E. J. M.
Forest Pathology Laboratory, Kenya Forestry Research Institute (KEFRI), Box 20412, Nairobi, Kenya
CABI Bioscience Centre Kenya, Africa Regional Centre, Box 633, Village Mkt., Nairobi, Kenya
Prunus africana, formerly known as Pygeum africanum, is widely distributed in moist, tropical Africa and produces durable timber. Extracts from its bark are used in treatment of prostate disorders. Powdery mildew was observed on nursery-grown seedlings of P. africana in Kenya (Nyeri, Kiambu, and Kericho districts) in the dry seasons of 1998, 1999, and 2000. White ectotrophic mycelial growth was observed on leaves. The fungus caused stunting, distortion of leaves, surface necrosis of invaded tissues, and general decline in growth of seedlings that led to premature leaf fall and death. Invaded leaflets wilted and dropped, leaving behind a bare stem. The primary mycelium was hyaline, with no secondary brown mycelium. The conidial state was conspicuous, with conidia produced in chains. Appressoria were unlobed and nipple shaped. Conidiophores were straight and three-celled, measuring 75 to 112 μm. Conidiophore foot cells were topped by a longer cell and one or two shorter cells measuring 35 to 77 μm. Conidia had fibrosin bodies, were ovoid, and measured 27 to 30 × 17 to 18 μm. The fungus was identified by the International Mycological Institute IMI (W6496) as Podosphaera leucotricha (Ellis & Everh.) E. S. Salmon. Infected leaves of P. africana were deposited at the East African Herbarium, National Museums of Kenya (Accession No. KM-KEFRI/446/2001). Pathogenicity was confirmed by inoculating seedlings of P. africana by gently pressing infected leaves with abundant sporulation onto healthy leaves. The plants were then incubated under moist conditions for 48 h and thereafter maintained in a glasshouse. After 15 days, powdery mildew symptoms developed on seedlings. Examination of leaves confirmed that they were infected with Podosphaera leucotricha. Uninoculated control plants were free of powdery mildew. To our knowlege, this is the first report of Podosphaera leucotricha as a pathogen of P. africana.
Reference: 1. H. J. Boesewinkel. Bot. Rev. 46:167, 1980.