Hort Update for April 19, 2021

1. 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. 

Lifecycle
Most rust diseases need two host plants to complete their life cycle. Distinctly different spores and symptoms are produced on each host. On Juniper species, such as cedar trees, infections of CAR and CHR cause 1/8 - 2" dimpled, reddish galls to form on branches. Infections of CQR cause cylindrical galls (often referred to as cankers) on small branches. Galls typically do not kill an entire branch or harm Junipers, though twig dieback may be evident with CQR. During wet spring periods, mainly in May, the galls form bright orange, gelatinous tendrils (teliohorns). These release spores that are blown to apples, crabapples, ornamental pear and Hawthorn. If a tree is susceptible, and environmental conditions supportive, CAR and CHR spores germinate and infect leaves, fruit and occasionally twigs. CQR primarily causes infection of thorns, twigs, and fruits with leaf infections being less common.

Symptoms
Rusty-orange spots or lesions develop at the points of infection. These spots may initially be yellow. Spores produced in these lesions then produce lesions on lower leaf surfaces that may appear hairy. As these lesions mature, they produce spores that are blown back to junipers during mid to late summer. The most common alternate host is eastern red cedar. Other hosts are Rocky mountain juniper, creeping juniper, and common juniper. Gall formation at the point of infection does not become evident until July of the next year. Galls increase in size until October and become fully mature the next spring when they produce telial masses and start the cycle over.

Once spent, galls no longer produce telial horns but remain attached to trees for several years. A main difference between these diseases is how long the galls remain active – G. globosum can produce telial horns for 3 to 5 years and G. clavipes galls may produce telia for up to 20 years. Another difference is that CHR on ornamental pear is often more of a visual nuisance than harmful to the trees.  Serviceberry and hawthorn are highly susceptible to CQR and numerous twig infections may result in dieback and a distorted canopy.

In high infection years, infected leaves can be covered with orange lesions and then turn yellow or brown and drop from trees. Early and severe defoliation, especially over a number of years in a row, will weaken and stress apple and crabapple trees.

Control
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

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3. Spruce Phomopsis browningTwig or branch cankers and dieback

Spruce issues are on the rise, especially as spruce continues to be overplanted. Spruce trees are affected by a number of diseases, including Rhizosphaera and Stigmina needle casts, Sirococus shoot blight, Cytosopora canker and others. These, along with a combination of stress from drought, weather extremes, poor planting, and incorrect care are most likely the cause of spruce dieback.

A newer issue could be Phomopis dieback, which causes dieback of branch tips that appears similar to cold temperature injury, root damage or Diploidia tip blight. Phomopsis usually causes tip blight on young spruce in plant nurseries but is now being observed causing cankers and branch dieback on mature spruce trees in landscapes. Norway, Englemann, and blue spruce are most commonly affected.

The disease begins in the lower canopy and moves upward, but in some cases progresses quickly, causing dieback of a large portion of the tree. Besides needle death and drop, there are few other external symptoms to indicate where the original infection took place. Cutting into and removing the bark will show the brown discoloration of the developing canker. Occasionally, resin may be found on the outside of a twig canker. Cankers can lead to older needle loss similar to needle cast diseases, while leaving terminal buds alive. However, once the pathogen girdles a branch, the branch dies all the way to the tip. The pathogen appears to infect younger tissue readily, but will also infect wounded older tissue, leading to cankers on larger limbs 

Pruning out infected branches will reduce potential inoculum for future seasons. We are not sure why this disease is becoming more prevalent in landscapes, but trees being exposed to more environmental extremes may be a factor. Fungicides are not recommended for homeowners but products containing thiophanate methyl can be used in nurseries dealing with this disease. Correct planting and care practices to maintain strong, healthy trees is most important. 

Phomopsis Dieback of Spruce, Purdue University

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4. Winter damage to trees and shrubsReport on commonly observed winter damage across Nebraska

Woody plants showing noticeable amounts of winter damage include the following. 

