky bluegrass lawns is due to iron chlorosis, the chlorosis is believed to be caused not only by high pH soil but also by a root dysfunction from hot and/or wet soils. Iron chlorosis will not respond to an application of nitrogen. When yellowing occurs, apply a foliar only application of iron, such as iron sulfate. Do not water it in.  Discuss with client correct irrigation practices to help avoid overly wet soils from excess or too frequent irrigation. 

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12. Turfgrass browning from inefficient or broken irrigation systemsScattered browning of turf often results from uneven irrigation or broken irrigation heads

When turfgrass turns brown, it is important to check for dry spots from inefficient or broken irrigation systems along with monitoring for disease or insects. Positively identify the true cause before applying a pesticide. Uneven irrigation or broken irrigation systems can result in browning of turfgrass.  When temperatures are cool and rainfall more abundant earlier in the season, turf browning from dry soil does not appear. As we move into hotter, dryer conditions of summer, signs become more evident. 

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13. Watering turf vs. treesLeaf scorch and early dropping of leaves common symptoms of water stress

Trees whose leaves have brown edges, or are curling, yellowing or dropping off may be suffering from dry conditions. Despite all of the rain in some areas; other areas have or had been quite dry. It is important to keep trees well-watered during dry conditions by deeply soaking the soil to a depth of about eight inches - at least twice a month; but only if your area is not receiving one inch of rain per week. Especially during dry weather, relying on lawn irrigation to water trees is not sufficient.

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