Fungi in the sooty blotch and flyspeck (SBFS) complex cause major economic losses on cultivated pome fruits in humid regions worldwide and also colonize many species of reservoir host plants. In 2007, 10 mature fruit of pawpaw (Asimina triloba), a native tree in eastern North America exhibiting SBFS colonies on the epicuticular wax layer, were collected from wild trees in eastern Iowa. Colonies of SBFS fungi on the fruit were described according to mycelial type (1). Isolates of representative colonies on acidified water agar were subcultured on potato dextrose agar and the morphological characters were observed. After DNA was extracted from cultures and amplified by PCR using primer set ITS-1F/ITS4, 470-bp sequences were compared with those of previously identified SBFS species using NCBI BLAST. The BLAST analysis showed 100% homology of the sequences with six species that had been previously confirmed to cause SBFS on apple fruit by fulfilling Koch's postulates (1): Colletogloeum sp. FG2, Dissoconium aciculare, Peltaster sp. P2.1, P. fructicola, Stomiopeltis versicola, and Stomiopeltis sp. RS1 (GenBank Accession Nos. AY598907, AY598874, AY5988888, AY598887, AY5160165, and AY598882, respectively). Using the NCBI bl2seq application, Dothideomycete sp. CS2, an additional previously confirmed SBFS species, was revealed by sequence homology. Morphology of some SBFS species on pawpaw differed from that on apple. For example, Colletogloeum sp. FG2, which produces the fuliginous mycelial type (1) on apple fruit, developed the ridged honeycomb mycelial type on pawpaw fruit. D. aciculare and Stomiopeltis sp. RS1 produced the compact speck mycelial type on pawpaw, but are known to develop discrete speck and ramose mycelial types, respectively, on apple. These differences may result from host species differences in the epicuticular wax layer of the fruit. To our knowledge, this is the first report of SBFS fungi on A. triloba in North America, although the SBFS species Zygophiala jamaicensis was reported on the same host in Japan (2). Identifying SBFS fungi on reservoir host plants is an important step toward improving disease management strategies.
References: (1) J. Batzer et al. Mycologia 97:1268, 2005. (2) H. Nasu and H. Kunoh Plant Dis. 71:361, 1987.
