First Report of Powdery Mildew of Greenhouse-grown Tomatoes in Alberta, Canada.

Abstract

In Alberta, powdery mildew disease of greenhouse-grown tomatoes (Lycopersicon esculentum Mill.) appeared for the first time in 1995 as circular white colonies on leaves, petioles, and stems. Since then it has been found every year and is becoming an economically important disease of tomatoes. This is the first report of the disease from Alberta, Canada. In Canada, powdery mildew on greenhouse tomatoes was previously reported in 1994 from the province of Quebec (1). The pathogen had unbranched conidiophores with average length and width of 62.3 μm and 8.5 μm, respectively. Conidia were clear or hyaline, elliptical to oval in shape, and were borne singly or in short chains. Average length and width of conidia were 36.0 μm and 17.6 μm, respectively. The conidia contained numerous vacuoles but fibrosin bodies were not observed. Germ tubes were straight and formed at the ends or very close to the ends of conidia. Rarely, a conidium produced two germ tubes. Cleistothecia were not found. Six-week-old, greenhouse-grown, healthy tomato cv. Trust plants were inoculated by shaking conidia from powdery mildew-infected plants onto the leaves of the healthy plants. The plants developed powdery mildew symptoms within 9 days after the inoculation. The symptoms on inoculated plants and morphological characteristics of the pathogen were similar to those for naturally infected plants. Based on the characteristics of the asexual stage, the pathogen was identified as an Erysiphe sp. until its identity is confirmed by the characteristics of its sexual stage. An Erysiphe sp. has also been reported to cause powdery mildew of greenhouse-grown tomatoes in Canada (1), the U.S. (3), and Spain (2). Optimal temperature and relative humidity for germination of conidia of the pathogen were 20 to 25°C and >90%, respectively. Myclobutanil, fenarimol, sulfur, triademefon, and triforine showed promise for effective management of this disease. Myclobutanil and sulfur are now registered for control of this disease in Canada. Since cleistothecium has not been found, there is a need to identify the sources of primary inoculum to understand the disease cycle and effective management of the disease.