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Hort Update for August 15, 2018

Nebraska Extension Hort Update for August 15, 2018,
Spider mite damage and webbing on tomato
Serious ConcernsMajor Symptom:
1. Bagworms Spray applications only effective if insects are still feeding; they are reaching a mature stage and many have stopped feeding
2. Nebraska's 4th EAB confimation - Mahoney State Park Found in June 2018; homeowners are encouraged to wait to begin treating their ash trees until the insect is confirmed within 15 miles of their location.
3. Dutch elm disease Remaining American elms still susceptible; often infected as they age
4. Late summer/fall weed control Prepare now for fall weed control
5. Vegetable garden diseases Diseases in full swing on tomatoes, cucurbits and others
6. Spider mites Active now in vegetable gardens, landscapes
Minor issuesMajor points:
7. Brown patch disease of turfgrass Minor foliar disease causing roughly circular reddish-tan patches in fescue or Kentucky bluegrass. Tan lesions with reddish margins will be found on grass blades. Fungicides best used earlier in season.
8. 3-year white grubs Very large white grubs found in mid-summer. Our common annual white grub, adult is a masked chafer, would not be this size by mid-summer. The adult of the 3-year grub are May/June beetles.
Timely TopicsMajor points:
9. Fall lawn seeding Late August into early September is the ideal time to seed/overseed cool season turfgrass lawns. Purchase quality seed, i.e. Blue Tag Certified and do proper seedbed preparation.
10. Fall tree watering In the absence of rainfall, fall watering is critical to reducing the risk of winter desiccation. Encourage clients to keep the soil of young trees and evergreens uniformly moist, not wet, up until the soil begins to freeze. Using a 2 to 4 inch deep layer of mulch also helps conserve soil moisture.
11. Stop pruning, fertilizing Cultural care practices that stimulate new growth can interfere with a plant hardening off for winter and increase the risk of winter injury. Stop pruning and fertilizing by early August.
12. Heavy fruit crops Fruit trees may have heavy fruit loads that can lead to branch breakage

1. BagwormsSpray applications only effective if insects are still feeding; Many have reached a mature stage and stopped feeding

Bagworm infestations have been high this year. As long as bagworms are still actively feeding, insecticides will reduce damage and overwintering populations. However, in eastern Nebraska many insects have reached the pupation stage and stopped feeding. When that occurs they will no longer be susceptible to control with contact spray applications - even if systemic products are used, since the product will not go through the bag and reach the insect inside.

Once the larvae have attached their needle-camouflaged bags firmly to twigs and move into the pupation stage, sometime in late August or early September, insecticide control will no longer be effective. See our NebGuide for bagworm information and control recommendations.

Bagworms, Nebraska Extension


2. Nebraska's 4th EAB confirmation - Mahoney State ParkFound in June 2018; homeowners are encouraged to wait to begin treating their ash trees until the insect is confirmed within 15 miles of their location.

Where is EAB Now?

In 2016, EAB was confirmed in three Nebraska locations – Pulaski Park in southeastern Omaha, Zorinsky Lake in western Omaha, and Greenwood, NE in Cass County. In 2018 a 4th confirmation was made at Mahoney State Park.  Nebraska EAB Detection Map

There’s Plenty of Time to Treat

EAB does not kill trees quickly; it takes a few years of continued infestation before trees begin to decline. Often insects have been in a tree for 2-3 years before signs of decline are noticed and 1-2 more years before the tree dies completely.

If treatment is begun on homeowners trees when 30% or less canopy dieback has occurred, an otherwise healthy vigorous tree can usually be expected to fully recover. Trees with over 50% canopy dieback, however, are less likely to recover. 

So even if, in the worst-case scenario, a tree is found to have EAB later this summer there is plenty of time to begin treating next year and have the tree make a good recovery.

What is the Best Time of Year to Treat?

Trees take in the systemic insecticides used against EAB best from April through early June. Research has shown that fall applications, although discussed on some product labels require double the amount of product to provide the same level of control as spring applications.

Considering the slow-moving nature of EAB, waiting until spring is the best choice – offering a balance between protecting the tree and preventing the introduction of extra insecticide in the environment.   

