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First Confirmation of Sphaerotheca macularis on Strawberry Plants in Southwestern Spain

In February 2001, in seven small tunnel production fields in southwestern Spain, symptoms and signs of powdery mildew were detected on strawberry (Fragaria × ananassa) cvs. Camarosa, Andana, and Carisma. On ‘Camarosa’ plants, white patches of mycelium developed on the lower leaf surface. Later, the leaf edges rolled upward, and reddish or necrotic irregular spots appeared on the lower leaf surface. Some immature fruits were abnormally hard and covered with a dense white mycelium. Symptoms were much less severe on ‘Andana’ and ‘Carisma’. Microscopic observations revealed conidia in chains, ellipsoidal to barrel-shaped, with fibrosin granules. Mean dimensions of conidia (N= 70) were 31.2 μm ± 2.8 × 20.39 μm ± 4.1. Cleistothecia were not detected. Based on these characteristics, the causal agent was identified as Sphaerotheca macularis (Wallr.:Fr.) Lind. (1). Pathogenicity was confirmed by inoculating leaves of 2-month-old potted ‘Camarosa’ plants. S. macularis obtained from field-grown plants was maintained on ‘Camarosa’ strawberry leaves in a growth chamber (20°C and 100% relative humidity). Inoculum was obtained from freshly sporulating infected leaves 11 days after inoculation. Conidia of S. macularis were gently brushed into 100-ml distilled water containing two drops of Tween 20. Ten strawberry ‘Camarosa’ plants were inoculated by misting with a conidial suspension (10⁴ conidia per ml) (2). The plants were incubated at 100% relative humidity for 48 h in the greenhouse (15 to 25°C). Colonies of powdery mildew were observed on leaves after 18 days. To our knowledge, this is the first confirmation of S. macularis on strawberry in Spain.

References: (1) K. G. Mukerji. No. 188 in: Descriptions of Pathogenic Fungi and Bacteria. CMI, Kew, UK, 1968. (2) R. Reuveni et al. Crop Prot. 19:335, 2000.