First Report of Twig Dieback on Poncirus trifoliata Caused by Diaporthe citriasiana , with Descriptions of Two New Species and a New Record of Phaeobotryon rhois in Korea

Poncirus trifoliata (trifoliate orange) is widely used in citrus breeding and as a rootstock. Although its horticultural and physiological traits have been studied extensively, research on diseases affecting this species and the fungal community associated with it remains scarce. Twig dieback symptoms were observed on P. trifoliata trees cultivated in Korea, representing the first report of such symptoms in this species. To investigate the fungal community associated with the diseased tissues, symptomatic twigs were collected, and isolates were obtained and subjected to morphological, molecular, and phylogenetic analyses. Multilocus phylogenetic analyses based on ITS, LSU, TUB2, and TEF1-α sequences identified four distinct fungal taxa: Diaporthe citriasiana, Basidiodesertica ponciri sp. nov., Sardiniella ponciri, and Phaeobotryon rhois. Among them, B. ponciri and S. ponciri are described here as new species. In addition, D. citriasiana and P. rhois are newly recorded in Korea. Pathogenicity tests revealed that D. citriasiana induced severe necrotic lesions on inoculated P. trifoliata branches, fulfilling Koch’s postulates, whereas S. ponciri caused only mild symptoms, and the other species were nonpathogenic under the tested conditions. Enzymatic activity assays demonstrated distinct extracellular enzyme profiles among the species, although no clear correlation with pathogenicity was observed under the tested conditions. This study provides the first record of twig dieback symptoms on P. trifoliata in Korea and expands the known fungal diversity associated with this host. It also provides detailed morphological and molecular characteristics of the newly identified taxa, two new species and two newly recorded species in Korea.