For more information - Nebraska Forest Service EAB website 


3. Dutch elm diseaseremaining American elms still susceptible; often infected as they age

Although there are not many Americal elms left in Nebraska, those that do remain often succumb to Dutch elm disease (DED) in the end. DED causes wilting of leaves and dieback of branches. While fungicide injections can be helpful, they work best when used as a preventive treatment. If symptoms are showing in over 10% of a trees crown, fungicides are much less likely to work. Promptly remove and destroy the wood of dead American elms. Infected branches should be pruned and the wood destroyed by burying, burning or chipping.

There are several disease‐resistant elms available now including American elm (Ulmus americana) cultivars ‘Princeton’ & ‘Jefferson’ that provide high‐canopy shade growing 60‐80’ x 60‐80’.  David Elm is a tough, adaptable and slower‐ growing species from Asia growing 30‐40’ tall and wide. Worthy hybrid elms include ‘Accolade’, ‘Cathedral, ‘Frontier’, ‘New Horizon’, ‘Triumph’, and ‘Vanguard’ that are mostly fast‐growing, drought tolerant and easy to establish. 

Dutch Elm Disease, Ohio State University


4. Late summer/fall weed controlPrepare now for fall weed control

Broadleaf Weeds

Fall-applied herbicides are preferred for broadleaf weed control because 1) winter annual weeds are smaller and more easy to control than when they mature in spring, 2) perennial broadleaf weeds are translocating stored energy (and properly applied herbicide) below ground, and 3) cooler temperatures reduce the likelihood of injuring turf or ornamental plants.

For best control that will be noticeable this fall, herbicide should be applied by mid to late October. Herbicides applied later in fall can still be effective provided that soil moisture isn’t limiting at the time of application, but control will likely not be perceivable until next spring. Herbicides are most effective when applied to actively growing weeds not stressed by extreme temperatures, drought, etc.

It is also generally recommended that turf is not mowed within 3 days before or after herbicide treatment – we are currently attempting to further quantify the effects of mowing on herbicide applications.

Summer Annual Grasses

The best way to control summer annual grasses is with dense, healthy turf. Provide adequate fertilizer and overseed thin turf areas this fall (unless you are managing warm-season turf). Preemergence herbicide applications beginning next spring will enhance the control of summer annual grasses. Foxtails and goosegrass germinate later than crabgrass, potentially making these grasses more difficult to control with a single application. Split-applications ensure a sufficient concentration of herbicide in soil to prevent the establishment of these grasses in mid to late summer. Foxtails tend to establish in thin turf, and goosegrass is common in compacted soils. So again, implement necessary cultural practices when environmental conditions are appropriate to encourage healthy turf. 

Herbicide Recommendations

Premixed herbicides containing 2,4-D, dicamba, and/or MCPP, are effective on most winter annual and perennial broadleaf weeds. For difficult-to-control weeds such as wild violets or ground ivy, herbicides containing triclopyr or fluroxypyr are most effective. Quinclorac is effective on field bindweed. Check herbicide labels for recommended rates and intervals for applications before or after establishment.


5. Vegetable garden diseasesDiseases in full swing on tomatoes, cucurbits and others

Tomato leaf spots, such as the fungal diseases early blight and septoria leaf spot, and bacterial leaf spot are appearing now in tomato gardewns. These diseases often begin as leaf spots on lower leaves, then work their way up the plant causing leaves to die; often leading to fruit sunscald. Tomato leaf spot diseases can be managed with regular applications of fungicides labeled for use on tomatoes. For best results, applications need to begin as soon as symptoms first appear on lower leaves and applications made about every 7 to 10 days. Avoid overhead irrigation and increase air circulation around plants with proper spacing and caging. Mulch the soil around tomatoes to reduce soil splash of fungus onto lower leaves. Plant resistant varieties and avoid planting tomatoes in the same area each year. Severely infected plants are best pulled and destroyed. Use fall sanitation to reduce the amount of overwintering fungus.

