Dothistroma needle blight has been diagnosed several times in the past week at the Plant Clinic. This fungal disease of pine occurs most often on Austrian and Ponderosa pine. Both Scotch and red pine are resistant. The disease causes spots and bands on needles, especially in the lower part of the tree. The problem is more intense in a monoculture, such as a nursery or plantation, than in a landscaped area, but we have seen a fair amount of this needle blight the past year or two in landscaped areas.
Symptoms of Dothistroma first occur in the fall but may go unnoticed. Look closely at the needles for yellow to brown bands or individual spots. Laboratory confirmation relies on finding the diagnostic fruiting bodies and spores within the spots. The fruiting bodies are black, pinhead-sized specks in the needle lesions. As the disease progresses, needle tips turn brown and fall from the affected needles, leaving green needle bases. Early drop of entire needles is not uncommon, with the period of greatest needle loss from Dothistroma in spring and summer.
Cultural controls to promote more rapid drying of foliage may help. Prune surrounding plants, control weeds in the area, and space plants properly. The copper fungicides, including fixed or neutral copper compounds and bordeaux mixtures, are registered for use on pine to control Dothistroma needle blight. Two sprays are required--one when needles are just emerging in mid-May and another when new needles are fully expanded. In the home landscape, some control may be attained by removing fallen needles and helping tree vitality through fertilization and watering practices.
Brown spot needle blight is nearly identical to Dothistroma blight. A different fungus is the pathogen and Scotch pine is the major host, but otherwise spores must be compared to distinguish these two diseases. There are more chemical options for preventing brown spot than for Dothistroma. These treatment options are listed in the pest management handbooks mentioned earlier. Applications should be made when needles are half grown and again 30 days later.
For both of these fungal needle blights, control measures are most successful when cultural controls are begun as soon as the disease is identified, with chemical controls started the following spring. For more information on pine needle blights, consult Report on Plant Diseases No. 624.