Tree Diseases: Dothistroma Needle Blight of Pines
Dothistroma needle blight, often referred to as red band disease in the western United States, is a devastating foliar disease of pines. The causal agent of the disease is the fungus, Dothistroma pini. The fungus infects, and kills susceptible pine needles. Severe infections can defoliate host trees, hindering their growth, and rendering them prone to failure.
Distribution & Habitat
Dothistroma needle blight has been reported in the Western, Central, and Eastern United States. It is particularly common in the states located east of the Great Plains. Infections have also occurred in southwestern Alaska, and the Western Provinces of Canada.
Dothistroma needle blight affects twenty nine pine species and hosts. Austrian, lodgepole, and ponderosa pines are the most commonly infected. Scots pine, and mugo are also infected, albeit with less frequency.
Dothistroma pini has a sexual and asexual stage. In each stage, the fungus is capable of infecting the needles of susceptible trees. In the United States, the sexual stage of the fungus is rare, having only been observed in Alaska, California, and Oregon. The fruiting bodies, or stromata, of the sexual stage produce ascospores, which are released into the air when conditions are sufficiently moist. The ascospores are disseminated by air currents, or splashes of rain to susceptible foliage, where they may initiate new infections.
During the asexual stage of the fungus, conidia, or spores, are borne in black fruiting bodies, or stromata, which develop below the epidermis of infected needles. The fruiting bodies generally require two growing seasons to reach maturity. During the first year, they develop on the infected needles. As the temperatures decline in fall, the fruiting bodies overwinter, and mature the following spring. Once mature, the epidermis of the fruiting bodies becomes raised, exposing the conidia. The epidermis splits longitudinally, and the conidia are released. The conidia are dispersed by air currents, or splashes of rain onto susceptible trees, where they initiate new infections. When conditions are favorable, infections may occur from May to October. In the Central United States, persistent rainfall can enable the stromata to mature within the first year of infection.
Symptoms of Infection
Dark green bands, and yellow to tan spots appear on infected needles. The dark green bands fade as the disease progresses, making them increasingly difficult to detect. The spots and bands later turn brown to reddish-brown. In California, Oregon, Washington, and Idaho, the bands tend to exhibit a brighter shade of red. The black fruiting bodies may be visible on the surface of the infected needles within the spots and bands. The ends of the infected needles become discolored, initially appearing light green before turning tan, and then brown. The base of the infected needles remains green. The needles may develop extensive necrosis two to three weeks after they have been infected. Infected needles are often shed from the tree, with second year needles dropping first. Needles that become infected the year they emerge often remain on the tree until late summer of the following year. Infection is generally most severe in the lower part of the crown. Successive years of infection can culminate in tree mortality.
- Registered fungicides may be utilized to prevent infections. Two applications should be performed during the growing season: one in late spring, to protect the older needles, and a second in early summer to protect the current year’s needles.
- When planting in urban settings, select resistant varieties, such as Mugo and Scots pine. Allow for ample spacing between trees.
- If a tree is planted in a landscape setting that has an irrigation system installed, ensure that they water does not splash onto the needles. Damp foliage is more prone to infection.
- Thin out dense canopies to promote air circulation, and sustain tree vigor.
- Maintain tree vigor through sound cultural practices. Ensure that trees are sufficiently watered, especially during periods of extended drought. Apply a layer of organic mulch around the base of trees to improve soil quality, moderate soil temperature, and retain soil moisture.
Photo courtesy of U.S. Department of Agriculture CC-by-2.0