Skip to main content

Hort Update for August 18, 2015

Lawns Major Symptom:
1. Overseeding for turf renovation  Early September ideal for cool season turfgrass
2. Fall fertilization September crucial for cool season grasses/stop fertilizing warm season grasses
3. Nimblewill Circular patches of grass going dormant and turning brown
4. Broadleaf weed control September into October most effective time for herbicide use
5. White grub control If 8 grubs can be found per square foot, consider treatment
Trees & Shrubs  
6. Ornamental pear leaf spot Reddish leaf spots on ornamental pear; cause unknown
7. Oak sawfly damage  Large leaf areas skeletonized and turning brown
8. Oak leaf flagging Browning of all leaves on small twigs throughout the tree
9. Spruce gall midge Swollen twigs and shoot dieback in spruce
10. Environmental stress buildup  Resulting in various symptoms. Wake up call to use good tree care practices.
11. Lacebugs Browning and yellowing of leaves; shiny black fecal spots on leaf undersides
12. Bagworm late season control Check bags to see if still feeding before applying insecticides
13. Fall webworm  Webbed nests in deciduous trees containing caterpillars & frass
14. Emerald Ash Borer, new tree hosts White fringed tree and devil wood trees now listed as EAB hosts
Landscape Ornamentals  
15. Japanese beetles - Omaha, Grand Island, Lincoln  Skeletonized leaves and numerous beetles
16. Dividing iris & peony Late august into September is the time to divide/transplant
Fruits & Vegetables  
17. Cucumbers wilting Sudden wilt of cucumber, squash, pumpkin can have several causes
18. Whitefly infestations Yellow leaves, presence of black sooty mold and sticky honeydew 
19. Sweet corn worms  Corn earworm, and occasionally cutworm, damage sweet corn ears
20. Grasshoppers Defoliation damage common in the late summer garden
21. Yellow jackets  Actively foraging for food. Aggressive, avoid
22. Spiders and webs in lawns/shrubs Sign of a beneficial insect, not a pest problem


1. Overseeding for turf renovation - Early September is the most effective time to overseed cool season turfgrass lawns for renovation. Overseed Kentucky bluegrass no later than September 30 and tall fescue no later than mid-September. Mow the turf as short as possible. Check the true thatch layer.  This is the reddish-brown, tightly intermingled mat made up of dead roots and rhizomes and is found between the soil and green turf. If thatch exceeds one-half inch, power rake and remove the debris. Prior to overseeding, core aerate in 2-3 directions to create openings to increase seed to soil contact. Select disease resistant cultivars and buy certified, blue tag seed from reputable retailers. Check cultivars on the National Turfgrass Evaluation Program (NTEP) website for disease resistance and other desired characteristics. Irrigate as needed to promote germination and establishment. Tall fescue should be evident in the coring holes 7-10 days after seeding and bluegrass 14-21 days after seeding.


2. Fall fertilization - While it is time to stop fertilizing warm season grasses like buffalograss and Zoysiagrass, September is a crucial time to fertilize cool season turfgrass. Fall fertilization encourages production of new tillers and/or rhizomes and stolons that increase turf density. This fertilization encourages rooting and production of storage products that help plants survive the stresses of winter and next year's growing season. Almost all turf areas should be fertilized with 1 lb N/1000 sq ft using a fertilizer with 25 to50% of the nitrogen as slow release (sulfur or polymer coated urea, urea formaldehyde, or natural organics). The next most important fertilization is near the last mowing.


3. Nimblewill is a warm season grass that forms circular patches in the lawn. At this time of year, it begins to go dormant and may first be noticed circular patches of tan grass. On close examination, nimblewill is a thin, wiry, pale green grass. The leaf blades are short and emerge at 45 degrees angles from the stems, which are slender, smooth and tend to lie flat on the ground. It spreads by short stolons, or above ground stems, that root at the nodes. Nimblewill can be controlled selectively with the herbicide Tenacity (mesotrione) if applied before the grass goes dormant. Several applications should be made on 3-4 week intervals for the best control. For nimblewill that is going dormant, wait until next season to treat or dig out the circular patches of grass and reseed the area in early September. Monitor the site for nimblewill in future years. If it returns, then Tenacity could be used at the correct time in the grass's life cycle.


4. Broadleaf weed control - The prime season for control of broadleaf perennial weeds like dandelions, clover, and violets starts about September 15. At this time of year, an increased amount of herbicide is likely to be translocated into roots as plants prepare for winter dormancy which increases success in killing weeds and not just killing foliage. Also, applications made now have less chance of affecting nearby trees and ornamentals, unlike spring applications made around non-target species that are just leafing out and/or blooming.

