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Hort Update for May 15, 2018

Nebraska Extension Hort Update for May 15, 2018,
Early hot temperatures, low humidity and high winds create the "perfect storm" for herbicide drift.
Serious ConcernsMajor Symptom:
1. Borers Prevention is best; otherwise identification and targeted control are necessary
2. Pine tip blight prevention Diplodia tip blight a common problem in Austrian, Ponderosa pines; treatment later this year than normal
3. Pine needle drop prevention Dothistroma needle blight control should be applied now
4. Dry spring - watering recommendations Dry spring conditions may warrant early watering; audit irrigation system to spot problems 
5. Spring herbicide applications & spray drift Early hot temperatures, low humidity and high winds create "perfect storm" for herbicide drift
Minor IssuesMajor Symptom:
6. Cedar-apple rust control Control needed now to prevent summer leaf drop diseases
7. Tree leaf spot diseases Rarely serious enough to warrant control; too late for fungicide treatments
8. Pre-emergent herbicide  heads-up Timing of 2nd application based on product residual & timing of 1st application
9. Grass control in ornamental flower beds Encourage clientele to use a combination of grassy weed control techniques
Timely topics
10. Pruning techniques - problems with shearing Encourage clientele to use a combination of grassy weed control techniques
11. Correct use of mulch Avoid common problems with mulch application
12. Subshrub winter dieback  Substantial winter dieback found in Buddleia, Caryopteris; coupled with slow emergence of spring growth
Product UpdateNotes:
13. Mesotrione update Over application will cause grass seedlings and existing turf leaves to turn white

1. BorersPrevention is best control; Otherwise identification and targeted control are necessary

Nearly all trees and most shrubs are subject to attack by wood boring insects, especially if they are injured or weakened by disease or environmental stresses. Borer infestations can be avoided by preventing bark wounding and by keeping plants healthy and vigorous through proper water and fertilization. 

Immature borers are cream-colored, worm-like larva of beetles or moths. Adult emergence holes are often the first sign of an infestation. Adult borers emerging in late May and early June include ash/lilac borer, banded ash borer, redheaded ash borer, emerald ash borer, bronze birch borer, elm borer, flatheaded appletree borer, honeylocust agrilus, twolined chestnut borer oak twig pruner and carpenterworm. Host trees should be inspected during this time of year for signs of emerging adult borers. 

Borer control requires 1) identification of the specific borer attacking a plant and 2) a tailored treatment targeting that specific insect at it's most vulnerable life stage. Refer to the publication below for pictures and more information on each insect, including a list of host plants for each. 

Insect Borers of Shade Trees and Woody Ornamentals, Nebraska Extension 


2. Pine tip blight preventionDiplodia tip blight common problem in Austrian, ponderosa pines; Treatment later this year than normal

SYMPTOMS - The main symptom is death of new growth shoots. In years with heavy disease pressure, later in the season entire branches die while the needles remain attached. Black pycnidia, or fungal structures, can be seen on the back side of cone scales. 

This disease can be controlled with fungicides. The first application is made at budbreak. Due to extended cool conditions this spring Austrian pine did not break bud in Lincoln and Omaha until May 10th, about 3 weeks later than normal.

If fungicides were applied at the usually time, around the third week of April, another application should be made now to provide protection for emerging new growth. Although normally only two fungicide application are recommended per year, this year a third application made 7 to 14 days after the second application may be needed for good control. The active ingredients of Thiophanate-methyl, Propiconazole, Copper Salts of Fatty & Rosin Acids, or Bordeaux mixture are recommended fungicides.

Diplodia Tip Blight of Trees, Nebraska Forest Service
Sphaeropsis Tip Blight of Pine, Nebraska Extension


3. Pine needle drop preventionDothistroma needle blight control should be applied now

Symptoms - This fungal disease causes the greatest amount of damage in Austrian and Pondera pines. Symptoms of infection begin in the fall. Yellow or tan spots appear on needles of the current year's or older growth. These spots darken and become brown or reddish-brown then spread to form a band around the needle. These bands are often bordered by a yellow, chlorotic ring on each side. The fungus grows within these tissues, killing that portion of the needle beyond the lesion.

