Last updated 6/30/98 |
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W. V. Sigler, R.F. Turco, Z.J. Reicher, and C.S. Throssell
The objective of this study is to investigate the effect of repeated fungicide applications on the microbial degradation of fungicides in the turfgrass canopy. By evaluating the rate of microbial degradation in the turfgrass canopy, the importance of turfgrass leaves as a pesticide sink and degradation site may be assessed.
Previous research describes microbial degradation in addition to the processes of enhanced biodegradation of fungicides in agricultural soils. However, the microbial activity within the seldom-studied turfgrass canopy may also prove to be important in the degradation of applied fungicides. The turf leaf surface has already been shown to be an important sink for applied fungicides. A dense turf canopy can intercept over 95% of applied pesticides and studies conducted at Purdue University have shown that a large bacterial population exists on the leaf surface of turfgrass plants. Thus, the turf canopy is a potentially important site for the microbial degradation of xenobiotic materials including fungicides.
Two studies were conducted simultaneously to determine the environmental fate of fungicides in the turfgrass canopy. The first study investigated field fungicide dissipation, which simply focused on the loss of the chemical, not necessarily on the mechanism of loss. A second experiment determined the amount of microbial fungicide degradation (biological mechanism) that occurred on harvested turfgrass clippings.
Dissipation Study
Three fungicides, triadimefon (Bayleton 25 WDG), metalaxyl (Subdue 2E), and iprodione (Chipco 26019) were applied to separate creeping bentgrass plots at a rate of 2 oz (fluid or dry, depending on the formulation of the fungicide)/1000ft2. Applications were made every two weeks for 16 weeks throughout the summer growing season resulting in a total of eight fungicide applications. Four different clipping harvests were made within each two-week window between fungicide applications. The clippings recovered at each sampling were analyzed for fungicide concentration in order to assess fungicide dissipation over time based on repeated fungicide applications.
Degradation Study
A portion of the first sampling of clippings collected after each fungicide application in the dissipation study was used to assess the amount of microbial fungicide degradation occurring on the turfgrass leaves. The degradation study was based on the principle of bacterial respiration; Bacteria that are capable of utilizing a fungicide as a growth substrate will metabolize the chemical and evolve CO2 as an end product of metabolism. This process is termed "mineralization". Clipping samples recovered from treated plots were incubated with the same fungicide in the laboratory; however, radiolabeled fungicides were used in the incubations instead of normally formulated fungicides. By using radiolabeled fungicides, the production of fungicide-based CO2 could be traced and the amount of microbial degradation on the turfgrass leaf surface assessed.
Dissipation Study
- Analysis of the harvested clippings indicated that dissipation rates on plots receiving iprodione, metalaxyl, and triadimefon applications were similar, regardless of the number of applications. Figure 1 shows the average of all fungicide concentration analyses from each of the eight application cycles. Notice the harvesting of clippings along with naturally occurring dissipation results in relatively short field half-lives of the fungicides.
- Although small changes in the dissipation rates were evident among the eight sampling cycles, the trend among application cycles was not conclusive support for the occurrence of enhanced biodegradation.
- The similarity of the dissipation curves suggests that repeated applications resulted in little change in the loss mechanism of the fungicides.
Degradation Study
- Radiolabeled CO2 evolution results for each fungicide studied suggests that very little microbial mineralization took place in the turfgrass canopy. The results indicate that only 1-3% of the applied radiolabeled fungicide was mineralized to CO2. (Fig. 2)
- The lack of mineralization of all three fungicides raises the question of possible fungicide transformation without mineralization. It is possible that microorganisms present on the leaf surface of the turfgrass could transform the fungicides into compounds closely related to the parent fungicide without metabolizing them completely to CO2.
- As in the dissipation study, the pattern generated by the cumulative evolution in radiolabeled CO2 gives little support for the possibility of enhanced biodegradation.
Although reports of enhanced biodegradation of both iprodione and metalaxyl in soil are abundant, we maintain that unfavorable conditions for microbial activity define the turfgrass canopy and greatly limit the magnitude of microbial degradation. Exposure to ultra-violet light, low relative humidity, and frequent vegetation removal all combine to create an inhospitable environment for microbial activity and population adaptation.
Further analysis of clipping samples did however result in findings that indicate microbial transformation of the fungicide materials without mineralization. The "incomplete" mineralization of these fungicides within the turfgrass canopy suggests that conditions in the canopy are unfavorable for microbial adaptation and/or the microbial population is utilizing only a portion of the degradative pathway to break down the applied fungicides.
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