Brown spot needle blight new in Michigan

Brown spot needle blight is new to Michigan. The browning and early needle loss caused by this fungus can make infected Scotch pine unsalable as Christmas trees.

Brown spot needle blight (Mycosphaerella dearnessii, syn. Scirrhia acicola) had not been reported in Michigan until the fall of 2010. This spring, we have seen a number of fields of Scotch pine in the Cadillac area showing symptoms of brown spot needle blight. On some trees, only the very top of the tree was still green.

Scotch pine showing typical symptoms of brown spot needle blight
Scotch pine showing typical symptoms of brown spot needle blight. Photo credit: Jill O’Donnell, MSUE.

The fungus causes two kinds of needle spots. These can appear on needles at any time of the year, but most commonly during August and September on Scotch pine. If you look closely at last year’s foliage, you will find reddish-brown, resin-soaked spots with yellow margin on green needles. On brown needles, you will find a lot of black fruiting bodies.

Black fruiting bodies on dead needles
Black fruiting bodies on dead needles. Photo credit: Jan Byrne, Diagnostic Services

Short-needled Scotch pine varieties such as Spanish and French-green are more susceptible to fungal attack than the long-needled varieties. It appears that brown spot can be controlled by fungicidal sprays. The first application should be applied when the new needles are about half grown (May-June) and a second spray three to four weeks later. In rainy conditions, the spray interval may need to be shortened. Several products with chlorothalonil or mancozeb are labeled for control.

Growers will also want to adjust their shearing practices to avoid moving spores into other plantations.

  • Do not shear infected trees during wet weather because spores released at this time may be carried from tree to tree on workers’ clothes or shearing tools.
  • Sterilize tools after shearing affected plantations by dipping in denatured alcohol for three minutes.
  • Shear healthy plantations first so pathogen spores will not be carried into them from affected plantations.

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