First Report of Powdery Mildew on Mulberry Caused by Phyllactinia guttata in the Eastern Mediterranean Region of Turkey

Mulberry (Morus spp.) is an increasingly important crop that is widely planted in the Hatay province, Turkey as a food source for silkworms. Production of silk products is the main income in this province. Serious damage caused by powdery mildew was discovered in the main mulberry production areas of Samandag and Harbiye regions in 2000. Disease incidence was 81-95% in infested orchards. Symptoms included white superficial mycelium with abundant sporulation on the lower leaf surfaces and chlorotic spots on the upper leaf surfaces. Mycelial growth was observed on leaves, petioles, and pedicels but not on fruit and branches. Symptoms were apparent on each plant, with disease severity ranging from a few individual spots to numerous lesions nearly covering entire leaves. Sporulating fungal structures were dissected from leaves and examined microscopically for morphological characters. The white mycelium had lobed appressoria. Conidiophores were straight and cylindrical. Single-celled, club-shaped conidia occurred usually singly on unbranched conidiophores. Conidia ranged in length from 55 to 85 m m (average 68.25 ± 2.88µm) and from 15 to 20 µm in width (average 16.65 ± 0.7µm). Cleistothecia with tapering appendages and a bulbous base were yellow when young and turned black at maturity. Each cleistothecium contained 9 to 13 unitunicate asci, broadly oval to ellipsoid, short stalked, and containing two ascospores. The size of cleistothecia ranged from 225 to 255 µm (average 235 ± 3.27µm) in diameter. Asci were 68 to 95 µm long (average 78.5 ± 2.1µm) and 35 to 40 µm wide (average 37.5 ± 0.9µm). Ascospores were 35 to 45 µm long (average 40 ± 1.29 µm) and 12.5 to 17.5 µm wide (average 16 ± 0.7µm). The length of appendages on cleistothecia ranged from 180 to 285 µm (average 242.6 ± 6.7µm).

Pathogenicity tests were conducted on one-year-old healthy mulberry seedlings. These plants were inoculated by shaking conidia from powdery mildew infected leaves onto the leaves of the healthy plants. Plants were incubated in a growth chamber at 26 ± 2°C with a 16 hr photoperiod. The plants developed powdery mildew symptoms within 9 days after 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 sexual and asexual states and host specialization, the fungus was identified as Phyllactinia guttata (Hanlin, 1992; Webster, 1979). We believe this is the first report of powdery mildew on mulberry in the Eastern Mediterranean region of Turkey. A combination of high inoculum pressure, low humidity, and an unusually hot and dry summer during 2000 favored fungal infection and disease development. The disease is becoming important because severe foliar lesions render plants unacceptable for silkworm production.