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Hort Update for August 14, 2020

Hort Update for August 14, 2020, Nebraska Extension, http://communityenvironment.unl.edu/update8142020
Severe cedar-apple rust infection of ornamental pear.
Serious ConcernsMajor Symptom:
1. Gray leaf spot in turf High heat and humidity favors rapid disease development; severe on perennial ryegrass
2. New emerald ash borer confirmations New confirmations in the city of Kearney, Washington & Seward counties; spring best time for treatment
3. Managing landscape drought Monitor landscape plants for signs of water stress; audit irrigation systems to spot problems; follow irrigation recommendations
4. Japanese beetles  Severe damage evident now on favored plants - rose, linden, grape and many more
Minor Issues
5. Dollar spot in turf Leaf blades exhibit straw-colored dead spots with reddish-brown margins; lesions pinched at center into an hour-glass shape; 4-6 inch dead spots in turf
6. Rust on pear trees Severe infection rates this year; reddish-orange leaf spots turn entire leaves orange-brown
7. Tree defoliators Be on the lookout for fall webworm, mimosa webworm and walnut caterpillars
8. Flagging in oak trees Small terminal twigs turn brown; variety of trees affected; culprits include twig girdlers and/or kermes oak scale
9. Lilac leaf browning Severe leaf browning caused by fungal leaf spot disease
Timely Topics
10. Winter annual weed control Preemergent herbicide application in September
11. Time for lawn renovation & overseeding Late August to mid-September
12. Time for lawn fertilization Late August to early September
13. Late season bagworm recommendations Chemical control no longer effective if insects have stopped active feeding
14. Leaf scorch on deciduous/shade trees Browning around leaf margins
15. Tree planting Time new plants for September & October; root starter products are not needed
16. Responding to tree storm damage Proper pruning cuts needed when removing storm damage to maintain tree health
17. Tomato diseases heavy this year Disease pressure high this year; infections severe
18. Grape harvest Harvest parameters; fruit storage; bird damage protection
19. GRO Big Red Virtual Garden & Landscape Learning Series Help your clients learn more about vegetable gardening and trees through this series of online programs
New Resource
20. Nebraska Forest Health Report New email newsletter from the Nebraska Forest Service



1. Gray leaf spot in turfHigh heat and humidity favors rapid disease development; severe on perennial ryegrass

This disease primarily affects perennial ryegrass and tall fescue. Kentucky bluegrass is resistant. Symptoms start as small, water-soaked lesions, progressing to leaf death. As leaves die, they twist and develop a shepard's crook appearance. Disease in mixed grass stands look sparse as leaves die, and is often confused with wilt or heat stress. Pathogen is most severe is highly fertilized turf or those stressed by environmental stress (soil compaction, drought, ect.) Leaf wetness favors infection so irrigation should be done in the morning to allow grass leaf blades to dry before evening. Strobilurin fungicides and thiophanate-methyl provide the most effective control. 

Be on the lookout for gray leaf spot, Nebraska Extension Turf iNfo
Gray Leaf Spot, Purdue University

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2. New emerald ash borer (EAB) confirmationsNew confirmations in the city of Kearney, Washington & Seward counties; spring best time for treatment

Three new EAB confirmations have been made this summer by Nebraska Department of Agriculgture and USDA APHIS. 

June 2020- City of Kearney
August 2020- Washington and Seward counties

EAB Detections in Nebraska, 8.3.2020

Counties previously under quarantine include Douglas, Dodge, Cass, Lancaster, Otoe, Sarpy and Washington. Homeowners in these locations should begin assessing their ash trees for health and whether or not they are good candidates for treatment. Encourage them to make plans to begin treatment next spring or remove the trees. Fall treatments are not recommended.

For trunk injections, mid-May through June is the ideal application timing for good control. For soil treatment, products containing imidacloprid are best applied in April just before trees leaf. Products containing dinotefuran are best applied mid-May to early June. Continuing treatment will be required for the life of the tree.