• Pine trees, specifically white and Austrian pine, but some others may also have symptoms - widespread desiccation (browning and drying) of needle tips throughout trees
• Arborvitae - desiccation of foliage
• Boxwood - desiccation and death of entire branches or sections of the shrub
• Forsythia - death of flower buds, except those protected by snow cover. Some plants may have better flower bud survival, if the shrub was in a very protected location away from winter wind and receiving some temperature buffering from homes or other structures. 
• Roses - extensive branch dieback, with large stretches of brown or black stems.

What should be done now? While pine needles may be partially brown, the buds on the branches may still be viable and capable of developing new growth. If twigs are not dry and brittle then leave them until late May to determine how much new growth will develop. Brown branches on arborvitae, boxwood and roses should be pruned back to living tissue. Forsythia flower buds may have died during winter, but in most cases the shrub itself survived. Be patient and it should bloom next year.

Resource to help clientele understand this issue: Winter Desiccation, Nebraska Extension

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5. Freeze damage to trees and ornamentalsDamage expected following cold night temperatures

Recent freezing night temperatures and snow in some locations, may result in damage to plants which are no longer fully dormant. Trees that have begun leafing out may suffer leaf death, but typically a second flush of leaves will follow in a few weeks. Ornamental trees currently in flower, such as magnolia, pear and crabapple, may lose their flowers. Potential damage to flower buds on tree fruits is a serious concern, since a significant crop reduction will occur if many buds are killed. For example, apple trees in the full pink stage - the point at which flower buds are fully swollen and showing color but not yet open - will suffer 10% flower death at 28°F and 90% flower death at 25°F.

The stage of bud development determines how susceptible any give fruit crop is when freezes occur.

• Picture Table of Fruit Freeze Damage Thresholds, Michigan State University
• Freeze Damage Depends on Tree Fruit Stage of Development, Michigan State University

Encourage clients to cover tender plants, including ornamentals and vegetables, they may have planted early. Bring containers inside a garage or shed. Early emerging perennials in the vegetable garden may also suffer damage. Rhubarb's highest freezing temperature is 30.3°F and asparagus 30.9°F. Tell clients to cut back frozen foliage and allow new foliage to emerge. 

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6. Lilac leaf spot preventionTime to prevent severe mid-summer leaf browning on plants with a history of infection

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 

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7. Soil temperaturesCheck soil temperature 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 tempertures 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

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8. Pine disease preventionPrepare to treat susceptible trees in late April & May

Diplodia (Sphaeropsis) blight and Dothistroma needle blight are two common fungal diseases of pine, especially Austrian and ponderosa, for which timing of fungicide applications is approaching.

Symptoms:

• Diplodia - dead and stunted needles on branch tips, black fruiting bodies (specks) on the bottoms of pine cones, and entire branches dying.
• Dothistroma - dark spots or bands on needles and needles turning brown from the tips back to a lesion. Branches near the lower half of the tree are usually infected first.

While neither disease rapidly kills a tree, repeated infections in consecutive years will affect aesthetics, windbreak value, and likely shorten a trees life. Now is the time to prepare to treat trees infected with either of these diseases. Positively identify a disease on a plant, and know when timing of fungicides will be effective, prior to treating.

Control:

• Diplodia - spray branch tips thoroughly when new growth starts (around the third week of April) just before needles emerge from sheaths. Reapply 7-14 days later according to fungicide label directions. 
• Dothistroma - spray trees as needles are emerging (mid-May) and after new growth has occurrecd (mid to late June). 

For fungicide recommendations, refer to Diseases of Evergreen Trees, Nebraska Forest Service

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9. Zimmerman pine moth controlPinkish pitch/sawdust masses at branch junction with trunk; ultimately results in branch death or breakage.

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

For a more complete list of common landscape insects and their GDD thresholds, visit Michigan State University GGD of Landscape Insects or GGD of Conifer Insects.  

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10. Winter annual weed control- henbit, speedwell and annual bluegrassManage lawn to compete; pre-emergence herbicides can be applied in September.

