Category Archives: Disease – Needled Woody Ornamental

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 fungi and algae. The fungi provide the physical structures of the lichens, as well as protection for the algae. The algae, in turn, produce food for the fungi via photosynthesis.

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.

Conifer Disease Quick Reference

Conifer - 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
Conifer Rhizosphaera Needle 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
Conifer - 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
Conifer - Spruce Needle Drop Spruce Needle Drop
Hosts:  Spruces
Pathogen:   Unknown (possibly Setomelannoma holmii)
Signs/Symptoms:  Needle loss and dieback at or near branch tips
Conifer - Cedar Apple 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
Conifer - Diplodia Tip Blight 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
Conifer - Dothistroma Needle_ 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
Conifer - 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
Conifer - 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
Conifer - 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
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. 

Weir’s Cushion Rust of Spruces – Pest Alert

What is Weir’s cushion rust?  Weir’s cushion rust is a needle disease that disfigures and reduces growth of spruce trees (Picea spp.) of all ages.  This disease has been known in both eastern and western regions of the United States, but was recognized in Wisconsin for the first time in 2002.

Yellow spots and bands in winter on spruce shoots affected by Weir's cushion rust.
Yellow spots and bands in winter on spruce shoots affected by Weir’s cushion rust.

What does Weir’s cushion rust look like?  Needles on current year’s shoots affected by Weir’s cushion rust may develop yellow spots or bands in the summer and fall.  These spots and bands may intensify to give needles a bright “green and gold” appearance the following spring, when tiny blister-like pustules (a type of fungal reproductive structure) develop in the yellow areas.  Microscopic examination of these pustules is required for diagnosis of the disease.  Affected one-year-old needles continue to yellow, turn brown, and fall off as the spring and summer progress.  Trees badly damaged by Weir’s cushion rust will have thin crowns due to repeated loss of the previous year’s needles.

Where does Weir’s cushion rust come from?  Weir’s cushion rust results from colonization of spruce needles by the fungus Ceropsora weirii (formerly Chrysomyxa weirii).  This fungus overwinters in needles infected during the previous growing season.  In late summer, or more typically the following spring, C. weirii produces spores in the pustules that develop on the needles.  These spores can be blown by wind or splashed by rain to newly emerging needles on the same tree or other trees.  Spore germination is followed by infection of young needles.

Can I save a tree affected by Weir’s cushion rust?  Apply fungicides containing chlorothalonil to trees affected by Weir’s cushion rust to prevent new needle infections.  Make the first application when 10% of the buds have broken and two additional applications at seven to 10 day intervals thereafter.  Fungicide applications do not kill the fungus in needles that are already infected, so be sure to begin applications promptly and complete the spray program, to ensure thorough coverage and protection of new foliage.  Please be sure to read and follow all fungicide label instructions to ensure that you use the product in the safest and most effective manner possible.  Needles infected by C. weirii eventually die.  The fungus does not continue to live or produce spores on these dead needles.  Therefore, destruction of dead needles is not necessary.

How do I avoid Weir’s cushion rust in the future?  DO NOT accept and plant landscape or nursery stock affected by Weir’s cushion rust.  Inspect established spruce trees (in both landscape and nursery settings) in late summer and fall for evidence of Weir’s cushion rust (e.g., yellow spots and bands on the current year’s needles).  Inspect suspect trees again in spring for these symptoms, as well as pustules of C. weirii on the previous year’s needles.  In nurseries, move affected trees to areas where the disease is not already present.  Use fungicide applications to prevent establishment of the fungus on new trees or in previously unaffected nurseries and landscapes.

For more information on Weir’s cushion rust:  Contact your county Extension agent. 

Swiss Needle Cast

What is Swiss needle cast?  Swiss needle cast is a fungal disease of Douglas-fir.  This tree is native to the Rocky Mountains and the Pacific Coast but has been grown in Wisconsin as a landscape ornamental and as a Christmas tree.  Swiss needle cast can limit the aesthetic appeal of Douglas-fir grown in landscape settings, as well as the marketability of Douglas-fir grown in Christmas tree production.

Fruiting bodies of the Swiss needle cast fungus on the undersides of needles.
Fruiting bodies of the Swiss needle cast fungus on the undersides of needles.

What does Swiss needle cast look like?  Infected needles become discolored (blotchy yellow-green or completely yellow) and then brown from the tips.  Older needles are more severely affected than younger needles.  Brown needles drop prematurely, leaving twigs with only the newest growth.  Using a hand lens, small, black reproductive structures of the Swiss needle cast fungus can be seen in two diffuse bands on the undersurface of infected needles.

Where does Swiss needle cast come from?  Swiss needle cast is caused by the fungus Nothophaeocryptopus gaeumannii.  Spore production, needle infection, and symptom development are favored by wet environmental conditions (e.g., rainy weather).  Infections primarily occur on new needles as they emerge and expand in the spring.  Fully expanded needles are less susceptible to infection.  Once infection has occurred, fruiting bodies (i.e., reproductive structures) form and produce spores on both discolored and green needles.  Spores may be produced for several seasons before needles drop.

