First report of powdery mildew caused by Erysiphe heraclei DC. on dill (Anethum graveolens L.) in Turkey

During annual dill (Anethum graveolens L.) disease surveys in September 2001 and May 2002, sporadic symptoms typical of powdery mildew were observed in many fields assessed in the eastern Mediterranean coastal regions of Hatay Province, Turkey. White, epiphytic mycelia and conidia characteristic of a powdery mildew, were present on leaves, stems and inflorescences. Severely diseased leaves became twisted, and the disease possibly contributed to premature senescence and drying of the plant.

Mycelial growth was amphigenous, forming thin, irregular white patches, sometimes effused to cover the whole leaf surface, stem and inflorescences, and had well developed multilobed appressoria. Simple straight conidiophores developed mostly singly from a hyphal cell, arising from the upper part of mother cells. Cylindrical foot cells (30-35 x 7.5-9 µm) were followed by one or two shorter cells (17.5-22.5 x 7.5-9 µm). Cylindric oval to fusiform conidia (30-38 × 12-15 µm) were produced singly without conspicuous fibrosin bodies. The length to width ratios of conidia ranged from 1.9 to 3. Conidia germinated at the ends by producing 4-5 germ tubes. Germ tubes formed lobed appressoria. The cleistothecia were found on leaves, stems and inflorescences and were spherical, gregarious and measured 80 to 125.5 µm in diameter. Appendages were myceloid with branched tips, brown to black and 88 to 145 µm in length and 4 to 7µm in diameter. Each cleistothecium contained 4-6 round to ovoid asci (45-62.5 x 25-37.5 µm). The ascus contained 3-5 ellipsoid to ovoid ascospores (22.5-25 x 10-15 µm).

Based on the morphological characteristics, the causal agent of dill powdery mildew was identified as Erysiphe heraclei DC. (Shin, 2000). Pathogenicity tests were conducted on 2 month-old greenhouse grown dill plants (Anethum graveolens variety 'Bereketli'). These plants were inoculated by shaking fresh conidia from naturally infected plants having abundant sporulation onto the leaves of healthy plants. Inoculated plants were kept in a moist chamber (100% RH) at 22°C for 2 days, and then moved into a growth chamber at 22 ± 2°C / 16 ± 2°C (day/night temperatures), 75% RH with a 16 h photoperiod. Inoculated plants developed powdery mildew symptoms 12 to 15 days after inoculation. The symptoms on inoculated plants and the morphological characteristics of the pathogen were similar to those observed on naturally infected plants. No symptoms developed on control plants. This is the first report of powdery mildew on dill in Turkey and also the first record of E. heraclei in this country. Previous reports list E. heraclei on dill in Iran (Sharifnabi & Nekoei, 1997), India (Upadhyaya & Gupta, 1992) and Taiwan (Tsay, 1990). E. heraclei was reported on several host plants belonging to Umbelliferae (= Apiaceae) such as parsley, celery, carrot and fennel (Shin, 2000).