First Report of Stevia as a Host of Sclerotinia sclerotiorum.

Stevia (Stevia rebaudiana Bertoni; Asteraceae), an annual plant originating from Paraguay, contains glucosides of a diterpenoid (2), which is used as a low-caloric sweetener in some South American and southeast Asian countries. The main active ingredient, stevioside, is 100 to 300 times as sweet as sucrose. Stevia has been experimentally grown under field conditions in central and western Canada and has the potential to become a commercially viable alternative crop. In August 1996, a previously undescribed stem rot disease was observed on stevia plants at the Crop Diversification Centre South, Brooks, Alberta. The disease was found in research plots where 4-month-old plants were growing in loam soil. Diseased stems showed dark brown lesions above and at soil level when plant height reached approximately 30 cm. Under dry conditions, mild stem lesions caused plant stunting with lower leaves turning black and curling downward. Wilted leaf symptoms gradually spread upward in affected plants. Partial wilting symptoms appeared when girdling was restricted to branches. The entire plant collapsed when girdling of the crown and roots occurred. Superficial white mycelium developed over the basal part of affected stems under moist conditions, especially after rainy periods. Black, round to oblong sclerotia, 3.5 to 10.1 mm in size, formed externally on the crown areas after plant death. Sclerotinia sclerotiorum (Lib.) de Bary (1) was consistently isolated from the diseased plants. To confirm pathogenicity, 4-week-old stevia seedlings were obtained from shoot cuttings and grown in 12-cm pots of soilless mix. Sclerotia produced on potato dextrose agar were inserted into the mix 0.5 cm deep and 0.5 cm from the stems of test plants. Plants were placed in a growth chamber at 22°C with a 12-h photoperiod and 95% relative humidity. Two weeks after soil infestation, plants wilted and S. sclerotiorum was reisolated from the diseased crown tissues. This is the first report on stevia of sclerotinia stem rot, a disease that could significantly reduce foliar growth and stevioside production in field plantings. References: (1) L. H. Purdy. Phytopathology 69:875, 1979. (2) T. Robinson. 1991. The Organic Constituents of Higher Plants: Their Chemistry and Interrelationships. 6th ed. Cordus Press, North Amherst, MA.