Category Archives: Disease – Needled Woody Ornamental

Plant Problems to Watch for in 2023

Lipstick Rust Lipstick Rust
Host:   Chinese juniper, apple, crabapple
Pathogen:   Gymnosporangium yamadae
Signs/Symptoms:   Brown blobs with orange gelatinous masses (juniper), red leaf spots (apple, crabapple)
Boxwood Blight Boxwood Blight
Host:   Boxwood
Pathogens:   Calonectria pseudonaviculata
Signs/Symptoms:   Circular, brown leaf spots followed by leaf drop and shrub death
For more information see:   UW Plant Disease Facts D0023
Late Blight Late Blight
Host:   Tomato, potato
Pathogen:   Phytophthora infestans
Signs/Symptoms:   Water-soaked spots on leaves, leathery areas on tomato fruits, rapid plant death
For more information see:   UW Plant Disease Facts D0068
Septoria Leaf Spot and Early Blight Septoria Leaf Spot and Early Blight
Host:   Tomato
Pathogens:   Septoria lycopersici and Alternaria solani
Signs/Symptoms:   Spotting and eventual total collapse of leaves, working from the bottom of the plant up
For more information see:   UW Plant Disease Facts D0100/46
Septoria Leaf Spot of Lilac Septoria Leaf Spot of Lilac
Host:   Lilac
Pathogen:   Septoria sp.
Signs/Symptoms:   Dead spots on leaves, potentially leading to complete leaf browning
Verticillium Wilt Vascular Discoloration Verticillium Wilt
Hosts:   Woody and herbaceous ornamentals, vegetables
Pathogens:   Verticillium sp.
Signs/Symptoms:   Wilting, branch dieback, plant death
For more information see: UW Plant Disease Facts D0121/D0122
Powdery Mildew Powdery Mildew
Hosts:   Herbaceous and woody ornamentals, fruit, vegetables, turf
Pathogens:   miscellaneous powdery mildew fungi
Signs/Symptoms:   Powdery white growth on leaves
For more information see:   UW Plant Disease Facts D0084/86/87
Rhizosphaera Needle Cast Rhizosphaera Needle Cast
Hosts:   Colorado blue spruce, other spruces
Pathogen:   Rhizosphaera kalkhoffii
Signs/Symptoms:   Browning/purpling of interior needles of lower branches, followed by needle drop
For more information see:   UW Plant Disease Facts D0093
Diplodia Shoot Blight and Canker Diplodia Shoot Blight and Canker
Hosts:   Austrian pine, other pines
Pathogen:   Diplodia spp.
Signs/Symptoms:   Dieback of brand tips with dead needles showing uneven lengths
For more information see:   UW Plant Disease Facts D0042
Chlorosis Chlorosis
Hosts:    Pin oak, red maple, birch, azalea, white pine, blueberry
Pathogen:   None
Signs/Symptoms:   Yellow leaves with dark green veins
For more information see:   UW Plant Disease Facts D0030
Improper Planting Improper Planting
Hosts:   Woody trees and shrubs
Pathogen:   None
Signs/Symptoms:   No root flare at the soil line, girdling roots, frost cracks, canopy thinning, early fall color, branch dieback, tree/shrub decline and death

For more information on plant problems to watch for:
See https://pddc.wisc.edu/ or contact your county Extension agent.

Plant Problems to Watch for in 2022

Boxwood Blight Boxwood Blight
Host:   Boxwood
Pathogens:   Calonectria pseudonaviculata
Signs/Symptoms:   Circular, brown leaf spots followed by leaf drop and shrub death
For more information see:   UW Plant Disease Facts D0023
Lipstick Rust Lipstick Rust
Host:   Chinese juniper, apple, crabapple
Pathogen:   Gymnosporangium yamadae
Signs/Symptoms:   Brown blobs with orange gelatinous masses (juniper), red leaf spots (apple, crabapple)
Late Blight Late Blight
Host:   Tomato, potato
Pathogen:   Phytophthora infestans
Signs/Symptoms:   Water-soaked spots on leaves, leathery areas on tomato fruits, rapid plant death
For more information see:   UW Plant Disease Facts D0068
Septoria Leaf Spot and Early Blight Septoria Leaf Spot and Early Blight
Host:   Tomato
Pathogens:   Septoria lycopersici and Alternaria solani
Signs/Symptoms:   Spotting and eventual total collapse of leaves, working from the bottom of the plant up
For more information see:   UW Plant Disease Facts D0100/46
Septoria Leaf Spot of Lilac Septoria Leaf Spot of Lilac
Host:   Lilac
Pathogen:   Septoria sp.
Signs/Symptoms:   Dead spots on leaves, potentially leading to complete leaf browning
Wood Rots Wood Rots
Hosts:   Woody trees and shrubs
Pathogens:   Miscellaneous wood rot fungi
Signs/Symptoms:   Shelf-like growths on trunks and branches
Canker Diseases Canker Diseases
Hosts:    Woody trees and shrubs
Pathogens:   Miscellaneous canker fungi
Signs/Symptoms:   Sunken areas on trunks/branches
For more information see:   UW Plant Disease Facts D0027, D0037, D0042, D0055, D0074, D0114
Virus Diseases Virus Diseases
Hosts:   All plants, particularly herbaceous ornamentals
Pathogen:   Miscellaneous plant viruses
Signs/Symptoms:   Blotchy leaf color, growth distortions
For more information see:   UW Plant Disease Facts D0036, D0063, D0067, D0115, D0116, D0130
Rhizosphaera Needle Cast Rhizosphaera Needle Cast
Hosts:   Colorado blue spruce, other spruces
Pathogen:   Rhizosphaera kalkhoffii
Signs/Symptoms:   Browning/purpling of interior needles of lower branches, followed by needle drop
For more information see:   UW Plant Disease Facts D0093
Chlorosis Chlorosis
Hosts:    Pin oak, red maple, birch, azalea, white pine, blueberry
Pathogen:   None
Signs/Symptoms:   Yellow leaves with dark green veins
For more information see:   UW Plant Disease Facts D0084
Improper Planting Improper Planting
Hosts:   Woody trees and shrubs
Pathogen:   None
Signs/Symptoms:   No root flare at the soil line, girdling roots, frost cracks, canopy thinning, early fall color, branch dieback, tree/shrub decline and death

