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Hort Update for May 2, 2022

Hort Update for May 2, 2022, Nebraska Extension,
May is crabapple blooming season.

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Serious ConcernsMajor Symptom
1. Understanding soil mycorrhizae An important contributor to plant health and nutrition
2. May 2nd growing degree days (GGD) Several Nebraska sites below, Understanding Growing Degree Days
Minor Issues
3. Cedar-apple rust, cedar-quince rust and apple scab control Time for control on apples; control not needed on junipers
Timely Topics
4. Emerald ash borer treatments May best time for injecting insecticides
5. Oystershell scale control Crawler hatch will begin soon; typically mid-May
6. Crabgrass PRE & POST control Germination just beginning; preemergence control still effective
7. White grub control timing Plan applications for turf with a history of infestation by mid to late June
8. Bagworm control timing Monitor evergreens; still time to hand remove old bags, apply controls in June
9. Evergreen disease control in pines and spruce Plan for control of Diplodia tip blight, Dothistroma needle blight and Rhizasphaera needlecast
10. Tick time! Tick identification and protection strategies

1. Understanding soil mycorrhizaeAn important contributor to plant health and nutrition

Mycorrhiza, which means "fungus-root," is a mutually beneficial relationship between the majority of plants and certain fungi that colonize their roots. Mycorrhizal fungi are provided sugars and carbon from plants. Fungal hyphae increase the plant's ability to take up water and nutrients (particularly nitrogen, phosphorus, zinc, manganese and copper) from soil.

Other reported benefits include increased pathogen resistance, increased drought and salinity stress tolerance, higher transplanting success, increased crop yield with enhanced flowering, increased water and nutrient uptake and improved soil structure.

Endomycorrhizae and Ectomycorrhizae
There are two categories of mycorrhizal fungi. Those whose root-like hyphae surround and occasionally penetrate root tissues (ectomycorrhizae) and those whose hyphae enter the root cells (endomycorrhizae). Ectomycorrhizae colonize the roots of many woody plant species and form an extensive hyphal network throughout mulch and topsoil.

Using Mycorrhizae Products
Due to increased understanding of the benefits of mycorrhizae, there are commercial products available for inoculating plants and soil. Since mycorrhizae can be found in most soils naturally, it might not be necessary to purchase them. However, most soilless media does not contain mycorrhizae, so they could be incorporated in container media.

Commercial products may be helpful where topsoil has been removed or soil structure damaged by construction; however, scientific studies show plants quickly become colonized by native mycorrhizal species. Good soil management is important to focus on.

Practices that promote native mychorrhizal fungi in soil include

  1. avoiding unnecessary soil disruption, like rototilling;
  2. preventing soil compaction;
  3. using correct irrigation;
  4. avoiding overfertilization by basing applications on a soil test;
  5. applying compost as a topdressing rather than working it into soil which harms mychorrihzae;
  6. using woody mulches which are good reservoirs for fungal spores; l
  7. imiting the use of fungicides; and
  8. planting for diversity as diverse landscape plantings favor mycorrhizal diversity.

According to the Oklahoma State article "Mycorrhizal Fungi" by Bruce Dunn, Richard Leckie, and Hardeep Singh, "commercial products can be found as granular, powder or in concentrated solution. Products vary in type, number and spore counts of fungi used as well as cost, which can range from a few dollars to several hundred dollars, depending on the product and amount needed.

Application of mycorrhizal fungi can be conducted as direct infection of cuttings or plugs during transplanting, incorporating into the media or soil or applied through irrigation.

Application rates vary by product and application area, but rates can be as little as 1 teaspoon or 50 milliliters, if using a liquid solution. Most commercial mycorrhizal fungi products do not require reapplication; however, others recommend additional applications after several weeks. Some fungi can colonize new roots within a week, while others may take as long as a month".

Mycorrhizal Fungi, Oklahoma State University
Mycorrhizae, University of Wisconsin- Madison Extension


2. May 2nd growing degree days (GDD)

Location Accumulated Growing Degree Days
Grand Island, NE - Airport 161
Lincoln, NE - Airport 157
Omaha, NE - Airport 154
Norfolk, NE - Airport 107
North Platte, NE - Airport 99
Scottsbluff, NE - Airport 85


3. Cedar-apple rust, cedar-quince rust and apple scab controlTime for control on apples; control not need on junipers

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


4. Emerald ash borer treatmentsMay best time for injecting insecticides

Injection treatment window for EAB is during May to greatly increase effectiveness. First consider if the tree is a good candidate for treatment and if it is within 15 miles from a confirmed site. Treatments used in May to apply insecticides will be about 70 percent more effective in controlling EAB than injection treatments used later in the growing season.

Adult EAB emergence begins at 400-500 GDD; peak adult activity 1000-1200 GDD. 

Where is EAB?, Nebraska Forest Service
EAB Trunk Injection Options for Professionals
, Nebraska Forest Service
Information for Homeowners, Nebraska Forest Service


5. Oystershell scale controlCrawler 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 begins at 350-500 GDD. 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


6. Crabgrass PRE & POST controlGermination just beginning; 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 for DIY homeowners to apply preemergence herbicides for crabgrass in eastern Nebraska is April 20 to May 5. Check soil temperatures, provided by Nebraska State Climate office. 

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. Mesotrione (Tenacity) provides both PRE and POST emergence control and can be used prior to and at seeding, but do not apply on newly germinated turf until it has been mowed twice or four weeks after emergence, whichever is longer. Review product labels for all application guidelines and restrictions.  

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


7. White grub control timing for DIY clientelePlan applications for turf with a history of infestation by mid to late June

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 Plus Turf Revitalizer (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.


8. Bagworm control timingMonitor 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. Caterpillar emergence begins at 600-900 GDD, usually 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.

Bagworms, Nebraska Extension


9. Evergreen disease control in pine and sprucePlan for control of Diplodia tip blight, Dothistroma needle blight and Rhizasphaera needlecast

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


10. Tick time!Tick identification and protection strategies

High tick season in Nebraska is generally April through June, but ticks can be active all year round when temperatures are above freezing. With the arrival of spring and an increased involvement in outdoor activities, Nebraskans must be prepared to practice tick safety to prevent tick-borne illnesses.

Tick Time - Check Yourself!, Nebraska Extension GRO Big Red
Tick Tag Go, University of Nebraska - Lincoln Dept. of Entomology


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.