Interactions between nematode and fungal pathogens of the citrus fibrous root cortex

The most commonly encountered association between nematodes and fungi in citrus occurs between the citrus nematode Tylenchulus semipentrans Cobb and the root rot fungus Phytophthora nicotianae(= parasitica) Dastur Breda de Hann. The fibrous root system of citrus trees in Florida and worldwide is commonly infected by both parasites. The efficacy of nematicides to manage T. semipenetrans in field experiments was directly related to the population density of P. nicotianae , implying significant competition between these two parasites. We initiated a study of this hypothesis by testing whether infection of root segments by the citrus nematode impeded root infection by P. nicotianae in vitro, and by determining the effect of the nematode on growth and pathogenicity of the fungus in whole plant experiments. Plants were infected by the fungus, the nematode, both organisms or neither organism. Infection of roots by T. semipenetrans reduced subsequent infection by P. nicotianae and damage to the plant. Citrus seedlings infected by both organisms grew larger and contained less fungal protein in the root tissues than did plants infected by only the fungus thus demonstrating antagonism of the nematode to the fungus.; Two hypotheses were investigated to explain the mechanism of the interaction between these pathogens: (1) indirect mediation through increased colonization of nematode feeding sites by microorganisms antagonistic to P. nicotianae , and (2) direct antibiosis by the nematode. To test the first hypothesis, a field survey was initiated to determine whether infection by T. semipenetrans changes the composition of rhizosphere inhabiting microorganisms, and to identify microorganisms that are consistently associated with the nematode. Results showed that T. semipenetrans altered the microbial community in the citrus rhizosphere by increasing propagule densities of bacteria and fungi. The dominant bacterial species isolated were Bacillus megaterium and Burkholderia cepacia. Both bacteria were used in whole plant factorial experiments with P. nicotianae and T. semipenetrans either alone or in combination. Neither bacterium inhibited growth of the fungus when inoculated alone. Nevertheless, the nematode and both bacteria increased the growth of citrus seedlings infected by P. nicotianae.; To test the second hypothesis, in vitro bioassays were conducted to determine the effects of eggs of two nematodes T. semipenetrans and Meloidogyne arenaria on mycelial growth of P. nicotianae and Fusarium solani. Tylenchulus semipenetrans eggs suppressed mycelial growth of P. nicotianae and F. solani in vitro, but M. arenaria eggs had no comparable effect on either fungus. This research showed an antagonistic effect of a plant parasitic nematode on a plant pathogenic fungus and shows potential mechanisms involving direct inhibition of the fungus by the nematode and indirect mitigation of fungal virulence mediated by complex microbial interactions in the citrus rhizosphere.