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Hort Update for May 17, 2019

Hort Update for May 15, 2019, Nebraska Extension,
Coming soon - a new generation of bagworms. Newly hatched bagworm larvae begin to build their protective bags right away. Image by Jim Kalisch, University of Nebraska Entomologist, retired.
Serious ConcernsMajor Symptom:
1. Bagworms Monitor evergreens; still time to hand remove old bags; apply controls in June
2. Evergreen tree diseases Time for control is now
3. Oystershell scale Monitor plants for crawlers; hatch begins soon, typically mid-May
4. Crabgrass control Crabgrass germination just starting, preemergence control still effective
5. Botrytis prevention in the spring greenhouse Water and humidity control critical to preventing common greenhouse diseases like Botryis gray mold
Minor IssuesMajor points:
6. Cedar-apple rust, cedar-quince rust and apple scab Time for control began in April, fungicide applications now will give limited results
7. Tree leaf spot diseases Rarely serious enough to warrant control; too late for fungicide treatments
8. Pythium infections - turf Sudden appearance of reddish-brown spots in low lying or wet turf areas; fishy odor
Timely TopicsMajor points:
9. Effects of winter injury  A variety of causes and symptoms; don't be too quick to assume plants are dead
10. Mushroom identification Critical to positively identify mushrooms before eating

1. BagwormsMonitor evergreens; still time to hand remove old bags; apply controls in June

Time to begin monitoring spruce, Juniper, Arborvitae and pine for the next generation of bagworms. Eggs typically hatch from mid-May through early June, but 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. 

Bagworms feed on a wide variety of hosts, including evergreen and broadleaf trees, shrubs, ornamentals and crops. Feeding on evergreen trees and shrubs does justify control since large populations build up on these plants and are capable of seriously damaging or killing evergreens. Bagworm feeding on broadleaf plants and ornamentals is more of a curiosity than a serious concern. 

After hatching, larvae spin protective cases or “bags” around themselves. Bags are constructed of silk and fragments of needles or leaves. Bags are initially one-eighth inch long. As larvae feed and grow, they enlarge the bag. By summers end, bags are up to two inches long.

Bagworms move around trees feeding on needles until early September. Early signs of damage are brown or stressed needles at branch tips caused by tiny, first-stage caterpillars etching needle surfaces as they feed. Heavy infestations of older bagworms can defoliate a tree or shrub. Less severe injury will slow growth and stunt plants. Bagworms are especially damaging to conifers because destroyed foliage is not regenerated.

Insecticides are most effective when applied from mid to late June targeting young caterpillars. Insecticidal spray applications require thorough coverage to penetrate the tree canopy. It’s best to use ground equipment capable of delivering higher spray volumes and pressure. Aerial applications may not provide thorough coverage. Insecticides containing Bacillus thuringiensis (Bt), spinosad, or neem oil (azadirachtin) and insecticidal soaps are effective against young larvae, but may require repeat applications. These products generally have minimal impact on beneficial insects.

Other insecticide options for bagworm control on conifers includes acephate, bifenthrin, chlorantraniliprole, carbaryl, cyfluthrin, dimethoate, esfenvalerate, fluvalinate, lambda-cyhalothrin, malathion, permethrin, and tebufenozide. When making an application, be certain the product is specifically labeled for both the target pest and plant species.

Battling Bagworms in Your Landscape, Nebraska Extension
Bagworms, Nebraska Extension


2. Evergreen tree diseasesTime for control is now

Diplodia Tip Blight is a fungal disease that commonly infects older Austrian, Ponderosa and other pines causing new growth to be stunted, black pycnidia to develop on the bottoms of cones and entire branches to die with needles turning brown and hanging straight down as if wilted.

