Twig Blight of Douglas-Fir: A New Disease Caused byDothiorella dothidea

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Abstract
D. dothidea was confirmed, by wound and nonwound inoculation of 7-yr-old seedlings, as the incitant of a twig blight of Douglas-fir. The most conspicuous symptoms were yellowing, wilting and browning of the needles on individual branches. This was followed by progressive dieback of branches from tip to the central leader and complete defoliation of some branches. The disease was most prevalent in rapidly growing leaders. The primary diagnostic feature was the presence of pycnidia on the leaf scars and bud scales of diseased branches. One to several black, globose to subglobose pycnidia were produced in stroma. Conidia were haline, 1-celled, and ellipsoid to clavate and measured 18.0-28.0 .times. 5.3-7.7 .mu.m. Conidia were borne individually on simple conidiophores lining the inner pycnidial wall. On potato-dextrose agar, the fungus grew well at 20-37.degree. C, slight growth occurred at 40.degree. C, and the optimum was 30.degree. C. The fungus produced white, cottony mycelium that turned gray and then black with age on potato-dextrose agar. The disease was found in 2 counties in Kansas [USA].

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