First report of powdery mildew caused by Leveillula taurica on leek (Allium porrum L.) in Turkey

In the winter of 2002, typical symptoms of powdery mildew were observed on leek plants (Allium porrum L.) in the Arsuz district, located in the Mediterranean region of Turkey. Initially symptoms consisted of the extensive growth of a white, superficial powdery coating on the abaxial leaf surface. Newly infected leaves had a sparse covering of powdery mildew. As the disease progressed, white mycelial growth completely covered both leaf surfaces. Three weeks after initial observation, 71.7% of leeks were affected.

The fungus produced mostly simple, straight, septate conidiophores (300-400 x 5-7 µm) arising from endophytic mycelium through stomata (Figure 1A and B). Hyaline, single-celled primary and secondary conidia were borne on conidiophores. Primary conidia were lanceolate with distinct apical points, 57.3-79.6 µm x 14.2-18.5 µm.

Secondary conidia were cylindric to long, obovate with rounded ends, 55.9-80.3 µm x 12.0-15.3 µm (Figure 2A). Globose, spherical, dark brown chasmothecia were scattered to occasionally gregarious, 160-210 µm in diameter, immersed in a dense mass of mycelium (Figures 1B and 2B). Unbranched appendages were light to dark brown, (4 to 7µm in diameter). Each chasmothecium contained 12 to 45 two-spored, stalked asci (75-98 x 18-32 µm). Ascospores were ellipsoid to ovoid, 37-40 x 15-18 µm in diameter.

Pathogenicity tests were performed by dusting conidia from naturally-infected leek leaves onto water-misted abaxial leaf surface of 40 day old healthy leek seedlings (cv. Uzun Bacak). Plants were enclosed in polyethylene bags for 2 days at 25°C and 80% relative humidity (RH), before being moved into a growth chamber at 25 ± 2°C, 75% RH with a 16 h photoperiod. Symptoms identical to that of naturally infected plants became visible 3-4 weeks after inoculation. No symptoms developed on control plants.

Based on the occurrence of the disease, the presence of endophytic mycelium, characteristics of asexual and teleomorph stages (Braun, 1987; Palti, 1988), the pathogen was identified as Leveillula taurica (Lév.) Arnaud [anamorph Oidiopsis taurica (Lév.) Salmon]. To our knowledge, this is the first report of Leveillula taurica on leek in Turkey. Powdery mildew infection on leeks and several other species including onion, garlic, carrot and fennel have been previously reported in northern Italy and Korea (Gullino et al., 1989; Shin, 2000).