Hort Update for October 3, 2023

Nebraska's drought status, 9/28/23Map updated weekly.

1. Magnolia scaleMulti-pronged approach needed for control

If clients have a saucer or star magnolia in their landscape, magnolia scale is a troublesome insect to watch out for. An integrated approach is necessary for control, targeting the most vulnerable life stages at specific times of year.

Plant Damage
This insect is the largest scale species found in Nebraska, but they can still be hard to spot. One way to identify an infestation, is to look for the symptoms they cause on the plant. Heavy infestations weaken plants, causing leaf yellowing or killing entire branches. But the first symptom most often noticed is a sticky, black moldy coating on the magnolia's leaves.

Image left - Drop of honeydew being excreted by an adult magnolia scale. Brian Kunkel, University of Delaware, Bugwood.org.

Insects feed by inserting their needle-like mouth into a stem and sucking up plant sap. They secrete excess plant sugars, which drop to foliage and branches beneath. This creates a sticky, shiny coating often quickly colonized by black sooty mold. Look for sticky leaves and branches, or plant parts with a black moldy coating.

The insects feed on plant stems, not on the foliage, so that's where to look for them. Females do not move once they have found a feeding site on a stem. Insects can build up to the point that stems are completely encrusted with scale. Usually at this point the stem dies. But these insects blend into the plant so well, many gardeners overlook them even after the plant starts to have visible symptoms.

What do They Look Like?
Adult females reach up to ½" diameter at maturity in late July and early August. Each female insect is covered by a soft, irregularly-shaped shell, shaped somewhat like a contact lens, which is shiny and light brown. By mid to late-August, the female's shell turns white as it is covered by a thin coating of wax. Mature males have a similar shell, although smaller. They pupate under their shell in late July and early August then emerge to resemble tiny flies, which fly to the females for mating.

Females give birth to tiny, dark nymphs which hatch out from mid-August through late September. These nymphs are called "crawlers" because at this point in their life they have legs and can move around on the plant to find a feeding site. Once nymphs begin feeding, they create a protective shell and stay in place until the males mature or the females die. But they are susceptible to insecticidal control during the crawler stage. If crawlers are not controlled, they will overwinter on plant stems and complete their lifecycle the following summer. Overwintering crawlers are small and dark-colored, greatly resembling normal lenticels, making them difficult to notice. 

Image right - Insects appear powdery-coated during summer and can easily be mistaken for mealybugs at this stage. 

Avoiding Problems
Stressed trees are more susceptible to attack, so keeping trees healthy and vigorous is a simple way to prevent problems. Proper pruning, watering, mulching and avoiding overfertilization of trees will help reduce their susceptibility to magnolia scale.

Control Strategies
Since scale insects are difficult to control, a multi-pronged approach is most effective.

Systemic soil drench - In early summer, a soil drench application of a systemic insecticide can be used. The same product is available under many different brands, but all are usually called Tree and Shrub Insect Control and contain the active ingredient imidacloprid. Measure the tree's trunk circumference at 4.5 feet up from the ground to determine the amount of product needed. Use 1 ounce of product per inch of trunk circumference. Once applied, plant roots will take in the chemical and it will be moved to all parts of the tree or shrub. To reach full strength within the plant takes time - one week for a small plant and up to 3 months for a very large tree. Application is easy - just mix the chemical in water and pour around the base of the trunk. Read and follow all label directions on the specifics of application.

Guidelines and Restrictions

• There should be no plants that bloom growing beneath treated magnolias. Blooming annuals and perennials should be relocated or all their flowerheads removed in years when this systemic product is applied.
• To protect pollinators, systemic products should not be applied to blooming plants like magnolia until AFTER blooming is done.
• When applying during dry periods, generously water the plant the night before treatment.
• Follow product restrictions on the total amount of product which can be applied per acre per year.
• Make only 1 application per year, no more.

Fall contact spray - When the next generation of crawlers hatches in mid-August, they can be killed with a contact insecticide. To determine when crawlers emerge, place double-sided tape around a few infested branches, then watch for the amber-colored crawlers which will get stuck on the tape.