 

Lichens

There are many types of lichens. Crustose lichens (left) are crust-like and adhere tightly to the surface upon which they grow. Foliose lichens (right) are leaf-like and composed of flat sheets of tissue that are not tightly bound.
There are many types of lichens. Crustose lichens (left) are crust-like and adhere tightly to the surface upon which they grow. Foliose lichens (right) are leaf-like and composed of flat sheets of tissue that are not tightly bound.

What are lichens? Lichens are organisms that arise from mutually beneficial interactions between certain filamentous fungi, algae and yeasts.  The filamentous fungi provide the physical structures of the lichens, as well as protection for the algae and yeasts.  The algae produce food for the fungi and yeasts via photosynthesis.  The yeasts are thought to produce compounds to fend off disease-causing organisms and insect pests.

What do lichens look like? Lichens come in four basic growth forms. Crustose lichens are crust-like and adhere tightly to the surface upon which they grow. Foliose lichens are leaf-like and composed of flat sheets of tissue that are not tightly bound together. Squamulose lichens are composed of scale-like parts. Fruticose lichens are composed of free-standing branching tubes.

Where do lichens come from? Lichens are everywhere. There are an estimated 13,500 to 17,000 species of lichens, and lichens can be found growing in tropical, temperate and polar regions throughout the world. Lichens will grow on almost any surface that is stable and reasonably well-lit. In temperate regions, lichens can often be found growing on the bark of trees or old fence posts. Others lichens grow in less hospitable places, such as bare rock surfaces or old headstones in graveyards, where they aid in the breakdown of rocks and the formation of soil.

There are many types of lichens. Crustose lichens (left) are crust-like and adhere tightly to the surface upon which they grow. Foliose lichens (right) are leaf-like and composed of flat sheets of tissue that are not tightly bound.
There are many types of lichens. Crustose lichens (left) are crust-like and adhere tightly to the surface upon which they grow. Foliose lichens (right) are leaf-like and composed of flat sheets of tissue that are not tightly bound.

How do I save a tree with lichens? DO NOT PANIC! Lichens do not harm trees; they are not pathogens or parasites, and do not cause disease. Lichens are self-reliant, with the algal component of the lichen producing food for the organism via photosynthesis. Lichens absorb water and minerals from rainwater and the atmosphere, and because of this, they are extremely sensitive to air pollution. As a result, the presence or absence of certain lichen species can be used as an indicator of levels of atmospheric pollutants. Information on the abundance and species of lichens growing in an area can give a good indication of the local air quality.

For more information on lichens: Contact your county Extension agent.