For ash trees that homeowners do not plan to treat, removal should be considered once EAB has been found within 15 miles of that tree, rather than waiting for the tree to become infested and die. At that time, ash trees can become brittle (more so than other species of trees that die from other causes) and they become a risk as well as more challenging to remove.

Selecting Trees for EAB Treatment, Nebraska Forest Service
Nebraska Forest Service - EAB Resource Center

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3. Managing landscape droughtMonitor landscape plants for signs of water stress; audit irrigation systems to spot problems; follow irrigation recommendations

Portions of Nebraska are currently experiencing dry to severe drought conditions. In turf, landscapes and gardens common affects of heat and/or dry conditions include those below. 

  • leaf scorch on ornamentals, especially shade plants getting too much sun
  • blossom drop/flower abortion in vegetable gardens caused by poor pollination due to high temperatures, low humidity or plant stress; home gardeners experience poor fruit set during summer periods when daytime temperatures are above 90°F
  • extended drought will result in Kentucky bluegrass turning brown as it goes dormant; even in the dormant state 1/4 inch of water every 4-5 weeks is required to keep the grass from dying
  • severe drought conditions may cause tall fescue lawns to begin browning indicating the plants are dying; tall fescue does not have the ability to go dormant like Kentucky bluegrass

Drought stress is often slower to appear in woody plants, but can have long-term consequences. Drought stressed trees are more susceptible to secondary attack by insect pests and disease problems, such as borers and canker diseases, which can cause tree death.

One common symptom of drought stress in woody plants is leaf scorch - a uniform yellowing or browning of leaf edges on broadleaf plants or the tips of evergreen needles. However, even trees that don't exhibit leaf scorch can be experiencing drought stress, and once a tree is stressed it takes 3-5 years of normal moisture conditions before the tree recovers its full vigor. Healthy trees, receiving adequate water, are much more resistant to pest problems.

Nebraska Drought Monitor
Keeping Your Landscape Healthy During Drought
, Nebraska Extension

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4. Japanese beetlesSevere damage evident now on favored plants - rose, linden, grape and many more

Adult Japanese beetles have been abundant for several weeks and feeding damage is severe on some plants. Linden trees, roses, grapes and soybeans are favorite foods. The beetles feed over a 4 to 6 week period beginning in late June. They eat green tissue between leaf veins leading to lacy leaves. Severe defoliation can stress trees and reduce yields in orchards and crops. 

Limited control will be achieved by insecticide applications now. Severely affected linden trees will not be killed. Only the leaves are affected, not the underlying twigs or buds. Trees will leaf out fine next year with little long-term damage. 

On smaller landscape ornamentals, reduced risk beetle control consists of collecting beetles (7 PM at night is a good time) and placing them in a bucket of soapy water or using plant covers to exclude them where feasible. Two organic sprays, Neem and Pyola, will protect plants for 3-7 days.

Chemically, adults can be controlled with pyrethroid products like Tempo and Bayer Advanced Lawn & Garden Multi-Insect Killer (cyfluthrin) or Ortho Bug B Gone (bifenthrin). Sevin (carbaryl) is another option. These all provide about 2 weeks of protection for foliage and flowers after thorough treatment. 

Because insecticides affect pollinators, try to spray in the evening only and after trees have finished blooming. Follow label instructions explicitly to avoid harming pollinators and damaging plants.

Japanese Beetles, GRO Big Red Gardening Podcast
How to ID and Manage Japanese Beetles, Nebraska Extension GRO Big Red Blog

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5. Dollar spot in turfLeaf blades exhibit straw-colored dead spots with reddish-brown margins; lesions pinched at center into an hour-glass shape; 4-6 inch dead spots in turf

This is a minor disease for most home lawns. The best way to manage dollar spot in residential lawns is with a June application of fertilizer to help it grow out of the damage. To identify dollar spot, symptoms appear as four to six-inch, straw-colored patches of blighted turf. A bleached lesion in the shape of an hour glass is present on the leaf blade. The lesion has a characteristic reddish-brown margin. In early, dewy mornings, a cobweb-like mycelium is visible in the affected area.

