Issue No. 7, Article 9/May 20, 2011
Dying Windbreaks? A Few Words from the Plant Clinic
Wet fields may have slowed down field work, but it has made some time for inspecting evergreen windbreaks. Most of what people are seeing is not good: dead branches, needles dropping or discolored, sap coagulating in branch and trunk cankers. This article addresses two of the most common fungal diseases that affect pine and spruce in Illinois windbreaks, but be aware that wind, winter, salt, chemical, environmental, insect and cultural issues can all cause damage as well. The past several springs have been particularly conducive to infection by several endemic fungal diseases on spruce and pine in Illinois.
Pines. The most common disease sees on pines in windbreaks is Sphaeropsis blight, also called Diplodia tip blight. The Plant Clinic is getting a lot of samples with this disease because it thrives in cool and wet weather. Though most pines are susceptible to Sphaeropsis blight, in Illinois we most often see it infect Scotch, Austrian, and Mugho pines. White pine is rarely affected.
The tip and needle blight symptoms of Sphaeropsis that you may be seeing now on your windbreak trees are often from last year's infections. The buds and new needles emerging at this time are very susceptible to infection. The Sphaeropsis pathogen infects healthy, unwounded needles as well as trees stressed from such events as drought, compacted soil, root injury, hail, and winter injury. A characteristic symptom of Sphaeropsis blight is the blighting of branch tips. Nearly all of the needles in the terminal 6 inches of growth will turn brown, dry out, and remain attached throughout the season. Frequently the tree will develop new growth below the dead area, causing the branch to grow in a zigzag pattern. Sphaeropsis can also cause cankers, which will noticeably ooze with sap, to develop on twigs. If the canker girdles the stem, the tissue beyond the canker will die. The disease is not known to cause tree death, but it can create some very unsightly damage.
Tip dieback on pine from Sphaeropsis infection.
It takes some commitment to control Sphaeropsis blight. Cultural control is an important part of management. To reduce disease inoculum, prune and remove dead wood or needles from the pine. To reduce disease spread, remove the dead tissue early in spring (before buds open), when the foliage is dry, or during the dormant season. Removing all infected cones will help deter the disease from overwintering and providing disease inoculum for the following spring. It is a good practice when possible to alleviate stress to your pines by fertilizing or watering during times of drought. Fungicides are available to manage Sphaeropsis blight, but they should be used in conjunction with the cultural controls just described. Available fungicides include propiconazole and thiophanate-methyl. Be sure to read and follow label directions. Fungicides should be applied at three different times: just as buds are expanding, just before new needles emerge from sheaths, and 10 to 14 days after new needles emerge. For more details about this disease, read the Report on Plant Disease no. 625, Sphaeropsis Blight or Diplodia Tip Blight of Pines.
Spruce. Windbreak spruce have had a hard go of it in the past couple of years. The most common disease we have seen by far this season is Rhizosphaera needle cast, or needle drop. Trees with purple/brown 1- and 2-year-old needles are suspect. Newly emerged growth will be green, while needles from last year's growth will be brown or purplish brown. Since evergreens do not refoliate along the branches, if untreated the disease will cause bare areas scattered throughout the tree. Norway spruces are considered fairly resistant to this needle cast, while Colorado blue spruce is a common and susceptible host.
Branch defoliation on blue spruce by Rhizosphaera needle cast.
Rhizosphaera needle cast is diagnosed by the tiny black fruiting bodies of the Rhizosphaera fungus. You may be able to see these lined up in rows along the bottom of the needle. Typically microscopic evaluation is necessary because Stigmina, a fairly new fungus affecting spruce in Illinois, looks the same on the needle. Rhizosphaera needle drop can be managed with fungicide applications of chlorothanonil, but there is currently no treatment for Stigmina needle cast. Treatment of Rhizosphaera usually needs to be done for 2 years. Foliar fungicide should be applied just after bud cap drop and again after needle elongation.
We would be happy to help with diagnosis of your windbreak problems and other plant issues. Find our Plant Clinic sample submission forms at plantclinic.cropsci.illinois.edu.--Suzanne Bissonnette