Rust diseases on pines
Pine trees in Minnesota are infected by several species of rust fungi. Rusts on pine all have similar symptoms and life cycles, but the primary and alternate hosts will differ. Most of these rust fungi require two different host plants to complete their life cycle.
Sweetfern rust, a fungal disease caused by Cronartium comptoniae, can kill young pine trees. The most seriously affected tree in Minnesota is jack pine (Pinus banksiana), but Austrian (P. nigra), mugo (P. mugo), ponderosa (P. ponderosa), and Scots pine (P. sylvestris) can also be damaged. This rust rarely affects red pine (P. resinosa) in Minnesota. Sweetfern rust requires the alternate hosts, sweetfern (Comptonia asplenifolia) or sweet gale (Myrica gale), to complete its life cycle.
During the summer, fungal reproductive structures develop on the underside of rust-infected sweetfern and sweet gale leaves. Spores produced by these structures are wind blown to current season pine needles in the summer or early fall. The fungus grows within the needle and eventually into branches and/or the trunk. Swelling of the infected area is observed about one year after infection. The following spring, cream colored structures develop on the surface of the swollen areas. These structures rupture through the bark, but may be hidden by loose bark scales. Orange colored spores are produced and dispersed by wind in the spring to sweetfern and sweet gale leaves, completing the life cycle.
Elongate cankers develop on infected areas of the trunk and branches. Cankers (sunken and discolored areas) are often inconspicuous, due to the scaly bark on some pine trees. Cankers may eventually girdle the branches or the trunk, causing yellowing and browning of needles, branch die back, or death of the tree. Cankered trees are susceptible to damage from wood decay fungi, rodents, and insects.
Sweetfern rust cankers differ from that of Stalactiform rust by being found near ground level on tree trunks. Canker margins of sweetfern rust are composed of raised ridges not seen in other rust cankers. Trees infected by sweetfern rust are usually younger than ten years old.
Stalactiform rust, also known as cow wheat rust, is caused by Cronartium coleosporioides. In Minnesota, jack pine is the primary host and the alternate hosts are cow wheat (Melampyrum spp.) and paintbrush (Castilleja spp.) The lifecycle is similar to that of Sweetfern rust.
Elongated cankers develop anywhere along the main stem of mature jack pine trees. Seedling infections cause swelling or curvature of the stem, often followed by death of the tree. Stalactiform rust cankers are narrower and longer (several meters) than the other rust cankers.
Comandra rust is caused by the fungus Cronartium comandrae. Jack and Scots pine are the primary hosts in Minnesota, and bastard toadflax (Comandra umbellata) serves as the alternate host. The lifecycle is similar to that of Sweetfern rust.
Mature pine trees develop resinous, oval to circular-shaped cankers on the trunk. The range and abundance of the alternate host is limited in Minnesota, as C. umbellata prefers open sites with moderately wet to damp soils. This restricts the distribution and the amount of damage caused by comandra rust.
White pine blister rust
White pine blister rust, a disease caused by the fungus Cronartium ribicola, occurs on eastern white pine (Pinus strobus). The most noticeable symptoms of this rust are branch dieback, cream-colored blisters on branches or the trunk in the spring, and oval-shaped cankers. This disease is discussed at length in another Yard and Garden Brief, White Pine Blister Rust (#P442W).
Control measures are the same for all pine rusts. Trees should be monitored for evidence of cankers, blisters, and swollen branches. Remove and properly dispose of cankered branches on trees. Severely infected trees should be removed. Removal of alternate hosts in the vicinity of the pines will reduce incidence of rust, but is not usually practical. Keep trees growing vigorously by properly watering, fertilizing, and mulching trees.