Isolation, identification and pathogenicity of two root rot pathogens Fusarium solani in citrus

Root rot is a prevalent soil-borne fungal disease in citrus. Citron C-05 (Citrus medica) stands out as a germplasm within Citrus spp. due to its complete resistance to citrus canker and favorable characteristics such as single embryo and easy rooting. However, Citron C-05 was found to be highly susceptible to root rot during cultivation, with the specific pathogens previously unknown. In this study, four candidate fungal species were isolated from Citron C-05 roots. Sequence analysis of ITS, EF-1α, RPB1, and RPB2 identified two Fusarium solani strains, Rr-2 and Rr-4, as the candidates causing root rot in Citron C-05. Resistance tests showed these two pathogens increased root damage rate from 10.30% to 35.69% in Citron C-05, sour orange (Citrus aurantium), sweet orange (Citrus sinensis) and pummelo (Citrus grandis). F. solani exhibited the weak pathogenicity towards trifoliate orange (Poncirus trifoliata). DAB staining revealed none of reddish-brown precipitation in the four susceptible citrus germplasm after infection with F. solani, while trifoliate orange exhibited significant H₂O₂ accumulation. Trypan blue staining indicated increased cell death in the four susceptible citrus germplasm following infection with these two pathogens but not in trifoliate orange. These findings provide a comprehensive understanding of citrus root rot and support future research on the mechanisms of root rot resistance in citrus.