The purple blooms of henbit are prevalent right now, along with the tiny blue flowers of field speedwll and the lush green blades of annual bluegrass. All of these weeds are winter annuals, meaning they germinated last fall. Small plants overwintered to grow aggressively in early spring with blooming in April to May. The plants will die with hot, dry weather in mid to late June. But seed production can be high right now and is one reason infestations may become worse each year.

Control of winter annual weeds with post emergent herbicides at this time of year is not very effective or recommended. Plants will die soon on their own. The best option at this time is to hand pull if possible to allow turfgrass to fill in these areas and to minimize seed production for next fall. If winter annual infestations are becoming unacceptable, then apply a preemergent herbicide in late summer to early fall. 

Other management options for winter annual weed control include maintaining a healthy and vigorously growing lawn to out-compete these annuals; or using a 2 - to 3-inch layer of mulch in landscape beds to reduce seed germination.

Annual Bluegrass Control - Nebraska Extension
Lawn and Turfgrass Weeds: Common Field Speedwell, PennState Extension

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11. Stop pruning oak & elmWait until next fall dormancy to prune to avoid attracting insects vectoring diseases specific to these two tree genera.

Now is the time to pause on pruning oak or elm trees.  Both are susceptible to diseases that are vectored by insects and these insects are attracted to fresh cuts.  Pruning should be avoided from April 15th to October 15th.  Learn more about Oak Wilt and Dutch elm disease below.

Oak wilt, Iowa State University
Dutch elm disesase, University of Minnesota Extension

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12. Too early to spray for bagwormsWay too early to spray yet. Insecticides, including systemics, will NOT go through bag wall to reach overwintering eggs

Clients may be calling soon asking if they should start spraying for bagworm control now. The answer is NO! Eggs do not begin to hatch until approximately mid-May. Spraying too early is a waste of chemical, time and money. Chemicals will NOT go through the bag covering to kill the overwintering eggs, not even systemic products like imidacloprid or orthene. Young larvae must be hatched out and actively feeding before spraying starts, but not all eggs hatch at exactly the same time. Give the insects about two weeks to hatch and plan to spray in early June. Then one thorough spray, soaking the entire plant should be all that is needed.

For now, continue to encourage clients to scout plants affected last year. Remove and destroy bags on high value, smaller evergreen trees will help reduce next year's bagworm population. Destroy bagworm eggs by removing bags from the plant and crushing or immersing them in soapy water. If bags containing eggs are discarded on the ground, eggs may survive winter fine then hatch and larvae return to the surrounding plants next summer. As many as 500 to 1000 eggs can overwinter in one female bagworm's bag.

Bagworms, Nebraska Extension
Bagworms, Backyard Farmer

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13. In the News - sudden apple decline (SAD) or rapid apple decline (RAD)NOT in Nebraska at this time

This disease has NOT been found in Nebraska, but it is in the news and may prompt questions from clients. It is being observed in Pennsylvania and is affecting young, dwarf apple trees with no known cause to date. The issue seems to revolve around the graft union and tree stress, such as severe winters or drought, appear to be a predisposing factor. Assure customers this issue is not in Nebraska but remind them of the importance of good plant care practices to grow healthier trees better able to withstand stress. Avoid planting too deep, irrigate correctly, avoid high nitrogen fertilization, mulch properly to prevent trunk damage from mowers/trimmers and to conserve soil moisture.

Sudden Apple Decline, PennState University 

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15. Irrigation auditsHelp clients identify and fix irrigation system problems. 

Automatic irrigation systems often do not water turf uniformly, resulting in areas that may turn brown. When this happens, homeowners respond by either watering more (and end up overwatering other areas) or assuming the brown spot is being caused by a pest that needs to be controlled. Irrigation systems can be audited for efficiency and uniformity to help avoid turf issues, water waste and unhappy customers. For information on how to conduct an audit, go to:

Increase Lawn Irrigation Efficiency, Nebraska Extension

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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 constitute endorsement by Nebraska Extension. Nor does it imply discrimination against other similar products.