Fall is Best for Broadleaf Weed Control, Nebraska Extension


5. White grub control - Late August into early September if typically the time we see white grub damage on lawns. If grubs are determined to be the cause of browning, check the population of grubs. If eight grubs can be found per square foot, this population size warrants insecticide control. In August or September, a rescue treatment of Dylox or carbaryl (Sevin) can be made.  Use correct irrigation to improve effectiveness. Read and follow label directions. If fewer than eight grubs per square foot are found, adequate irrigation and fall lawn care practices will help reduce damage and aid turf recovery.

White Grub Management 2015, Nebraska Extension Turf iNfo


6. Ornamental pear leaf spots have been fairly common this year, in part due to our rainy May and June. The spots were reddish and in some cases appeared to have teliospores on leaf undersides. Initially the cause was believed to be a rust disease. Rust has, for the most part, been ruled out and samples taken to the UNL Plant Diagnostic lab for further identification. Like most fungal leaf spot diseases, this disease will cause minimal long term damage to trees whose leaves are infected. Sanitation, raking and destroying infected leaves, should be practiced.


7. Oak sawfly damage - Several species of oak sawflies commonly attack oak tree leaves.  Sawfly larvae feed on the leaf undersides where they remove leaf material between the veins, leaving a network of veins. This damage is called "skeletonization".  A thin layer of the upper epidermis remains on the leaf and quickly dries and turns brown.

The oak sawfly is a wasp.  The larvae are sometimes called "slugs" because they resemble true slugs (shiny/slimy, nonsegmented, largest just behind the head and tapering toward the tail end).  Larvae are present on the trees through much of the summer. It is typical for damage not to be noticed until after larvae have finished feeding and dropped from the leaves.  In this case, especially in late summer, it is too late to take any effective action.

Defoliation, which may range from spotty to complete, is not usually fatal to healthy, well-established trees.  In these cases, control is not justified.   Small, newly transplanted and stressed trees may warrant protection from severe defoliation and this needs to take place while the larvae are still small, earlier in the season. On small trees, oak sawfly larvae can be handpicked from trees. If justified and when large numbers of larvae are present, most garden insecticides could be applied if the specific host is listed on the label.  Any application made after larvae have left the leaves will not provide any control.


8. Oak leaf flagging appears as small twigs with a cluster of leaves turning brown. These are often scattered throughout the tree and there may be a few to many ‘flags'. Leaf flagging typically will not lead to tree death; depending on the cause. This year, along the Missouri river, leaf flagging may be due to egg-laying by the 17 year cicada. Twig girdlers, an insect that lays eggs in small twigs on branch ends often causes the ‘flagging' symptom at this time of year. Rarely do twig girdlers cause severe tree damage. One cause that may result in continuing tree decline is Cytospora canker which results in browning of leaves on individual branches. Canker fungi infect the cambium of the trunk or branches, restricting flow of water and nutrients. There is no control for cankers except to remove infected branches and keep trees healthy and reduce tree stress.


9. Spruce gall midge causes affected twigs to be swollen and needles to drop off or become stunted. The galls often kill the shoot they are on. Spruce trees rarely die but tree appearance is affected. Adults are small, orange-brown flies.  After mating, females lay bright orange eggs between the scales of new buds. After hatching, larvae move to the base of developing needles and begin to feed. Their feeding alters growth so needles grow around the larvae. The needle base swells as larvae grow creating the gall. Larvae overwinter in spruce shoots, and so pruning to remove the galls can reduce populations. This should be done prior to April 1.

Spruce Gall Midge, Michigan State University Extension


10. Environmental stress buildup - We continue to see a variety of tree issues that are not biotic, meaning the cause is not a disease or insect.  Environmental stress over previous years is resulting in a variety of symptoms ranging from branch dieback to bark sloughing off trees to tree death. In some cases, the environmental stress lowered a trees natural chemical defense, opening the tree up to attack by pests. Canker diseases, crown or root rots and wilt diseases like verticillium are fairly common a few years after a major stress. There are no control for these diseases. An increase in borers is often seen one to a few years after or during years of environmental stress.

The increase in tree issues is a wake-up call. With climate change, extremes in weather will be more of the norm and now is the time to prepare. Plant a variety of hardy, quality trees. Plant at the correct depth, mulch and water correctly; and in most cases, stop fertilizing trees growing in landscape settings with nitrogen. Trees benefit from lawn fertilizers and excess nitrogen only increases a trees susceptibility to environmental stress and to disease and insect attack.


