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Hort Update for May 17, 2021

Hort Update for May 17, 2021, Nebraska Extension,
Spring turf freeze damage, including whitening of the leaf blades from the tip back.
Serious ConcernsMajor Symptom
1. White grub control Plan applications to turf with a history of infestation
2. May growing degree days (GGD) Lincoln Airport 5/16/21 GGD -  369,  Understanding Growing Degree Days
3. Spring turf freeze damage Leaf whitening from tip down; no leaf spot lesions present
4. Boxwood winter damage Dry brown to light tan foliage, twigs & branches; in some cases severe dieback
5. Ips & bark beetle damage Increasing in western Nebraska; browning of affected branches
Minor Issue
6. Rust galls on juniper Alarming to homeowners but seldom damaging to trees; prune out
7. Black knot Hard black elongated branch galls
Timely Topics
8. Bagworms Treatment window approaching - caterpillar emergence 600-900 GGD50; early treatments are NOT effective
9. Tree bark damage Death of bark, followed by bark falling from trunk to reveal inner wood
10. Fruit tree flower death Freeze damage varies by stage of bud development and tree type
11. Deciduous shrub winter injury Bare branches or poor/slow leafing out 

1. White grub controlPlan applications to turf with a history of infestation

White grubs are among the most destructive insect pests of turfgrasses. They feed below the soil surface on the roots and rhizomes of all commonly grown turfgrass species and cultivars, and are capable of destroying the entire root system of the plant. When abundant, white grubs can rapidly destroy large areas of turf.

The term "white grub" actually encompasses the larval stage of several scarab beetles, the most common, and most damaging, being the June beetle or masked chafer and the Japanese beetle

  • Adult June beetles are stout bodied, oval-shaped insects, about 1/2 inch in length, and dark yellow to light brown in color. They are most active at night and, unlike other scarab beetles, do not feed on plants as adults. 
  • Japanese beetle adults are slightly smaller, only 3/8 inches in length, with a dark metallic green head and coppery-brown body.  They also have 5 tufts of white hairs on the sides of their abdomen. 
  • Both masked chafers and Japanese beetles have a 1-year lifecycle.

Less well-known are the May/June beetle, green June beetle, Asiatic garden beetles, oriental beetles, European chafers and black turfgrass ataenius. Each of these scarab species has a unique biology and life cycle requiring a specific management approach, but they all have a grub larval stage that can cause damage to turfgrass. 

The grubs are off white, with six legs located just behind their reddish-brown head and are usually found curled into a "C" shape in the soil.

Injury Symptoms
White grubs feed on turf roots and other organic matter in the soil. They damage grass by destroying roots and eliminating plants’ ability to pull up water from the soil. Damage symptoms are usually at their worst in late July and early August if high insect numbers are present and not controlled.

Initially small patches of grass, usually in hot sunny areas, turn brown and die. Damage may appear to be drought injury, or even a disease such as summer patch.  But close inspection of affected areas show turf can be pulled back easily, like a carpet, and numerous white grub larvae are found. Later in the season, September and October, birds and other types of wildlife can cause further damage as they rip up turfgrass to find juicy, fat mature grubs.

Effective white grub control depends on proper timing of the application and moving the insecticide down to the root zone where the grubs are feeding. Apply control in mid to late June.

Preventive Control - Most of the preventively-applied insecticides are systemic in nature and will be taken up by the plant and translocated to the root zone where the grubs are active. All the following products are very effective against young grubs. Labels: c=commercial product, h=homeowner product.

  • Chlorantraniliprole – Acelepryn(c), Scotts GrubEx (h)
  • Clothianidin – Arena (c)
  • Halofenozide - Mach 2 (c)
  • Imidacloprid – Merit (c), Bonide Grub Beater (h), BioAdvanced Season Long Grub Control + fertilizer (h)
  • Thiamethoxam – Meridian (c)

Curative Control (rescue treatments) - If grub control is needed in August or September, carbaryl (Sevin) or trichlorfon (BioAdvanced Grub and Insect Control) provide the best control due to their higher kill rate against mature white grubs. These products must be watered in for acceptable control. Moving the insecticide into the root zone involves applying ½ inch of water immediately after application.

Do not use products that contain ONLY bifenthin, deltamethrin, cyfluthrin, permethrin, lambda-cyhalothrin or gamma-cyhalothrin for soil-applied grub control. These chemicals bind with organic matter at the soil surface and will not move down into the soil to provide effective grub control.