Initially, the tip of the needle dies while the base remains green, but eventually as the disease progresses, the base of the needle also dies, and the entire needle drops off the tree. Typically, clusters of needles within a shoot are infected. Lower branches of trees are most severely infected although the entire tree can be affected. Usually the greatest amount of needle drop is seen in the late spring or early summer following infection.

Older needles are infected and fall from the tree prematurely, resulting in a thin tree canopy. Lower branches in trees are most heavily infected. 

The first application should be done in mid May, and protects the existing needles from infection. The second application, which protects the current season's new growth, is made after considerable new growth has taken place, usually around mid June. This spring's new growth is initially resistant to infection and will not become susceptible until midsummer, around July.

Dothistroma Needle Blight of Pine, Nebraska Extension


4. Dry spring - watering recommendationsdry spring conditions may warrant early watering; audit irrigation system to spot problems

Dry winter and spring conditions have left Nebraska landscapes struggling. Unless your area of the state has received good precipitation in the last few weeks, landscapes would benefit from a deep soaking. Focus on woody plants - trees & shrubs - but turf, annuals and perennials would also benefit. 

To water properly, its important to know where, how much and how often water should be applied to established landscape trees.  Water should always be placed where the roots are growing. Research shows more than 70 percent of tree roots are in the top 24 inches of soil, where the water, oxygen and nutrients they need for healthy growth are available. Water placed below 24 inches cannot be absorbed by the roots.

How Often?
One mistake frequently made by homeowners with in-ground irrigation systems, is watering too often.  Applying a light application of water daily or every other day results in a shallow area of continually saturated soil.  It also drives oxygen out of the underlying soil, resulting in severe root decline or death.  Frequent, light applications of water is one of the most common causes of tree death. 

Water requirements for trees vary by species, size of tree and soil type.  Of these, soil type is the greatest factor in watering frequency and amount.  Sandy soils absorb water quickly and release it easily to plants, but the amount of water sandy soil can hold at any one time is low. Clay soils can hold a lot of moisture, but absorb water slowly.

Sandy Soil
It is almost impossible to overwater trees in sandy soil, though it can be wasteful. When too much water is applied to sandy soil it drains quickly through the soil and isn't available to tree roots.  Frequent watering in low amounts is the best way to water trees in sandy soil. Ideally, soil should be moist 18 inches into the ground for as long as the tree is growing. Watering every five to seven days may be necessary for trees in sandy soils to maintain adequate moisture in high temperatures and high winds.

Clay Soil
Proper watering can be difficult in clay soils because water often doesn't easily enter the small pores spaces in clay soils, so percolates into the soil slowly. Water applied too quickly to clay soils runs off and is wasted.  However, once clay soils are wet they hold moisture for long periods of time.  Deep, infrequent applications of water are recommended for clay soils.  To avoid water runoff, apply water slowly.  Use a sprinkler set very low, letting it run in one area until the top 8-10 inches of soil is moistened then move it as needed to water the entire area underneath the tree's canopy.  A deep soaking every 2 weeks is adequate for most trees in un-irrigated landscapes.

Be sure to check client's irrigation system for good function with the first watering. 

Irrigation: Inspecting and Correcting Turf Irrigation System Problems, Colorado State University Extension


5. Spring herbicide applications & spray drift

early hot temperatures, low humidity and high winds create "perfect storm" for herbicide drift

Drift occurs in two ways, particle or vapor. Particle drift occurs when small spray droplets travel long distances during periods of high wind and blow droplets from the targeted site. To avoid this, use larger spray droplets with low pressure, and apply herbicides only when wind speed is low.

Vapor drift occurs when products volatilize or evaporate and move off the application site. The volatility of some products increases as temperatures rise into the upper 80s and 90s. The product label will provide information on when it's not safe to apply the product based on certain temperatures. The highest potential for drift is when it's hot and dry.

