Key Takeaways

• Fungicide efficacy depends on getting the right product to the right place. A good understanding of the pathogen’s location (foliar vs. root) and how different types of fungicides move in the plant, the turf canopy or through the soil is essential for effective control.
  
  
• Contact fungicides need good leaf coverage and dry conditions after application, while acropetal or localized penetrant fungicides benefit from post-application irrigation when controlling fungal diseases in the crown or roots.
  
  
• Carrier volume, the amount and timing of post-application irrigation, and nozzle selection all influence fungicide placement. Higher volumes and larger droplets help move fungicides into the soil, while lower volumes and finer sprays are generally better for foliar disease control.
  
  
• Soil properties like organic matter content and fungicide properties like K_OC (a measure of mobility) can affect movement in the turf canopy, thatch and soil. Selecting products that are mobile enough to reach the roots but won’t move through the target zone is key for controlling soilborne pathogens.
   

Any fungicide application should have an intended placement and target pathogen (or pathogens) it is attempting to control. The distance acropetal penetrants translocate in the plant varies greatly. DMI fungicides are moderately mobile in the xylem but thiophanate-methyl and mefenoxam are highly mobile. Knowing how a fungicide moves in the plant is a vital part of disease suppression. For example, summer patch (Magnaporthiopsis poae) is a disease that affects turfgrass roots, so delivering an acropetal penetrant to the roots is necessary to suppress the disease, as delivery to the foliage would not be beneficial. However, many common fungicides for summer patch, like DMI, SDHI and QoI fungicides are also capable of targeting foliar diseases like brown patch. The difference is that irrigation immediately following application will greatly reduce the efficacy against brown patch, whereas watering the same products in is necessary for summer patch control. Understanding the disease you are trying to control and where it impacts the plant is critically important.

Acropetal penetrants, localized penetrants and systemic fungicides will take longer to curatively control diseases. When treating active foliar disease, it is a good idea to include a contact fungicide for quicker control.

Understanding how fungicides interact with the soil

For fungicide applications targeting soilborne pathogens, it is important to consider a fungicide’s affinity for binding to the soil. One value that is helpful in assessing how a pesticide interacts with the soil is the soil organic carbon-water partition coefficient or K_OC. A higher K_OC leads to less pesticide mobility in the soil. If a fungicide has a higher K_OC, it has a higher affinity to bind to soil organic matter and thatch rather than remaining in solution to be absorbed by roots. Higher levels of soil organic matter and thatch can influence how much active ingredient reaches the roots. Elevated organic matter levels contribute to increased disease pressure not only due to excessive moisture retention, but also because it can inhibit fungicide movement to turfgrass roots.

K_OC values are measured in milliliters per gram which indicates the volume of water (mL) that would contain the same amount of chemical as one gram of organic carbon. Values greater than 5,000 are considered immobile in soil, 5,000 to 2,000 slightly mobile, 2,000 to 500 low mobility, 500-150 moderate mobility, 150-50 highly mobile and less than 50 very high mobility (Swan et al., 1983). Most modern fungicides are not of great concern to leach through an entire soil profile as they are classified as having a low to slight mobility in soil and organic matter (Table 1).

Moving fungicides into the soil

Wetting agent applications, in conjunction with supplemental irrigation, may allow for more fungicide distribution deeper in the soil profile (Hutchens et al., 2020). However, research on the influence of wetting agents on fungicide movement has produced variable results, and the wetting agent may not aid in fungicide movement as much in a turfgrass stand compared to bare soil (Latin & Ou, 2018).

As shown in Table 1, most acropetal penetrant fungicides currently on the market have a low soil mobility classification. Applying irrigation to move these products into the soil is very unlikely to move them beyond the rootzone because a large majority of the active ingredient adheres to the plant parts, thatch and rootzone before it is able to reach deep into the profile.

To control soilborne diseases, it is recommended to increase carrier volumes (refer to label recommendations for specific carrier volumes) and apply post-application irrigation as quickly as possible. Maxwell and Gannon (2021) found that irrigation applied immediately after an azoxystrobin application effectively dislodged more active ingredient from the foliage than irrigation applied four hours after application, which dislodged more active ingredient than irrigation applied six days following application. Applying approximately 0.25 inch of irrigation is a good rule of thumb, but the optimal amount of water will vary based on environmental factors (like amount of thatch) and label recommendations. Using the right amounts of carrier volume and post-application irrigation are essential to move the active ingredient to the target, especially when applying acropetal and localized penetrants that are not able to move downward through the plant. Some additional recommendations for moving fungicides deeper into the soil profile include:

• Aeration with small-diameter solid tines to encourage infiltration and minimize adsorption to aboveground plant parts, especially if compaction or thatch are concerns.
  