Ten Common Plant Diseases/Disorders You Can Diagnose by Eye

Powdery Mildew Powdery Mildew
Hosts:   Herbaceous and woody ornamentals, fruits, vegetables, turf
Pathogens:   Miscellaneous powdery mildew fungi
Signs/Symptoms:  Powdery white growth on leaves
For more information see:       UW Plant Disease Facts D0084/86/87
Tar Spot Tar Spot
Hosts:  Maples
Pathogen:   Rhytisma spp.
Signs/Symptoms:  Tarry areas (either solid spots or clusters of small spots) on leaves
For more information see:       UW Plant Disease Facts D0110
Peach Leaf Curl Peach Leaf Curl
Hosts:  Peach
Pathogen:   Taphrina deformans
Signs/Symptoms:  Light-green, yellow or purplish-red puckered areas on leaves
For more information see:       UW Plant Disease Facts D0076
Sooty Mold Sooty Mold
Hosts:  Any plant
Pathogen:   Miscellaneous sooty mold fungi
Signs/Symptoms:  Powdery black growth on leaves or needles
For more information see:       UW Bulletin A2637
Chlorosis Chlorosis
Hosts:  Oak, red maple
Cause:   Iron or manganese deficiency, often induced by high soil pH
Signs/Symptoms:  Yellow leaves with dark green veins
For more information see:       UW Plant Disease Facts D0030
Gymnosporangium Rusts Gymnosporangium Rusts
Hosts:  Juniper, apple, crabapple, hawthorn, quince
Pathogen:   Gymnosporangium spp.
Signs/Symptoms:  Brown blobs with orange gelatinous masses (juniper); yellow/orange leaf spots (other hosts)
For more information see:       UW Plant Disease Facts D0058
Black Knot Black Knot
Hosts:  Prunus spp. (plum and cherry)
Pathogen:   Apiosporina morbosa
Signs/Symptoms:  Black poop-like growths on branches
For more information see:       UW Plant Disease Facts D0018
Elderberry Rust Elderberry Rust
Hosts:  Elderberry
Pathogen:   Puccinia sambuci
Signs/Symptoms:  Light yellow, powdery growths on branches
For more information see:       UW Plant Disease Facts D0049
Golden Canker Golden Canker
Hosts:  Pagoda dogwood
Pathogen:   Cryptodiaporthe corni
Signs/Symptoms:  Gold-colored branches with orange spots
For more information see:       UW Plant Disease Facts D0055
Dog Vomit Slime Mold Dog Vomit Slime Mold
Hosts:  Any plant and on mulch
Cause:   Fuligo septica
Signs/Symptoms:  Scrambled egg-like masses on mulch or at the base of plants
For more information see:       UW Plant Disease Facts D0102

For more information on common plant diseases:  See https://pddc.wisc.edu/ or contact your county Extension agent.

Conifer Disease Quick Reference

Root and Crown Rot Root and Crown Rots
Hosts:  All conifers
Pathogens:   Assorted root rot fungi/water molds
Signs/Symptoms:  Poor growth, branch dieback, discolored and deteriorated roots
For more information see:       UW Plant Disease Facts D0094
Rhizosphaera Needle Cast Rhizosphaera Needle Cast
Hosts:  Colorado blue spruce, other spruces
Pathogen:   Rhizosphaera kalkhoffii
Signs/Symptoms:  Browning/purpling of interior needles of lower branches, followed by needle drop
For more information see:       UW Plant Disease Facts D0093
Cytospora Canker Cytospora Canker
Hosts:  Colorado blue spruce, other spruces
Pathogen:   Cytospora kunzei
Signs/Symptoms:  Branch dieback with milky-white patches of dried sap on affected branches
For more information see:       UW Plant Disease Facts D0037
Spruce Needle Drop Spruce Needle Drop
Hosts:  Spruces
Pathogen:   Unknown (possibly Setomelannoma holmii)
Signs/Symptoms:  Needle loss and dieback at or near branch tips
Gymnosporangium Rust Gymnosporangium Rusts
Hosts:  Juniper, apple, crabapple, hawthorn, quince
Pathogen:   Gymnosporangium spp.
Signs/Symptoms:  Brown blobs with orange gelatinous masses (juniper); yellow/orange leaf spots (other hosts)
For more information see:       UW Plant Disease Facts D0058
Phomopsis Tip Blight Phomopsis Tip Blight
Hosts:  Junipers
Pathogen:   Phomopsis juniperovora
Signs/Symptoms:  Browning and dieback of branch tips  in spring and early summer as new growth emerges
For more information see:       UW Plant Disease Facts D0077
Diplodia Shoot Blight and Canker Diplodia Shoot Blight and Canker
Hosts:  Austrian pine, other pines
Pathogen:   Diplodia spp.
Signs/Symptoms:  Dieback of branch tips with dead needles showing uneven lengths
For more information see:       UW Plant Disease Facts D0042
Dothistroma Needle Blight Dothistroma Needle Blight
Hosts:  Austrian pine, other pines
Pathogen:   Dothistroma pini
Signs/Symptoms:  Needle tip browning and death with a distinct break between live and dead tissue
For more information see:       UW Plant Disease Facts D0043
Drought Stress Drought Stress
Hosts:  All conifers
Cause:   Insufficient water
Signs/Symptoms:  Purpling/browning of needles near branch tips or higher up in plants during the summer
Winter Injury Winter Injury/Winter Burn
Hosts:  All conifers, particularly yew and juniper
Cause:   Insufficient water
Signs/Symptoms:  Needle browning/bleaching over winter or in spring as plants come out of dormancy
For more information see:       UW Plant Disease Facts D0127
Herbicide Damage Herbicide Damage
Hosts:  All conifers
Cause:   Exposure to herbicides
Signs/Symptoms:  Twisted or otherwise distorted growth, needle yellowing or browning, plant death
For more information see:       UW Plant Disease Facts D0060

For more information on conifer diseases:  See https://pddc.wisc.edu/ or contact your county Extension agent.

