The Plant Disease Clinic has been receiving an assortment of conifer samples. The browning of needles observed cannot be explained by a single disease or environmental factor.
The dry conditions of last year, the mild winter, and dry conditions this spring have been problematic for conifers. Symptoms of browning have been especially noticeable on last year's growth, or the growth at the ends of the branches. Owners often describe these trees as appearing healthy in the fall, but beginning to show browning in the late winter and spring. In many cases the symptoms appear on the entire tree. Samples submitted have included white pine, concolor fir, spruce, and arborvitae.
Pinpointing the primary cause of browning is important. Fungicides can be used as a tool to control needle diseases. However, no benefit is gained by applying a fungicide to a conifer showing browning from drought stress.
With close observation, fungal diseases can be distinguished from environmental stress problems.
- Needle diseases often work from the lower branches upward over a period of time (usually years). When stresses such as drought are to blame, the symptoms often appear on the entire tree.
- A close look at diseased pine needles will show spots or banded areas on the needles. Death of the needles beyond the banded areas eventually occurs. Brown spot on Scots pine and Dothistroma needle blight on Austrian pine are common in Iowa.
- Diplodia (Sphaeropsis) tip blight on Austrian pine is recognized by the development of stunted, brown shoots with short needles.
- Drought stress can rob trees of energy, causing them to be susceptible to canker fungi such as Cytospora and Diplodia (Sphaeropsis). Canker fungi encircle twigs and branches, causing individual branches to die, usually in a scattered pattern on the tree. White patches of sticky resin are usually associated with cankered areas.
Pm-1528 "Common Diseases of Conifers in Iowa" is a helpful reference when diagnosing conifer diseases. Control measures are also discussed. Note that mid-May is the time when the first fungicide application is made for several of the common disease problems. Timing and good coverage are important.
In addition to fungal needle diseases and environmental factors, other agents may also cause browning of conifer trees. Beetle injury on the main trunk can cause rapid dieback of trees and well as pine wilt, caused by a nematode. These factors are especially problematic on Scots pine.
For assistance is diagnosing disease problems, samples can be submitted to the Plant Disease Clinic through county Extension offices or mailed directly to the Clinic. A $10 free is assessed per sample for problem diagnosis. Plant Disease Clinic 323 Bessey Hall
Iowa State University
Ames, IA 50011
This article originally appeared in the May 12, 2000 issue, p. 53.
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