Mycosphaerella dearnessii (Scirrhia acicola)
(1 = rare 5 = annual)
(1 = very little damage 5 = plants killed)
Mainly affects Scots and Ponderosa pines. Sometimes affects Jack, red, mugo, and eastern white as well as several other pines. Brown spot occurs most often in Christmas tree farms and nurseries; rarely occurs in the landscape.
Look for symptoms throughout the summer into fall. The disease can have two different appearances at times. Lesions start as yellow spots often with a small drop of resin associated with each spot. The yellow spots turn light brown with a dark border by late summer. Or the lesions can be brown spots with yellow halos instead of the dark border. Eventually, the needles turn all brown from the tip to the base and may fall off, leaving the branch tip naked. New twig growth may or may not die by fall. Lower branches are more likely to be infected first due to less air circulation and infected needles falling on ground. Disease can affect trees of any age but seedlings are most susceptible.
The time from infection to symptoms takes several weeks. Infection occurs early in growing season - on newly emerging needles. By August fruiting bodies form in the needles. The pathogen overwinters as fruiting bodies in the infected needles. This fungus tends to infect pines from seedlings to eight years of age. Warm wet weather is best for infection. Disease usually takes several years to reach epidemic conditions. Thus allowing enough time to prevent an epidemic outbreak.
In Christmas tree plantations, shearing of wet trees can spread spores of this fungus. Do not shear trees or work among trees when the foliage is wet. Practices to improve air movement between trees will discourage infection. If symptoms and fruiting bodies are identified in the fall, consider fungicide application the next spring when needles are half grown and again thirty days later. You may need to shorten intervals between sprays in wet weather. In new plantings, look for resistance found in some of the long-needled Scots pine varieties from Central Europe. Do not plant seedlings near established pine windbreaks. Follow good sanitation practices. Plant the pines in sun with good air ciculation.