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Hort Update for April 1, 2023

Image of grassy and broadleaf weeds in a lawn.
Using good weed control strategies will help prevent resistance development in new weed species.
Serious ConcernsMajor Symptom:
1. Herbicide resistance in turfgrass weeds Use strategies to reduce resistance development.
2. April 1st growing degree days (GDD) Several Nebraska sites below, Understanding Growing Degree Days.
3. Pest update Pests to watch for based on Growing Degree Days (GDD).
Timely Topics
4. Rust diseases or apple, pear and ornamentals Yellow or rusty orange spots on infected leaves. Begin fungicide applications now to prevent infection. 
5. Fireblight Complete annual pruning before blooming begins; prune only to remove infected branches in spring; streptomycin applications.
6. Soil temperatures Check soil temperature to determine planting dates.
7. Evergreen diseases Plan for control of Diplodia tip blight, Dothistroma needle blight, and Rhizosphaera needle cast. 
8. Zimmerman pine moth control Pinkish pitch/sawdust masses at branch junction with trunk; ultimately results in branch death or breakage. GGD - 25-100 and 1700.
9. Spring lawn care timing Advising customers and do-it-yourselfers.
10. Spring lawn seeding Complete as soon as possible or wait until mid-August into September.
Minor Issues
11. Lilac leaf spot prevention Time to prevent severe mid-summer leaf browning on plants with a history of infection.
Heads Up: For Your Information
12. Commercial/Non-commercial pesticide applicator certification New license and recertification options.
13. Digital Diagnostic Network - Need help with diagnostics? Submit pictures and questions for diagnosis by Nebraska Extension experts.

1. Herbicide resistance in turfgrass weedsUse strategies to reduce resistance development

Herbicide resistant weeds are plants able to survive labeled application rates of an herbicide that had previously been lethal to the weed. While there may not be a large number of resistant turf weeds yet, there are some and the likelihood of additional weeds becoming resistant is fairly high if specific weed management and herbicide application practices continue to be used.

Turfgrass weeds now known to have resistance include annual bluegrass, smooth and large crabgrass, goosegrass, spotted spurge and buckthorn plantain, according to the International Herbicide-Resistant Weed Database.

Weed management practices that can lead to herbicide resistance include:

  • Using herbicides as the sole means of weed control.
  • Applying only one type or group of herbicide/s and making frequent and repeated applications over an extended period of time, i.e. multiple seasons.
  • Not following recommended label rates, such as using less than the recommended rate.

How Does Resistance Occur?
A weed develops resistance when a few weeds survive after an herbicide application. When the surviving weeds complete their life cycle and develop seed, resistance can then be transferred to the next generation. If the same herbicide or a different herbicide with the same mode of action (MoA) is applied, additional weeds survive until the resistant population becomes dominant.

Mode of action is the effect an herbicide has on a weed that leads to its death. It is based on the biological process the herbicide disrupts to then affect plant growth and development. To help reduce resistance, herbicides have been categorized into numbered groups based on MoA. For example, Group 1 herbicides inhibit lipid synthesis, Group 2 inhibit amino acid synthesis, Group 4 are growth regulators, Groups 5, 6 and 7 inhibit photosynthesis and so on. The group number is listed on herbicide labels.

Strategies to Reduce Herbicide Resistance in Weeds

  • Rotate herbicides with different MoAs or group numbers to avoid frequent and repeat application of herbicides having the same MoA. Even if herbicides have different active ingredients (AIs), they can have the same MoA. Rotate herbicide MoAs from year to year or within the season.
  • Herbicide rotation also helps by reducing the chances of survival and reproduction of already herbicide resistant weeds. If a weed survives an application with one MoA because it is resistant, the problem can be controlled if the surviving weed is treated with an herbicide having a different MoA to which it is not resistant. This will work best if herbicide MoAs are rotated within the season, such as with sequential applications.
  • Manage turfgrass and turfgrass soils to maintain a healthy, dense turf to increase competition with weeds.
  • Follow good sanitation practices with equipment like mowers, aerators, and power rakes (de-thatchers) to avoid introducing new weeds.
  • Encourage clients to use some mechanical controls, such as hand-digging blooming weeds before they go to seed.

Examples of MoA Rotations

  • Rotate halosulfuron (Group 2) with mesotrione (Group 27) or sulfentrazone (Group 14) or bentazon (Group 8) for postemergence yellow nutsedge control.
  • Rotate pendimethalin, prodiamine, dacthal, dithopyr, benefin, oryzalin (Group 3) with mesotrione (Group 27) or oxadiazon (Group 14) or bensulide (Group 8) or siduron (Group 7) for pre-emergence annual grass control.
  • Rotate 2,4-D, dicamba, MCPA, clopyralid, fluroxypyr (Group 4) with carfentrazone (Group 14) or mesotrione (Group 27) or quinclorac (Group 26) for postemergence broadleaf weed control.