Dollar Spot Disease, Nebraska Extension Turf iNfo

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6. Rust on pear treesSevere infection rates this year; reddish-orange leaf spots turn entire leaves orange-brown

Cool May and wet June conditions have resulted in severe cedar-apple rust infections of pear trees this year, both ornamental and fruiting cultivars. Symptoms of cedar-apple rust are bright yellowish-orange leaf and fruit spots, which often have a band of red or yellow around the outer edge. 

Fungal infections occur during spring as trees are leafing out and rains provide leaf wetness needed for infection. It is during the infection period (April into June) that fungicides should be applied. Fungicides should not be applied now. Cedar-apple rust will not kill the tree; but repeated severe infections over several years can reduce tree vigor and health. 

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

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7. Tree defoliatorsBe on the lookout for fall webworm, mimosa webworm and walnut caterpillars

Several tree defoliators are active in late summer, including mimosa webworm, fall webworm and walnut caterpillar. Homeowners often spot the silken webs created by the first two insects at this time of year. The webbing provides protection from some predators and the caterpillars feed inside the web until all leaves are devoured, then additional leaves are encased in the web. Webbed areas of leaves grow larger as the caterpillars mature, becoming a messy, ugly eyesore as it is filled with shed skins, excrement and leaf fragments.

Fall webworm attacks many hosts, over 85 known species of deciduous trees, including elm, hickory, pecan, plum, chokecherry, poplar, walnut and willow. In fact, almost all fruit, shade and ornamental trees, except conifers, can be affected by fall webworm. Adults of this native insect are white moths, with reddish-orange front legs and a 1.25 inch wingspan. Immature insects are pale yellowish caterpillars with red heads and reddish-brown spots. An alternate color variation among the larva is yellow-green caterpillars with black heads a broad dark stripe on the back and black spots. The caterpillars have many long, fine hairs on their backs. There are one to two generations per year in Nebraska.

Mimosa webworms are similar, but attack honeylocust and mimosa trees. As adults, mimosa webworms are dull colored moths. They have a half inch wingspan and are grey in color with small black dots on their wings. Their eggs are small, oval, and white but will turn rose-colored as they near hatching. The caterpillars are about an inch long when mature. In terms of color they are gray to dark brown with five white stripes and a brown head. If disturbed they will drop down from trees on a silken line.

Walnut caterpillar does not create a web, but feed in large groupings in walnut, butternut, pecan, hickory and on occasion oak, willow, birch, honeylocust, and apple trees. Newly hatched larvae have black heads and are generally light green, gradually changing as they grow to become reddish brown or purple with white stripes. As worms approach maturity, they darken, becoming almost black and at that time are covered with long, fuzzy, white hairs. Caterpillars eventually reach a length of up to 2 inches. Larvae behave oddly when threatened, arching their fore- and hind-legs in a defensive posture.

All these caterpillars feed on the tree's foliage, reducing it's photosynthetic capacity. Although unsightly, feeding by these defoliators is rarely seriously damaging to large trees; however, several years of defoliation for small ornamental trees can weaken them. The web impedes most insecticides from reaching the insects, unless you can catch it early. One of the best ways to get rid of them is by taking a rake and breaking up the web. Or you can try a heavy stream of water to break up the webbing. Many of the caterpillars will be knocked out of the web onto the ground, and will be killed by predatory insects.

Biological insecticides such as Bacillus thurengiensis, B.T., or Dipel are effective. Other insecticides, such as spinosad, permethrin and bifenthrin, will also provide good control. Thoroughly cover leaves next to the nest, and as the larvae ingest the insecticide they will be killed.

