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Some aspects of the association between Rhabdogloeum pseudotsugae Sydow and Rhabdocline pseudotsugae Sydow Parker, Arthur Kneeland

Abstract

Rhabdogloeum pseudotsugae Sydow has been suspected of being the imperfect stage of Rhabdocline pseudotsugae Sydow since their original description by Sydow in 1922. Because Rhabdogloeum has been reported so seldom in North America, and never in Europe, this possibility has been considered slight by most investigators. Investigation of the problem of Rhabdocline pseudotsugae in British Columbia revealed several new aspects of the association between the two leaf-cast diseases. Inspection of Rhabdocline and Rhabdogloeum collections from the interior of British Columbia indicated that the association of Rhabdogloeum pseudotsugae and Rhabdocline pseudotsugae is more prevalent than was formerly believed. Collections from the coast, and on Vancouver Island in particular, indicated that the association is probably less prevalent there than in the interior. In relation to the frequent and widespread occurrence of Rhabdocline throughout the Douglas fir range however, Rhabdogloeum is seldom found. If Rhabdogloeum pseudotsugae is the imperfect stage of Rhabdocline pseudotsugae, then it appears that frequently the Rhabdogloeum stage and occasionally the Rhabdocline stage is completely suppressed, as both stages have been found dissociated from each other. Frequent observation of a group of tagged Rhabdocline-infected trees on Vancouver Island over a period of two years did not reveal the presence of a Rhabdogloeum stage. Tissue cultures of lesions from these tagged trees, however, resulted in the growth of a fungus quite similar in appearance to that produced in tissue cultures by lesions typical of those produced by Rhabdogloeum pseudotsugae. This supports the view that Rhabdogloeum pseudotsugae is the imperfect stage of Rhabdocline pseudotsugae. Observation of these trees indicated that Rhabdocline pseudotsugae is capable of vegetating two years before producing apothecia. Apparently it is also capable of vegetating more than two years or of infecting leaves other than those of the current year.

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