Leaf Blight of Aucuba japonica Var. Variegata (Spotted Laurel) Caused by Neocalonectria aucuba sp. Nov. in China

Abstract

Aucuba japonica ‘variegata’ commonly known as spotted laurel, is a dioecious evergreen shrub widely used in cities in south China. Leaf blight on spotted laurel has recently been observed in Nanchang, Jiangxi Province, with incidence rates ranging from 20% to 75% in landscape belts. Symptoms initially appeared as black lesions on the leaf tips; lesions then expanded and coalesced, progressing to the stems and causing severe foliar necrosis, dieback, and eventual plant death. A fungus was consistently isolated from the margins of advancing lesions and preliminarily identified as a species of Neocalonectria based on colony and conidial morphology. Pathogenicity was confirmed by fulfiling Koch's postulates, whereby three representative isolates induced identical symptoms on inoculated, healthy A. japonica ‘Variegata’ plants. The conidia are one septate, straight to slightly curved, and measured 31.3–47.5 × 2.5–2.9 μm (average 38.7 × 2.6 μm), narrower than those of the type of Neocalonectria, N. tristaniopsidis (39–46 × 3–3.5 μm). Phylogenetic analysis of five nuclear loci confirmed the three isolates as a novel species, Neocalonectria aucuba sp. nov., closely related to N. tristaniopsidis, the causal agents of leaf blight on Aucuba japonica.