Horticultural oils, also known as summer oils, are a good product to use. Since they are not traditional insecticides, but instead are highly refined oils, they are very safe to use around humans, pets, wildlife and other beneficial insects. Oils can be applied from mid-August until freezing temperatures occur in fall. Crawlers must come in contact with the oil to be killed, so thorough spray coverage in the tree or shrub is essential.

But remember, just because a pesticide, like horticultural oil, is considered safe and low toxicity doesn't mean it can't damage plants. Use caution when applying oils; they can burn leaves if applied when temperatures are too hot or when applied to drought-stressed plants. Never spray landscape plants with a pesticide if air temperatures will reach 85° F or above during that day. Water plants well a day or two before application and wait until moderate temperatures occur to make your applications. Read and follow all pesticide label directions before use.

Dormant season contact spray - Horticultural oil can also be reapplied in early spring before the flower buds begin to swell, to smother some of the overwintering nymphs. For good control, it's important to get thorough oil coverage on plant stems.

Physical removal - Heavily infested branches can be pruned out at any time of year and can help greatly to reduce the overall insect population. On smaller plants, use a plastic dish scrubber to scrape scale shells away from the bark, being careful not to damage the bark itself.

Because magnolia scale is tough to control, at least 2-3 years of treatment are often needed to reduce insect populations to a manageable level.

Predatory Insects
Several species of lady beetles, and their larvae, and other predators feed on magnolia scale. They may help provide some level of control. 

• multi-colored Asian lady beetle
• signate lady beetle larvae
• parasitic wasps

More information, including pictures.

• Magnolia Scale, The Morton Arboretum
• Be on the Alert for Magnolia Scale and It's Predators, The Ohio State University

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2. Drought and woody plantsCorrect watering during summer and fall is most important to tree health

Nebraska continues to experience drought conditions ranging from abnormally dry to exceptional drought. See Nebraska Drought Monitor map. Clients often believe turf irrigation is sufficient for trees and shrubs. Turf irrigation usually does not moisten soil deep enough to benefit trees and shrubs fully. Correct fall watering is important when soils are dry. Along with an increased risk of winter desiccation and cold temperature injury, drought stressed trees and shrubs are more susceptible to attack by harmful pests like borers, canker disease and Verticillium wilt. Plant owners often believe their landscape plants survived a drought year, but then dieback shows up three to five years after a drought.

While most plants benefit from fall watering in the absence of rainfall, the priority should be evergreens, newly planted trees and shrubs and younger woody plants. Soils need to be kept moist to an 8" to 12" depth from near the trunk/stems to at least the dripline of trees and preferably well beyond. Homeowners can measure depth of irrigation with a long screwdriver or dowel rod.

If plants are not mulched, a 3" to 4" deep layer of organic mulch, like wood chips, should be applied in at least a 4' to 6' diameter ring around plants to help conserve soil moisture. Be sure mulch is not piled against tree trunks and avoid continuously saturated soils.

Winter watering is needed in the absence of rain/snowfall. Water can be applied early in the day when soils are not frozen and air temperatures are above 40 to 45 degrees Fahrenheit. Correct watering during summer and fall is most important to tree health. Remind clients that overwatering is also harmful to plants.

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3. Prostrate knotweedNovember preemergence applications for spring control

Heat and drought thinned turf has allowed annual weeds like prostrate knotweed to invade lawn areas. Prostrate knotweed is a summer annual broadleaf weed. It has wiry stems with small, narrow leaves and forms a mat in the turf.

Prostrate knotweed germinates earlier than most summer annuals, typically in late February and March. It is often found in low-oxygen soils, such as compacted areas next to sidewalks or overwatered turf. Premergence control with herbicides containing isoxaben can be applied in late fall, usually November.

Postemergence control is not recommended now. Next year, use combination herbicides such as 2,4-D combined with triclopyr or dicamba. 

