Detection, Quantification, and Vegetative Compatibility of Verticillium dahliae in Potato and Mint Production Soils in the Columbia Basin of Oregon and Washington.

Soil samples from 87 fields intended for potato production in the Columbia Basin of Washington and Oregon and 51 fields intended for mint production in Washington were assayed on a semiselective medium to quantify populations of Verticillium dahliae. The pathogen was isolated from 77 (89%) of the fields intended for potato production and 41 (80%) of the fields intended for mint production. Population densities ranged from 0 to 169 propagules/g of air-dried soil in fields intended for potato production and 0 to 75 propagules/g of air-dried soil in fields intended for mint production. Isolates of V. dahliae were recovered from soil assay plates and pure cultures were prepared to provide a collection of isolates for vegetative compatibility analysis. Among fields intended for potato production from which isolates of the fungus were assigned to a vegetative compatibility group (VCG), 93% of the fields were found to contain one or more isolates assigned to VCG 4A, nine (23%) contained one or more isolates assigned to VCG4B, and only one (3%) contained any isolates assigned to VCG 2B. In the case of fields planned for mint production in 1999 from which isolates of the fungus were assigned to a VCG, 13 fields (81%) were found to contain one or more isolates assigned to VCG 4A, 7 (44%) contained one or more isolates assigned to VCG 4B, and 5 (31%) contained one or more isolates assigned to VCG 2B. VCG 4A isolates of V. dahliae are widespread and numerous, particularly following potato production, but cause only mild to moderate symptoms in mint; therefore, this pathotype is unlikely to seriously endanger subsequent plantings of mint. However, planting potato in a field recently used to produce mint may pose a significant risk to the potato crop if high populations of the VCG4A pathotype (highly aggressive to potato) predominate. Preplant assessment of soil populations of V. dahliae without regard for the relative populations of various pathotypes present in a particular sample may lead to information not fully useful in integrated pest management systems.

Report of Colletotrichum coccodes Associated with Mentha.

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.

Vegetative Compatibility and Pathogenicity of Verticillium dahliae from Spearmint and Peppermint.

The vegetative compatibility of 128 isolates of Verticillium dahliae from spearmint and peppermint in the western and midwestern United States was determined. Nit mutants were used to assign isolates to vegetative compatibility groups (VCGs). All isolates were assigned to VCG 2B except for one assigned to VCG 2A and two assigned to VCG 4A. VCG 2 isolates were found in all commercial mint growing regions, while the two VCG 4A isolates originated from southern Idaho. Pathogenicity assays on mint were performed using isolates from mint and other hosts, and pathogenicity assays were conducted on the potato cultivar Russet Norkotah using mint and potato isolates. Isolates originating from mint were significantly more aggressive on mint than were other host isolates, indicating that mint isolates were host-adapted. VCG 4A isolates from mint and potatoes were significantly more aggressive on potato than VCG 4B potato isolates and VCG 2B mint isolates. We speculate that the low VCG diversity of mint isolates may be due to the introduction of a single aggressive strain into Washington State mint fields via infected rhizomes.