Turfgrass Herbicides: Mode of Action and Resistance Management, University of Florida
Look Out for Herbicide Resistant Weeds in Turf, Purdue University


2. April 1st growing degree day(GDD)

Location Accumulated Growing Degree Days
Grand Island, NE - Airport 8
Lincoln, NE - Airport 10
Omaha, NE - Airport 3
Norfolk, NE - Airport 1
North Platte, NE - Airport 6
Scottsbluff, NE - Airport 1


3. Pest updatePests to watch for based on Growing Degree Days (GDD)

GGD (base 50) InsectLifestage present at this GGD
25-100 Zimmerman pine moth 1st larvae
45-100 Eastern tent caterpillar Egg hatch
150 Eastern tent caterpillar Tents apparent
100-195 European pine sawfly 1st larve
150-175 Spruce spider mite 1st egg hatch
220-250 Honeylocust spider mite Egg hatch
245-440 American plum borer Adult flight and egg laying
250 Codling moth 1st generation control stage
400-500 Emerald ash borer (peak adult emergence at 1000-2000, see below) 1st adult emergence
400-575 Euonymous scale 1st generation
400-600 Bronze birch borer Adults, eggs, new larvae
440-700 Ash sawfly 1st larvae appear
600-900 Bagworm Larvae appear

For a more complete list, visit Michigan State University GGD of Landscape Insects or GGD of Conifer Insects.


4. Rust diseases of apple, pear and ornamentalsYellow or rusty orange spots on infected leaves. Begin fungicide applications now to prevent infection.

Rusty orange spots on the foliage of ornamental pear, apple and crabapple were prevalent last summer. Here’s an in-depth look at the three diseases responsible for these symptoms and subsequent leaf drop. For clients who wish to control either disease, we are in the time period when labeled fungicides will need to be applied.

Although these diseases have almost identical life cycles and similar symptoms, the causal fungi are three different species.

  • Cedar-apple rust (CAR) is caused by Gymnosporangium juniper-virginianae and infects apples and crabapples.
  • Cedar-hawthorn rust (CHR) is caused by G. globosum and can infect ornamental pear, hawthorn, serviceberry and quince.
  • Cedar-quince rust (CQR) is caused by G. clavipes and can infect many rosaceous plants including quince, apple, crabapple, pear, quince, hawthorn, serviceberry, cotoneaster, and others.

Control of these diseases on the different trees is the same. Once infection occurs on leaves, fruit, or twigs fungicides are not effective; hence fungicides need to be applied to the deciduous host during the spring infection period. Control is not recommended on Juniper hosts.

  • A good fungicide to use is one with the active ingredient myclobutanil. Other labeled fungicides will also reduce infections. A benefit of myclobutanil is it will kill rust spores up to four days after they germinate. Most fungicides must be present on the foliage before the spores germinate and infect leaves or they are ineffective. As a rule, the first application is made just as leaf buds are opening and leaves are emerging. Follow label directions for repeat application instructions.
  • The best way to manage any of these rust diseases is to select and plant resistant cultivars of susceptible trees. On resistant cultivars, fungicide applications are rarely needed.

Cedar Apple Rust and Related Rusts of Ornamentals, University of Nebraska-Lincoln
Pear Rust, Kansas State University


5. FireblightComplete annual pruning before blooming begins; prune only to remove infected branches in spring; streptomycin applications.

Fireblight is a bacterial disease that can kill branches and entire plants. It attacks plants in the rose family, including apple, pear and crabapple. Symptoms include dead branches, water-soaked blossoms, light brown to blackened leaves, discolored bark, black "shepherd's crook" twigs, and dried fruits.

Management includes resistant varieties, cultural practices, pruning and preventive chemical sprays. Ideally major fruit tree pruning for size and structure was completed in March. Now, as trees begin to bloom, pruning should only be done to remove branches infected with fireblight. Cut each branch eight to 12 inches below the canker to be sure all infected growth is removed. Clean pruning equipment between each cut to avoid spreading infections throughout the tree. Dip tools in a 10% bleach solution or ethyl alcohol. Chemical controls are best applied during blooming. Streptomycin applications should begin when 25% of flowers are open, and be repeated at 4-5 day intervals until petal fall.

Fire Blight of Apples and Pears, Ohio State University
Fruit Spray Schedules for the Homeowner, University of Missouri Extension


6. Soil temperaturesCheck soil temperatures to determine planting dates.

Many gardeners are anxious to start planting their vegetable gardens, but planting too early is not a good idea. Using specific planting dates, such as planting potatoes on Good Friday, is also not the best practice due to the variable temperatures we experience in any given Nebraska spring. Measuring and planting based on soil temperatures is a better practice.  