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8. Flagging in oak treesSmall terminal twigs turn brown; variety of trees affected; culprits include twig girdlers and/or kermes oak scale

Many oak trees can be seen in the late summer landscape with many small dead twigs at the end of larger branches. There are two potential causes of this damage.

Twig girdler causes terminal leaves to turn brown; a symptom called 'flagging'. It also causes twig dieback and the girdler can attack oak, elm, linden, hackberry, honeylocust, poplars, hickory, pecan, persimmons and some fruit trees like apple. The girdler is a long-horned beetle that emerges in late summer. As part of egg laying, the female girdles the twig to kill it because the larvae cannot develop in healthy wood. The dead tip may fall to the ground or hang in the tree until wind knocks it out. While damage is obvious, it is rarely severe, and there is usually no need for control. Larvae overwinter inside twigs. Pick up and discard dead twig sections that fall to the ground to reduce this insect. Squirrels clipping tree twigs can be confused with girdler damage.

Twig Girdler, Kansas State University

Another possible cause is Kermes oak scale. Pale brown, hemispherical scales appear as large growths attached to leaf midribs and twigs. Mature scales are very tough and gall-like. Leaves become stressed, yellow, or withered, and honeydew secretions are evident. Infested trees can suffer serious branch dieback, but infestations are usually isolated to specific trees and are rarely widespread.

This scale produces a profuse amount of honeydew that covers leaves and becomes blackened by sooty mold. Ants and many other insects feed on the honeydew, and there are a number of natural enemies that help restrain populations. Homeowners often complain of sticky sap falling from infested trees.

Nebraska records have confirmed the presence of Kermes Scale since 1921. Hosts affected have been red, pin, and bur oaks, but a wide range of oaks can be infested. Counties involved in the records include Gage, Otoe, Richardson, Pawnee, Lancaster, Douglas, Dodge, Saline, Sarpy, Platte, and Buffalo counties, but this pest is undoubtedly widespread across the state wherever oaks occur.

There is one generation per season, with females reaching maturity in June. Crawlers emerge in September then migrate to buds being formed for the following year where they spend the winter.  A dormant-season spray oil from March through mid-April is an opportunity to treat. Crawlers are also susceptible to control in September with a topical insecticidal spray.

Kermes Oak Scale, Kansas State Research and Extension

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9. Lilac leaf browningSevere leaf browning caused by fungal leaf spot disease

In the last few weeks, 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.

Fungicides are not effective at this time on plants already infected. Next year, fungicide should be applied in the spring when the leaves first emerge. 

Lilac Pseudocercospora Leaf Spot, Iowa State University Extension

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10. Winter annual weed controlPreemergent herbicide application in September

Time for control of winter annuals in lawns. Common winter annuals include little barley, henbit and corn speedwell. Winter annuals germinate in fall, survive the winter, then grow, bloom, go to seed and die the following spring/summer. Maintaining a dense turf and tall mowing height to help lawns shade out and compete with weeds, along with the use of a preemergence herbicide applied in early September provides the most effective control.

Little Barley, Nebraska Extension Turf iNfo

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11. Time for lawn renovation & overseedingLate August to mid-September

For cool season grasses, Kentucky bluegrass and tall fescue, late August into mid-September is the best time for seeding. Now is the time to prepare the seed bed, whether doing a complete renovation or overseeding. The key to success is seed to soil contact. See information links below for step by step seed bed preparation information. When purchasing seed, buy from a reputable retailer and look for blue tag certified seed to avoid planting a problem.

Improving Turf in Fall, Nebraska Extension
Establishing Lawns from Seed, Nebraska Extension

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12. Time for lawn fertilizationLate August to early September

Late August to early September is one of the best times to fertilize cool season turfgrass. You do not need to use a “winterizer” fertilizer is not needed; a general fertilizer is fine. Apply slow release granular products at 1.0 lbs/1000 sq. ft. Aim for a product with 50% slow release nitrogen or less. If additional nitrogen fertilization is required later in the fall (such as on younger lawns), use products with more quick release nitrogen.

