A New Disease of Flowering Dogwood Caused by Colletotrichum acutatum.

In 1992 symptoms consisting of twig dieback, canker-like deformities, and often, mortality of all sizes of container-grown flowering dogwood trees (Cornus florida L.) was frequently observed in northern Florida where nursery production of dogwood is concentrated. Twigs, branches, growing points, and portions of main trunks up to 3 cm in diameter were killed. On affected portions, leaves drooped downward but did not wilt severely; they later became silvery, gray-green and eventually died but did not drop. Affected trees produced flower buds, but the flower buds did not open. Many trees died and others were destroyed because of unsightly dead limbs and deformed trunks. The inciting pathogen was not satisfactorily determined. After 1994, disease incidence subsided, but in 1999, it reappeared and caused serious economic damage. In 1999, we followed the disease from propagation through several stages of nursery production, including growth in large containers. A Colletotrichum species was consistently isolated from acervuli produced in abundance near the soil line on diseased, rooted cuttings and small plants. The same fungus was consistently isolated from acervuli produced on diseased leaves and twigs of larger plants and small trees. Conidia of the isolated pathogen were used to inoculate small dogwood trees kept at 100% RH for 2 days then grown in a greenhouse. Within 23 weeks, a slowly developing leaf spot was produced on inoculated plants. After 3 months, numerous acervuli were produced on inoculated leaves and on adjacent small twigs from which the same pathogen was consistently reisolated. The symptoms and pathogen were not consistent with descriptions of anthracnose incited by Discula destructiva. On potato-dextrose agar (PDA), spore masses were bright red-orange on lawn plates inoculated with large numbers of conidia. Conidiomata did not produce setae in culture nor did acervuli on diseased plant material. On PDA, mycelial growth was tufted and pale-gray. The reverse side of colonies was buff to cream-colored or pale-gray to tan but never dark. With age, a pale pink or orange-pink pigment often formed within the agar media (1,3). When produced on PDA, most conidia were elliptical and elongated with abruptly tapering ends. An average of 18% had both ends slightly rounded; none had only one end abruptly tapered or rounded. Conidia measured 15.3 × 4.78 μm; the length/width ratio was 3.20 (1,3). The teleomorph was not found on diseased plant material or in culture. Analysis of DNA by polymerase chain reaction (PCR) using the CaInt2-ITS4 and CgInt-ITS4 primer pairs (1,2) and by comparison of PCR products with those produced by C. acutatum pathotypes isolated from leatherleaf fern, lime, post-bloom fruit drop-affected Navel orange fruit, and strawberry fruit, and with isolates of C. gleosporiodes from citrus, Camellia, Nandina, holly, and strawberry indicated that the fungus was C. acutatum. The amplified PCR product (approximately 490 bp) obtained from all dogwood isolates using primers CaInt2 and ITS4 was consistent with the size of product expected from C. acutatum. No products were produced with the CgInt-ITS4 primer pair. Based on the morphology of conidia, growth in culture and PCR results, the pathogen was identified as C. acutatum and represents the first report of this pathogen on flowering dogwood. References: (1) J. E. Adaskaveg and R. J. Hartin. Phytopathology 87:979, 1997. (2) A. E. Brown et al. Phytopathology 86:523, 1996. (3) B. C. Sutton. Pages 1-27 in: Colletotrichum: Biology, pathology and control. Brit. Soc. For Plant Pathol., CAB International, 1992.