5 Things to Know to Avoid Herbicide Drift, Nebraska Extension


6. Cedar-apple rust controlControl needed now to prevent summer leaf drop in apples/crabapples

If customers ask what they can do to prevent apple or crabapple trees from dropping numerous leaves by mid to late summer, they may be referring to one of two foliar diseases – cedar apple rust or apple scab. They may mention that the tree dropped almost all of its leaves and that they applied a fungicide when the trees were dropping leaves and that it did no good.


  • Fungal infections by these two diseases mainly occur during spring as trees are leafing out and spring rains are providing the moisture needed for infections to occur. It is during the infection period (April into June) that fungicides need to be applied to reduce summer leaf drop.
  • Fungicides applied after leaf drop have very little affect. Encourage customers to wait until the following spring to apply a fungicide.
  • Cedar-apple rust and apple scab will not kill a crabapple or apple tree; but repeated infections will reduce tree vigor and health.
  • The best control of these two diseases is to plant resistant cultivars of apple and crabapple.
  • To confirm if one of these two diseases is the cause of leaf drop, ask the customer to describe leaf symptoms or bring in a sample when leaves begin to drop during summer.

SYMPTOMS - Cedar-apple rust causes bright yellowish-orange leaf and fruit spots, which often have a band of red or yellow around the outer edge. Apple scab causes olive to greenish-black leaf spots. Similar cracked, scabby spots appear on the fruits with heavily infected fruits becoming misshapen.

Spring Disease Control – Apple Scab & Cedar-apple Rust


7. Tree leaf spot diseasesRarely serious enough to WARRANT control; too late for fungicide treatments

Foliar diseases such as anthracnose of sycamore, maple, ash and walnut and rust of ash and Hawthorne, are similar to apple scab and cedar apple rust. The key points in #6 above apply here as well. The infection period is typically April into June; and the level of infection each year is weather dependent.

While leaf drop can occur following infection, leaf spot diseases rarely kill a tree. Fungicide applications should no longer be applied this season since most infections have already occurred; hence fungicides will no longer be effective. If a tree suffers serious repeated infections with major leaf drop, fungicide controls applied in early spring may be beneficial.

SYMPTOMS - Anthracnose symptoms are irregular shaped brown areas, often following leaf veins with some leaf curling and distortion. Ash rust symptoms are bright orange curved and raised areas, often only one or two per leaf, and some leaf distortion. Hawthorne rust symptoms are bright yellowish-orange leaf and fruit spots.


8. Pre-emergent herbicide heads-upTiming of 2nd application based on product residual & timing of 1st application

On the lawns of clients where the first application of a preemergence herbicide was applied from late March into late April, a 2nd application made 6 to 8 weeks after the first application (late May to mid-June) is needed for season long control of crabgrass and other annual weeds.

If a customer is a do-it-yourselfer, educate them about applying fertilizers containing preemergence herbicides for crabgrass during the window of April 20 to May 5. By applying at this time, only one application of herbicide is needed. This can save customers money, improve control, and increase client trust in you.


9. Grass control in ornamental flower bedsencourage clientele to use a combination of grassy weed control techniques

Grassy weeds in flower beds quickly become a problem if hand weeding and correct mulching are not maintained. As with all weeds, there are four control methods. Encouraging homeowners to use a combination works best.

  1. Planting flowers densely enough to help shade out weeds is the first method.
  2. Using organic mulch correctly will reduce weed seed germination and smother tiny seedlings. Mulch, like wood chips, is used correctly if placed on moist soil, kept to a depth of about three inches, and not piled against flower stems.
  3. Regular hand-weeding is always helpful. Young grassy weeds are easy to pull when young, and if there is no weed mat beneath the mulch for the roots of weeds to become entwined.
  4. If grassy weeds get ahead of the grower, herbicides such as Grass-Be-Gone or Over the Top can be applied to grassy weeds without killing ornamental flowers. Be sure all label directions are read and followed.



As plants begin vigorous growth, pruning shears come out to keep them in bounds. Unfortunately, shearing is a common pruning method used for shrubs. Shearing is repeatedly and uniformly reducing the height and width of shrubs without thought given to where the pruning cuts are made on the stem.

