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Background   Ipomoea cairica  (L.) Sweet is a destructive invasive weed in South China but rarely infected with pathogens in nature. Its pathogen resistance mechanism is largely unknown at present. Some non-pathogenic isolates of  Fusarium oxysporum  and  Fusarium fujikuroi  are prevalent on many plant species and function as pathogen resistance inducers of host plants. The objective of the present research is to investigate whether the symbiosis between the both fungi and  I. cairica  is present, and thereby induces pathogen resistance of  I. cairica.Methods  Through field investigation, we explored the occurrence rates of  F. oxysporum  and  F. fujikuroi  on leaf surfaces of  I. cairica  plants in natural habitats and compared their abundance between healthy leaves and leaves infected with  Colletotrichum gloeosporioides , a natural pathogen. With artificial inoculation, we assessed their pathogenicity to  I. cairica  and studied their contribution of pathogen resistance to  I. cairica  against  C. gloeosporioides.Results  We found that  F. oxysporum  and  F. fujikuroi  were widely epiphytic on healthy leaf surfaces of  I. cairica  in sunny non-saline ,  shady non-saline and sunny saline habitats. Their occurrence rates reached up to 100%. Moreover, we found that the abundance of  F. oxysporum  and  F. fujikuroi  on leaves infected with  C. gloeosporioides  were significantly lower than that of healthy leaves. With artificial inoculation, we empirically confirmed that  F. oxysporum  and  F. fujikuroi  were non-pathogenic to  I. cairica . It was interesting that colonization by  F. fujikuroi ,  F. oxysporum  alone and a mixture of both fungi resulted in a reduction of  C. gloeosporioides  infection to  I. cairica  accompanied by lower lesion area to leaf surface area ratio, increased hydrogen peroxide (H 2 O 2 ) concentration and salicylic acid (SA) level relative to the control. However,  NPR1  expression, chitinase and β-1,3-glucanase activities as well as stem length and biomass of  I. cairica  plant only could be significantly improved by  F. oxysporum  and a mixture of both fungi but not by  F. fujikuroi . In addition, as compared to colonization by  F. oxysporum  and a mixture of both fungi,  F. fujikuroi  induced significantly higher jasmonic acid (JA) level but significantly lower β-1,3-glucanase activity in leaves of  I. cairica  plants. Thus, our findings indicated the symbiosis of epiphytic fungi   F. fujikuroi  and  F. oxysporum  induced systemic resistance of  I. cairica  against  C. gloeosporioides .  F. oxysporum  played a dominant role in inducing pathogen resistance of  I. cairica . Its presence alleviated the antagonism of the JA signaling on SA-dependent β-1,3-glucanase activity and enabled  I. cairica  plants to maintain relatively higher level of resistance against  C. gloeosporioides.

Epiphytic fungi induced pathogen resistance of invasive plant Ipomoea cairica against Colletotrichum gloeosporioides