Don’t apply nitrogen after mid to late-October depending on your location in Nebraska. Later applications will linger in soil and promote excessive spring growth. This increases mowing requirements in spring and depletes carbohydrates prior to summer. Apply potassium and phosphorous based on soil test recommendations.

Rethinking Fall Fertilization, Nebraska Extension

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13. Late season bagworm recommendationsChemical control no longer effective if insects have stopped active feeding

Bagworm control will no longer be effective in most areas.  If bagworms have closed their bags and attached to twigs, (eastern Nebraska), then feeding has stopped and insecticides will no longer be effective. Products do not control them through contact since their tightly woven bag protects them. Insecticides, even systemic products, do not penetrate through the bag covering.

In central and western Nebraska, bagworms may still be feeding. However, many are at a size where they are difficult to control with insecticides. The large caterpillar would have to eat a fair amount of needles with insecticide on them to be controlled; and at this time of the year feeding is slowing down. Clients need to be made aware of this.

Insecticides are most effective against bagworms when the bags are about one-half inch long, typically from late June into early July. At this time of year, hand-picking bagworms and stepping on them or placing them in the trash in a sealed bag is a control option.

Bagworms, Nebraska Extension

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14. Leaf scorch on deciduous/shade treesBrowning around leaf margins

If leaf margins are uniformly brown, this is leaf scorch. It is due to roots not replacing moisture as quickly as leaves are losing moisture via transpiration. On larger leaved trees, or young trees with less established root systems, growing in windy, exposed sites, it may simply be due to the heat and winds we’ve had this year which increases transpiration. A deep watering, about 8” deep from the tree trunk to well beyond the dripline, and the use of 2” to 4” deep layer of organic mulch can decrease drought stress. Know that most lawn irrigation systems do not moisten the soil deep enough to benefit trees, especially in abnormally dry years. Leaf scorch can also be a sign of an issue with tree roots or within the trunk. If this is the case, the tree may continue to decline but there is not much that can be done other than mulching and correct irrigation. Do not recommend nitrogen fertilization to clients in these cases.

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15. Tree plantingTime new plants for September & October; root starter products are not needed

September and October are good times to plant trees. A common mistake made is planting too deep. Once planted too deep, a tree grows slower and has stress issues over its lifetime. To avoid planting too deep, always find the root flare (where the first lateral root grows off of the trunk and the trunk flares out slightly) before ever digging the hole. Once found (you’ll likely have to remove soil on top of the root ball to find the root flare), only then dig the hole just deep enough so the first lateral root will be just below the soil line and/or the trunk flare is visible above ground. If the soil in the hole is loosened beneath the root ball, it will sink after planting and end up being too deep. As little as two inches too deep can cause issues. Always set the root ball on undisturbed soil. It is more important to dig a wider hole to loosen soil around the sides of the root ball because tree roots grow outward. Backfill the hole with the same soil removed. Do not use amended soil in the planting hole only.

Too much phosphorous from root starter fertilizers can be harmful. If clients ask if a starter fertilizer is needed at planting, the answer is usually no. Only a soil test can determine if phosphorous, the common nutrient found in starter fertilizers, is needed. However, as a general rule, landscape soils in Nebraska has plenty of phosphorous (P). This nutrient binds to soil and does not readily leach out of soil like nitrogen does. And P is readily available in the warm soils found with fall planting. The buildup of phosphorus in soil can cause plants to grow poorly and even die. Excessive soil phosphorus reduces the plant’s ability to take up required micronutrients, particularly iron and zinc, even when soil tests show there are adequate amounts of these nutrients in soil. Phosphorus buildup is caused by excessive use of inorganic fertilizer or the use of composts and manures high in phosphorus. High soil phosphorus levels also can threaten streams, rivers, lakes and oceans.

