First Description of the Causal Agent of Fairy Ring of Cranberry and Its Novel Dispersal Method.

Fairy ring is an economically important disease of cranberry in the northeastern United States, yet its causal agent has remained unresolved for more than a century. Field sampling, targeted isolation, morphological and molecular characterization, pathogenicity tests, and population analyses consistently associated the disease with the genus Helicobasidium (Pucciniomycetes), a known teleomorph of the violet root rot fungus. Internal transcribed spacer and large subunit rDNA sequences placed all isolates from New Jersey and Massachusetts in a distinct lineage within H. longisporum, designated here as H. longisporum III. Pathogenicity was confirmed in greenhouse inoculations, and infection cushions characteristic of violet root rot pathogens were consistently present on symptomatic vines. Tuberculina isolates were recovered from aecial rust pustules of Puccinia macrospora and P. minutissima on Smilax spp. and Decodon verticillatus, respectively, near cranberry plantings and were genetically identical to Thanatophytum isolates from cranberry, supporting a holomorphic life cycle involving a rust-parasitic stage. Mycelial compatibility group and genotyping-by-sequencing analyses revealed high diversity among rings, suggesting that new ring formation results largely from independent mating events during the mycoparasitic phase, with limited clonal spread within beds. These findings demonstrate that cranberry fairy ring is caused by a Helicobasidium-Thanatophytum-Tuberculina complex with both soilborne and airborne components. Understanding this life cycle provides new opportunities for management, including targeting the rust-parasitic stage and associated alternate hosts.