Common Conifer Diseases

Many conifer samples showing needle diseases have been arriving in the Plant Disease Clinic. Most of these diseases can be diagnosed by carefully examining needles. Control of these diseases requires the use of protectant fungicides when the new growth is emerging (May).

The new bulletin Pm-1528 "Common Conifer Diseases in Iowa" contains color photos of common conifer disease symptoms, descriptions of the diseases, and control measures. The bulletin is available from county Extension offices or from the Extension Publications Distribution office, 515-294-5247. The cost is $1.50.

Rhizosphaera needle cast, is a needle disease of spruce, most commonly on blue spruce. The disease causes older needles (those closest to the tree trunk) to turn purplish-brown and fall from the tree. If you look closely at these needles using a hand lens or magnifying glass you can see small black structures (fruiting bodies of the fungus that causes the disease) in the pore-like openings that run the length of the needle. Symptoms usually begin on lower branches and spread to upper branches.

Diplodia tip blight affects primarily Austrian and Scots pine in Iowa. The most obvious symptom is stunted, brown shoots with short needles. Small black fungal fruiting bodies usually can be seen at the base of infected needles in the fall. Small black fungal fruiting bodies may also be seen on the scales of infected cones. The disease causes annual destruction of buds and shoots.

Brown spot needle blight and Dothistroma needle blight produce similar symptoms. However, brown spot occurs mainly on Scots pine and Dothistroma needle blight on Austrian and ponderosa pine. Both diseases usually begin on lower branches and spread upward. The diseases cause yellow to red spots or bands on needles. Death of the needle from this banded area to the top eventually occurs. Eventually small black fruiting structures develop on dead needles.

Disease control consists primarily of protectant fungicide sprays, but certain cultural practices will also help prevent disease problems.

Inspect trees carefully for disease symptoms before planting and reject diseased trees.

  • Do not plant susceptible species next to infected trees.
  • Promote good air circulation by adequate tree spacing and weed control.
  • Improve tree vigor through mulching and watering when needed.
  • Do not shear trees when foliage is wet.
  • If symptoms appear, use an appropriate fungicide.

Rhizosphaera needle cast-Bordeaux mixture or chlorothalonil (e.g. Daconil 2787) in the last two weeks of May and repeat four to six weeks later.

Diplodia tip blight-thiophanate methyl (e.g. Cleary's 3336), Bordeaux mixture, benomyl (e.g. Benlate), copper fungicides (e.g. Tenn-Cop 5E) when buds start to swell, one week later, then two to three weeks later.

Brown Spot-Bordeaux mixture or chlorothalonil (e.g. Daconil 2787) when needles are half-expanded (mid-May and three to four weeks later.)

Dothistroma needle blight-Bordeaux mixture or other copper-containing fungicides (e.g. Tenn-Cop 5E) in mid-May and four to six weeks later.

This article originally appeared in the May 4, 1994 issue, p. 63.


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