To view current soil temperatures, visit Crop Watch

Vegetable Seed Storage and Germination Requirements, Nebraska Extension
Determine Soil Temperatures Before Planting Vegetables for Improved Results, Michigan State University Extension


7. Evergreen disease controlPlan for control of Diplodia tip blight, Dothistroma needle blight and Rhizosphaera needle cast

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, 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


8. Zimmerman pine moth controlPinkish pitch/sawdust masses at branch junction with trunk; ultimately results in branch death or breakage. GGD - 25-100 and 1700.

The first signs of infestation is the appearance of soft, pinkish pitch masses on the trunk or branches. These pitch masses, which form where larvae are feeding beneath the bark, may be found anywhere on the tree. After larvae finish feeding, pitch masses dry and become light yellow to cream colored, hard, and brittle. Mostly affects ponderosa and Austrian pines.

Signs of damage are broken or dead branches or tops of trees may be broken or dead. Larvae hatched last fall and spent the winter under loose bark scales or in old tree wounds and are now susceptible to control. 

To control, spray bark with a drenching spray of permethrin or bifenthrin the second week of April and the second week of August, or at the growing degree days listed below. Remove heavily infested trees.

  • 1st larve - 25-100 GDD
  • Adult flight - 1700 GDD

Insect Pests of Evergreens, Nebraska Forest Service
Zimmerman Pine Moth, Backyard Farmer


9. Spring lawn care timingAdvising customers and do-it-yourselfers

For do-it-yourselfers, lawn care should not begin until after the grass breaks dormancy and begins to grow and soils have warmed. Mowing is not needed until after the grass begins to grow and requires mowing. Then maintain the same height of 3 to 3.5 inches from the first to the last mowing of the season.

Preemergence herbicides targeted at controlling crabgrass and other warm season annual weeds should not be applied until soil temperatures reach 50 degrees Fahrenheit. Soil temperatures across the state can be found at: CropWatch - Soil Temperature Update.

Fertilization should not begin until soil temperatures are even warmer, ideally sometime in May. A well-timed fertilizer application supplies the turf with nutrients when they are needed by the plant and when the soil isn't already supplying the nutrient. See Cool Season Lawn Calendar - Eastern Nebraska


10. Spring lawn seedingComplete as soon as possible or wait until mid-August into September.

Fall remains the best time to seed cool season turfgrass. If spring seeding needs to be done, the sooner seed bed preparation and seeding can be done, the better. There are challenges to spring seeding. This year, we may go from cool conditions to hot weather very quickly which will make seedling survival tough.

One of the largest issues with spring seeding is weed pressure. UNL Turfgrass specialists recommends the use of Scott's Turf Builder Starter Food for New Grass. It contains mesotrione which provides PRE and POST control of weeds. While Siduron has been the recommended in the past as a preemergence to use on new seedings, this product can be difficult to find and has no post emergence control of weeds.


11. Lilac leaf spot preventionTime to prevent severe mid-summer leaf browning

For the last few years in late July and early August, many lilacs have suffered severe leaf browning. This is caused by the fungal disease Pseudocercospora. It shows up as brown spots on the leaves, moving from the edge of the leaves inward, sometimes splotchy in appearance. The fungus is favored by moderate summer temperatures and high humidity. It is common when temperatures are around 76 degrees but the infection occurs at least 7 days before any symptoms are seen on the plant.

Because high humidity favors disease development, increasing airflow around and through lilac stems will help reduce disease severity by decreasing leaf wetness time following rain or a heavy dew. Prune affected plants by cutting out 1/3 of stems, removing the largest canes and those canes that are cankered, girdled or completely dead.

The fungus can survive for at least 2 years on plant debris, so fall cleanup of the infected leaves will also help reduce disease pressure next year. Apply fungicide now, as leaves emerge, to prevent infection. 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


12. Commercial/Non-commercial pesticide applicator certificationNew and recertification options

If you have a pesticide applicators license expiring April 2023 or you need to get a new license — classes begin soon. Make plans now to attend the training option that fits your needs.

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.

Commercial/noncommercial applicators have several options to recertify or get a new license.

Traditional In-person Classes
Classes begin in late January. Visit for dates, locations and registration. Preregistration is required; cost $95 per on-line registration.

In-person trainings are a supplemental learning opportunity; they DO NOT replace pre-class studying of category manuals or flipcharts for test preparation. Study materials for all commercial categories must be purchased online

On-line Self-Paced Option - Recertification ONLY
This is a 100% online training, which includes watching the General Standards recertification video, plus a video for each additional category to be renewed. Cost is $80.00 per on-line registration. Visit to register, beginning December 19, 2022. 

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 -
  • 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.)


13. 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.


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.