Management of Diplodia/Sphaeropsis shoot blight of Austrian, Red and Scotch pine

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.  

This disease is different from the needle cast diseases in that it kills the current year’s infected needles and stems. The fungal pathogen, Diplodia sapinea, (was known as Sphaeropsis) will also kill seedling trees in nurseries, whereas the needle cast diseases do not infect seedling trees because their needles dry so fast. With Diplodia shoot blight and canker disease, mature trees usually require a stress event to trigger a susceptible reaction, but seedlings can be infected if a source of the pathogen is available. A typical stress event last year was hail damage in localized areas.

This fungus infects growing and elongating shoots in the spring and kills the main terminal of the seedlings. The black fruiting bodies are relatively large and can be seen easily with a hand lens and even the naked eye. These black dots will be on the needles, usually under the sheath, as well as on the stems and cones.

Diplodia shoot blight is currently widespread, ravaging many landscape pines. You should consider applying fungicides every two weeks, up to four times to prevent the spores that are now being produced from infecting the succulent new growth. During rain and windstorms, these spores will disperse over long distances and conditions for infection will remain favorable during rainy and humid days. If wounds are created by hail, you should consider spraying to protect the trees from severe disease. Also, if trees surrounding your nursery have severe blight this year, you may want to reduce the number of susceptible species, such as Austrian or red pine in the nursery for a couple of years, as this infection period will probably be extended, even with management. If you are planting mature Austrian and red pine, consider the fact that even though they have been inspected, they may still have come from nurseries or plantations with latent infections, and more than likely, spores will have infected some of these trees. If the trees are stressed, shoot blight will appear and you should be ready to manage this disease next spring. If, later in the summer, more than 10 percent of the trees have severe shoot blight, begin a spray program next spring. Do not shear trees that have infections, or may have come from nurseries with infections, as the shears will carry the spores to each new cut. Also, in landscape situations, consider raking up cones, which can be a major source of spores.

Manage this disease with a package of treatments

Consider turf removal under the tree and apply mulch and water. By eliminating the grass under the tree, it will remind you not to apply fertilizer to the tree. Use no nitrogen fertilizer. Water during dry periods to reduce stress. Prune out dead tips and clean pruners with a bleach mixture. Use Cleary’s 3336 thiophanate methyl or chlorothalonil for chemical control if needed.

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