Phlyctema vagabunda Causes Coin Canker of Ash (Fraxinus spp.) in North America

During the spring of 2001, nursery-grown ash trees in Michigan and Ontario, Canada displayed coin cankers that were previously described (1). Cankered cultivars included Cimmaron®, ChampTree®, and Urbanite® (Fraxinus pennsylvanica), and Autumn Purple® (F. americana). Tissue from surface-sterilized cankers (0.06% sodium hypochlorite or 70% ethanol, 3 min) was placed onto one-half strength potato dextrose agar amended with 200 ppm of streptomycin sulfate (½ SPDA). Plates were incubated at 20 and 4°C in the dark. Phlyctema vagabunda (anamorph of Neofabraea alba (E.J. Guthrie) Verkley, (1999)) was isolated on the 4°C plates from most tissue pieces from all cultivars. P. vagabunda causes stem cankers of apple (Malus spp.) (1) but its pathogenicity to ash has never been demonstrated (2). Inoculations with P. vagabunda were made to green and white ash to fulfill Koch's postulates. Apple trees were also inoculated with the ash isolate. In Corvallis, OR, 2-year-old, bare-root ash trees and 15 1-year-old apple seedlings were inoculated in January, 2003. In Michigan, 10 trees each of cvs. Cimmaron® and Autumn Purple® were inoculated in December, 2002. The trees were inoculated with a wound-freezing method (3). Wounded sites received a 5-mm-diameter plug of ½ SPDA on which a 30-day-old culture derived from a single conidium of P. vagabunda AR3664 was actively growing, with sterile ½ SPDA used as a negative control. The plugs were held in place with Parafilm. Each ash in Oregon received three inoculations per stem. Six trees each of cvs. Autumn Purple®, ChampTree®, and Urbanite® and four trees of cv. Cimmeron® were inoculated with the fungus. Four additional trees of each cultivar were treated with sterile ½ SPDA (three sites per tree). Ten seedling apple trees were also inoculated with the fungus (three sites per tree) and five additional trees (five sites per tree) were inoculated with negative control plugs. In Michigan, each tree received two plugs of inoculum and two negative control plugs. All trees were evaluated 4 to 6 months after inoculation. Cankers similar to the originals formed on all ash cultivars in Oregon and Michigan; 97 of 106 inoculated wounds developed cankers on ash, and 29 of 30 inoculated wounds on Malus spp. developed cankers. P. vagabunda was recovered from cankers on each of the inoculated ash and apple trees. Five of 88 controls developed necrotic areas, but Phlyctema spp. were not recovered from any of these wounds. To our knowledge, this is the first report that P. vagabunda causes cankers on Fraxinus spp. and the first to show that Malus spp. can be infected with an ash isolate. References: (1) T. D. Gariépy et al. Mycol. Res. 107:528, 2003. (2) A. Y. Rossman et al. Plant Dis. 86:442, 2002. (3) R. Scorza and P. L. Pusey. Phytopathology 74:569, 1984.