The fate of fungicide

Post-application management practices (i.e., post-application mowing timing and irrigation timing) can influence fungicide fate and performance. For example, previous research found that up to 34 percent of azoxystrobin was removed in tall fescue clippings following a single mowing event one day after application.

The fate and efficacy of fungicides as influenced by post-application management practices have not been evaluated on golf course putting greens. We assessed the fate of pyraclostrobin, triadimefon and penthiopyrad following various post-application irrigation and mowing treatments. These evaluations were on a putting green constructed to USGA specifications and planted with A-1 creeping bentgrass.

Plots were treated with a single application of the fungicides and irrigated either immediately (zero hours) or six hours after fungicide application with 0.25 inch (0.64 cm) of water. We then mowed the plots at either zero, one or three days after treatment (DAT). Daily mowing resumed after three DAT.

Cores were harvested using a standard 4.25-inch cup cutter and dissected into four subsections: remaining above-ground vegetation (verdure and thatch), a depth of 0-1 inch (0-2.5 cm), a depth of 1-2 inches (2.5-5.1 cm) and a depth of 2-3 inches (5.1-7.6 cm). We homogenized samples using a FitzMill and then analyzed them using high-performance liquid chromatography-mass spectrometry.

For the first year of data, pyraclostrobin recovered in turfgrass clippings at zero, one and three DAT did not exceed 2.5 percent of total fungicide applied (Figure 1). Delaying mowing events post pyraclostrobin application did not significantly influence the amount of fungicide removed with clippings.

A large amount of fungicide remained bound in the verdure and thatch. However, we did observe significant differences between irrigation treatments at and past one DAT at the 0-1-inch depth. We detected more fungicide in the 0-1-inch and 1-2-inch depth for plots irrigated immediately compared to those irrigated six hours after application.

We only detected pyraclostrobin at the 2-3-inch depth at 14 DAT for the immediate irrigation plots. A greater total amount of pyraclostrobin was in plots for later DAT (five, seven, 14 DAT) irrigated immediately compared to those irrigated six hours after treatment. Immediate irrigation may have implications in residual disease control with this compound.

The movement and distribution of triadimefon over time, following different irrigation and mowing treatments, were similar to pyraclostrobin (Figure 2). Less triadimefon was removed in turfgrass clippings when compared to pyraclostrobin.

We found smaller differences between irrigation treatments in the verdure/thatch and 0-1-inch depth compared to pyraclostrobin. However, detection of triadimefon in the 1-2-inch depth occurred earlier and in higher amounts than pyraclostrobin. Detection in the 2-3-inch depth was only present at 14 DAT for plots irrigated immediately.

Post-application irrigation timing greatly influenced penthiopyrad movement (Figure 3). We saw significantly less fungicide bound in the verdure/thatch and more fungicide moving into the 0-1 inch depth for plots irrigated immediately compared to those irrigated six hours after penthiopyrad application.

The movement and distribution of penthiopyrad over time, following different irrigation and mowing treatments, was similar to the other compounds (Figure 3). Penthiopyrad was detected in the 1-2-inch and 2-3-inch depth earlier and in higher amounts for immediate irrigation plots.

It is important to note that for all three fungicides evaluated, fungicide recovery was more than 90 percent at zero DAT, which confirms the methodology used in this experiment.

Post-application irrigation timing, and, to a lesser extent, mowing timing, can influence fungicide movement through the soil profile and fungicide removal with clippings. In our experiment, delayed mowing events did not result in less fungicide removed in clippings.

The preliminary results suggest that most of the fungicide is retained in the foliage and thatch regardless of post-application irrigation timing. However, immediate irrigation following fungicide application can move fungicide deeper into the soil where target soil-borne pathogens reside.

Our data suggest that a small difference in fungicide movement may significantly impact potential field efficacy. For example, an in vitro evaluation of the ectotrophic root infecting fungi (Gaeumannomyces graminicola) on pyraclostrobin-amended media showed that 1.0 ppm is the concentration required to inhibit fungal growth (Figure 4).

When irrigated immediately after application (zero hours), the converted percent of applied (1.1 ppm) from a sample core collected on 14 DAT provides an adequate amount of fungicide to the 0-1-inch depth and only 0.5 ppm when irrigating six hours after application.

Research takeaways

• We found very little fungicide within removed clippings when delaying mowing events.
• It is challenging to move fungicide past the remaining verdure and thatch, regardless of irrigation treatment.
• Immediate irrigation resulted in more fungicide movement past the above-ground vegetation.
• Small differences in fungicide movement could dramatically impact the efficacy of fungicides on golf course putting greens.

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