First Report of Verticillium dahliae on Stevia (Stevia rebaudiana) in North America.

Stevia (Stevia rebaudiana Bertoni, Asteraceae) is the source of stevioside, a sweet, low-calorie sugar substitute. Acreage of stevia in California has been increasing in recent years. In October 1999, stevia plants in a commercial field exhibited stunting, leaf necrosis, and vascular discoloration. Verticillium dahliae was consistently isolated from diseased root and stem pieces plated on water agar and acidified potato dextrose agar (APDA). Colonies became dark with age on APDA, formed single celled conidia on hyaline verticillate condiophores, and produced dark microsclerotia. Stevia (cv. R-set 1) plants were grown from seed in soilless potting mix in 20-cm-diameter pots. Roots of eight 4-week-old stevia plants were submerged in a 10⁶ conidia per ml suspension of one of the isolates for 2 min. Eight control plants were dipped in sterile water. The plants were then repotted in soilless potting mix in 10-cm-diameter pots. Four weeks later, inoculated plants exhibited stunting, necrotic lower leaves, and discolored vascular systems. Control plants remained healthy. V. dahliae was reisolated on water agar from diseased plants. The experiment was conducted twice and the results were identical. This is the first report of V. dahliae on stevia in North America.