Fungus on pine trees

Tree Diseases: Pine-Oak Gall Rust


Pine-oak gall rust, also referred to as eastern gall rust, is a fungal disease that affects various pine and oak trees. The disease is native to North America. It is caused by the fungus, Cronartium quercum. The disease produces oblong to rounded galls on the stems or branches of infected trees. The galls increase in size each year, growing up to several inches in diameter. Fruiting structures develop on the galls, and rupture, covering the bark surface with a mass of powdery, orange-yellow spores. These spores are dispersed by air currents or splashes of rain to susceptible oaks, where they initiate new infections. Large galls can stunt plant growth, resulting in significant foliar dieback. Infections of the main stem often result in tree mortality.

Distribution & Habitat

Pine-oak gall rust occurs on susceptible pine and oak trees throughout the United States. It is most prevalent in the Southern and Western United States.


Pine-oak gall rust infects a variety of pines. Scots, Austrian, ponderosa, Jack, mugo, red, and Virginia pine are the most commonly infected. Alternate hosts include northern pin, bur, pin, and northern red oak. Red and black oaks are especially prone to infection.

Disease Cycle

Pine-oak gall rust has a complex disease cycle that requires two hosts to achieve completion: a pine, and an oak. In spring or early summer, spores are released from fruiting structures on infected oak leaves. These spores are disseminated by the wind or rain to the needles and shoots of pine trees. The fungus invades susceptible pines through the needle stomata, or through wounds in the bark. The fungus germinates in the needles, or bark for up to forty eight hours. Once it has become established in the host, the fungus moves from the needles, or bark, into the vascular cambium of the branches, or stem. The presence of the fungus in the vascular cambium stimulates an increase of cell size, resulting in the formation of a gall. The galls continue to grow until the following spring, when they reach maturity. Throughout the growing season, the galls become enlarged. This produces a girdling effect, which inhibits the flow of water and nutrients to the infected branch, or stem. Infected branches often become brittle, rendering them prone to ripping, or snapping during snow or ice storms. Once mature, the bark on the galls cracks, and sloughs off, releasing an abundance of bright yellow-orange powdery spores. The spores are dispersed by air currents or splashes of rain to the leaves of nearby oak trees, which they readily infect.

During summer, clumps of powdery orange spores develop on the underside of infected oak leaves. The spores are released when conditions are sufficiently moist. However, they are only capable of infecting additional oak leaves. In late summer, dark brown to black hair-like structures appear on the lower leaf surface of infected oak leaves. These spore-producing structures, called telia, contain a specialized spore type that is capable of infecting pines. These spores are released in spring of the following year, once conditions are favorable. They are carried by the wind or rain to nearby pines, where they initiate new infections.

Symptoms of Infection

Small galls may be visible after the first year of infection. The galls appear as round to oblong swellings along pine stems or branches. They are initially small, resembling peas. As the galls enlarge, they may reach up to the size of a pomegranate, or grapefruit. After the second year of infection, yellow-orange fruiting structures, referred to as aecia, will form on the surface of the gall. In spring, bright yellow to orange spores can be observed on this region of the gall. Bark collars occasionally form on either end of the gall, where the branch and gall intersect. Infected needles may turn yellow, and curl upwards. Infected branches and twigs may be stunted. Infected oak leaves usually develop chlorotic spots or blotches. Telia form on the underside of oak leaves.


  • When planting, avoid fields or landscapes adjacent to forest stands of oaks or pines that exhibit symptoms of infection. Select healthy plants that are resistant to the disease.
  • Cull heavily infected trees to reduce the incidence of sporulation.
  • Remove branches with galls prior to active sporulation in late spring.
  • Registered fungicides can be used to prevent the infection of susceptible pines. To ensure success, applications must be performed in early spring, prior to sporulation.
  • Maintain tree vigor through sound cultural practices. Ensure that trees are sufficiently watered, especially during extended periods of drought. Apply a layer of organic mulch around the base to improve soil quality, moderate soil temperature, and maintain soil moisture.

If you have any questions about pine-oak gall rust, or you are interested in one of our tree services, contact us at 978-468-6688, or [email protected] We are available 24/7, and can accommodate any schedule. All estimates are free of charge. We look forward to hearing from you.

Photo courtesy of Mary Kelm CC-BY-NC-SA 2.0

Orange Gooey Fungus on Trees

branch of pine-tree image by Maria Brzostowska from

When you’re the owner and caretaker of a tree, it’s upsetting when you discover something wrong with it. In some cases, and on some trees, certain fungi can grow on the tree. Two species of pine trees are especially subject to an orange fungus that grows on swollen trunks and branches, but it can attack some types of oaks and other trees as well.

