ProHort Update - May 2024

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Nebraska's drought status, 5/9/24 Map updated weekly.

1. Reducing the risk of pesticide resistanceUsing alternative modes of action

Pesticide resistance occurs when a pest is no longer affected by a class of pesticides. Resistance most often occurs when repeated applications of pesticides with the same mode or site of action are made during the growing season or on an annual or preventive only basis.

Resistance begins to develop when not all of the targeted pests are killed by a pesticide. Those not killed typically have a genetic change or mutation that allows them to tolerate a class of pesticides. When these pests (insects, diseases or weeds) reproduce, they pass this pesticide tolerance onto their offspring. The cycle can continue until pesticides with the same mode of action are no longer harmful to the pest population.

What is a Mode of Action?
Pesticides have different modes of action or chemistries based on how they kill a pest or the site of action. For example, some insecticides are stomach poisons, some act on an insect’s nervous system, and others prevent molting which halts metamorphosis.

If pesticides used are alternated, but the different products have the same mode of action or chemistry, this will not work to reduce pest resistance. Rotating modes of action or chemistries is needed. These are designated by a number found on the front of pesticide labels. For example, organophosphate insecticides are classified as Group 1 while Pyrethroids and Pyrethrins are Group 3.

To guide users, the Insecticide Resistance Action Committee (IRAC) classifies insecticides, the Herbicide Resistance Action Committee (HRAC) classify herbicides, and the Fungicide Resistance Action Committee (FRAC) classify fungicides.

Developing a Pesticide Rotation Plan
As we move into the growing season, responsible pesticide use can help prevent pest resistance, harm to non-targets like pollinators, and environmental harm.

A pesticide should not be applied until the true cause of the problem is positively identified. Is it a pest? If so, what type of pest and can it cause long term harm or only short-term aesthetic injury? Does the pest need to be managed to prevent serious damage? Is the pest population high enough to cause damage?

If it is determined management is needed, what is the best method or combination of methods to use? Pesticides are an important tool in pest management, but should be one of the last methods turned to whenever possible.

If it is determined a pesticide application is justified to prevent serious or economic damage, select a low risk pesticide but one that will be effective and apply it at the correct time in the pest’s life cycle, which may not be when signs or symptoms are first noticed.

Use the lowest recommended label rate recommended. If less than the recommended rate is used, the targeted pest may recover and this can build resistance. If higher than the recommended rate is used, the plant may be damaged, money wasted, and there could be environmental or human risks.

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2. May 7th growing degree days (GGD)

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

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

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4. Tree stabilization: Current products and practicesAlternatives to traditional staking

Products and systems used to stabilize trees at transplant should be prescribed based on site conditions, tree characteristics, and planting and maintenance practices. Alternatives to traditional aboveground trunk staking and guying methods exist, generally consisting of products that anchor tree root balls rather than supporting tree trunks. When assessing the need for tree stabilization at transplant, several factors should be considered, including material costs, time required for installation and maintenance, product persistence in the landscape, and aesthetics.

Tree Stabilization: Current Products and Practice. Authors Bonnie L. Appleton, Carolyn M. Cannella, P. Eric Wiseman, and Alexis A. Alvey

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5. Prevent landscape materials contributing to nonpoint source water pollutionTips for reducing pollution potential

As it flows over surfaces like pavement, rainwater collects materials such as grass clippings, pesticides, fertilizers, and sediment from bare soil areas. These materials are then carried to surface water through storm drains. This is nonpoint source pollution, sometimes referred to as runoff pollution, and is considered a main cause of surface water pollution leading to impairment of streams, rivers, and lakes. The growth of toxic blue-green algae is one example of impairment promoted by excess nitrogen and phosphorous. Each residential, business and public property contributes a small amount to runoff pollution. While the amount from an individual property may be small; when combined with pollution from other properties and with upstream and downstream pollution, it can add up to more than a water ecosystem can handle.

Property managers can do their part by keeping pollutants out of the path of rainwater runoff. When managing landscapes, sweep fertilizer and pesticide granules and grass clippings from driveways, sidewalks and other impervious surfaces into a container such as a dust pan and place them on the lawn. Instruct clients to water in products as recommended after applying, and avoid application if heavy rainfall is in the forecast. Unless a soil test indicates the need for the nutrient phosphorous, apply phosphorous-free fertilizers. Do not blow grass clippings or dirt from pavement into the street or storm drains where the next rainfall will carry them to surface water. In many communities, there are city ordinances that ban this practice. Seed bare or thin turf areas do plant roots help hold soil in place to reduce erosion from these sites.

Stormwater Management: Water Pollution From Our Own Yards, Nebraska Extension

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6. Storm damaged treesInformation series from the Nebraska Forest Service

With recent storms and tree damage, many landscape have been left with severe tree damage. Below are links to the Nebraska Forest Services publications addressing storm damage recovery for trees.

• Immediate Care of Storm Damaged Trees
• How to Select an Arborist or Tree Service
• Find a Forestry Contractor
• Large Tree Pruning & Care
• Tree Hazard Awareness
• Trees to Plant in Nebraska
• Tree Planting Guide
• Care of Newly Planted Trees

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7. Reasons for tree failureFactors predisposing trees to failure

Severe damage is expected when trees are in the direct path of a tornado, but often trees on the edges of a storm may fail as well. Tree failure - splitting or falling over - during a storm leads to question about why one tree fails, when another next to it does not. There can be a variety of reasons, ranging from tree species with weak or brittle wood to issues like decay, poor structure from lack of or incorrect pruning, stem girdling roots and site problems such as shallow, compacted or saturated soils; or confined root areas. For some prevention tips, see

Storm Damage to Landscape Trees: Prediction, Prevention and Treatment, University of Minnesota

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8. Protect storm damaged evergreens from fungal infectionsDamage to needles and bark from severe winds and hail open a pathway for fungal infections; fungicide applications needed to protect trees 

Trees damaged by strong winds or hail are very susceptible to infection by common fungal diseases following the storm - even trees which have not had significant disease problems in the past. 

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

Diseases of Evergreen Trees, Nebraska Forest Service

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9. Commercial/Non-commercial pesticide applicatorsObtaining a new license or updating an expired license

If you have a pesticide applicators license which expired in April 2024 or you need to get a new license, commercial/noncommercial applicators have several options to recertify or get a new license.

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 - https://pested.unl.edu/.
• 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.)

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.

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10. 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.

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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 constitue endorsement by Nebraskas Extension. Nor does it imply discrimination against other similar products.