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Lea Corkidi, Jeff Bohn and Mike Evans

The insecticide bifenthrin is a synthetic pyrethroid required by regulation for the production of nursery crops to suppress the red imported fire ant (Solenopsis invicta) in Orange and Riverside counties in California. We conducted a greenhouse experiment to analyze the effects of different rates of bifenthrin on the growth and mycorrhizal colonization of ‘Silver Queen’ corn (Zea mays) inoculated with VAM 80®, a mycorrhizal inoculum with spores, hyphae, and root pieces colonized by Glomus spp., used to inoculate California native plants in containers. Corn was used because it is the standard indicator plant used for mycorrhizal inoculum potential assays and it is a good host for arbuscular mycorrhizal fungi propagation. The application of bifenthrin had no detrimental effects on mycorrhizal colonization of corn. There were no significant differences in the root length colonized by arbuscules, vesicles, or in the total percentage of mycorrhizal colonization obtained in the plants grown with the different bifenthrin rates 6 weeks after transplanting. However, there were significant interactions on the effects of bifenthrin and mycorrhizal colonization on plant growth. The addition of 12, 15, and 25 ppm of bifenthrin reduced corn biomass of nonmycorrhizal plants, but had no effect on the growth of mycorrhizal plants. There were no significant differences between the mycorrhizal and nonmycorrhizal plants grown with 0, 10, and 12 ppm of bifenthrin. In contrast, inoculation with VAM 80® increased the shoot dry weight of plants grown with 15 and 25 ppm of bifenthrin. This study showed that mycorrhizal colonization can be helpful to overcome some of the negative effects of bifenthrin on the growth of corn.

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Lea Corkidi, Donald J. Merhaut, Edith B. Allen, James Downer, Jeff Bohn and Mike Evans

Our goal was to investigate the effects of mycorrhizal colonization on nitrogen (N) and phosphorus (P) leaching from plants grown in nursery containers. We compared the growth response and the content of nitrate (NO3), ammonium (NH4), and orthophosphate, in leachates collected from mycorrhizal (AM) and nonmycorrhizal (NonAM) plants of the fast-growing perennial, Encelia californica Nutt. (california sunflower), and the slow-growing woody shrub, Rhus integrifolia (Nutt.) Brewer & S. Watson (lemonade berry). Plants were grown for 8 weeks with no fertilizer or with 0.88 g (half rate) and 1.76 g (full rate) of 18N–2.6P–9.9K Osmocote (18-6-12, 6–7 month longevity at 26 °C, Osmocote® controlled-release fertilizer; Scotts Co., Marysville, OH). Mycorrhizal colonization increased the growth and nutrient uptake of E. californica and R. integrifolia but was more effective at decreasing nutrient leaching from containers with E. californica. Mycorrhizal colonization contributed to reduce the content of NO3, NH4, and orthophosphate by up to 65% in leachates from E. californica grown with half rate of Osmocote and up to 70% to 80% in those from plants grown in full rates of Osmocote. In contrast, only the leachates from AM plants of R. integrifolia grown without fertilizer had generally lower nutrient content than those from NonAM plants. Leachates collected from AM plants grown in half rates of Osmocote had less P but no less N, and there were mostly no significant differences in the leachate content of NO3, NH4, and orthophosphate from AM and NonAM plants of R. integrifolia grown in full rates of Osmocote. However, mycorrhizal colonization reduced the fertilizer requirement to achieve maximum growth in both species. AM plants of E. californica and R. integrifolia grown with half rates of Osmocote had greater dry weight than the NonAM ones grown in full rates of Osmocote. Our study shows that mycorrhizal colonization can reduce N and P leaching either by increasing nutrient uptake or by allowing the use of lower fertilizer rates.