Types of Fungi

The University of California at Davis website describes several types of nuisance fungi. From mushrooms that grow in lawns to slime molds such as the “dog vomit” fungus that form gooey masses, not all types of fungi are desirable when they grow on your plants. Although they often help the decomposition of organic materials in soil, some fungi are poisonous to humans and pets. Fusiform gall rust results when the fungus known as Cronartium fusiforme finds a suitable habitat on host trees. It is the most common fungus that causes an orange, gooey mass on trees.

Trees Affected

Two types of pine trees are the most common trees to become victims of the fusiform gall rust–the slash pine and the loblolly pine. The slash pine, or Pinus elliottii, is native to the southern United States, according to North Carolina State University. This university also reports that the loblolly pine, or Pinus taeda, occurs in the wild in the southern United States, but its range extends farther north than the slash pine and it does not occur as far as southern Florida, as opposed to the slash pine, which does grow in more tropical environments. The longleaf and shortleaf pines do not develop this fungus, according to the University of Mississippi, but the black oak is susceptible, according to The U.S. Department of Agriculture (USDA) website. Other trees that can contract this rust disease include willows, laurel oaks, bluejack oaks and turkey oaks.


The orange spores of this form of gall rust often begin to become noticeable in early spring. March and April are the typical months when this fungus begins to form a colony. If you live near the Gulf Coast or south Atlantic, your trees are more likely to develop this disease. Orange spores begin to form on the leaves of pine and oak trees beginning in late February and continuing until April. Wherever the spores land, due to being blown by wind, an infection begins to occur. Within 10 days, small orange pustules form on the undersides of leaves. When the infection spreads, it grows into the tree’s stems and branches, causing them to become weak and eventually die back.


When the rust attacks young trees under the age of 10 years, they are more likely to succumb than when the rust attacks older trees. Older trees, however, can develop cankers as a result of the rust, deforming them and slowing growth. Defoliation can occur and branches can become weak, which increases the danger that they might break off in high winds and damage not only the tree but nearby structures and vehicles.


The USDA website suggests planting varieties of trees that are resistant to the fusiform gall rust. Another suggestion is to spray trees with silvicultural and chemical products designed to control fungal diseases. You can prune affected branches of young trees to stop the infection from growing into their trunks if you catch the symptoms early. In commercial groves, the USDA recommends planting trees more closely than they might normally be planted; this practice will keep the number of healthy trees high when some trees die as a result of infection from the fusiform gall rust disease.

Pine Gall Rust

United States Department of Agriculture

Prepared by Forest Service Alaska Region Leaflet R10-TP-37 6/93


The pine gall rust disease is common wherever lodgepole pine or shore pine (Pinus contorta) grow in southeast Alaska. It may also be found on Mugo pine and other exotic pines grown as ornamental trees in Alaska. No other tree species or vegetation is known to be directly affected by this disease. Although pine gall rust is quite noticeable, it is usually not very damaging and frequently is easy to control. This leaflet will familiarize you with the tree disease, briefly explain the biology of the causal fungus, and provide ideas for reducing damage to pine.

Pine gall rust is caused by the rust fungus Endocronarlium harknessii. The disease is simple to identify because it causes swollen, spherical growths on the branches or main bole of pine trees (cover) These galls are composed of pine tissue and range in size from less than an inch to about one foot in diameter. In spring, the rust fungus produces abundant orange spores in the fissures of the gall (Fig. 1) which ensures proper diagnosis of the disease.

Life History

The rust fungus spreads by means of its microscopic spores. Spores are produced in orange masses on galls in spring and are dispersed on air currents. Spores then germinate and infect pines through needles or young twigs. No other host plants are needed for the rust fungus to complete its life cycle. Once the fungus has grown inside pine branches, it does not immediately kill tissues. Rather, it takes control of the tree’s growth hormones and causes extra growth in the vicinity of the infection. After several years, this results in the formation of a woody gall with a diameter larger than the surrounding branch or bole. You can see this by cutting through the gall to observe the swollen growth rings compared to those rings in the branch. Each spring, the fungus produces more microscopic orange spores which can infect other pines, thereby continuing the life cycle of the fungus.

Figure 1. During spring, the rust fungus produces its infectious spores in an orange dry mass in the fissures of the pine gall.

Figure 2. Gall rust on the main bole of a pine tree.


The amount of damage caused by pine gall rust depends upon the number, size, and location of galls on the tree, and how unsightly you think they are. Generally, pines can tolerate several galls without tree health being adversely affected. Galls on the main bole (Fig. 2), however, are more disruptive to the tree’s physiology and have more effect on tree growth and survival.