This disease can be controlled with fungicides. The first application is made at budbreak (around the third week of April), a second just before needle emerge (early May), and a third 7 to 14 days later. 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

Dothistroma  Needle Blight is also a fungal disease, causing the greatest amount of damage in Austrian and Pondera pines. 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

Rhizosphaera needle cast is a common fungal disease affecting Colorado blue spruce and other spruces.  Trees in eastern Nebraska are more commonly affected than those in the west. Needles are infected in spring, but symptoms do not become evident until a year later when the needles turn yellow, then a reddish brown which is being seen in trees now.  Older needles on the interior of the branch are affected.  Black fungal fruiting structures can be seen with a hand lens protruding from the stomata of infected needles. 

Infections can be high due to extended wet weather last season and now this spring. Saturated soils increase air humidity around the tree's lower canopy and also contribute to good conditions for disease development.   The disease can be controlled with an application of chlorothalonil in spring when new growth is one-half to two inches long.  Follow-up applications should be made every 3-4 weeks if frequent rains occur during spring and early summer.

Rhizosphaera Needle Cast of Spruce, Iowa State University


3. Oystershell scaleCrawler hatch will begin soon; typically mid-May

Oystershell is a species of armored scale common on many trees and shrubs, including lilac, cotoneaster, poplar, willow, ash and aspen. Feeding injury from high populations causes branch or stem dieback, or in severe cases, plant death. These tiny insects, 1/8 inch a maturity, are bronze or grayish in color and attached to branch surfaces. Their shells look like long, flat oystershells.

Insects overwinter in the egg stage beneath old scales; hatching usually begins around mid-May. The time for control is immediately after hatching. Newly hatched crawlers appear as yellowish specks on new growth or on a piece of dark colored paper if a branch is tapped over the paper. Black tape can be loosely wrapped near the top of infested branches with the sticky side facing out. As scales hatch and move to younger growth, they stick to the tape.

Pruning and destroying infested branches will reduce scale numbers. Horticultural oils or insecticidal soaps can be effective organic controls. Insecticides recommended for homeowner use are acephate or malathion. Make the first application when crawlers are present, usually starting in late May, and repeat in 7 to 10 days. 

There can be a second generation of scale in August. Monitor infested shrubs during this time for signs of crawlers and apply insecticides if needed.

Oystershell Scale, Colorado State University Extension


4. Crabgrass controlCrabgrass germination just starting, preemergence control still effective

Crabgrass, a summer annual, begins germination when soil temperatures reach and sustain 55 degrees F at a two to four inch depth for a few consecutive days. In most years, this typically does not occur until May in Nebraska. The targeted window to apply preemergence herbicides for crabgrass in eastern Nebraska is April 20 to May 5.

This year, with the long, cool spring temperatures and cold soils, crabgrass germination has been slow. We are just now observing crabgrass emerging along curbs and sidewalks where the soil temperatures are elevated. Applying preemergence herbicide now will still give good control. Dithiopyr (Dimension) also provides some post emergence control, killing newly germinated crabgrass seedlings up to the 3-leaf stage.

Timing of a second preemergence application should be based on the residual length and label recommendation of the product used. 

Post emergence crabgrass control can be achieved when crabgrass weeds are present and actively growing with products such as Tenacity (mesotrione), Drive (quinclorac), Q4 (quinclorac, sulfentrazone, 2,4-D and dicamba), One Time (quinclorac, mecoprop and dicamba) and Acclaim Extra (fenoxaprop-ethyl). 

Crabgrass Control in Home Lawns, Nebraska Extension Turf iNfo 
Lawn Care Pro Series: Crabgrass and Other Summer Annual Grassy Weeds, Nebraska Extension Turf iNfo


5. Botrytis prevention in the spring greenhouseWater and humidity control critical to preventing common greenhouse diseases like Botryis gray mold

High greenhouse humidity and low air movement can occur in the spring greenhouse when temperatures outside are still cool. Vents are closed to maintain warmth. Botrytis cinerea or gray mold infections are common under these conditions and affects many ornamental and vegetable crops. Botrytis infection produces large masses of gray fuzzy-appearing fungal spores and quick tissue collapse of flowers and leaves. Stem cankers of older stock plants and damping off of seedlings is also common.  