Diplodia Shoot Blight and Canker

Diplodia shoot blight and canker killing branch tips of Austrian pine.
Diplodia shoot blight and canker killing branch tips of Austrian pine.

What is Diplodia shoot blight and canker?  Diplodia shoot blight and canker (formerly Sphaeropsis shoot blight and canker) is one of the most common fungal diseases of Austrian pine in Wisconsin.  This disease can also affect other pines including red, jack, Scots and mugo pine, as well as other conifers including cedars, cypresses, firs, junipers and spruces.

What does Diplodia shoot blight and canker look like?  Initially, affected branch tips may ooze a large amount of resin.  Eventually, these branch tips brown and die, with dead needles on these branches having varying lengths.  As the disease progresses, sunken or swollen, discolored areas (called cankers) may form on infected twigs.  Diplodia shoot blight and canker can be distinguished from damage from boring insects (where there also may be heavy resin flow) by an absence of any tunneling.

Where does Diplodia shoot blight and canker come from?  Diplodia shoot blight and canker is caused by several fungi in the genus Diplodia.  These fungi include Diplodia sapinea (formerly known as Diplodia pinea and Sphaeropsis sapinea), which has historically been cited as the cause of the disease on Austrian pine.  However, this fungus is only one of several Diplodia species that can cause problems on this host.  Diplodia fungi survive in infected shoots and pinecones where they form small, black fruiting bodies (i.e., reproductive structures) that produce brown-colored spores.

How do I save a tree with Diplodia shoot blight and canker?  Immediately remove and destroy diseased branch tips.  Also, where possible, remove and destroy pinecones that have fallen from infected trees.  Dispose of these materials by burning (where allowed by local ordinance) or burying them.

When pruning, cut branches six to eight inches below the point where they are obviously infected.  Prune only in dry weather.  Between cuts, decontaminate pruning tools by treating them for at least 30 seconds with 70% alcohol (e.g., rubbing alcohol, certain spray disinfectants), or a 10% bleach solution (i.e., one part of a disinfecting bleach and nine parts water).  Decontaminating tools is important to help prevent accidental movement of Diplodia fungi from branch to branch during pruning.  If you decide to use bleach, be sure to rinse your tools thoroughly after pruning and then oil them to prevent rusting.

Needles of different lengths (left) and production of excessive resin on branch tips (right) are typical symptoms of Diplodia shoot blight and canker.
Needles of different lengths (left) and production of excessive resin on branch tips (right) are typical symptoms of Diplodia shoot blight and canker.

How do I avoid problems with Diplodia shoot blight and canker in the future?  Avoid planting Austrian pines; plant other types of evergreens instead.  Minimize any stresses on established Austrian pines.  Water trees adequately, particularly during dry periods.  Established trees should receive approximately one inch of water per week from the time that the ground thaws in the spring, through the summer and into the fall up until the ground freezes or there is a significant snowfall.  New transplants (i.e., conifers planted within three years) require approximately two inches of water per week.  During periods with insufficient rain, apply water at the drip lines of trees (i.e., the edges of where the branches extend) using a drip or soaker hose.

When planting trees, be sure to allow ample space for roots to grow, avoid compacting the soil around trees, and make sure there is adequate soil drainage.  Mulch trees to at least their drip lines with a high-quality mulch (e.g., shredded oak bark mulch, red cedar mulch).  Use one to two inches on heavier (e.g., clay) soils.  Use three to four inches on lighter (e.g., sandy) soils.  Keep mulch approximately four inches from tree trunks.  DO NOT fertilize new transplants, and fertilize established conifers only when a soil fertility test indicates that fertilization is needed.  DO NOT overfertilize, particularly with nitrogen.

Finally, you may want to apply fungicides to help prevent infections.  Apply fungicides only after you have pruned out diseased branches as described above.  Fungicides prevent infections but do not cure existing infections.  Alternate use of fungicides containing thiophanate-methyl and chlorothalonil that are labeled for use on conifers.  Start applications at bud break and continue at 14 day intervals until full shoot elongation.  DO NOT use thiophanate-methyl alone.  Overuse of thiophanate-methyl can potentially select for variants of Diplodia that will no longer be controlled by this active ingredient.  Be sure to read and follow all label instructions of the fungicides that you select to ensure that you use the products in the safest and most effective manner possible.

For more information on Diplodia shoot blight and canker:  Contact your county Extension agent.

Rhizosphaera Needle Cast

Browning of interior spruce needles caused by Rhizosphaera needle blight.
Browning of interior spruce needles caused by Rhizosphaera needle blight.

