It is important to determine whether the browning evident on evergreen trees is caused by an infectious disease or by winter injury. Fungicide sprays are effective against needle diseases, but will not solve winter injury problems.
Many evergreen samples have been arriving in the Plant Disease Clinic showing browning of needles. In a majority of the cases, an infectious needle disease fungus is not the culprit. Winter injury appears to be the cause of browning. White pine, arborvitae, and yew have been especially affected. In many cases, bud tissue appears succulent and green, suggesting recovery should occur. It other cases complete death is evident. Recently planted as well as established trees have been affected.
We have observed infectious needle diseases as well. Rhizosphaera needle cast on spruce, brown spot on Scots pine, Dothistroma needle blight on Austrian pine, and Diplodia tip blight on Austrian pine and Scots pine have been observed. These diseases are pictured and discussed in Pm-1528 "Common Diseases of Conifers in Iowa". As stated in this bulletin, fungicide sprays beginning in the spring are necessary for control. They protect newly emerging needles from infection. The timing of sprays is dependent upon the disease. Rhizosphaera needle cast on spruce, for instance, is controlled by applying a fungicide beginning when new shoot growth is 1/2 to 2 inches in length. This usually occurs in mid-May in Iowa. (Because of the cooler temperatures this year, some trees have been slow putting out new growth.)
If you need assistance in diagnosing a possible disease problem, submit a sample to your local county extension office or directly to the Plant Disease Clinic, 323 Bessey Hall, Ames, IA 50011.
This article originally appeared in the May 17, 1996 issue, p. 77.