11. Lacebugs are fairly common on oak, hackberry and sycamore. Lacebug feeding causes leaves to discolor, usually turning a dirty yellow color. Lacebugs are tiny, flat, square-shaped insects with white lacy wings. They feed on leaf undersides by sucking sap from the plant. Each species of lacebugs has specific host plants and will not feed on other plants. Control of lacebugs, or other sapsucking insects such as aphids, is usually not necessary when damage occurs so late in the season. Most lacebugs have completed feeding and insecticide applications at this time of year are not effective, or needed.

LaceBUGS are not the same as LaceWINGS. Lacewing larvae and adults are beneficial insects that prey on aphids and other insect pests. They can also be found on plants infested with lacebugs, especially if aphids are present too. If you have lacewings on your plants you should not use a chemical pesticide to kill lacebugs or their aphid hosts.


12. Bagworm late season control - Bagworms have been feeding since early June. Homeowners notice them now, after they reach one to two inches in size. This pest warrants control, either by handpicking and destroying bags or by applying an insecticide. However, chemical control and biological control with Bacillus thuringiensis are most effective if applied earlier in the season. When large bagworms are found now, cut open some of the bags to see if larvae are still active and feeding. Insecticides do not penetrate the bags and larvae must be feeding to ingest the insecticide, or it will not be effective. "Just in case" applications should not be applied as the risk to beneficial insects, like pollinators, is too high. This is an irresponsible use of a pesticide. Bagworm larvae have or will soon stop feeding to pupate inside the bag. Once this occurs, damage for this year will stop, and insecticides will no longer be effective.

Bagworms, Nebraska Extension


13. Fall webworms have been appearing as webbed nests on the ends of branches in cottonwood, crabapple, walnut and other trees. Caterpillars hide in the webbed nests during the day and feed at night. The nests are unsightly, but caterpillars cause little harm to otherwise healthy trees. Tree health is usually not affected until more than 50 percent of the foliage is eaten. If there are enough nests, almost one on every branch, a tree could be completely defoliated. If you can safely reach the nest, use a broom to break up the bag of webworms. Follow up by spraying with a strong stream of water or an insecticide such as permethrin or Spinosad. Prune out the webbed nests when feasible. There are natural parasites and pathogens of fall webworm that often bring these infestations under control.


14. Emerald Ash Borer, new tree hosts - While Emerald Ash Borer has yet to be found in Nebraska, it eventually will be. Be aware that two new host trees have been added to the list of susceptible trees. Neither are common species found in Nebraska. Both are related to ash. These two trees are white fringetree (Chionanthus virginicus) and devilwood tree (Osmathus americanus).


15. Japanese beetles are showing up on ornamentals in Omaha, Lincoln and Grand Island. Japanese beetles are aggressive feeders on the foliage and fruit of nearly 300 species of landscape plants. Japanese beetles feed on the upper leaf surface, removing the soft green tissue and leaving the veins in a lace-like pattern. Japanese beetles release a strong aggregation pheromone that attracts additional beetles to a food source. The larva (grub) of Japanese beetles feeds on the roots of plants from late summer into early fall. Grub control in lawns is the same as for annual white grubs.


16. Dividing iris and peonies - September is a good time to divide peonies. The tops can be cut back after September 1 or wait until after the first frost if the foliage is green and healthy. Old peony clumps which are blooming well should not be dug and divided unless there is a good reason to do so. It generally takes three years after dividing and replanting for a peony to return to desirable size and flower display.

When dividing, lift and divide the roots after the plants go dormant (September 1). Before lifting, cut the leaves and stems off to the ground. Carefully dig around and under the plant, taking care not to break off the roots or eyes. Wash off soil. Use a sharp, sterilized knife to cut the roots into divisions containing three to five strong buds and a generous portion of fleshy root. Shorten roots to four to six-inch stubs and remove the smaller, threadlike roots. Be sure to replant at the correct depth (the buds, eyes, should be no deeper than one to two inches) or peonies my fail to bloom in the future.

Iris rhizomes multiply rapidly and may require dividing every two to five years. Divide iris plants any time after blooming is completed, but for best results divide in late summer to early fall. Before dividing iris, cut the leaves to about one third of their full height. Remove rhizome clumps from the soil and wash away some of the soil to examine rhizome health. Cut away older, damaged rhizomes. The most vigorous rhizomes will be those on the outer edge of the clump. Cut rhizomes into sections, with each section having minimally one to two leaf fans and healthy white roots, and replant. Larger divisions will produce flowers more rapidly, but will require division more often than smaller divisions. Newly planted irises may need to be winter mulched. Use clean hay, straw, evergreen boughs or other non-packing material. Put mulch in place in late fall, just after the soil freezes. If there is snow on the ground, mulch over the snow.