What is Rhizosphaera needle cast?  Rhizosphaera needle cast is the most common disease of Colorado blue spruce in Wisconsin, making Colorado blue spruce unsightly and unusable in many landscape settings.  The disease also affects other conifers including black, Engelmann, Serbian, Sitka, and white (e.g., Black Hills) spruce; Austrian, mugo and eastern white pine; Douglas-fir, balsam fir and western hemlock.

What does Rhizosphaera needle cast look like?  The first noticeable symptom of Rhizosphaera needle cast is purpling or browning and loss of the innermost needles on lower branches of spruce trees.  Often, the youngest needles at the tips of branches remain healthy.  Rows of small, black spheres form along the length of infected needles and are visible with a 10X hand lens.  These black spheres are fruiting bodies (i.e., reproductive structures) of the fungus that causes the disease and are diagnostic.

Where does Rhizosphaera needle cast come from?  Rhizosphaera needle cast is typically caused by the fungus Rhizosphaera kalkhoffii, although other species of Rhizosphaera can be involved depending on the host.  Infected needles, including those that are still attached to branches and those that have fallen to the ground, produce spores that can be blown or splashed to healthy needles.

How do I save a tree or shrub with Rhizosphaera needle cast?  Consider treating affected trees with fungicides labeled for use on evergreens and containing copper or chlorothalonil.  Treatments will not cure existing infections, but can prevent additional infections.  Apply treatments every three to four weeks starting as new needles emerge each spring.  Continue applications through periods of wet weather.  For fungicide treatments to be effective, thoroughly cover all needles.  This may be difficult on large trees.  Be sure to read and follow all label instructions of the fungicide that you select to ensure that you use the product in the safest and most effective manner possible.

How do I avoid problems with Rhizosphaera needle cast in the future?  The easiest way to avoid Rhizosphaera needle cast is to avoid planting Colorado blue spruce.  If you do plant this tree, consider using dwarf varieties, and allow adequate spacing between trees so that branches will not overlap when they are full size.  Dwarf varieties and properly spaced larger spruce varieties will have better air penetration and needles will dry more quickly.  Dry needles are less likely to be infected.  Check existing spruce trees for the disease, and remove and destroy any diseased branches and needles by burning (where allowed by local ordinance), burying or hot composting.

For more information on Rhizosphaera needle cast:  See UW Bulletin A2640, Colorado Blue Spruce and Other Conifers Disorder:  Rhizosphaera Needle Cast (available at https://learningstore.extension.wisc.edu/) or contact your county Extension agent.

Wood Mulch and Tree Health

What are the benefits of wood mulch?  Wood mulch is typically available as chipped wood, or shredded or chunked bark, and can contribute to tree health in many ways.  When high quality, composted mulches are applied two to four inches deep in a ring three to six feet in diameter (or greater) from the trunk of a tree, mulch can help preserve moisture, control weeds, limit damage to the trunk from mowers and string trimmers and moderate the soil temperature.  Use four inches of mulch when soils are light and well-drained, and two inches of mulch on heavier, clay soils.

Use of properly composted mulches can be beneficial to trees and shrubs in the landscape.
Use of properly composted mulches can be beneficial to trees and shrubs in the landscape.

Can wood mulch harm trees?  Use of improperly composted mulches (some-times called “sour mulches”), can lead to tree nutrient deficiencies.  Sour mulches can also produce gases like methane and ammonia that can be toxic to plants.  Foliage on trees surrounded by sour mulches may initially turn yellow, then brown, die and fall off.  If your mulch smells like vinegar, ammonia or sulfur, it is likely a sour mulch and should be removed.  Replace the sour mulch with a high quality, composted mulch and consult with your local Extension agent about testing the soil for nutrient deficiencies.  Fertilize appropriately based on the results of these tests.

Improper application of mulch can also lead to problems.  Piling wood mulch up against the trunk of a tree can keep the bark underneath excessively wet.  This wetness can contribute to bark decay.  In addition, use of thick mulch layers (greater than four inches) can lead to overly wet soils that are favorable for development of root rots (see UW Plant Disease Facts D0094, Root and Crown Rot).  To avoid these problems, make sure mulch is applied at least four inches away from the trunk of a tree and that the mulch layer is the appropriate thickness for the soil type in your landscape (see above).

Does woody mulch harbor or attract insects?  Insects such as earwigs [see UW Bulletin A3640, Controlling Earwigs (available at https://learningstore.extension.wisc.edu/)], centipedes (see University of Wisconsin Garden Facts XHT1113, Centipedes), millipedes (see University of Wisconsin Garden Facts XHT1108, Millipedes) and sowbugs (see University of Wisconsin Garden Facts XHT1110, Sowbugs) can feed on decaying organic matter in mulches.  While these insects are often only nuisances, earwigs can feed on and cause damage to a variety of ornamentals, particularly to flowering plants.  If mulch is used near entrances to a home or around basement windows, these unwanted insects may get inside.  Termites ingest wood and can be attracted to wood mulch, but new termite colonies are not likely to become established due to use of wood mulches.  Typically, termites are not a problem in Wisconsin, and when colonies are found, they occur only in the southern half of the state.

