First Report of Powdery Mildew on Spiraea japonica Caused by Podosphaera spiraeae in the United States

Abstract

Spiraea japonica is a perennial ornamental plant that belongs to the family Rosaceae. It is grown as an ornamental plant throughout the USA. In the summer of 2017, Japanese Spiraea plants growing at Washington State University Puyallup Research and Extension Center (47.1882oN, 122.3290oW) showed signs of powdery mildew. Signs of powdery mildew include dense patches of white to greyish mycelia on leaf surfaces. The fungus formed superficial branched hyphae with septa. The foot cells of the conidiophore were straight followed by one to three short cells bearing conidia in short chains. The size of the foot cell ranged from 55 to 105 μm with an average of 93.76 μm (n=30). Appressorium was nipple shaped and the germ tubes of the conidia were terminal. Conidia were formed in chains of up to 5-7 hyaline, broad ellipsoidal spores which measured 25 × 15 (21 to 28 × 12 to 19) μm. Chasmothecia did not form during the time of observation (July- October of 2017). These features were consistent with the description of...