First report of powdery mildew caused by Podosphaera phaseoli (syn. Sphaerotheca phaseoli) on cowpea (Vigna sinensis L.) in Turkey

During the summer of 2003, typical symptoms of powdery mildew were observed in many cowpea fields assessed in Hatay Province, Turkey. White, epiphytic mycelia and conidia, characteristic of a powdery mildew, were present on leaves, stems and inflorescences. The plant tissue underneath the mycelial patches was purplish in colour (Fig.1).

Mycelial growth was amphigenous, thick, forming irregular white patches, sometimes effused to cover the whole leaf surface and had poorly developed nipple-shaped single appressorium. Simple straight conidiophores (115-190 x 10-13 µm) developed mostly singly from a hyphal cell, arising from the upper part of mother cells, having the basal septum at the branching point of the mycelium with a sharp constriction. Each conidiophore had three to eight barrel-shaped conidia formed in chain (Fig. 2A). Conidia with fibrosin bodies were 28-42 x 15-18 µm in size and germinated below the shoulder by producing a simple germ tube. Dark brown ascomata, found on leaves and stems as embedded in the mycelial felt, were spherical, gregarious to subscattered and measured 85 to 105 µm in diameter (Fig. 2B). Appendages (6 to15) were myceloid arising from the lower half of the ascomata, brown, paler upward and 6 to 8 µm wide. The ascomata contained single ascus (65-95x 55-67 µm). The ascus contained 8 ellipsoidal to ovoid ascospores (18-24 x 12-16 µm). On the basis of morphological characters of the conidial stage and teleomorph, the fungus was identified as Podosphaera phaseoli (syn. Sphaerotheca phaseoli) (Braun & Takamatsu, 2000; Shin, 2000).

Pathogenicity tests were conducted on 4 week-old cowpea plants, cv. Libye, by shaking fresh conidia from naturally infected plants onto the healthy leaves. Inoculated plants were kept in a moist chamber (100% RH) for 2 days and then maintained a growth chamber at 22 ± 2°C, 75% RH with a 16 h photoperiod. After 7 to 10 days, inoculated plants developed powdery mildew symptoms, which were similar to those observed on naturally infected plants.

This is the first report of powdery mildew on cowpea in Turkey. Previous reports list P. phaseoli on Vigna spp. in Korea (Shin & La, 1992; Lee et al., 2002). P. phaseoli was also reported on several related host plants such as Phaseolus spp. and Rhynchosia volubilis (Shin, 2000). Although bean is the one of the alternative host of the disease agent and grown in nearby cowpea plants in the same field, no disease symptoms was observed on bean plants.