  
• Use fungicides with lower K_OC values (more mobile) to encourage downward movement if infiltration is a concern.
  
  
• Apply a soil surfactant that may encourage penetration into the soil profile.
  

Tips for successful fungicide placement

Nozzles and carrier volume

Droplet size and carrier volume requirements will vary based on desired placement of a fungicide. If a foliar application is desired, finer droplet size is needed to increase coverage and uniformity. This is particularly true with contact fungicides, which require direct contact with the fungal spores to be effective. For example, flat-fan nozzles with a finer droplet size would be a good choice for chlorothalonil applications targeting dollar spot. For applications targeting soilborne diseases, a larger droplet with a higher carrier volume is beneficial to encourage infiltration of the active ingredient into the soil. Newer technologies like air-induction nozzles can be beneficial for minimizing drift by creating bigger droplets. Using a larger droplet size is always preferred if smaller droplet sizes are moving off target and are not reaching the plant due to wind or volatilization.

Concerns with pH

Though it is primarily a problem for organophosphate and carbamate insecticides, using water with a high pH to mix fungicides can result in alkaline hydrolysis. Acidifiers can be used if water has a high pH, but most modern fungicides are not impacted by alkaline hydrolysis due to improved formulation. Still, active ingredients like iprodione, mancozeb and propiconazole can have issues with alkaline hydrolysis at higher pH ranges (Schilder, 2018).

Presence of dew

If dew is present, it can have a negative effect on the application of contact or localized penetrant fungicides because dew will dilute the active ingredient and potentially cause it to run off the leaf tissue. In contrast, applying acropetal penetrant or systemic fungicides while dew is present likely increases efficacy as less active ingredient is stuck in the verdure (Clarke et al., 2024).

Spray intervals

When deciding reapplication intervals, it is important to remember that fungicide residues only remain for seven to 10 days in an ideal world. Effective fungicides will provide control for longer than that time frame because 21 or 28 days often pass before the pathogen regrows to populations capable of providing injury to the turf (Latin, 2021). Population regrowth will only occur if the environment is suitable for the pathogen. Remembering that fungal development requires an adequate environment, susceptible host, and presence of the pathogen is vital to deciding whether a reapplication is necessary. Consider environmental factors and perhaps even using disease models to plan your fungicide applications rather than relying on the calendar alone.

Summary

Getting fungicides to the proper target to control disease is vital, and control is improved when plant uptake and fungicide mobility are considered. Effective disease control depends on matching application practices to the target. For root pathogens, strategies like light irrigation, higher spray volumes or using surfactants can improve fungicide placement. For foliar diseases, allowing fungicides to remain on the leaf surface is critical.

If you are unsure about which pathogen is infecting your turf, sending a sample to a turf pathology lab is recommended to avoid wasting time and resources. Once the pathogen has been identified and a fungicide selected, placement and amount of post-application irrigation can be determined. “Chemical Control of Turfgrass Diseases” is an excellent, comprehensive resource produced by several leading turf pathologists that can help you identify turfgrass diseases and select control products.

Fungicide applications can be costly, so ensuring that the maximum amount of active ingredient is reaching the desired area of the plant is important to get the results you want.

References

Clarke, B.B., Vincelli, P., Koch, P.L., & Chou, M.Y. (2024). Chemical control of turfgrass diseases 2024. Cooperative Extension Service, College of Agriculture, Food and Environment, University of Kentucky. https://uknowledge.uky.edu/anr_reports/185

Hutchens, W.J., Gannon, T.W., Shew, H.D., Ahmed, K.A., & Kerns, J.P. (2020). Soil surfactants influence fungicide movement in United States Golf Association putting green soil. Journal of Environmental Quality, 49(2), 450-459. https://doi.org/10.1002/jeq2.20021

Latin, R. (2021). A Practical Guide to Turfgrass Fungicides, Second Edition. The American Phytopathological Society.

Latin, R., & Ou, L. (2018). Influence of irrigation and wetting agent on fungicide residues in creeping bentgrass. Plant disease, 102(11), 2352-2360.

Maxwell, P.J., & Gannon, T.W. (2021). Post‐application irrigation timing affects dislodgeable azoxystrobin foliar residue. Crop, Forage & Turfgrass Management, 7(1), e20098.

Schilder, A. (2018). Effect of water pH on the stability of pesticides. Michigan State University Extension.

Swann, R.L., Laskowski, D.A., McCall, P.J., Vander Kuy, K., & Dishburger, H.J. (1983). A rapid method for the estimation of the environmental parameters octanol/water partition coefficient, soil sorption constant, water to air ratio, and water solubility. In Residue reviews: Residues of pesticides and other contaminants in the total environment (pp. 17-28). Springer New York.