Carpenter ants [see UW Bulletin A3641, Controlling Carpenter Ants (available at https://learningstore.extension.wisc.edu/)] and powderpost beetles (see University of Wisconsin Garden Facts XHT1053, Powderpost Beetles) are unlikely to utilize mulch as a food source because conditions required for their development would not be satisfied by wood mulch.  Carpenter ants do not ingest wood as a food source; instead, they chew non-living wood (in trees or landscape timbers, etc.) to excavate galleries in which they live and raise their young.  Since wood mulch is composed of small wooden pieces, it would not serve as a home.  To avoid potential insect problems, keep mulch as far away from the foundation of your home as possible and seal all holes and crevices that insects might use as entry points.  Also, periodically inspect landscape timbers and the house for termites.

Does woody mulch harbor tree pathogens?  Wood mulch may come from many sources, including trees and shrubs that have died from a wide range of diseases.  To be harmful to your trees, disease-causing organisms (pathogens) would have to survive in mulch and these organisms would have to move from the mulch either directly, or through the soil, to their new host – your tree.  There is currently very little research on this topic.

Elm trees killed by Dutch elm disease (see UW Plant Disease Facts D0045, Dutch Elm Disease), can serve as breeding areas for native and European elm bark beetles.  Bark beetles that breed in logs or firewood from these trees can pick up the fungi that cause Dutch elm disease (Ophiostoma ulmi and Ophiostoma novo-ulmi) and carry these fungi from tree to tree.  Chipping infected elm trees creates an unfavorable environment for bark beetles yet there is no scientific literature that describes the level of risk of transmitting the Dutch elm disease fungi from wood chips or bark chunks to healthy elms.

Oak trees killed by oak wilt (see UW Plant Disease Facts D0075, Oak Wilt) can be attractive to several sap-feeding beetles that can potentially pick up the oak wilt fungus (Bretziella fagacearum) and move it in the landscape.  This process is affected by moisture and temperature and would likely be disrupted by the chipping and composting process yet there is no scientific literature that describes the level of risk of transmitting the oak wilt disease fungus from wood chips or bark chunks to healthy oaks.

Research at the University of Wisconsin-Madison suggests that wood chip mulches produced from trees suffering from Verticillium wilt (see UW Plant Disease Facts D0121, Verticillium Wilt of Trees and Shrubs) can serve as a source of the fungus (Verticillium dahliae) that causes the disease.  These studies show that Verticillium can survive for at least one year in mulch and that use of this contaminated mulch can lead to Verticillium wilt in both woody and herbaceous plants.  Therefore use of mulches produced from trees with Verticillium wilt should be avoided.

For more information on wood mulch and tree health:  Contact your county Extension agent.

Winter Burn

What is winter burn?  Winter burn is a common problem of evergreens including those with broad leaves (e.g., boxwood, holly, rhododendron), needles (e.g., fir, hemlock, pine, spruce, yew) and scale-like leaves (e.g., arborvitae, false cypress, juniper) grown in open, unprotected locations and exposed to severe winter conditions.  Evergreen plants that are marginally hardy in a location (i.e., not well-adapted to local winter conditions) are at increased risk for winter burn.  Winter burn can be so severe that affected plants may die and/or require replacement.

Browning due to winter burn on dwarf Alberta spruce.
Browning due to winter burn on dwarf Alberta spruce.

What does winter burn look like?  Winter burn symptoms often become apparent as the snow melts and spring temperatures rise.  Foliage starts to brown at the tips of branches with browning progressing inward toward the center of the plant.  On broad-leaved evergreens, leaf edges typically brown first, followed by browning of entire leaves.  Foliage facing south, southwest or west is most often affected.  Symptomatic foliage often begins to drop off starting in spring and continuing through mid-summer as new foliage is produced.  In extreme cases, entire plants can brown and die.

What causes winter burn?  There are many factors that can contribute to winter burn.  In general, plants with shallow or poorly-developed root systems that do not efficiently take up water (e.g., recent transplants) are more prone to winter burn.  Warm fall temperatures that delay the onset of plant dormancy can also contribute to winter burn.  Under such conditions, plants are not prepared for the subsequent rapid onset of freezing winter temperatures, and as a result damage to foliage occurs.  Similar cold injury can occur mid-winter when temperatures drop sharply at sunset causing foliage that has warmed during the day to rapidly cool and freeze.  In addition, on sunny winter days, foliage (particularly foliage facing the sun) can begin to transpire (i.e., naturally lose water through the foliage).  Because the ground is frozen, plant roots cannot take up water and replace the water that has been lost from the foliage.  As a result, foliage dries and browns.  Foliage under snow or facing away from the sun and direct winds is usually not damaged.  Strong winter winds can lead to additional water loss making winter burn more severe.  Colder than normal winter temperatures and longer than normal winters can also be factors in the development of winter burn, especially if below normal temperatures occur into April (the time of year when plants normally come out of dormancy and are most susceptible to winter injury).  Finally, exposure of plants to salt used to de-ice roads, driveways and sideways during the winter can make plants more prone to winter burn injury.

