Rhizoctonia Web Blight-A New Disease on Mint in Israel.

Mentha longifolia is produced in and exported from Israel with annual revenue of US$16 million. In 2010, a severe epidemic of unknown etiology reduced growers' returns up to 50%. Disease symptoms included water-soaked lesions, necrosis, and web-like mycelia on plants. Two isolates (JV-1 and BS-1) from randomly selected symptomatic plants were identified as members of Rhizoctonia solani anastomosis groups 1-IB and 4HG-I, respectively. The fitness of JV-1 and BS-1 interacted with temperatures between 17 and 35°C: JV-1 grew faster and was more aggressive (P < 0.05) at lower temperatures (<24°C), BS-1 grew faster and was more aggressive at higher temperatures (>30°C), and the two isolates performed similarly at intermediate temperatures. Disease developed fastest at 24 to 28°C. Yield was reduced between 46 and 100%. In all, 77.5% fewer plants recovered from disease developing at 24 to 28°C than at 17 to 23 or 30 to 35°C. The relationship of disease to relative humidity (RH) fit a quadratic model (P < 0.0015, R² = 0.98). Disease was most severe at 100% RH, decreasing by 1.3-, 1.9-, 3-, 4.5-, and 10.5-fold with the reduction of RH from 100% to 88, 76, 69, 55, or 49%, respectively. This is the first report of Rhizoctonia web blight in mint in Israel.