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16. Responding to tree storm damageProper pruning cuts needed when removing storm damage to maintain tree health

Recent high winds caused storm damage to trees in some areas of Nebraska. Refer to the Nebraska Forest Service publication series below for recommendations in dealing with storm damage. One issue we often see, in the hurry to deal with safety and removal of broken branches following a storm, is improper pruning cuts. Often, branch stubs are left. If this is done, stubs should eventually be removed to eliminate this avenue of wood rot entry into the tree.

A good time to prune shade trees is late winter through spring. Remind clients not to treat pruning wounds with any kind of dressing/paint. These products interfere with the trees own response to sealing wounds.

Storm Damaged Trees, Nebraska Forest Service
Weather –Ready Landscapes Infographics, Nebraska Extension

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17. Tomato diseases heavy this yearDisease pressure high this year; infections severe

Septoria leaf spot and early blight infections on tomatoes are high this year; symptoms below.  Both start near the bottom of the plant and progress upwards.

  • Early blight - Starts with small brown spots on lower leaves. Lesions develop a "bulls-eye" pattern of concentric rings. Infected leave yellow and die. Infection progress up the plant. These symptoms appear about 10 days after infection. Occurs in midsummer during warm, humid periods and can spread very rapidly.
  • Septoria leaf spot - begins as tiny black dots on the leaves, enlarging to small circular spots with a dark margin and gray center. Infected leaves turn yellow and die. Elongated lesions develop on stems and petioles. 
  • Bacterial spot/speck - Begins as very small black spots on leaves and fruits. Severe infection causes leaf loss and unmarketable fruits. 

Fungicide control this late in the season may not be very effective for home gardeners. Home gardeners should use good sanitation and clean up the garden this year. Next year, select resistant tomato cultivars, avoid overhead irrigation, mulch around the base of plants immediately after planting to avoid soil splash and use crop rotation.  Controlling tomato leaf spot diseases, Nebraska Extension

Commercial growers should refer to the product guide like below for fungicide recommendations. 
Midwest Commercial Vegetable Production Guide

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18. Grape harvestHarvest parameters; fruit storage; bird damage protection

Most grapes are ready for harvest and it is looking to be a good year. See links for information on how grape growers in the wine industry determine when grapes are ready to harvest and tips for homeowners harvesting small fruits.

Harvesting and Storing Small Fruits, Iowa State University Extension
Harvest Parameters, University of Nebraska Viticulture Program

Bird damage is often more severe on red grape varieties. Netting is the best method for protecting grape clusters from bird feeding. Netting cannot simply be draped over the vines. It needs to be placed on a frame and anchored to the ground to be effective. In small plantings, individually bagging grape clusters with white paper bags or small netted bags made of tulle is another option. Don’t wait too long to bag.

Managing Bird Damage on Fruit Farms, Michigan State University

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19. GRO Big Red Virtual Learning SeriesHelp your clients learn more about vegetable gardening and trees through this series of online programs

GroBigRed flyerNebraska gardeners can join the experts from Nebraska Extension to learn about vegetable gardening and tree issues. Each session will include a short presentation on the topic and time for discussion and question/answer. Programs are free. Program flyer Registration 

7/21 Fall Gardening
7/28 Tomato Troubles
8/4 Preserving the Harvest
8/11 Tree Problems
8/18 Growing, Harvesting, Drying, and Cooking with Herbs
8/25 Tree Selection
9/1 Tree Planting
9/8 Tree Care
9/15 Incorporating Cover Crops
9/22 Composting
9/29 Putting the Garden to Bed

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20. Nebraska Forest Health ReportNew email newsletter from the Nebraska Forest Service (NFS)

Created by NFS forest health specialists, it provides information on timely tree problems along with growing degree day calculations for sites across the state. This enables tree managers to time their pesticide applications to insect development stages customized for their location. 

Subscribe at nfs.unl.edu/newsletters. Click on "Tree pest and disease."

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