Moving fungicides down in soil

Mixing ¹⁴C-fungicide in water before applying to soil. (Photo: Wendell Hutchins)

Crown- and root-infecting pathogens cause many of the most devastating disease outbreaks in all species of turfgrass. Diseases caused by them include Pythium root rot (Pythium spp.), take-all root rot (Gaeumannomyces spp.) and spring dead spot (Ophiosphaerella spp.).

Many of the more problematic plant parasitic nematode species belong to this group of pathogens as well. Damage caused by crown- and root-infecting pathogens can be economically and aesthetically detrimental to golf courses, athletic fields, sod farms and home lawns. Therefore, effectively managing these pathogens preventively or therapeutically through cultural and chemical practices is paramount to maintaining healthy turfgrass.

Fungicides are the most common means of crown- and root-disease suppression. However, many effective fungicides are costly, so optimizing the efficacy of the product is economically advantageous as well as beneficial to turfgrass health.

A majority of the fungicides superintendents use in turfgrass management are contacts, localized penetrants or acropetal penetrants. None of these topical modes of action have the capability to move down through the phloem to the basal portions of the plant. Therefore, application methods that aid downward movement of fungicides have the capability to increase efficacy against crown- and root-infecting pathogens.

Soil surfactants used in fungicide movement studies

Post-application irrigation has been shown to increase downward movement of fungicides and efficacy against summer patch. Research has determined that applying fungicides at a higher carrier volume increases deposition and coverage of basal portions of the turfgrass plant, thereby increasing efficacy against large patch (Rhizoctonia solani). Moreover, soil surfactants could offer the potential for increased distribution of fungicides in the soil. The purpose of our studies was to examine the effect of three commonly applied soil surfactants (Table 1) on myclobutanil distribution in bare soil, as well as the influence of the soil surfactant Revolution (Aquatrols, Paulsboro, N.J.) on movement of azoxystrobin and propiconazole in bare soil.

Two lab studies

We conducted two laboratory studies in Raleigh, N.C., in 2018 to determine the influence of soil surfactants on fungicide distribution. All fungicides tested in each study were tagged with a ¹⁴C-isotope to track the movement of the fungicide through the soil column. For each study, we applied ¹⁴C-fungicides and soil surfactants with a pipette to a bare 90-percent sand/10-percent peat moss by volume United States Golf Association putting green soil mix contained within a lysimeter (2.5 inches by 8 inches). We then irrigated the soil with a pipette immediately after application with ¼ inch of water.

Effect of soil surfactants on ¹⁴C-myclobutanil movement in a bare 90-percent/10-percent sand/peat moss (v/v) soil. Bars represent mean percent recovery from three replications. Bars with the same letters within each sampling depth are not significantly different, according to Fisher’s LSD t-test (P < 0.05).

In the first study, three treatments included a positive control and negative control. The treatments were as follows: ¹⁴C-myclobutanil plus Aquifer, ¹⁴C-myclobutanil plus Fleet, ¹⁴C-myclobutanil plus Revolution, ¹⁴C-myclobutanil alone (positive control) and no ¹⁴C-myclobutanil or soil surfactant (negative control). We applied soil surfactants four weeks before ¹⁴C-myclobutanil treatment, two weeks before ¹⁴C-myclobutanil treatment and at ¹⁴C-myclobutanil treatment. We sampled columns 14 days after ¹⁴C-myclobutanil treatment (Photo 2), and the data are presented in Figure 1.

The second study followed the same methods as the first, except we treated lysimeters with ¹⁴C-azoxystrobin or ¹⁴C-propiconazole and only used the soil surfactant Revolution. Moreover, we applied Revolution six, four and two weeks prior to ¹⁴C-fungicide application as well as at ¹⁴C-fungicide application. The data for this study are presented in Figures 2 and 3.

Results

¹⁴C-myclobutanil plus soil surfactants study

For this study, recoveries of ¹⁴C-myclobutanil per lysimeter ranged from 74 percent to 116 percent of the original applied amount. Data were analyzed as percent of ¹⁴C-myclobutanil of the total amount recovered.

Sectioning soil into 1-inch increments to recover applied ¹⁴C-fungicide. (Photo: P. Maxwell)

At least 32 percent of the recovered ¹⁴C-myclobutanil remained in the top inch of soil for every treatment (Figure 1), highlighting the limited mobility of ¹⁴C-myclobutanil. However, soil not treated with one of the three soil surfactants retained at least 19.4 percent units more ¹⁴C-myclobutanil in the top inch of soil than soil treated with a soil surfactant, suggesting that soil surfactants increased the downward movement of ¹⁴C-myclobutanil. Recovery of ¹⁴C-myclobutanil at the 1-inch to 2-inch sampling depth was similar among all treatments, yet at the 2-inch to 3-inch sampling depth, recovery increased by more than 14 percent of units when soil was treated with one of the three soil surfactants compared with nontreated soil. At the 3-inch to 4-inch sampling depth, ¹⁴C-myclobutanil recovery increased by at least 4.9 percent units for soil that was treated with surfactants compared to nontreated soil. We recovered no ¹⁴C-myclobutanil below 4 inches for any treatment. Moreover, ¹⁴C-myclobutanil recoveries were the same among the three soil surfactants tested at every sampling depth.