Thatch plays an important role in reducing the efficacy of turf insecticides applied for white grub control. If the thatch layer exceeds ½ inch, light aeration and increased post-treatment irrigation will enhance insecticide penetration and should improve white grub control. In problem areas, such as those with thick thatch layers, repeated irrigations may be necessary every three to four days to continue moving the insecticide into the soil. When white grubs are deeper in the soil, curative treatments are more effective if a retreatment irrigation of 1/2 inch is applied 48 hours before the insecticide application. This will encourage grubs to move closer to the soil surface and enhance the level of white grub control.

Common Homeowner Questions

  1. Do all lawns have grub problems? No. Newly established lawns and low maintenance lawns usually have few problems with grubs.  Turf-type tall fescue lawns also have few problems and seldom need preventive treatment.  Kentucky bluegrass lawns maintained at a high level with frequent fertilizer and water applications are most prone to attack. 
  2. Do all lawns need grub protection? No, only those with a history of problems.
  3. I found a couple grubs in my lawn. Do I need to apply control? No. Masked chafers are Nebraska native insects, so a few white grubs are natural and common in the spring lawn or landscape during planting. At this level, control is not needed. The turf damage threshold by masked chafer larva is 8-10 white grubs per square foot of lawn; for Japanese beetle larva it's 10 grubs per square foot.


3. Spring turf freeze damageLeaf whitening from tip down; no leaf spot lesions present

Fescue lawns and bluegrass lawns have been showing leaf whitening from the tip down. While it appears very similar to Aschochyta leaf blight, in many cases the damage has been found to be tip burn from frost injury. Following a warming trend, temperatures of 28 degrees or lower in the spring can damage young succulent new grass growth.  The turf should grow out of the damage. No treatment is needed at this time. Nitrogen is not needed to promote growth. Know that excessive nitrogen and irrigation and excessive thatch can exacerbate frost injury.

turf with freeze damage


4. Boxwood winter damageDry brown to light tan foliage, twigs & branches; in some cases severe dieback

Boxwood browning, in some cases very severe browning, is a common sight this spring and was caused by winter desiccation. The problem is planting the wrong plant in the location and winter dryness.  Boxwood planted in full sun locations and/or exposed to wind, and certain cultivars, are showing the most damage. Some cultivars, like Green Mountain and Green Velvet, planted in the right location and not pruned after July are not showing as much damage. Site boxwood in the right location. Remind homeowners of the importance of correct summer and fall watering, to avoid pruning after July, and the possible need for winter screening with burlap/snow fence and the application of anti-transpirants two to three times during winter.

Boxwood Blight, as of this date, has only been found (and eradicated) in nursery stock in Nebraska.  It is not established in landscapes. Unlike winter desiccation, boxwood blight causes circular, tan leaf spots and long black lesions on the stem.  It is good to be aware of this disease and know what to watch for. See the link below from Purdue University. If suspect boxwood are seen, take a sample to your local Extension office or send it into the Plant and Pest Diagnostic clinic.

Boxwood Blight, Purdue University
UNL Plant and Pest Diagnostic Clinic


5. Ips & bark beetle damageIncreasing in western Nebraska; browning of affected branches

Ips bark beetle damage on pines is increasing in western Nebraska mainly due to drought stress. Ips beetles begin emerging in early spring. Adults tunnel into branches or trunks, producing sawdust. Tunnels are constructed beneath the bark in a Y, X or H pattern. Larvae hatch from eggs laid along the tunnels and excavate smaller tunnels leading from the main tunnel. Two or more generations occur per year. Symptoms including "fading" or browning of foliage on branches where beetle damage has occurred. Repeated infestations can cause a tree to decline and die. Look for very small, shot-hole sized, adult beetle exit holes, typically 1/4-3/8 inches wide.

The key to preventing bark beetle damage is to maintain tree health with correct planting, watering and mulching. High value trees can be treated with insecticides applied to the trunk and branches in spring.

Ips Beetles, Colorado State University Extension


6. Rust galls on juniperAlarming to homeowners, but seldom damaging to trees; prune out

Cedar-apple rust requires both an apple and cedar or juniper to complete its life cycle. On the cedar, the fungus produces reddish-brown galls that are up to golf-ball size on young twigs. During wet weather these galls swell and begin to push out bright orange gelatinous tubular structures. Wind carries fungal spores from these gelatinous structures to susceptible apple or crabapple cultivars.