Galls on branches or the main bole frequently remain alive for many years. A more serious condition occurs when galls die. This can result from growth of the rust fungus or when galls are attacked by another fungus, Nectria macrospora (Fig. 3). The Nectria fungus is known to kill fast-growing nutritious tissues such as pine rust galls. When a gall dies, nutrients are no longer carried to the outer portions of the branch. The branch dies from the gall outward and needles turn brown (Fig. 4). The top of a pine tree sometimes dies when a gall along the main bole is killed. If your pine has dead branches or a dead top, look carefully along the branch or the bole and you will probably find a pine rust gall. Although the Nectria fungus is not present for long, you may see the clustered bright reddish fruiting bodies (each less than 1/32 inch diameter) of this damaging fungus (Fig. 3).

Figure 3. The combination of pine gall rust and another fungus, Nectria macrospora, can kill the pine galls. This results in branch death, so commonly seen on pines.

Figure 4. When the gall dies from the rust fungus or attack by secondary fungi, the branch or top of the tree from the gall outward or upward is sure to die.


The simplest form of control is to prune away branches that have galls. Tissues far from the gall do not have to be removed as the rust fungus does not penetrate very far from the swollen gall. Pruning infections in this way may decrease future disease in pines by slightly reducing the number of infectious fungal spores produced each year. For controlling the disease, these branches need not be disposed of in any particular way because the fungus will die as soon as the pine branch material dies. When pruning away dead galls, however, the pine material should probably be burned or moved away from pines because it may contain the Nectria fungus, which can survive and produce infectious spores on dead pine material.

Controlling a large gall on the main bole of pines will present more of a problem. Removing the gall and the top of the tree will probably cause the tree to appear malformed, even many years after pruning.

Pesticides may be effective for controlling the spread of both the rust and Nectria fungus, but they are probably unnecessary and techniques for their use on this disease have not been determined.

Many diseases can be reduced by enhancing tree growth and vigor by adding fertilizer or extra watering. Evidence from British Columbia suggests, however, that the rust fungus is more likely to attack the fastest growing pines. Thus, fertilizing cannot be relied upon to control the disease and may actually aggravate the problem.

Doing nothing to control pine gall rust might be best for the pine tree. Unless galls are very numerous or on the main bole, they may have only negligible effect on the health of pines.

Pine Gall Rust, by Paul E. Hennon Forest Pathologist, USDA Forest Service Alaska Region, State and Private Forestry
August 1993

Additional information on this disease can be obtained from your local USDA Cooperative Extension Service office, Alaska State Forestry office, or from:

Forest Health Protection
State and Private Forestry
USDA Forest Service
2270 Sherwood Lane, Suite 2A
Juneau, AK 99801
Phone: (907) 586-8811

Controlling Pine Tree Diseases – Symptoms Of Pine Gall Rust Disease

Both western and eastern pine gall rust are caused by fungi. You can learn more about these destructive pine trees diseases in this article.

Rust Pine Tree Diseases

There are essentially two types of pine gall rust diseases: western pine gall and eastern pine gall.

Western Pine Gall Rust (Pine-Pine)

Also known as western pine gall rust or as pine-pine gall rust for its proclivity to spread from pine to pine, pine gall rust disease is a fungal disease that affects two- and three-needle pine trees. The disease, caused by a rust fungus known as Endocronartium harknesii, affects Scots pine, jack pine and others. Although the disease is found across much of the country, it is especially widespread in the Pacific Northwest, where it has infected nearly all lodgepole pines.

Eastern Pine Gall Rust (Pine-Oak)

Eastern pine gall rust, also known as pine-oak gall rust, is a similar disease caused by Cronartium quercuum rust. It affects a large number of oak and pine trees.

Although there are some differences between the two diseases, both types of gall rust are easily recognized by round or pear-shaped galls on branches or stems. Although the galls are initially less than an inch across, they grow year by year and can eventually reach several inches in diameter. In time, they may become large enough to girdle stems. However, they often they aren’t noticeable until about the third year.

In spring, the surfaces of mature branches are typically coated with masses of orange-yellow spores, which can infect nearby plants when they are dispersed in the wind. Western pine gall rust requires only one host, as spores from one pine tree can directly infect another pine tree. However, eastern pine gall rust requires both an oak tree and a pine tree.

Pine Gall Rust Treatment

Maintain proper care of trees, including irrigation as needed, as healthy trees are more disease resistant. Although some professionals advise regular fertilization, evidence indicates that the fungus is more likely to affect fast-growing trees, which suggests that use of fertilizer may be counter-productive.

Western pine gall rust generally doesn’t present a serious danger to trees, unless the galls are large or numerous. Fungicides may help prevent the disease when applied at bud break, before spores are released. Control measures are generally not recommended on oak trees.

The best way to control pine gall rust disease is to prune affected areas and remove galls in late winter or early spring, before they have time to produce spores. Remove the galls before they grow too large; otherwise, extensive pruning to remove the growths will affect the shape and appearance of the tree.

