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Extension > Garden > Yard and Garden > Trees and Shrubs > Pine-oak and pine-pine gall rusts

Pine-oak gall rust and pine-pine gall rust

Rebecca Koetter and Michelle Grabowski

Importance

M. Grabowski, UMN Extension.

Pine Oak gall releasing powdery orange fungal spores.

Pine-oak and pine-pine gall rusts are diseases that cause round to oblong tumor-like galls to form on branches and trunks of 2-3 needle pine trees. Galls result in poor branch growth, death of branches or death of young pine trees. Even though these diseases can cause death of pine seedlings and young trees, well established trees undergo some disfigurement but overall health is not affected. These two diseases cannot be separated without microscopic examination.

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.

Pathogen and susceptible plants

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 Pinaceae (pine) family and the other from the Fagaceae (oak) family.

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

Table 1: Tree species affected by pine-oak gall rust and pine-pine gall rust

Pine-oak gall rust Pine-pine gall rust
Pine (Pinus) Pine (Pinus)
Austrian (P. nigra) Jack pine (P. banksiana)
Jack pine (P. banksiana) Scots pine (P. sylvestris)
Mugo pine (P. mugo)  
Red pine (P. resinosa)

 

Ponderosa (P. ponderosa)

 

Scots (P. sylvestris)

 

Oak (Quercus)

 

Northern pin oak (Q. ellipsoidalis)

 

Bur oak (Q. macrocarpa)

 

Pin oak (Q. palustris)

 

Northern red oak (Q. rubra)

 

Identification

M. Grabowski, UMN Extension.

Young galls developing on a pine branch.

Biology

Pine-oak gall rust

In spring or early summer, airborne spores are released from last year’s infected oak leaves and infect young needles and shoots of pine trees. The fungus moves from the needles into branches or stems and stimulates an increase of cell size and quantity which results in the formation of a gall. Once galls are formed and growing, movement of water and nutrients to the outside of the gall is eliminated so the branch eventually may die. Infected branches become brittle and snap off from extreme winds or snow loads. Galls are not typically 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 lower leaf surface of the infected oak. These specialized spore producing structures, called telia, produce a different spore type that infects nearby pines the following spring.

Pine-pine gall rust

The fungus overwinters and 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. These spores infect succulent new growth of the same pine tree or neighboring pine trees in the spring. Swelling of the infection on the stem is not apparent until later in the growing season or even during the following spring.

Management


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