Peppermint (Mentha spp.)-Spotted Wilt

Cause The Impatiens necrotic spot virus (INSV) and the Tomato spotted wilt virus (TSWV) infect many economically important plants, including both dicots and monocots, and have been found in mint. A number of weedy hosts have been identified and include lamb's-quarters (Chenopodium album), chickweed (Stellaria media), shepherd's purse (Capsella bursa-pastoris), and purslane (Portulaca oleraceae). TSWV and INSV are transmitted by at least five species of thrips. The onion thrips and the flower thrips are known to be in the Pacific Northwest. The INSV is associated with the most severe symptoms in mint. New mint plantings show the worst symptoms, but they fade or disappear in later years.

Symptoms At first, tips and margins yellow on individual leaflets. Symptoms appear first on terminals and move progressively toward the plant base. Yellow leaf areas soon become necrotic, eventually resulting in the leaf's death, followed by death of the entire aboveground portion of the plant. Occasional ringspots are on leaflets. SWV/INSV symptoms in the field are inconsistent, frequently showing only one or even no diagnostic symptoms other than reduced growth. PCR lab testing rather than Elisa testing is highly recommended, because virus titer seems highly erratic in plants.

Early disease stages may be mistaken for powdery mildew. In the field, the underground portion of the plant regrows in the fall. Losses caused by this virus are unknown.

Cultural control

  • Plant virus-tested stock.
  • Propagators should check mother blocks for the virus and should control thrips in greenhouses.