Common Vegetable Diseases, Backyard Farmer on YouTube
Tomato Leaf and Fruit Diseases & Disorders, Kansas State University

Anthracnose of cucurbits and melons is a destructive disease that occurs during warm, moist seasons. Significant damage can occur to cucumber, muskmelon, and watermelon unless resistant varieties are grown. All aboveground plant parts can be infected. Symptoms vary somewhat among the cucurbits. On watermelon, leaf spots are irregular and turn dark brown or black. On cucumber and muskmelon, leaf spots are brown and can enlarge considerably Stem lesions on muskmelon can girdle the stem and cause vines to wilt. The most striking symptoms are on the fruit and appear as circular, black or brown, sunken areas. A combination of crop rotation and fungicide applications are necessary for controlling this disease on susceptible varieties. Protective spray applications should be made when vines start to run and continue at 7 to 10 day intervals during periods of humid or rainy weather.

Powdery & Downy Mildew can also be problems in Cucurbits.  Powdery mildew appears as a white flour-like dusting growing on leaf surfaces. Downy mildew first appears as pale green areas on upper leaf surfaces which change to yellow irregular shaped spots. A fine white-to-grayish downy growth soon appears on lower leaf surfaces. Infected leaves generally die but may remain erect while the edges of the leaf blades curl inward. Usually, the leaves near the center of a hill or row are infected first. The infected area spreads outward, causing defoliation, stunted growth, and poor fruit development. The entire plant may eventually be killed. Spores are carried by wind so crop rotation is not as effective. Plant resistant cultivars. Avoid overhead irrigation. Space correctly. Improve air circulation. Copper based fungicides are recommended for mildew in cucurbits, but can damage plants. Read and follow label directions.

Managing Downy Mildew in Cucurbits, Cornell University
Powdery Mildew of Cucurbits, University of Minnesota Extension
Anthracnose of Cucurbits,University of Minnesota Extension


6. Spider mitesActive now in vegetable gardens, landscapes

Spider mites on tomatoes can cause problems in the late summer garden, particularly on tomato, watermelon and muskmelon growing in light or sandy soil.  Damage symptoms progress from stippling to yellowing, wilting, browning, and eventually to death of the leaves or whole plant. Mites may move from soybean fields into vegetable gardens, as the soybean plants begin to turn yellow and dry out. Mites can also be a problem in landscapes, particularly burning bush and spruce. 

To check for spider mites, place a white piece of paper beneath the branch or leaves and tap several times. The mites will appear as very small, bits of dust that are crawling across the page.

Controlling spider mites is difficult because they reproduce so rapidly. One method to try involves spraying the plant with a strong jet of water once or twice a day to dislodge some of the insects and to create an environment that is cooler, more humid and less favorable for spider mite reproduction. Several days or even weeks of this treatment will be required to make a noticeable difference in spider mite populations.

In the late summer garden chemical control may not be needed as plants near the end of their harvest season. Removal of infested plants may be the best option.  Refer to the publications below for additional chemical control options. Be sure any chemical you use is labeled for use in the vegetable garden.


12. Heavy fruit cropsFruit trees may have heavy fruit loads that can lead to branch breakage

To help avoid this, branches can be propped up with boards or tied up with a wide belt-like material. To support limbs with a board, cut a ‘V’ into the end of a one-inch thick board. Place the board under limbs with heavy fruit loads so the branch is resting in the “V”. Long branches may require two boards, one near the center of the limb and one near the end.

To support a branch by tying, use plastic “belt-like” material that is two inches wide. Tie it around the limb to be supported and then tie it to a larger limb above the branch, or to the trunk if feasible. Again, longer branches may need to be tied in two or more locations. Check fruit trees twice a week to see if adjustments need to be made.

To avoid too heavy fruit loads and alternate year bearing in the future, hand thin excess fruit on trees in June. (Source: Wade Upham, K-State)

Prop Up Fruit Tree Limbs if Needed, Rural Messenger

Reference to commercial products or trade names is made with the understanding that no discrimination is intended and no endorsement by Nebraska Extension is implied. Use of commercial and trade names does not imply approval or constitute endorsement by Nebraska Extension. Nor does it imply discrimination against other similar products.