  • Reduce greenhouse humidity
  • Keep plants and greenhouse surfaces as dry as possible
  • Avoid overwatering plants
  • Space plants further apart to increase air movement around them
  • Sanitation - keep the greenhouse as clean as possible. Remove fading leaves, flowers and other plant debris regularly. 
  • Use preventive fungicides if necessary


6. Cedar-apple rust, cedar-quince rust and apple scab Time for control began in April, fungicide applications now will give limited results

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 diseases


There are a number of fungal diseases that cause various colored, shaped and sized leaf spots on plant leaves. Unless these diseases repeatedly cause severe defoliation, they have a minor impact on otherwise healthy established plants. Fungi require moisture on leaf surfaces for a certain amount time to infect leaves. With increased moisture last season and this spring, there will be an increase in leaf spot diseases leading to some leaf drop.

Be aware fungicides work best by preventing infections. On plants with a history of serious leaf spot disease, fungicides are best applied just as plants are leafing out in spring. While fungicides applied now may prevent new leaf infections, they will not cure current infections and they will have missed the main infection period. If homeowners are concerned about leaf drop, have them note how much foliage the plant has retained which is still doing the job of photosynthesis; and assure them leaf drop diseases are often more cosmetic than harmful. The best management practices for foliar diseases is selecting resistant plants, avoiding overhead irrigation and sanitation by raking and removing fallen leaves.

Diseases of Broadleaf Trees, Nebraska Forest Service


8. Pythium infections - turfgrasssudden appearance of reddish-brown spots in low lying or wet turf areas; fishy odor

This fungal disease occurs during wet weather and in high maintenance landscapes that are frequently watered, especially on creeping bentgrass and perennial ryegrass turf. Key symptoms are gray spots or streaks that appear water soaked and form suddenly. Affected turf may have a fishy odor, especially if a sample is bagged overnight.

Control - ensure good soil drainage of low areas in the landscape, fill depressions where water stands, avoid mowing when turfgrass is wet and keep traffic to a minimum. Preventive fungicides may be needed in areas with a history of infection. 

Pythium Blight of Turfgrass, Nebraska Extension 


9. Effects of winter injurya variety of causes and symptoms; don't be too quick to assume plants are dead

The extent and severity of winter injury seen in spring is determined by many factors, including 1) plant species or cultivar, 2) plant site or location, and 3) timing and severity of weather extremes or fluctuations. Evergreens like Japanese Yew, Arborvitae, junipers, boxwood, and dwarf Alberta spruce are especially prone to winter injury.  Symptoms are often uniform browning of needles/leaves on the ends of branches, one side of the plant or from the top down. This is in comparison to fungal diseases that often begin near the bottom half of a plant and do not follow a uniform pattern.

Potential injury includes the following. 

Key Points

  1. Positively identify the cause of browning before using a control method. With increased moisture the last few years, we are seeing an increase in needle blights. The browning these cause will not be as uniform and pycnidia (black specks) can be found on twigs or needles.
  2. If winter injury is suspected, wait until June 1 to see if new growth occurs. The foliage may be brown, but if branches and leaf buds are not affected, new growth will mask the injury.
  3. Do not apply nitrogen to plants showing signs of winter injury.
  4. The best prevention is planting the right plant in the right place, i.e. the south side of a light colored home is not a good site for ornamental evergreens. Also, adequate summer and fall irrigation while avoiding overwatering; and avoiding pruning from mid-August into early winter. 

Winter Injury to Trees and Shrubs, The Morton Arboretum 


10. Mushroom identification 

Critical to positively identify mushrooms before eating

Submit mushroom pictures or samples to the University of Nebraska - Lincoln Plant and Pest Diagnostic Clinic for proper identification. Pictures should include 1) a view of the entire mushroom, 2) the underside of the cap, and 3) the top of the cap. Make sure pictures are clear, focussed and have another object in the picture to convey size. If microscopic examination is needed, there is a $15.00 per sample. 

Plant & Pest Diagnostic Clinic
448 Plant Science Hall
Lincoln, NE 68583-0722
(402) 472-2559

Website - find the sample submittal form here and additional basic information. 


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