Identification, Characterization, Pathogenicity, and Host Range of Slafractonia indigofera sp. nov. (Dothidotthiaceae) Causing Leaf Spot Disease of Indigofera pseudotinctoria in China.

Abstract

Indigofera pseudotinctoria is an economically important perennial shrub in southwestern China. In recent years, leaf spot disease on I. pseudotinctoria has been frequently observed in commercial fields and the forage germplasm repository in Shuanghe County, Rongchang District, Chongqing Municipality, China. The typical symptoms, primarily on leaflets, appeared as sunken, necrotic, oval, circular, or elongated dark brown lesions, with an average incidence of 37.46% in 2023 and 47.81% in 2024, respectively. Three representative isolates were obtained and identified based on morphological characteristics and phylogenetic analysis of the combined LSU, SSU, and ITS sequences. The results revealed that Slafractonia is a new member of the family Dothidotthiaceae, and the isolates belong to a novel species closely related to Slafractonia leguminicola. Morphologically, the species is characterized by branched hyphae with septa and a lack of conidia, significantly differing from S. leguminicola in growth rate, hyphal size, the presence of sclerotia or microsclerotia, and spherule-like structures. Pathogenicity tests were confirmed by inoculating representative isolates on living leaves from seedlings and developing plants, as well as on detached leaves. Pathogenicity tests on four locally grown perennial legumes suggest a broad host range for the pathogen, not restricted to I. pseudotinctoria alone. As a result of the phylogenetic, morphological, GCPSR, and pathogenicity analyses, we propose Slafractonia indigofera as the name for this pathogen of I. pseudotinctoria in Chongqing, China.