Conifer samples with needle diseases are among the most common samples submitted to the Plant Disease Clinic. Cultural practices, such as not planting susceptible species next to infected trees, promoting good air circulation by tree spacing and weed control, maintaining tree vigor by mulching and adequate watering, and shearing when needles are dry can help prevent disease problems. However, once a needle disease is confirmed, control mainly consists of protective fungicide sprays.
Timing for applying protective fungicides is very important for effective management of needle diseases. Protective fungicides work best when applied in the spring, before fungi can infect the new needles. Rhizosphaera needle cast, Diplodia tip blight (Sphaeropsis tip blight), brown spot needle blight, and Dothistroma needle blight are some of the most common needle diseases in Iowa.
Below are symptoms to look for when diagnosing conifer needle diseases.
Rhizosphaera needle cast is a common disease of spruce, especially blue spruce. Older needles (those closest to the tree trunk) turn purplish-brown and fall from the tree. Symptoms usually begin on lower branches and spread upward. A hand lens or magnifying glass can help you see small black structures, fruiting bodies of the fungus that causes the disease. These fruiting bodies emerge from pore-like openings that typically appear in rows along the length of the needles.
Rhizosphaera needle cast fungicides and timing - chlorothalonil (e.g. Daconil) or Bordeaux mixture during the last two weeks of May and repeat four to six weeks later.
Diplodia tip blight, also known as Sphaeropsis tip blight, can kill buds and shoots of primarily Austrian and Scots pine in Iowa. Look for stunted, brown shoots with short needles. Small black fungal fruiting bodies usually can be seen on infected needles and scales of infected cones in the fall.
Diplodia tip blight fungicides and timing - thiophanate methyl (e.g. Cleary's 3336), Bordeaux mixture, or copper fungicides when buds start to swell, one week later, then two to three weeks later.
Brown spot needle blight and Dothistroma needle blight have very similar symptoms. Brown spot occurs mainly on Scots pine, while Dothistroma needle blight occurs on Austrian pine. Both diseases usually begin on lower branches, spreading upward. The diseases cause yellow to red spots or bands on needles. Infected needles typically die from the banded area to the tip of the needle. Eventually small black fruiting structures develop on dead needles.
Brown Spot fungicides and timing - Bordeaux mixture or chlorothalonil (e.g. Daconil) when needles are half-expanded (mid-May and three to four weeks later.)
Dothistroma needle blight fungicides and timing - Bordeaux mixture or other copper-containing fungicides in mid-May and four to six weeks later.
Common Conifer Diseases in Iowa (ISU Extension Bulletin Pm-1528) contains color photos of common conifer disease symptoms, descriptions of the diseases, and control measures. The cost is $1.50. The bulletin is available from county Extension offices or: Extension Distribution Center 119 Printing and Publications Building Iowa State University Ames, Iowa 50011-3171 Telephone: (515) 294-5247 Email: pubdist@iastate.edu
This article originally appeared in the April 5, 2002 issue, p. 37.
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