If a formal hedge is desired, then shearing by an experienced pruner is the method to use. But for most landscape shrubs, shearing can create issues. The most common being a shrub that has a shell of tight, dense growth on the exterior and is bare of foliage on the interior; or what is referred to as umbrella shrubs. These have foliage near the tips of the branches, but are bare of leaves on interior sections of the branches. Once a shrub is sheared a few times, it can be difficult to repair.

Repeated shearing can also result in ugly witches‘ brooms; clumps of short branch stubs originating from a node

For a more natural appearance, use a combination of heading back and thinning cuts on shrubs. And whenever a pruning cut is made, make it just above an outward facing bud. Heading back cuts are used to shorten the height of a shrub or the length of a branch. Thinning cuts are the removal of an entire stem back to another stem or at ground level. Thinning opens up a shrub to sunlight penetration to help prevent it becoming a shell of leaves.

Pruning Ornamental Shrubs, University of Missouri Extension


11. Correct use of mulch

avoid common problems with mulch application

The use of organic mulch, like wood chips, in perennial flower beds, shrub borders and around trees is an excellent maintenance practice, but often overdone.

Using too deep of a mulch layer or piling mulch against plant stems are common mistakes. Both can harm landscape plants. If too deep, some roots grow in mulch instead of soil and die from drying or heat. Deep mulch layers reduce oxygen levels in soil which weakens or kills roots. Plant roots need soil oxygen as much as soil water.

Mulch piled against plant stems holds moisture against stems and leads to rot or decay. It provides a haven for insects or small rodents to feed on plants.

During spring planting, the mulch in many beds and trees is replenished. When doing so, use it correctly. Mulch, like wood chips, is used correctly if placed on moist soil, kept to a depth of about three inches, and not piled against plant stems. Also monitor mulch for matting. Some mulches, like heavily shredded mulch, will mat down and act like roof thatch, repelling moisture and oxygen. Rake to fluff mulch as needed.


12. Subshrub winter diebackSubstantial winter dieback found in Buddleia, Caryopteris; coupled with slow emergence of spring growth 

Subshrubs are plants whose above ground stems appear to be woody but are not as hardy as the roots. These plants are also known as suffrutescent. They perform more like herbaceous perennials in landscape plantings, with portions of stems dying back each winter. Such shrubs include butterfly bush (Buddleia), blue mist spirea (Caryopteris), beautyberry, perennial hibiscus and others.

Each spring, dead wood occuring in subshrubs needs to be removed. After a mild winter, the amount of pruning needed may be minimal. After a cold winter, such as we just experienced, subshrubs can die all the way to the ground. When subshrubs do this, they are often slower to begin new growth in spring. Homeowners may assume the plant was winter killed and remove it. However, it could still be alive. Give subshrubs until June 1st before removing and replacing them.


13. Mesotrione updateOverapplication will cause grass seedlings and existing turf leaves to turn white 

In the April 2018 Hort Update, we discussed mesotrione most commonly sold as Tenacity by Syngenta and available to homeowners as  Scotts Turf Builder Starter Food for New Grass. It's a newer herbicide that can be used on cool season grass and buffalograss lawns to control both broadleaf and grassy weeds. 

Mesotrione is a group 27 herbicide, killing weeds by disrupting pigment development.  It's mode of action is through the inhibition of the HPPD enzyme (p-hydroxyphenylpyruvate dioxygenase) which is involved in carotenoid pigment synthesis. These pigments protect chlorophyll from decomposition by sunlight. A sign mesotrione is working following application, is seen when affected weeds lose their green color, created in plants through the presence of chlorophyll, turn white and die. 

Overapplication of mesotrione causes these same symptoms to appear in desirable turfgrasses. Turfgrasses will recover and turn green again, but this is an important reminder to use the proper herbicide rate to avoid cosmetic damage and concern by homeowners. Tenacity is applied at very low rates of product per acre, so take care to measure and apply the product accurately. Homeowners using granular products many cause overapplication symptoms to appear if they hand-apply Scotts Turf Builder Starter Food for New Grass and exceed the amount of herbicide specified by the label rate.


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.