Colletotrichum coccodes (Wallr.) Hughes is a pathogen of tomato and potato and occurs worldwide on plants primarily from the Solanaceae. It has not been previously reported for Mentha. C. coccodes was isolated from rhizomes and lower portions of above ground stems of symptomless Mentha × piperita L. (peppermint) plants collected from commercial fields in central Washington and Wisconsin (central sands). Three isolates from mint were evaluated for pathogenicity on mint and potato. Rooted cuttings of eight plants each of M. × piperita, M. spicata L. (native spearmint), and M. × gracilis Sole (Scotch spearmint) were dipped into a conidial suspension (1 × 106 conidia per ml) of each isolate of C. coccodes for 10 min. Plants were transplanted into a sterilized potting mix and moved to a greenhouse. After 50, 90, and 185 days, pieces of rhizomes and roots were thoroughly washed in running water, soaked in 1.5% NaOCl for 7 min, and plated on NPX agar medium (1). C. coccodes was reisolated from 25 of 36 plants at 50 days, 14 of 18 plants at 90 days, and 10 of 18 plants at 185 days. Disease symptoms were not observed on roots, rhizomes, or aboveground plant parts. C. coccodes was not isolated from rhizomes and roots of eight noninoculated plants of each mint species. In pathogenicity tests on potato, the isolates from mint produced cortical root rotting and sclerotia on cv. Russet Burbank and did not differ in their aggressiveness from three potato isolates. (Conidial suspensions at 106 were applied to soil surface of potted plants at 10 cm of shoot growth and assessed for disease severity at plant maturity.) All tests were repeated with similar results. Mint is often rotated with potato in central Washington and appears to be a bridging host for C. coccodes.
Reference: (1) E. J. Butterfield and J. E. DeVay. Phytopathology 67:1073, 1977.