How do I save a plant with winter burn?  For evergreens such as arborvitaes, boxwoods, junipers and yews, prune out dead, brown, damaged or dying tissue in mid-spring after new foliage is produced.  If new foliage has not yet emerged by spring, scratch the bark on affected branches and look for green tissue underneath.  Also gently peel back the bud scales to look for inner green bud tissue.  If the stem or bud tissue is green, buds on the branch may still break to form new foliage.  If the tissue is brown, the branch is most likely dead and you should prune the branch back to a live, lateral bud or branch.  Such buds and branches may be far back inside the canopy and pruning may remove a substantial amount of the plant.  Pines, spruces and firs typically produce new growth at branch tips in spring that will replace winter burn-damaged needles, and thus pruning may not be required on these evergreens.  After a couple of growing seasons, new foliage will fill in the areas that were damaged.  If an entire evergreen is brown, recovery is unlikely and the plant should be replaced with something (e.g., a deciduous shrub or tree) that is better-suited to the site.

How do I avoid problems with winter burn in the future?  Use a variety of strategies to prevent winter burn before winter arrives.

Plant the right plant in the right place.  Buy plants that are rated as cold hardy for your location and are well-adapted to local growing and soil conditions.  Plants exposed to drying winter sun and winds are more likely to be injured.  Therefore, avoid planting winter injury sensitive evergreens, particularly those that require shade or that are marginally cold-hardy, in exposed, sunny, windy areas.  Plant them on the northeast or east side of a building or in a protected courtyard.  Plant boxwoods, hemlocks, rhododendrons, and yews in partial shade to provide them added protection from winter sun and wind.

Plant evergreens at the right time of year.  Optimally, plant evergreens either in early spring (before buds break) or in late summer (i.e., August through September).   Evergreens can be planted in the summer if you provide supplemental water.  Avoid planting after early October in northern Wisconsin and after mid-October in southern Wisconsin as this will not allow sufficient time for roots to grow adequately before the ground freezes.

DO NOT prune evergreens in late summer or early fall.  Late season pruning of some non-native evergreens may encourage a flush of new growth that will not harden off properly before winter.  See University of Wisconsin Garden Facts XHT1013, Pruning Evergreens, for details on when and how to prune specific evergreen trees and shrubs.

Mulch evergreens properly.  Apply two inches (on clay soils) to four inches (on sandy soils) of loose mulch (e.g., shredded hardwood, pine, or cedar bark; leaf compost; or wood chips) around the base of evergreens out to at least the drip line (e.g., the edge of where the branches extend).  Keep mulch at least four inches away from the trunks of trees and the bases of shrubs.  Proper mulch insulates roots from severe fluctuations of soil temperatures and reduces water loss.  It also helps protect roots from injury due to heaving that occurs when soils go through cycles of freezing and thawing during the winter.  Heaving can especially be a problem for shallow-rooted and newly planted evergreens.  DO NOT mulch excessively or too close to plants as this can lead to damage by providing shelter for mice and voles (which can girdle trunks and branches) and by providing a favorable environment for disease development as well as insect activity and feeding.

Water plants properly.  Plants that are well-hydrated are less prone to winter burn.  In particular, newly planted or young evergreens, especially those planted in open, exposed sites, those planted under eaves, or those planted in dry falls may suffer severe moisture loss during the winter and consequently severe winter burn.  Established evergreens should receive approximately one inch of water per week and newly transplanted evergreens up to two inches of water per week during the growing season up until the soil freezes in the autumn or there is a significant snowfall.  If supplemental watering is needed, use a soaker or drip hose to apply water near the drip lines of plants rather than using a sprinkler.

Avoid late summer or fall fertilization.  Applying quick-release, high-nitrogen fertilizers in late summer or fall could potentially stimulate growth of new foliage (particularly on some non-native evergreens) as well as inhibit proper onset of dormancy which can lead to damage over the winter.  If you are concerned that your evergreens may need to be fertilized, submit a soil sample from around your plants to a professional soil testing lab that can provide specific information on what fertilizer to use and when to fertilize.

