Needle Diseases of Conifers

A number of Austrian pine samples have been submitted to thePlant Disease Clinic recently showing Dothistroma needle blight,Diplodia tip blight, or both diseases. Theses diseases are acommon problems on Austrian pine and are fairly easy to recognize.

Dothistroma needle blight, which occurs primarily on Austrian,ponderosa, and mugo pines in Iowa usually occurs first on lowerbranches. Infected needles show yellow to reddish-brown spots orbands that extend around the needles. Needles die from the bandedarea to the tip. Diseased needles drop prematurely.

Diplodia tip blight is most common on Austrian and ponderosapines in Iowa but also occurs on Scots, mugo, and red pine. Themost obvious symptom of tip blight is stunted, brown shoots withshort needles. In the fall, small black fruiting structures of thefungus usually can be seen on needle bases. Small black fruitingstructures also appear on scales of infected cones. The annualdestruction of buds and shoots gradually causes tree decline.There are a number of measures that can be taken to prevent andcontrol needle diseases:

  1. Promote good air circulation by adequate spacing and weedcontrol.
  2. Improve tree vigor through good cultural practices such asmulching and watering as needed.
  3. Do not shear trees when foliage is wet.
  4. Do not plant Austrian pine.
  5. Do not plant susceptible species next to infected trees.
  6. If symptoms appear, fungicides applied in the spring willprotect the newly emerging growth.

For Dothistroma needle blight, apply Bordeaux mixture or othercopper-containing fungicides in mid-May and 4 to 6 weeks later.

For Diplodia tip blight, spray thiophanate-methyl, benomyl,Bordeaux mixture, or copper fungicides when buds start to swell, 1week later, then 2 to 3 weeks later.

For further information on symptoms, disease development, andfungicide products, consult Pm-1528 "Common Diseases of Conifers inIowa". This recently published bulletin also contains color photosof Dothistroma needle blight, Diplodia tip blight, and the othercommon conifer diseases that occur in Iowa.

This article originally appeared in the December 8, 1993 issue, p. 168.


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