How can I save a tree with Swiss needle cast?  Infected needles cannot be cured.  However, fungicide sprays containing chlorothalonil or mancozeb may help prevent additional infections on small trees where complete, uniform coverage is possible.  Fungicide treatments are not recommended for large trees.  For plantation and landscape trees, apply two fungicide applications, the first when buds are ½ to two inches long, and the second about three weeks later.  In the more stressful environment of nurseries, trees may require sprays every two weeks through mid-August.  Using preventive treatments, infected trees may regain their lush, full look within two years.  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 can I prevent Swiss needle cast in the future?  Use Douglas-firs grown from seed originating in Pacific Coast areas rather than those grown from seed originating in the Rocky Mountains.  Pacific Coast trees appear to be less susceptible to Swiss needle cast.  Also, use resistant and tolerant varieties where available.  Purchase stock from a reputable nursery; Nothophaeocryptopus gaeumannii is often spread through infected nursery stock.  When planting Douglas-firs, choose a site that has well-drained, but moist soil with a neutral to slightly acidic pH and allow adequate spacing between trees to promote increased airflow and quicker drying of needles.  This drier environment is less favorable for infection and disease development.  Make sure trees have sufficient water and are fertilized properly (based on soil and needle nutrient tests) to reduce stress that may increase trees’ susceptibility to Swiss needle cast.

For more information on Swiss needle cast:  Contact your county Extension agent.

Root and Crown Rots

Discoloration of maple crown and roots typical of Phytophthora root/crown rot.
Discoloration of maple crown and roots typical of Phytophthora root/crown rot.

What is root/crown rot?  Root/crown rot is a general term that describes any disease of woody ornamentals where the pathogen (causal organism) attacks and leads to the deterioration of a plant’s root system and/or lower trunk or branches near the soil line.  Root rots can be chronic diseases or, more commonly, are acute and can lead to the death of the plant.

What does root/crown rot look like?  Gardeners often become aware root/crown rot when they see above ground symptoms.  Affected plants are often slow-growing or stunted and may show signs of wilting.  Often the canopy of an affected tree or shrub is thin, with foliage that is yellow or red, suggesting a nutrient deficiency.  Careful examination of the roots/crowns of these plants reveals tissue that is soft and brown.

Where does root/crown rot come from?  Several soil-borne water molds (i.e., fungi-like organisms) and true fungi can cause root/crown rots, including (most frequently) Phytophthora spp. and Pythium spp. (both water molds), and Rhizoctonia solani and Fusarium spp. (both true fungi).  These organisms have wide host ranges, and prefer wet soil conditions.  Water mold root rot organisms such as Pythium and Phytophtora produce thick-walled spores (called oospores) that can survive for long periods (years to decades) in soil.

How do I save a plant with root/crown rot?  REDUCE SOIL MOISTURE!  Provide enough water to fulfill a plant’s growth needs and prevent drought stress, but DO NOT over-water.  Remove excess mulch (greater than four inches) around trees and shrubs.  Excessive mulch can lead to overly wet soils.

A thinning canopy with red or yellow leaves can indicate a root/crown rot problem.
A thinning canopy with red or yellow leaves can indicate a root/crown rot problem.

Chemical fungicides (e.g., PCNB, mefenoxam, metalaxyl, etridiazole, thiophanate-methyl and propiconazole) and biological control agents (e.g., Gliocladium, Streptomyces, and Trichoderma) are labeled for root/crown rot control.  However, DO NOT use these products unless you know exactly which root/crown rot pathogen(s) is(are) affecting your trees and shrubs.  Contact your county Extension agent for details on obtaining an accurate root/crown rot diagnosis and for advice on which, if any, fungicides you should consider using.

How do I avoid problems with root/crown rots?  Buy plants from a reputable source, and make sure they are root/crown rot-free prior to purchase.  Establish healthy plants in a well-drained site, and when planting, place the root collar just at the soil surface.  To moderate soil moisture, add organic material (e.g., leaf litter or compost) to heavy soils to increase soil drainage, and DO NOT over-water.  Also, DO NOT apply more than three inches of mulch around trees and shrubs, and keep mulch from directly contacting the base of trunks and stems.  Prevent physical damage (e.g., lawnmower injury) that can provide entry points for root/crown rot pathogens.  Finally, minimize movement of root/crown rot fungi in your garden.  DO NOT move soil or plants from areas where plants are having root/crown rot problems.  DO NOT water plants with water contaminated with soil (and thus potentially with root/crown rot organisms).  After working with plants with root/crown rot, decontaminate tools and footwear by treating for at least 30 seconds with a 10% bleach solution or 70% alcohol (e.g., rubbing alcohol, certain spray disinfectants).  If you use bleach to decontaminate metal tools, be sure to thoroughly rinse and oil your tools after you are done gardening to prevent rusting.

For more information on root/crown rots:  See UW Bulletin A2532, Maple Decline:  Collar Rot and Basal Canker Complex (available at https://learningstore.extension.wisc.edu/), or contact your county Extension agent.