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4. Lilac leaf browningFall sanitation reduces 2024 disease pressure

Many lilacs have suffered severe leaf browning this summer, caused by the fungal disease Pseudocercospora. It shows up as brown spots on leaves, moving from the edge of leaves inward, sometimes splotchy in appearance.

This fungus can survive for at least 2 years on plant debris, so fall cleanup of the infected leaves is the most effective way to reduce disease pressure next year. Regular pruning of older canes is also effective at reducing disease pressure. Fungicide applications in the spring, as leaves emerge, may be effective in preventing infection, however no research has been conducted. Repeated applications are necessary until the spring rainy period has passed. Follow product label guidelines for repeat application timing.

Use fungicides labeled for use on ornamental shrubs, such as those listed below.

• Chlorothalonil: Daconil, Bonide Fung-onil, Ortho Garden Disease Control
• Thiophanate-methyl: Cleary's 3336
• Myclobutanil: Spectracide Immunox
• Propiconazole: Bonide Infuse

Lilac Pseudocercospora Leaf Spot, Iowa State University Extension and Outreach

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5. Mimosa webworm on thornless honeylocustLeaf feeding damage aesthetic; not a serious problem for tree health

These caterpillars web together leaves near the ends of twigs. Their feeding skeletonizes leaves which then turn brown. There are two generations per year, noticed in June and September.

In most cases, and this late in the season, damage is more aesthetic than harmful.

According to Purdue Extension, the many insects and birds that feed on the caterpillars are likely to prevent the injury from harming tree health. In June when webbing becomes apparent, if the affected branches can be reached from the ground, break up large webs with a rake or broom. In higher branches, webs can be broken up with a strong jet of water.

To prevent unsightly amounts of webbing, trees can be sprayed with insecticides that target young caterpillars in June and in August, however it's too late for insecticidal control now to provide any benefit.

When selecting pesticides, see table at link below, it is important to consider that early season use of long lasting, broad spectrum pesticides can cause honey locust spider mites to become a problem later in the year. Avoid this problem by using a biorational insecticide such as: acelepryn, Bacillus thuringiensis, indoxacarb, spinosad, or tebufenozide. These materials do not kill mite predators and are not likely to cause spider mite outbreaks. Use the other insecticides to rescue trees when problems are severe and webbing is more extensive.

Mimosa Webworm, Purdue University

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6. Lilacs blooming - and other flowering plantsEnvironmental triggers initiate off-season blooming

Reports of lilacs blooming have been made across the state. As spring blooming plants, lilac develop flower buds in mid to late summer. Environmental stress, like heat and drought, are the most likely triggers of flower buds opening during fall. Flowering is not harmful to the shrub and other flower buds will still bloom next spring. If needed, deep water lilacs to reduce drought stress.

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7. Avoid pruning spring flowering shrubsPrune in spring after blooming is finished

Shrubs that bloom in spring have already developed their flower buds during this past summer. For this reason, general management pruning, such as thinning or heading back, is best done in spring after these shrubs bloom to avoid removing flower buds and reducing or preventing spring blooms.

If a shrub needs renovation pruning, such as cutting it near to ground level, this is best done while it is dormant in late winter or early spring. The shrub will not bloom that year but blooming will return in future springs.

Common spring blooming shrubs include lilac, forsythia, some Spirea like Bridal wreath, Weigela, some dogwoods, oakleaf and bigleaf Hydrangea, mockorange and spring blooming Viburnums.

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8. Mowing tree leaves into turfMulched leaves do not contribute to thatch; add organic matter and nutrients to soil

Many professional turf managers mulch fallen tree leaves by mowing them into the turf. Mulch mowing can be easier than raking and returns complex organic matter and nutrients to the soil. According to research at Kansas State Extension, fallen leaves to a depth of 6-inches or more can be chopped by the mower and returned to the soil without causing damage, if done properly.