Protect plants during the winter.  Use burlap, canvas, snow fencing or other protective materials to create barriers that will protect plants from winter winds and sun.  Install four to five foot tall stakes approximately two feet from the drip lines of plants especially on the south and west sides (or any side exposed to wind) and wrap protective materials around the stakes to create “fenced” barriers.  Leave the top open.  These barriers will deflect the wind and protect plants from direct exposure to the sun.  Remove the barrier material promptly in spring.  DO NOT tightly wrap individual plants with burlap as this can collect ice, trap moisture and make plants more susceptible to infection by disease-causing organisms.  Use of anti-transpirant products to prevent water loss from foliage over the winter has been shown to have limited benefit.  These materials degrade rapidly, require reapplication after each significant rain or snow event, and may not be effective in preventing water loss that can lead to winter burn.

For more information on winter burn:  Contact your county Extension agent.

White Pine Blister Rust

What is white pine blister rust?  White pine blister rust is a serious, tree-killing disease of eastern white pine and its close relatives (pines with needles in bundles of five).  This disease is caused by the exotic (non-native) fungus Cronartium ribicola, which was introduced into North America in the early 1900’s.  This pathogen is now found in most regions where pines grow in the United States, including Wisconsin.  White pine blister rust affects trees of all ages, but perhaps most frequently and severely damages seedlings and saplings.

White pine blister rust cankers on branches and stems often have a cracked or blistered appearance and may bear yellow to orange spores in fruiting bodies of Cronartium ribicola.
White pine blister rust cankers on branches and stems often have a cracked or blistered appearance and may bear yellow to orange spores in fruiting bodies of Cronartium ribicola.

Where does white pine blister rust come from?  White pine blister rust is caused by the fungus Cronartium ribicola, which requires two different plant hosts to complete its complex life cycle.  Spores of the fungus produced on white pine are blown to alternate host plants in the genus Ribes (gooseberries and currants).  After infecting gooseberry and currant bushes the fungus multiplies rapidly throughout the summer, repeatedly producing spores that germinate to result in additional gooseberry and currant infections.  Spores produced in late summer on gooseberries and currants are spread by the wind to white pines, where needles that are moist from rain, fog, or dew are infected.

What does white pine blister rust look like?  Infection of needles by Cronartium ribicola results in development of yellow to brown spots and bands.  The fungus slowly grows through pine needles and bark to eventually form cankers on twigs, branches, and trunks of trees.  A canker is a localized diseased area (either swollen or sunken) that is surrounded by healthy tissues.  A typical white pine blister rust canker has resinous margins, and may appear “blistered” before rupturing to expose fruiting bodies (reproductive structures) with yellow to bright orange spores of Cronartium ribicola.  As a canker expands to completely encircle stems, all parts of the plant beyond the canker are killed.  Dead white pine branches may temporarily retain their orange to red dead needles to form a bright “flag” that is a common symptom of this disease.

Can I save a tree affected by white pine blister rust?  There is no “cure” for a tree with a white pine blister rust canker on its main trunk or stem.  As the canker expands to completely encircle the trunk, all parts above the canker will die.  Branch cankers that extend to within four inches of the trunk, probably indicate that the trunk already is colonized by the fungus and future development of a main stem canker is likely.  Pruning off branches on which cankers are located farther from the trunk, however, can prevent the fungus from growing into the trunk.  These branch cankers can be removed by pruning at least six inches beyond any visible symptoms on the trunk side of the cankered branch.  Removal of branch cankers and trees with main stem cankers prevents production of spores that are spread to gooseberries and currants.  The fungus quickly dies in discarded branches and trunks and presents no further danger to either of its hosts.

How do I avoid future problems from white pine blister rust?  Infection of pine is most frequent near diseased gooseberries or currants.  Therefore, removing these plants (especially within 200 feet) can greatly reduce the incidence of white pine blister rust.  Gooseberries or currants can be killed by uprooting them or by application of herbicides in accordance with label directions.

Conditions that promote or prolong needle wetness should be avoided.  These include use of sprinklers that wet the needles of ornamental trees, dense plantings, growth of weeds surrounding young trees, or planting white pines in low-lying, chronically moist areas near water or sites that lack good air movement.  Growing white pines under a hardwood overstory that will intercept evening dew can help keep needles dry and reduce infection.

Regular inspection allows prompt detection and removal of cankered branches on young trees.  Also, because infection very often occurs on needles of low branches, pruning to remove healthy branches (if aesthetically acceptable) will reduce the likelihood of this disease.  When trees are five to seven years old, starting close to the ground, prune off all branches up to no more than one-third to one-half the height of the tree.  Branch pruning can be continued every other year until the lower eight to nine feet of trunk is free of branches.

Planting or measures to encourage natural regeneration of conifer species other than white pines (including red or jack pines, spruces, firs, arborvitae, hemlock, and junipers) might be considered, especially where gooseberries or currants are prevalent and where moisture conditions favor infection.

Application of protectant fungicides may be a useful management practice in white pine production nurseries.  Nursery stock should be carefully inspected, particularly just prior to sale, because the planting of diseased white pine seedlings is one way Cronartium ribicola has been spread into previously disease-free locations.

For more information on white pine blister rust:  Contact your county Extension agent.