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Cylindrocarpon didymum Pathogenic on Apple Seedlings in Washington State. F. M. Dugan, USDA, ARS Tree Fruit Research Lab; Washington State University Tree Fruit Research and Extension Center, Wenatchee 98801. G. G. Grove, Washington State University Tree Fruit Research and Extension Center, Wenatchee 98801.. Plant Dis. 78:1219. Accepted for publication 3 October 1994. Copyright 1994 The American Phytopathological Society. DOI: 10.1094/PD-78-1219B.

In March 1993, a Cylindrocarpon sp. was isolated from sporodochia on extensively cankered upper boles of several cultivars of 1-yr-old apple (Malus x domestica Borkh.) trees that had been in storage at 2-7 C since November 1992 at a nursery in Quincy, WA. Symptoms appeared as early as 1 mo into storage; rootstocks were unaffected. Pathogenicity was confirmed by stab-inoculation of apple twigs (cv. Rome) with a conidial suspension (104 conidia per milliliter) of a subcultured strain (ATCC 90768), incubating the twigs at 20-22 C for 30 days, and reisolating the fungus. Mean canker elongation after 21 days was 10 cm. ATCC 90768 was characterized as follows: colony diameter approx. 25 mm after 7 days at 20 C on potato-dextrose agar; aerial mycelium cream to buff to tawny brown, floccose, with reverse yellow-brown to dark red-brown. Conidia 0-l(rarely 2-3) septate; O-septate conidia (7-)10-17(-20) x (2.5-)3-4.5 (-5) µm; 1-septate conidia predominant, (l4-)18-25(-28) x (2.5-)3.5-4.5(-5) µm; no clear division into micro- and macroconidia. Phialides 12-25(-5O) x 2.5-3 ?m, borne singly or 2-3, lateral or terminal; chlamy-dospores terminal, intercalary, clustered or catenulate, hyaline to yellow-brown, irregular to spherical, 12-16 ?m diameter. Red-brown sporodochia present on potato-sucrose agar. No teleomorph was observed. The strain most closely conforms to Cylindrocarpon didymum (Harting) Wollenweb. on morphological criteria, but rare 3-septate conidia demonstrate affiliation with C. destructans (Zinssmeister) Scholten, as does colony coloration; other characters show affiliation with C. magnusianum Wollenweb. (1). C. didymum, C. destructans, and C. magnusianum are members of a group of allied species attacking Malus (1,2). This is the first official report for C. didymum as a pathogen of Malus in the United States.

References: (1) C. Booth. Mycological Paper No. 104, 1966. (2). D. Brayford. My-copathologia 100:119, 1987.