Dividing Iris & PeonyBackyard Farmer on YouTube


17. Cucumbers wilting – Sudden wilting of plants in the cucurbit family, such as cucumber, squash and pumpkins, can have several causes including bacterial wilt, fusarium wilt and squash vine borer.

Bacterial wilt is transmitted by cucumber beetles, small greenish-yellow beetles with black stripes or spots. They migrate into Nebraska on southerly winds in early July. They are most destructive when they transmit bacterial wilt to plants while feeding. One to three generations of cucumber beetles are produced each growing season. Infestations of cucumber beetles are quite high in late summer and fall, but beetles perish with winter freezes. There is no control of bacterial wilt. Plant resistant varieties and control cucumber beetles with insecticides. Because cucurbits rely on honey bees and other insect pollinators for fruit production, select insecticides with low persistence and treat crops when pollinators are not active, such as in the early morning or late evening.

Bacterial Wilts of Cucurbits, Nebraska Extension

Fusarium wilt in cucurbits typically begins with wilt symptoms. Plants can be infected at any stage. The fungus infects the plant's vascular system, which carries water from roots to leaves. To help confirm Fusarium, cut the lower main stem and look at the cross section of the stem for a pale brown discoloration. If found, pull and destroy infected plants. There are no fungicides are labeled for control of wilt diseases. Select varieties with resistance to Fusarium (they will have the initials VFN on their label) and use crop rotation. However, even resistant plants can become infected if disease pressure is high.

Fusarium Diseases of Cucurbits, Cornell University Vegetable MD Online
Common Vegetable Diseases, Backyard Farmer on YouTube

Squash vine borer larvae tunnel into plant stems (mainly squash, pumpkins, and gourds) and their feeding restricts translocation of water and nutrients. The point where a borer enters a stem, usually at the plant base, may have a sawdust-like frass around it and be decayed. Infested plants are weakened or die; depending on the number of borers. Control borers by practicing good sanitation, physically removing borers by slitting stems when borer activity is noticed, or applying insecticides labeled for vegetables during egg laying, usually about the time vines begin to run, and re-apply every 7 to 10 days for 3 to 5 weeks.

Squash Vine Borer Pictures, Nebraska Extension
Squash Vine Borer Management in Home Gardens, Minnesota State University


18. Whitefly infestations occasionally occur in the home vegetable garden, particularly on tomato, eggplant and cucurbits.  They are closely related to aphids and feed by sucking sap from plant foliage. Whiteflies are very small, only 1/10 to 1/16 inch long, and are completely white.  They look like tiny white gnats. Multiple generations occur each summer.

Whiteflies congregate on the undersides of leaves and fly up when disturbed. Symptoms of a whitefly infestation include stunted plants that grow poorly due to the reduction in vigor caused by the insect feeding. Leaves often turn yellow, or appear dry and drop early. The insects secrete excess plant sap, called honeydew, which gives the leaves below a shiny, sticky coating. The honeydew is often colonized by a black sooty mold, which causes the leaves to turn black. Whiteflies can also transmit plant viruses.

Removal of heavily infested plants is a good way to slow their spread through the garden. Whiteflies have many natural enemies, such as lady beetles, lacewings and spiders, so avoid the frequent or widespread use of insecticides that would kill off your beneficial insects. If chemical control is necessary, make a minimum of four to five applications of insecticide on 5 to 7 day intervals.  Products labeled for whitefly control include neem oil, insecticidal soap, horticultural oil, pyrethrins, and permethrin. Make sure the product you choose is labeled for use on vegetables and follow the product directions related to the post harvest interval following application. 


19. Sweet corn worms are commonly found when corn is harvested. The unwanted intruder is usually the corn earworm, a caterpillar that eventually grows to 2 inches long, although occasionally cutworms are a problem, too. Corn earworms vary in color, and may be green, brown, pink, black or various shades between these colors, with light and dark stripes along its sides and back. The head is always a yellow or light brown color, without any spots.

The adult corn earworm is a grayish-brown, night-flying moth with a wingspan of about 1 1/2 inches. Female moths prefer to lay their eggs on fresh corn silks, but will also choose buds and growing tips of young corn if silks aren't available. The tiny, light yellow eggs are laid singly. By the time the eggs hatch in a few days, they have turned to a dark brown. The little caterpillars crawl down the silks to the end of the ear to feed on the maturing corn kernels. Since they are cannibalistic, you typically will only find one in each ear of corn.

