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First Report of Powdery Mildew of Greenhouse Pepper Caused by Leveillula taurica in British Columbia, Canada

In 2003, powdery mildew of greenhouse pepper (Capsicum annuum L.) was reported simultaneously in two commercial greenhouses at two separate locations near Langley in southern British Columbia. Trace amounts of mildew on the foliage of cv. Zamboni in one operation, and 10 to 80% of the foliage of cv. Triple Four was affected in an organic operation, with losses in fruit yield of 2 to 4 kg/m², were noted. The greenhouse pepper industry in British Columbia in 2001 comprised 76 ha with sales of $41.3 million (Canadian). Affected areas appeared as a white, powdery coating on the abaxial leaf surface of older foliage while diffuse chlorotic spots were present at corresponding locations on the adaxial surface. In some cases, pale yellow spots appeared on the adaxial portion of the affected foliage during later stages of disease development (2). Hyaline, single-celled pyriform and cylindrical conidia were present in 40:60 proportion, respectively. Both conidia had a network of crests and granules sometimes with tiny thorn-like projections on the surface. These projections were evident also on the lower half of conidiophores. Pyriform conidia ranged in length from 53.8 to 79.5 μm (mean = 68.0, SE = 0.8), and in width from 12.9 to 28.0 μm (mean = 20.1, SE = 0.6, n = 50) with a mean length to width ratio of 3.5. Cylindrical conidia had a length from 48.2 to 84.0 μm (mean = 66.0, SE = 0.9), and in width from 13.4 to 25.2 μm (mean = 18.6, SE = 0.4, n = 50) with a mean length to width ratio of 3.6. Short conidial chains borne on conidiophores consisted of a basal pyriform conidium followed by cylindrical conidium. No cleistothecia were observed. Stomatal penetration and extensive endophytic mycelial growth in the mesophyll layer were evident in foliar tissue cleared with glacial acetic acid/EtOH (1:2) and stained with lactophenol cotton blue. To confirm pathogenicity, a suspension of 2 × 10⁴ conidia/ml from infected pepper leaves was applied onto the abaxial and adaxial leaf surfaces of 60-, 42-, and 28-day-old greenhouse pepper cv. Cubico, tomato cv. Trust, and cucumber cv. Corona, respectively. Plants were maintained in a growth chamber at 25/21°C day/night temperature and 80% relative humidity. Control plants in the same chamber were treated identically but not inoculated. After 21 days, inoculated pepper plants developed foliar powdery mildew symptoms on the abaxial surface, including sporulation similar to that of naturally infected plants. Occurrence of the fungus on the abaxial surface of pepper foliage, presence of endophytic mycelium, and the morphological characteristics of the imperfect state confirm the identity of the fungus (2). To our knowledge, this is the first report of Leveillula taurica (Lév.) G. Arnaud on pepper in British Columbia. The disease was first reported in Ontario, Canada in 1999 (1).

References: (1) R. Cerkauskas et al. Plant Dis. 83:781,1999. (2) J. Palti. Bot. Rev. 54:423, 1988.