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

Abstract

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.