¹⁴C-azoxystrobin and ¹⁴C-propiconazole plus Revolution study

Recoveries ranged from 73 percent to 88 percent of the total applied for ¹⁴C-azoxystrobin, and 73 percent to 96 percent of the total applied for ¹⁴C-propiconazole (Figure 2 and Figure 3). Similar to the ¹⁴C-myclobutanil experiment, data were analyzed as the percent of ¹⁴C-fungicide of the total recovered.

Influence of post-application irrigation on azoxystrobin efficacy (average turf quality) against summer patch of creeping bentgrass. Bars with the same letter are not significantly different, according to Fisher’s LSD t-test (P < 0.05).

In the ¹⁴C-azoxystrobin study, Revolution had no effect on fungicide movement in the top 2 inches of soil. However, from the 2-inch to 4-inch depths, there was more ¹⁴C-azoxystrobin recovered in the soil treated with Revolution compared with the nontreated soil. There was no ¹⁴C-azoxystrobin recovered below 4 inches in the soil. Similar trends to the ¹⁴C-azoxystrobin study existed in the ¹⁴C-propiconazole study, with more ¹⁴C-propiconazole recovered from 2 to 4 inches when treated with Revolution compared with the nontreated soil. As with the ¹⁴C-azoxystrobin, we recovered no ¹⁴C-propiconazole below 4 inches in the soil.

Increased downward movement

Our data from these studies indicate that soil surfactants increase the downward movement of fungicides in soil. Previous research has found results similar to our study with the nematicide abamectin (Avid, Syngenta Crop Protection). Furthermore, a programmatic approach to soil surfactant applications could be more beneficial than only applying soil surfactants at or a few days before fungicide application. It’s been shown that one Revolution application a day before fungicide treatment did not increase downward movement of the fungicide. In our study, we applied soil surfactants three to four times prior to treatment and saw increased downward movement of the tested fungicides.

Influence of post-application irrigation on azoxystrobin efficacy (root length) against summer patch of creeping bentgrass. Bars with the same letter are not significantly different, according to Fisher’s LSD t-test (P < 0.05).

Implementation of strategies to increase downward distribution of fungicides could be beneficial for turfgrass managers facing crown and root diseases. Our recommendation for turfgrass managers applying fungicides for crown and root diseases is to apply with a water carrier volume of 2-4 gal/1,000 ft², irrigate with 1/8-1/4 inch of water immediately after fungicide application and programmatically spray wetting agents before and at fungicide application.

Wendell Hutchens conducted the research discussed in this article while earning his MS degree at North Carolina State University and is now a Ph.D. candidate at Virginia Tech University. Travis Gannon, Ph.D., Dave Shew, Ph.D., Khalied Ahmed and Jim Kerns, Ph.D., are at North Carolina State University. You may reach Wendell at wendelljh@vt.edu for more information.

Acknowledgements

The authors thank Sam Greene from Aqua-Aid, Ron Hall from Divots and Syngenta for donating products and materials for the studies, as well as the North Carolina State University Turfgrass Center for Environmental Research and Education for providing financial support for the project.

References

Benelli, J.J. 2016. Improved Fungicidal Control of Large Patch through Optimal Use of Surfactants and Spray Rate Volume. PhD diss. University of Tennessee, Knoxville, TN.

Gannon, T.W., M.D. Jeffries, and K.A. Ahmed. 2017. Irrigation and Soil Surfactants Affect Abamectin Distribution in Soil. Crop Sci. 57.2:573-580.

Gardner, D.S., and B.E. Branham. 2001. Effect of Turfgrass Cover and Irrigation on Soil Mobility and Dissipation of Mefanoxam and Propiconazole. J. Environ. Qual. 30:1612-1618.

Hutchens, W.J., Gannon, T.W., Shew, H.D., and Kerns, J.P. 2019. Effect of post-application irrigation on fungicide movement and efficacy against Magnaporthiopsis poae. Crop Prot. 122:106-111. https://doi.org/10.1016/j.cropro.2019.04.027

Latin, R., and L. Ou. 2018. Influence of Irrigation and Wetting Agent on Fungicide Residues in Creeping Bentgrass. Plant Dis. 102:2352-2360.