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Endophyte-mediated disease suppression induced by application of metalaxyl-m to potato foliage

A 2-year field study was undertaken to examine the effect of the systemic fungicide metalaxyl-m (the R enantiomer of metalaxyl) on endophytic populations of bacteria in tubers of potatoes (Solanum tuberosum) 'Russet Burbank'. The frequency (population density) of bacterial endophytes with antibiosis activity against Phytophthora erythroseptica and Fusarium avenaceum was not independent of fungicide regime, according to Fisher's exact test (two-tailed; P = 0.0032). Significantly more strains with antibiosis activity (in vitro) were recovered from the tubers of plants treated with metalaxyl-m. A weak, but statistically significant (P = 0.01) increase in the strength of antibiosis activity of tuber bacterial endophytes against selected phytopathogens occurred in plants that received foliar fungicide spray programmes incorporating metalaxyl-m (Ridomil Gold® MZ 68 WP (wettable powder)) in addition to chlorothalonil (Bravo® 500), as compared with plants that received Bravo 500 applications only. Disease development was significantly (P = 0.01) reduced in tubers from plants receiving metalaxyl-m treatments, following tuber inoculation with P. erythroseptica or F. avenaceum. Indices for evenness of distribution and for diversity of bacterial species recovered from potato tubers were significantly higher (P = 0.05) in plants receiving metalaxyl-m treatments. It would appear that, in stimulating a more biodiverse endophyte community, metalaxyl-m may be inducing a form of defensive mutualism amongst bacterial endophytes (a chemically induced endophyte-mediated disease suppression), thereby reducing phytopathogen colonization, including that of the fusaria, against which metalaxyl-m has no direct fungicidal activity.