MAJOR DISEASES OF RED PINE

Disease

Armillaria shoestring root rot caused by the fungus, Armillaria obscura, a part of the complex of Armillaria species.

Importance

This fungus is common throughout the Lake States in red pine plantations. Damage is especially severe on trees under stress and trees growing in cutover hard-wood stands. Armillaria causes a decay, seldom extending more than a few feet above ground. It kills trees by girdling at the root collar.

BIOLOGY

Figure 1

Armillaria usually lives as a saprophyte and obtains its food from dead stumps and roots and the heartwood of living trees. But it may also live in roots as a parasite on living tissue. The disease causes abrupt or gradual reduction in growth, yellowing and then browning of foliage of the entire tree, and finally, death (fig. 1). Resin exudate may be present on roots or near the root collar. Veined, white mycelial fans form between the bark and wood at the ground line where the cambium has been killed (fig. 2). Black fungal strands (fig. 3) (rhizomorphs) also form on tree roots, on or under the bark, or in severely decayed wood. These strands may radiate into the soil where they may cause infection if they come in contact with the roots of other red pine. Honey-colored mushrooms form on the lower trunk of dead or dying trees usually in the fall (fig. 4). The fungus may spread either by windborne spores produced by the mushrooms or, more commonly, by rhizomorphs. The fungus may also spread from infected tissue to living trees through root contact.

Figure 2	Figure 3

Figure 4

Management Guidelines

Preventive measures that maintain conditions favorable for rapid and vigorous tree growth will usually minimize the impact of this disease.

• Choose a site well suited for red pine to maintain healthy, vigorous trees. This reduces stress and trees will be more resistant to Armillaria infection.

• Avoid planting on cutover sites, especially those with many hardwood stumps that serve as an Armillaria reservoir.

• If practical, remove dead trees, and infected stumps and large roots before planting red pine. These items can serve as a source of Armillaria for at least 10 years after the tree dies. This is also true of herbicide-killed trees.

Leaphart, C.D. 1963. Armillaria root rot. For. insect and Dis. Leafl. 78 Washington, DC: U.S. Department of Agriculture, Forest Service. 8 p.

Pronos, T.; Patton, R.F. 1977 Armillaria root rot of red pine planted on oak sites in Wisconsin. Plant Disease Reporter. 61:955-958.

Pronos, T.; Patton, R.F. 1978. Penetration and colonization of oak roots by Armillaria mella in Wisconsin. European Journal of Forest Pathology. 8:259-267.

Whitney, R.D. 1988. Armillaria root rot in softwood plantations in Ontario. Forest Chronicle. 64: 345-351.