Verticillium dahliae, a Cause of Wilt in Echinacea purpurea

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.