To evaluate whether the location of bacterial endophyte communities contributes to disease resistance in potato tubers (Solanum tuberosum), the population density, biodiversity and antibiotic activity of endophytic bacteria was examined from the tuber peel (periderm plus top 3 mm of tissue) of four cultivars (Russet Burbank, Kennebec, Butte and Shepody). There were no significant differences for population density of bacteria among the layers of peel examined and no cultivar × peel layer interaction. Endophytic bacteria from several layers of peel were challenged in in vitro bioassays to the soil-borne plant pathogens Fusarium sambucinum, Fusarium avenaceum, Fusarium oxysporum and Phytophthora infestans (mating types A1 and A2). In general, antibiosis of bacterial endophytes against these pathogens was significantly higher (P ¼ 0·01) in isolates recovered from the outermost layer of tuber peel and decreased progressively toward the centre of the tuber. Antibiosis against P. infestans was variable, with a progressive decrease in antibiotic activity from outer to inner layers of peel occurring in cvs Russet Burbank and Kennebec only. For antibiosis there were significant cultivar × peel, and cultivar × pathogen interactions (P ¼ 0·01). In all cases the inhibitory activity of endophytic bacteria was significantly greater (P ¼ 0·01) against the A1 than the A2 mating type of P. infestans. In four of seven cases, where the same species of bacteria were recovered from all three peel layers, antibiosis to pathogens decreased significantly (P ¼ 0·01) with depth of recovery (from the periderm to inside the tuber), indicating that in certain communities of endophytic bacteria, defence against pathogens may be related to bacterial adaptation to location within a host and may be tissue-type and tissue-site specific.

A 2‐year study investigated the relationship between the nematicides aldicarb and fosthiazate and populations of plant growth‐promoting (PGP) and plant‐growth‐inhibiting (PGI) bacteria, and root‐lesion nematodes and bacteria‐feeding nematodes in the root‐zone soils of potatoes. Fewer (P < 0.05) bacterial genera and species were recovered from aldicarb‐treated soils than from the fosthiazate and untreated soils but bacterial population densities were greatest in the aldicarb‐treated soils. In potato plantlet bioassays using root‐zone bacteria from the three soil sources, bacteria from the aldicarb‐treated soils reduced (P < 0.01) the mean wet and dry weight of shoots and roots compared with those from the fosthiazate and untreated soils. In the field, fosthiazate (but not aldicarb) significantly increased tuber yields and reduced nematode population densities of Pratylenchus penetrans in roots and soil. Population densities of bacteria‐feeding nematodes (primarily Diplogaster lheritieri) were not affected by either nematicide. Aldicarb appeared to suppress the populations of plant‐growth‐promoting bacteria that contribute to enhanced growth in potatoes. This is attributed to the long‐term use of aldicarb at the trial site, which may have encouraged the selection, or adaptation, of soil microorganisms that degrade carbamate pesticides but do not enhance potato growth.

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