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In July 1997, mature (flowering) purple coneflower plants exhibiting severe stunting, interveinal leaf bronzing, leaf twisting, and vascular discoloration of the stem were abundant in a 2-ha commercial field in central Oregon. Isolations from vascular tissues onto water agar yielded Verticillium dahliae. A single conidium isolate was obtained and used to inoculate 2-month-old, greenhouse-grown E. purpurea plants raised from seed to complete Koch's postulates. Conidia were washed from 25-day-old cultures grown on potato dextrose agar and filtered through cheesecloth. The resulting conidial suspension was adjusted to 8 × 106 spores per ml. Plants (n = 15) were washed free of soil and the roots immersed into either the conidial suspension (n = 10) or sterile, deionized water (n = 5) for 5 min, then potted into a pasteurized soil mix (1:1:1, clay loam:sand:perlite). Plants were maintained under natural light on a glasshouse bench from February to August (21–30/12–20°C day/night over the course of 6 months). Forty days postinoculation a heat wave caused temperatures in the glasshouse to peak at 42°C, which killed all but one inoculated plant and four control plants. Six months postinoculation the inoculated plant exhibited symptoms similar to those originally observed in affected field-grown plants including leaf bronzing, severe stunting, and vascular discoloration. The control plants were asymptomatic. Isolation from the vascular tissues of the inoculated plant yielded V. dahliae; no fungus was recovered from control plants. This is the first report of Verticillium wilt in purple coneflower. |
