First record of Erysiphe diffusa causing powdery mildew on Bergera koenigii in the USA

Abstract

Bergera koenigii L., commonly known as the curry tree, belongs to the family Rutaceae (Singh et al., 2014). Its leaves are used as a spice for flavor in Indian cuisine and are a good source of vitamins and antioxidants (Rao et al., 2011). In March 2023, powdery mildew was observed on the young leaves of a potted B. koenigii plant in Raleigh, North Carolina (Fig. 1). The incidence of the disease on the plant was 30–40%; the mycelium mostly present on the adaxial surface. Hyphal appressoria were lobed, the foot cells of the conidiophores ranged from 23 to 60 μm in length. Hyaline conidia were formed singly, ellipsoid to cylindrical, without fibrosin bodies. The size of the conidia (n = 40) ranged from 14–38 × 11–18 μm, germ tubes were terminal. Mature chasmothecia were dark brown, globose, 92–100 μm in diameter, with most appendages unbranched, except one appendage with loosely dichotomously branched apex, tips of the ultimate branchlets straight. Each chasmothecium contained up to six asci that were broad ellipsoid-obovoid, with a short stalk. Each ascus contained four to six ellipsoid to ovoid amber-coloured ascospores (17-23 × 5–9 μm) (Fig. 2). Morphological features were consistent with the genus Erysiphe (Braun & Cook 2012). A few infected compound leaves were deposited in the herbarium at North Carolina State University (Catalog No. 000024426).