MICROBIAL-POPULATIONS, FUNGAL SPECIES-DIVERSITY AND PLANT PATHOGEN LEVELS IN-FIELD PLOTS OF POTATO PLANTS EXPRESSING THE BACILLUS-THURINGIENSIS VAR TENEBRIONIS ENDOTOXIN

The environmental release of genetically engineered (transgenic) plant s may be accompanied by ecological effects including changes in the pl ant-associated microflora. A field release of transgenic potato plants that produce the insecticidal endotoxin of Bacillus thuringiensis var . tenebrionis (Bit) was monitored for changes in total bacterial and f ungal populations, fungal species diversity and abundance, and plant p athogen levels. The microflora on three phenological stages of leaves (green, yellow and brown) were compared over the growing season (sampl e days 0, 21, 42, 63 and 98) for transgenic potato plants, commercial Russet Burbank potato plants treated with systemic insecticide (Di-Sys ton) and commercial Russet Burbank potato plants treated with microbia l Btt (M-Trak). In addition, plant and soil assays were performed to a ssess disease incidence of Fusarium spp., Pythium spp., Verticillium d ahliae, potato leaf roll virus (PLRV) and potato virus Y (PVY). Few si gnificant differences in phylloplane microflora among the plant types were observed and none of the differences were persistent. Total bacte rial populations on brown leaves on sample day 21 and on green leaves on sample day 42 were significantly higher on the transgenic potato pl ants. Total fungal populations on green leaves on sample day 63 were s ignificantly different among the three plant types; lowest levels were on the commercial potato plants treated with systemic insecticide and highest levels were on the commercial potato plants treated with micr obial Btt. Differences in fungal species assemblages and diversity wer e correlated with sampling dates, but relatively consistent among trea tments. Alternaria alternata, a common saprophyte on leaves and in soi l and leaf litter, was the most commonly isolated fungus species for a ll the plant treatments. Rhizosphere populations of the soilborne path ogens Pythium spp., Fusarium spp. and V. dahliae did not differ betwee n the transgenic potato plants and the commercial potato plants treate d with systemic insecticide. The incidence of tuber infection at the e nd of the growing season by the plant pathogen V. dahliae was highest for the transgenic potato plants but this difference was related to lo nger viability of the transgenic potato plants. This difference in lon gevity between the transgenic potato plants and the commercial + syste mic insecticide potato plants also made comparison of the incidence of PVY and PLRV problematic. Our results indicate that under field condi tions the microflora of transgenic Btt-producing potato plants differe d minimally from that of chemically and microbially treated commercial potato plants.