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The ability of endophytic bacteria to influence Erwinia carotovora var. atroseptica (Eca) growth and disease development was examined in potatoes. Bacterial populations isolated from within the tubers of five potato (Solanum tuberosum L.) cultivars (Kennebec, Butte, Green Mountain, Russet Burbank and Sebago) showed antibiosis toward Eca in an in vitro assay. Sebago was host to the highest percentage of bacterial isolates inhibiting Eca growth in vitro (49.5%), followed by Green Mountain (33.3%), Kennebec (29.3%), Russet Burbank (12.9%) and Butte (1.8%). Of these, Curtobacterium luteum was the most common species. Few endophytic bacteria from Butte were inhibitory to Erwinia; all were from Pantoea agglomerans. Significantly higher populations of Erwinia-inhibiting bacteria were recovered from Kennebec (1.89 x 106 cfu fresh weight tuber tissue) as compared to the other cultivars; the lowest populations were recovered from Butte (0.01 x 106 cfu per g fresh weight tuber tissue). Published levels of cultivar disease resistance to blackleg did not correspond to actual bacterial soft rot development (induced by Eca) in an in vivo (tuber) assay. However, bacterial soft rot development was negatively correlated with the density of tuber populations of endophytic bacteria found able to inhibit Eca growth in vitro (R = -0.879, p = 0.05).

Clover and potatoes, in a crop rotation, were found to share specific associations of bacterial endophytes. Twenty-five bacterial species from 18 genera were common to both clover and potatoes and represented 73% of all the bacteria recovered from clover root tissues and 73% of all the bacteria recovered from potato tubers. Endophytic bacteria tested in potato plant bioassays were predominantly plant growth neutral (56%). The remainder were either plant growth promoting (21%) or plant growth inhibiting (24%)(P < 0.05). Of the plant growth promoting bacteria, 63% increased shoot height, 66% increased shoot wet weight, and 55% increased root wet weight. The effects of plant growth inhibiting bacteria were restricted to reductions in plant height (86%) and shoot wet weight (36%); root weight was not affected. Of the bacteria tested, 74% showed some degree of in vitro antibiosis to the clover and potato pathogen Rhizoctonia solani. Such endophytic intercrop bacterial associations appear to be complementary in nature and support the view that there are microbial benefits to be gained from clover in crop sequences with potatoes, beyond those of the residual nitrogen left in the soil and the organic matter added.Key words: Trifolium, allelopathy, endophytic, plant growth promoting, antifungal, crop rotation, Solanum.

Isolates of Phytophthora erythroseptica, causal agent of pink rot of potato, were obtained from diseased tubers collected in Prince Edward Island (PEI), Canada. In an in vitro assay of field and single-zoospore isolates, all isolates of the pathogen from PEI were sensitive to metalaxyl (EC 50 , 0´5 mg mL 21 ), unlike moderately or highly resistant reference isolates obtained from Maine, USA. Allozyme-banding patterns at the glucose-6-phosphate isomerase (Gpi) locus were identical (91/91) for all isolates of P. erythroseptica examined from PEI and Maine, but could be used to distinguish isolates of P. erythroseptica from local isolates of Phytophthora infestans. Inoculation of potato tubers from plants treated with foliar applications of metalaxyl in the field indicated that compounds inhibitory to metalaxyl-sensitive isolates of P. erythroseptica were present in the periderm, even after 4 months' storage. By contrast, moderately or highly resistant isolates of the pathogen caused significantly (P # 0´05) more disease than sensitive isolates in tubers from plants treated with metalaxyl. The effectiveness of metalaxyl in controlling pink rot in a particular region will depend on resistance levels of local populations of P. erythroseptica. Preventing the development and spread of resistant strains of P. erythroseptica will be critical in maintaining metalaxyl as an effective chemical for control of this pathogen.

The influence of plant growth promoting (PGP) activity of bacterial communities recovered from each of six weed species (barnyard grass (Echinochloa crusfalli (L.) Beauv.), corn spurrey (Spergula arvensis L.), goldenrod (Sonchus sp.), Italian ryegrass (Lolium multiflorum L.), lamb's-quarters (Chenopodium album L.), and quack grass (Agropyron repens (L.) Beauv.)) was examined in relation to the effect it had on the growth of the potato cultivar Russet Burbank. Bacterial species composition and community structure were compared, species-abundance relationships were determined, and those members conferring positive benefits for potato growth and development were identified. Of the genera identified, Bacillus, Arthrobacter, Stenotrophomonas, Acinetobacter, and Pseudomonas were the most common, and Stenotrophomonas maltophilia was the most frequent species recovered across all sources. Significantly higher population densities were found in the root zones of quack grass, compared with Italian ryegrass and lamb's-quarters. There were no significant differences in species richness among the root zones; however, evenness indices (species distribution) were significantly lower in corn spurrey (P = 0.05). Significantly higher diversity indices (Hill-1 and Hill-2 numbers) (P = 0.05) were found in the root zone soil communities of potato and goldenrod, indicating a decrease in the proportional abundance of common and very abundant species, respectively, while in barnyard grass, corn spurrey, and Italian ryegrass the reverse was the case. In both years of the study, Italian ryegrass and corn spurrey were consistently better sources of PGP rhizobacteria for potatoes, significantly (P < 0.001) increasing the mean wet weight of shoots and roots in in vitro bacterization studies. Barnyard grass was a consistently poor source of such isolates. Species-abundance measures of root zone bacterial biodiversity were not found, in this instance, to be a particularly good predictor of the presence or absence of PGP rhizobacteria. We consider that the study of complementary crops and soil-conditioning treatments should not preclude the examination of weed species as possible beneficials, as alterations in rhizobacterial biodiversity and functional versatility can influence the numbers and types of PGP bacterial strains, and consequently may serve to improve soil quality.

The culturable component of bacterial communities found in the endoroot and associated exoroot (root zone soil) was examined in potatoes (Solanum tuberosum L.) grown under either conventional or minimum tillage systems. Bacterial species--abundance relationships were determined and in vitro antibiosis ability investigated to discover whether tillage practice or bacteria source (endo- or exoroot) influenced bacterial community structure and functional versatility. Antibiosis abilities against Phytophthora erythroseptica Pethyb. (causal agent of pink rot of potatoes), Streptomyces scabies (Thaxt.) Waksm. and Henrici) (causal agent of potato common scab), and Fusarium oxysporum Schlecht. Emend. Snyder and Hansen (causal agent of fusarium potato wilt) were selected as indicators of functional versatility. Bacterial community species richness and diversity indices were significantly greater (P = 0.001) in the exoroot than in the endoroot. While both endo- and exoroot communities possessed antibiosis ability against the phytopathogens tested, a significantly greater proportion (P = 0.0001) of the endoroot population demonstrated antibiosis ability than its exoroot counterpart against P. erythroseptica and F. oxysporum. Tillage regime had no significant influence on species-abundance relationships in the endo- or exoroot but did influence the relative antibiosis ability of bacteria in in vitro challenges against S. scabies, where bacteria sourced from minimum tillage systems were more likely to have antibiosis ability (P = 0.0151). We postulate that the difference in the frequency of isolates with antibiosis ability among endoroot versus exoroot populations points to the adaptation of endophytic bacterial communities that favour plant host defence against pathogens that attack the host systemically.

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