There are a number of options for controlling corn earworm in the home garden. There are some sweet corn varieties that show resistance to corn earworm damage. These include Country Gentleman, Stay Gold, Victory Golden, Silver Cross Bantam, and Silvergent. You may choose to plant mid-season varieties that will mature between moth flights so the ears will be less likely to be injured.

Chemical insecticides can also be used to protect the ears, but once the caterpillar has entered the ear, there is no effective control. To be effective, you need to completely cover the end of the ear so that when the eggs hatch, the young caterpillars will immediately contact a lethal dose of insecticide. Treatments must be reapplied every 3 to 4 days from when silks first appear until they become brown. At this time of year, its better to simply cut off the damaged parts of infested ears, as the portion not fed on by the caterpillar is still perfectly good.

There are a number of natural enemies that attack corn earworm eggs. A number of tiny wasps parasitize the eggs, while minute pirate bugs eats the eggs and there is a virus that may infect and kill the eggs. There are also wasps that attack corn earworm larvae and pupae, however, these beneficial insects are not numerous enough to provide acceptable control.

Corn earworm has a wide range of tastes besides just corn. Other vegetables it will consume include tomatoes, beans, cabbage, and soybeans. It is referred to as the tomato fruitworm when found on tomato. Corn earworm prefers corn, but late in the season when corn plants are not as attractive, it may damage tomatoes and snap beans by eating into the fruits or pods.


20. Grasshoppers - The potential for grasshopper damage increases as summer progresses. Adult grasshoppers are more likely to move into yards and gardens to find new food sources in July and August when adjacent agricultural crops or grasslands mature or are harvested.

Grasshoppers often damage yard and garden plants by defoliation. Grasshoppers show a preference for flowers and some garden vegetables, such as lettuce, beans and sweet corn. However, when populations are high they will feed on nearly all garden vegetables, as well as trees and shrubs. Small trees and shrubs are the most seriously affected, as defoliation of larger trees will have little impact unless it continues for more than one year.

Grasshopper management can be effective if the area to be protected is relatively small and isolated. Protecting a garden from grasshoppers moving out of a large area of adjacent grassland or cropland may be impossible.

Chemical control often is the most effective practice to eliminate heavy infestations of grasshoppers. The best time to control grasshoppers is when they are one-half to three-fourths of an inch long. At this stage, grasshoppers are concentrated in their hatching areas, and can be controlled more effectively than when dispersed later in the summer.

Weedy, untilled areas, such as vacant lots, ditches and poor pastures with mixed grass and broadleaf plants serve as the preferred egg laying and early season feeding areas for grasshoppers. Dense grass growth or regular tillage of these areas will reduce grasshopper numbers. If grasshoppers have removed much of the foliage in these areas, when possible, control them before they can move to lawns or garden plots.
In areas where grasshopper populations are expected to be extreme, consider not planting a garden for a year or plant early maturing vegetables or varieties that are less attractive to grasshoppers, such as tomatoes or squash.

Leaving border areas around gardens and yards unmowed also delays grasshopper movement. Attractive plants, such as zinnias, also can be planted around the edge of the garden to attract and hold grasshoppers. These areas can be sprayed to reduce populations.

Row covers and screens also can help protect more valuable plants. Grasshoppers can eat through most fabric screens, causing damage to storm door and window screening. These screens should be replaced with aluminum screening.

A Guide to Grasshopper Control in Yards and Gardens, Nebraska Extension


21. Yellow jacket wasps become a nuisance during fall as they scavenge for food. If yellowjackets are disturbed, give them plenty of room as they are capable of inflicting multiple stings. If yellow jackets become excited and appear about to attack, do not panic; retreat slowly and calmly. Yellow jackets construct paper nests, usually in underground cavities. Favorite nesting sites include rodent burrows, compost piles, wood piles and wall voids. Occasionally, yellow jackets build aerial nests in garages, crawl spaces or other enclosed areas.

They feed on insects, spiders and a variety of food items. Most stinging wasps and bees are beneficial and should be preserved unless they pose a direct hazard to humans. Colonies of yellow jackets are annual and nests are not reused. Freezing temperatures in November and December kill all stinging workers and only fertilized queens survive the winter. If yellow jackets pose a hazard, recommendations for treating the nests safely are available in the following publication.

Stinging Wasps and Bees, Nebraska Extension


22. Spider webs in lawns and on ornamental plants are a sign of beneficial arthropod activity, not a damaging pest.  Chemical control of spiders associated with lawns and shrubbery is not needed. If webs appear unsightly, remove them with a broom.