On Austrian pine, the damage caused by these two diseases is worse than it has been for at least a decade. Injury to ponderosa pine is also severe, but this species is less common in the Iowa landscape. Paula Flynn did a brief article on these and other conifer diseases in the May 4, 1994 issue.
Diplodia tip blight is properly called Sphaeropsis tip blight because the fungus that causes the damage has been renamed from Diplodia pinea to Sphaeropsis sapinea. Whatever you call it, it is an insidious disease. Most trees escape infection for the first 15 to 20 years of life, only to develop symptoms after they reach maturity and begin to develop cones. The reason is that the fungus colonizes the female cones (the woody ones) and innumerable fruiting bodies appear on the cone scales. These fruiting bodies shower the tree with spores each time it rains. If a spore lands on a young, expanding shoot in the springtime, it can infect and kill the shoot, typically before the new needles have reached their full length. The result is one of the most distinctive symptoms of any plant disease: sprays of stunted, dead needles at the tips of branches, usually on lower branches. At first, the damage is often inconspicuous, but after unusually wet spring weather it can spread rapidly. Lower branches begin to die, and the ornamental value of the tree can be reduced to zero - or lower. Long before the tree dies, you will wish it were dead. Eventually, most of the branches on the affected tree can die.
Dothistroma needle blight can attack susceptible pines (especially Austrian and ponderosa pines) at any age. Spores of the causal fungus, Mycosphaerella pini (also known as Dothistroma pini), infect needles, producing yellow-orange to brown bands around the needles. When the fungus girdles the needle, it dies and turns brown from the point of infection to the tip. This development results in needles that are brown at8 the tip and green at the base. Typically, bands are also present on the still-green portions of infected needles. Infected needles often fall from the tree, creating a denuded look. Like Diplodia tip blight, Dothistroma needle blight can result in dramatic browning of the foliage, beginning on the lower branches. Also like Diplodia, repeated annual cycles of infection can result in dead limbs and eventual loss of any meaningful ornamental value.
Why are these diseases so prevalent in 1994? The short answer is the very rainy conditions in 1993, but we have actually had several wet spring seasons in the last 4 years, and these diseases crept up further each time.
Many homeowners and property managers are now facing a "kill or cure" situation with heavily infected pines. Fortunately, it is possible to rescue a tree with considerable damage. A protectant fungicide spray program, repeated over several years, will eventually allow new, undamaged needles to replace the diseased ones. For Diplodia tip blight, spray when buds begin to swell, then repeat 7-10 days later and about 2 weeks after that. For Dothistroma needle blight, spray about mid-May (coincides with the final Diplodia spray) and again about July 1; the first spray protects the previous year's needles and the second spray protects the current year's needles as well. When symptoms of the diseases have disappeared, you can discontinue spraying. . . but remember to scout the trees at least twice per year, so you can resume spraying if problems redevelop. Several copper-based fungicides, including Bordeaux mixture, are labeled for Dothistroma needle blight; these can be used (if labeled) for Diplodia tip blight, or you can use Benlate or Cleary's 3336.
An advisable, long-term strategy that fits into the Plant Health Care approach is to avoid Austrian pine when selecting species for planting. Little genetic resistance is available in nursery stock, and these two diseases are both expensive and pesticide-intensive to deal with - exactly the characteristics that clients DON'T want in their landscapes.
This article originally appeared in the June 24, 1994 issue, p. 99.