There have been some reports of cedar branches dying from rust galls. This would be rare and in most cases, dieback in cedar would be due to wildlife damage to branches, cankers, or possibly Cercospora blight, which causes browning and needle loss from the bottom up. It is important to positively identify the cause of plant damage prior to any treatment.

Cedar-apple and Related Rusts of Apples and Ornamentals, Nebraska Extension


7. Black knotHard black elongated branch galls

Black knot causes hard, black, elongated galls on branches and twigs. It is a fungal disease affecting plum and cherry and occasionally other plants in the Prunus genus. As the galls grow, they eventually cut off water and nutrient flow to branches, leading to stunting, wilting, and dieback. Smaller twigs can die within a year. Larger branches may live several years before being girdled and killed. Pruning is the most important control measure and can reduce infection by 80%. Knots should be pruned in late winter or early spring before growth starts with branches pruned at least 2 to 4 inches below each knot. Sterilize pruning tools between cuts. Bury, burn or compost pruned plant material. Fungicides offer protection against black knot, but will not be effective if pruning and sanitation are ignored. Fungicides provide the greatest benefit if applied before rainy periods when temperatures are greater than 55 degrees F.

Control Black Knot Through Pruning, Nebraska Extension


8. BagwormsTreatment window approaching - caterpillar emergence 600-900 GGD50

Eggs typically hatch from mid-May through early June, depending on weather conditions, but Lincoln has not yet reach the level of growing degree days to indicate hatching has started.

Remember, not all eggs hatch at exactly the same time so it's best to wait until the majority have hatched before making any insecticide applications. The recommended time for control is mid to late June. Systemic products will NOT go through the bag covering to reach unhatched eggs inside. Early treatments before hatching has occurred are a waste of time and pesticide.


9. Tree bark damageDeath of bark, followed by bark falling from trunk to reveal inner wood

Bark damage/blast has been seen on some trees due to temperature extremes. For example, some Redbuds have bark flaking off. There is nothing that can be done when this occurs. Pruning or wound paint or dressing should not be applied to the area and the area should not be wrapped or covered during summer as this holds in moisture and can attract insects. During winter, using tree wrap on young tender barked trees might help reduce bark blast.


10. Fruit tree flower bud deathFreeze damage varies by stage of bud development and tree type

Peach and other fruit tree blossoms were damaged by frost. The extent of damage depends on the stage of the flower bud and how cold of temperatures they were exposed to. Peaches are very vulnerable because the first growth that emerges in spring are blossoms. Freeze damage to fruit crops can occur when temperatures dip below 31 degrees F. However, various fruit types can withstand temperatures lower than 31 depending on the species and stage of development. The killing temperature (also called critical temperature) is defined as the temperature that buds and developing fruit can withstand for a half-hour before permanent damage occurs. Often the freeze will only damage part of the flowers such as the most developed ones or flowers in the bottom of the tree.

The stage of bud development determines how susceptible any give fruit crop is when freezes occur.


11. Deciduous shrub winter injuryBare branches or poor/slow leafing out

Many shrubs are having problems leafing out this spring, such as burning bush and doublefile viburnum. In some cases, only branches at the bottom of the shrub where foliage was protected by snow cover this winter have developed leaves properly and bloomed as expected. Upper branches are completely bare or very slow to leaf. While these shrubs are hardy, fall, winter and spring conditions have offered many challenges, resulting in damage to many plants.

  • Last fall's drought stress, October through December.
  • Sharp temperature drop last November 10 to 13. Night temperatures had been in the 20s and 30s, then suddenly dropped into single digits for four nights.
  • Record low temperatures in mid-February (all-time record low of -31 on Feb 16 in Lincoln).
  • Warm March and early April conditions, mid 70s to nearly 90.
  • Mid- April freezing night temperatures.
    • April 14-15, Lincoln nighttime temperatures dip to 27°F and 26°F degrees, respectively.
    • April 19-22, Lincoln nighttime temperature 31°F, 32°F degrees, 29°F and 23°F, respectively.

Suffrutescent shrubs (definition: subshrub, slightly or partially woody) like Butterfly bush, Caryopteris, and beautyberries died back quite far this year. When they do this, they are slower to begin growth in spring.

In both of these cases, tell homeowners to wait until about June 1st before removing shrubs or pruning out bare branches to see how much, if any, growth occurs.   


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.