Pine gall rusts

Quick facts

  • Pine-oak and pine-pine gall rusts cause round to oblong, tumor-like galls to form on branches and trunks of 2-3 needle pine trees.

  • Galls cause poor branch growth, death of branches or death of young pine trees.

  • Well established trees may be disfigured but overall health is not affected.

  • These two diseases cannot be separated without microscopic examination.

How to identify pine-oak and pine-pine gall rusts

Pine oak gall releasing powdery orange fungal spores

  • Galls (tumor like growths) form along the stems or branches of pine trees. These galls are:

    • Woody, round, oblong or irregularly shaped.

    • At first, galls are pea size (1/4 inch) but can grow to the size of a grapefruit (4 inches across).

    • On main stems, galls may reach 2-1/2 feet across.

  • In spring, bright yellow to orange spores form in cracks on the gall surface, making the galls very noticeable.

  • Sometimes, bark collars form on either end of the round or oblong gall where the branch and gall meet.

  • Needles may become yellow and stunted, and twig or branch growth may become stunted on the branch beyond a gall. Eventually the branch may die.

  • Infected oak leaves develop yellow spots or blotches, and brown hair-like structures on underside of oak leaves.

Trees affected by pine-oak gall rust

Pine-oak gall rust is a native fungal disease caused by Cronartium quercuum f.sp.banksianae. Two different host plants are required to complete the pathogen’s life cycle. One plant from the pine family (Pinaceae) and the other from the oak family (Fagaceae).

Trees commonly affected by pine-oak gall rust include:



  • Northern pin oak (Q. ellipsoidalis)

  • Bur oak (Q. macrocarpa)

  • Pin oak (Q. palustris)

  • Northern red oak (Q. rubra)

Trees affected by pine-pine gall rusts

Pine-pine gall rust is a native fungal disease caused by the fungus Peridermium harknessii (syn. Endocronartium harknessii), which affects only pine trees.

Trees commonly affected by pine-pine gall rust include:

  • Jack pine (P. banksiana)

  • Scots pine (P. sylvestris)

Where do pine gall rusts occur in Minnesota?

On Jack pine trees, pine-pine gall rust is most common in the northernmost counties of Minnesota including Cook, Lake, St. Louis, Koochiching, Lake of the Woods, Roseau, Itasca and Beltrami.

Pine-oak gall rust is most common in counties south of those listed above where red oaks are present.

How do pine gall rusts survive and spread?

Gall girdling a young pine tree

Pine-oak gall rust life cycle

  • Pine-oak gall rust is caused by Cronartium quercuum f.sp.banksianae.

  • In spring or early summer, airborne spores are released from last year’s infected oak leaves.

  • These spores infect young needles and shoots of pine trees.

  • The fungus moves from the needles into branches or stems and stimulates the tree to produce more and unusually large cells. This causes a gall to form.

  • Water and nutrients don’t move easily through the gall. As a result, needles beyond the gall may turn yellow or brown and the branch eventually may die.

  • Infected branches become brittle and snap off from extreme winds or snow loads.

  • Galls are not usually visible for several months to a year after infection.

  • When the gall reaches maturity, the bark on the gall cracks and flakes off, releasing bright yellow-orange, powdery spores in spring.

  • These spores infect the leaves of nearby oak trees.

  • During the summer, clumps of powdery orange spores develop on the underside of oak leaves. These spores can only infect other oak leaves.

  • Later in the season, dark brown-to-black, hair like structures appear on the underside of leaves on the infected oak. These specialized spores, called telia, produce a different spore type that infects nearby pines the following spring.

Pine-pine gall rust life cycle

  • Pine-pine gall rust is caused by the fungus Peridermium harknessii (syn. Endocronartium harknessii)

  • The fungus survives from year to year in the living galls on pine trees.

  • Airborne, white-to-orange spores form on galls that are two years old or older.

  • In spring, these spores infect succulent, new growth of the same pine tree or neighboring pine trees.

  • Galls are not usually visible until later in the growing season or even until the following spring.

How to manage pine gall rusts

Young galls developing on a pine branch

  • Inspect pine trees before you buy them. Purchase only trees with no branch swellings or galls.

  • Prune out galls:

    • Pruning out galls reduces the amount of spores produced that will infect nearby pine and oak trees.

    • Prune to remove infected pine branches with galls in late winter or early spring.

    • Galls do not need to be pruned if you choose to maintain the pyramidal shape of your pine tree.

  • There are no practical or recommended management practices to reduce pine oak gall rust on oak trees.

  • No pesticides are recommended on landscape trees.

Rebecca Koetter and Michelle Grabowski, Extension educator

Reviewed in 2019

Leave a Reply

Your email address will not be published. Required fields are marked *