Occurrence of Rhabdocline Taxa in Douglas-Fir Christmas Tree Plantations in Michigan. J. G. O’Brien, Graduate Research Assistant, Department of Plant Pathology, University of Minnesota, St. Paul 55108. H. L. Morton, Professor, Forest Resources Program, School of Natural Resources, University of Michigan, Ann Arbor 48109. Plant Dis. 67:661-663. Accepted for publication 29 November 1982. Copyright 1983 American Phytopathological Society. DOI: 10.1094/PD-67-661.

A survey to determine the identity of taxa causing Rhabdocline needle cast in six plantations of Douglas-fir (Pseudotsuga menziesii) Christmas trees in northern Lower Michigan was undertaken in 1979. Rhabdocline weirii subsp. oblonga was the predominant taxon present, occurring on 83.9% of the infected needles examined. R. pseudotsugae subsp. pseudotsugae, the only other Rhabdocline taxon encountered, was present on 17.3% of the infected needles examined. The two taxa could be separated with 94% accuracy by characteristic methods of rupture of the host epidermis by the apothecia. The overall level of infection (percentage of needles infected) in one stand was significantly higher (P = 0.05) in the interior of the stand than on the margins; it was also significantly lower (P = 0.05) on the north sides of the trees.

Keyword(s): epidemiology.