The Plant Clinic sees these two diseases on pine fairly regularly. The two fungal needle diseases are very similar. Even with aid of a microscope, we find it difficult to distinguish the spores of the causal fungi. Symptom expression is also similar. Infected pines may appear yellowed and sparsely foliated, especially near the bottom. Needles may have brown spots with yellow borders, and pinhead-sized fungal fruiting structures may appear embedded in the brown spots. Infection occurred last summer or fall, and fungal fruiting bodies were formed at that time. This spring, as weather warms, the spores will be released and the infection cycle continue. Fungicides are available to use in managing these diseases, but only in a protective mode. You need to identify the disease now and plan to apply the fungicides to newly emerging needles next spring to help manage the diseases. Meanwhile, concentrate on cultural controls. Here are some tips on identification.
Dothistroma needle blight occurs most often on Austrian and Ponderosa pine. Both Scotch and red pine are resistant. The disease causes spots and bands on needles, especially on 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 last several years in landscaped areas.
Symptoms of Dothistroma first occur in the fall but may go unnoticed. Look closely at the needles now for yellow to brown bands or individual spots. If the disease is a problem, you won’t have any trouble finding these lesions. Laboratory confirmation relies on finding the diagnostic fruiting bodies and spores within the spots. The fruiting bodies are pinhead-sized, black specks in the needle lesions. You can see them with a hand lens. If you just see brown needles--without spots, bands, or fruiting bodies--then Dothistroma blight and/or brown spot are not present. In that case, I would suspect root injury or moisture extremes. As Dothistroma progresses, needle tips turn brown and fall, leaving green needle bases. Early drop of entire needles is not uncommon.
Brown spot needle blight symptoms are nearly identical to those of Dothistroma blight. Scotch pine is the major host, and we generally make the distinction based on the presence of spores and the host species. Brown spot may begin in July with yellowing of needles, development of brown spots in the yellowed areas, and sometimes the presence of a drop of resin on each spot. Brown bands with yellow halos are more often observed in the fall.
Management of these diseases may involve pruning surrounding plants for better air flow, thus allowing more rapid needle drying after rain, weed control, and proper plant spacing for the same reason; using resistant varieties where available; working with trees in dry weather; and using registered fungicides. Consult the 2003 Illinois Commercial Landscape & Turfgrass Pest Management Handbook or the Illinois Homeowner’s Guide to Pest Management for chemical options. Read carefully the label of the product you choose. Generally, applications are made twice; and generally, the label says to spray when needles are half grown and again 30 days later. Still, each product differs slightly in timing of sprays. Also, there are more chemical options for preventing brown spot than for Dothistroma. Copper fungicides work for both.
Keeping disease names straight can be a problem. Dothistroma is most common on Austrian pine, while brown spot is most common on Scotch pine. I like to remember that the long disease name goes with the long host name. One master gardener shared his method of remembering this information. He simply remembers that scotch is brown (the drink, that is).
For more information, consult Report on Plant Disease (RPD), no. 624, “Needle Blights and Needle Casts of Pine,” available in Extension offices and on the Extension VISTA Web site.