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Volatile loss rates of pesticides from turfgrass were measured using the Backward-Time Lagrangian Stochastic Dispersion model (Flesch et al., 1995). Solar radiation, ambient temperature, surface temperature, relative humidity, wind direction, and wind speed were monitored continuously. Growth regulator was applied to the turf plot several days before pesticide application to maintain a constant grass height and aerodynamic roughness length during the experiment. No irrigation occurred following application. Pesticides were applied as mixtures to allow direct comparison of evaporative loss. Mixtures studied were chlorpyrifos + triadimefon + ethofumesate and triclopyr (acetic acid) + propiconazole + cyfluthurin. Airborne flux estimates correlated with temperature, solar radiation, wind speed, time, and vapor pressure of the active ingredient. A log vapor pressure vs. 1/Temperature (K) relationship was observed between flux and surface temperature over a single day for most pesticides. An exponential attenuation of flux was observed over a period of several days and correlated with attenuation of dislodgeable surface residues for two of the pesticides. A fugacity-based model for predicting initial evaporative loss rates from turf grass is presented. Input parameters include pesticide vapor pressure, molecular diffusion coefficient, surface temperature, wind speed profile, atmospheric stability, surface roughness, and average upwind fetch. The GC retention method (Jensen, 1966) was used to estimate pesticide vapor pressures over an environmentally relevant temperature range. The model predicts fluxes that are an order of magnitude greater than measured values. This bias may be due, in part, to deviation from the assumption of pesticide saturated vapor density at the foliar surface. In addition, sensitivity analysis suggests improved estimates of leaf surface temperature and pesticide vapor pressures have the greatest potential to improve model performance. / Graduation date: 2003

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The status of Curvularia lunata as a primary pathogen of creeping bentgrass (Agrostis palustris) has been debated in the literature for many years. The most recent and most complete evidence indicates that this fungus is a weak pathogen, colonizing only senescing tissues. In the present study, four groups of experiments were conducted: 1) The effects of five herbicides on the growth of C. lunata in vitro were determined. 2) Acute high temperature stress was evaluated as a predisposing factor in the susceptibility of creeping bentgrass to C. lunata. 3) The effects of acute high temperature stress and exposure to five different herbicides on creeping bentgrass were evaluated together and separately. 4) The potential of chronic, moderately high temperature stress and exposure to five herbicides when occurring separately and together to serve as predisposing factors on the susceptibility of creeping bentgrass to C. lunata were evaluated. It was found that effects of the herbicides on the growth of C. lunata depend on both the specific herbicide and the concentration. High temperature stress and herbicide exposure increase the colonization of creeping bentgrass by C. lunata only to the extent that these treatments produce moribund plant tissue. / Master of Science

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Sclerotinia homoeocarpa F.T. Bennett, causal agent of dollar spot on creeping bentgrass (Agrostis palustris Huds.); and Rhizoctonia solani Kihn, causal agent of brown patch on tall fescue (Festuca arundinacea Schreb.), are important pathogens of turfgrass. This research evaluated the ability of twenty bacterial strains of the genus Pseudomonas as potential biological agents for the control of these diseases. Year 1 dollar spot field trials resulted in the identification of five strains performing statistically as well as the recommended fungicide, chlorothalonil. Year 2 trials, using the top strains from Year 1, employed different application schedules and inclusion of a commercial spray adjuvant, Agri-Dex™, Results from Year 2 dollar spot trials indicated that eight strains performed statistically better than the non-treated control. Results from Year 1 and Year 2 of brown patch field trials provided three strains that performed statistically better than the nontreated control during Year 1, but only one strain during Year 2. Laboratory tests performed with all strains and both pathogens showed that demonstration of agar-based inhibition is more difficult to obtain with R. solani. Six strains evaluated using AgriDex™ and S. homoeocarpa in laboratory and greenhouse tests, determined that application with Agri-Dex™ can be more effective than with bacteria alone. Supporting greenhouse tests using the top strains from the S. homoeocarpa field trials to evaluate application timing in controlling dollar spot, indicated less disease with earlier and more frequent applications. With further research, several strains have the potential for disease control on turf, particularly dollar spot, and may reduce the reliance on chemical fungicides. / Master of Science

Rapid blight is a new disease of cool season turfgrasses that affects several important turfgrasses used for overseeding Bermuda in Arizona such as Poa trivialis and Lolium perenne (perennial rye). It is caused by Labyrinthula terrestris, an unusual organism that causes collapse of susceptible hosts and usually occurs in turfgrasses irrigated with moderate to high salinity irrigation water (EC>2.0). Rapid blight has not been observed in the field in warm season grasses such as Cynodon sp. (Bermudagrass) although laboratory trials have shown that common Bermudagrass and other grasses may be hosts. Because the survival mechanism of Labyrinthula terrestris in the absence of cool season turfgrass hosts was unknown, this study was initiated to determine if L. terrestris "over-summers" in different Bermudagrass systems. Results of assays of several different Bermudagrass systems show that Bermudagrasses are good hosts for L. terrestris and explains why rapid blight is a perpetual threat year after year in the same locations.