Colletotrichum species can function as plant pathogens, saprobes or endophytes on a wide variety of plant hosts and are considered amongst the ten most significant genera of plant pathogens globally. China contributes almost half the walnut production in the world. However, Colletotrichum species occurring on walnut remain largely unresolved in China. To explore the Colletotrichum species found on walnut in China, 470 walnut fruit or leaf samples with anthracnose were collected from 14 main walnut-producing regions across seven provinces. A total of 165 Colletotrichum strains were isolated from these samples. The Colletotrichum isolates were identified, based on morphological characteristics and sequence analyses of ACT, CHS-1, GAPDH, ITS and TUB2. Twelve species, including 11 known Colletotrichum species (C. boninense, C. citrulli, C. fioriniae, C. fructicola, C. godetiae, C. juglandicola, C. karsti, C. mengyinense, C. pandanicola, C. peakense and C. siamense) and a novel species (C. chinensissp. nov.) were identified. The species distribution revealed regional prevalence as follows: C. mengyinense was the most dominant species in Gansu, C. mengyinense and C. siamense in Shandong, C. chinensis in Beijing, C. pandanicola in Shaanxi and C. godetiae in Yunnan. Colletotrichum siamense was the sole species isolated in Sichuan and Xinjiang Provinces. Koch’s postulates were fulfilled, demonstrating that all 12 species cause anthracnose on walnut. This is the first report of C. boninense, C. citrulli and C. karsti as pathogens of walnut anthracnose worldwide.