However, it is best to mow a thin layer, 1- to 2-inches or so of leaves, each time they accumulate on the turf. Do not wait until six inches have piled up and then mow them all at once. This process of frequent mowing can continue as long as the shredded leaves do not start to pile up on top of the turf and shade out the grass.

Ground tree leaves will not increase thatch, which is mainly a layer of dead roots and rhizomes. Tree leaves can fall fast and quickly pile over the lawn. If needed, rake and bag excess leaves, then compost them or use as garden mulch.

Do not blow leaves into the street or onto other concrete surfaces. Leaves can leach nutrients that pollute waterways; or be carried to surface water through storm drains where they release nutrients leading to algal problems.

Mulch Mowing Fall Leaves, K-State Research and Extension

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9. Inner needle yellowing on evergreensNatural needle drop in fall is normal and not a health problem for the tree

Evergreen conifers do not keep their needles forever. Older needles on the interior of trees are shed each fall after they turn yellow, brown or reddish tan in color. This natural process is usually subtle and goes unnoticed, but some years it is very evident, especially in white pine. However, natural needle drop is a natural process and is not harmful to trees. The fallen needles will also not make the soil too acid.

Pine trees hold needles for 2-5 or more years, depending on the species. Spruce trees generally hold needles longer than pine trees, approximately 5-7 years, which serves to hide the browning needles. Firs and Douglas-fir needles last 3-4 years. Japanese Yew needles last about 3 years and tend to yellow and drop in spring rather than fall. Branchlets of Arborvitae turn brown and drop off, rather than individual needles. The brown branchlets remain on the tree for some time before falling.

Natural Needle Drop, Nebraska Extension

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10. ProHort Lawn & Landscape Update- Lunch & LearnFinal conversation - September 22

Lunch and Learn zooms are something new. It is an informal opportunity for Extension staff and greenspace professionals to share what’s happening in landscapes, management recommendations and helpful resources. Mark your calendars for this year's final program - September 22, 12:00 pm to 1:00 pm. Please join us. Be ready to share! Program is free and open to all green industry professionals. Register now to receive the free zoom link.

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11. Commercial/Non-commercial pesticide applicatorsObtaining a new license or updating an expired license

If you have a pesticide applicators license which expired in April 2023 or you need to get a new license, testing options are listed below.

Testing-only Options

• Closed-book exams are given by the Nebraska Department of Agriculture (NDA). Preregistration is not required an there is no cost. Visit the link below for a list of available test-only dates, times and locations - https://pested.unl.edu/.
• NDA computer-based testing is provided through the Pearson-Vue company. Click here for a list of testing sites, categories available, dates, and registration information. Cost $55 per exam. (For applicators with multiple categories on their license, each category is charged the full testing fee.)

Commercial/noncommercial applicators are professionals who apply restricted-use pesticides for hire or compensation. Anyone who applies pesticides to the property of another person, either restricted- or general-use products, for control of pests in lawns, landscapes, buildings or homes must also have a commercial pesticide applicators license. Public employees (those employed by a town, county, state) applying mosquito control pesticides whether restricted- or general-use, must also hold a commercial or noncommercial certification.

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12. Digital Diagnostic Network - Need help with diagnostics?Submit pictures and questions for diagnosis by Nebraska Extension experts

Do you or your clients have questions you need help answering? Maybe you are a lawn care person and they're asking about trees, shrubs, or flowers? While you can refer them to their local Extension office, another option is Digital Diagnostic Network. Homeowners, lawn care professionals, pest control operators and others are invited to submit questions and photos through this website or with the assistance from an Extension professional at any Nebraska Extension office. All offices are equipped with high-resolution digital image capturing technology. Whether the question is about a lawn weed, insects on a plant, diseases in a shrub border or other, an expert panel of Extension professionals will review and respond to the question. To get started, create an account so the question can be reviewed and responded to via email. For more information and to create an account, go to Digital Diagnostic Network.

Bugging Out With Your Camera Phone - Tips on how to get a good picture.

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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 constitue endorsement by Nebraskas Extension. Nor does it imply discrimination against other similar products.