Key Takeaways:

• Microdochium patch can be suppressed using a rotation of mineral oil, phosphorous acid, and elemental sulfur to achieve control that is equivalent to traditional fungicides.
• Applying iron sulfate heptahydrate tank-mixed with phosphorous acid every two weeks leads to thinning of annual bluegrass putting greens. However, this tank-mix may be a viable option for other areas of the golf course – such as tees and fairways – or in areas where pesticide restrictions occur and control options are limited.
• It is recommended to apply no more than 1.5 pounds of elemental sulfur per 1,000 square feet per year on annual bluegrass putting greens because of an increased risk of anthracnose in the summer.
   

Turfgrass managers will correctly recognize Microdochium nivale as the pathogen causing pink snow mold. To avoid confusion, we only use the name “pink snow mold” when referring to the disease when it occurs under snow cover. Many areas that deal with pink snow mold may only need to make one or two fungicide applications per year to suppress the disease. In these environments, snow cover will often extend the residual of timely fungicide applications. In cool and humid regions with little to no snow cover, like parts of the Pacific Northwest, Dwyer et al. found that Microdochium patch can occur anytime the humidity is 90% or greater for 20 hours or more and the temperature is above freezing – but it is most likely to occur when the temperature is between 46 and 63 degrees Fahrenheit (Dwyer et al., 2017). In cool and humid regions with little to no snow cover, like parts of the Pacific Northwest, these conditions commonly occur. Indeed, it is common to see Microdochium patch develop during the last weeks of summer on putting greens growing in the shade. As a result, it may be necessary to make repeated fungicide applications from late summer into the spring to suppress Microdochium patch.

Repeated fungicide applications and the prolific spore production associated with Microdochium nivale increases the chances of developing fungicide resistance. Strains of Microdochium nivale have already shown resistance to methyl benzimidazole carbamates (thiophanate-methyl) and dicarboximides – e.g., iprodione. Fortunately, in most areas of the U.S., there are multiple fungicides that effectively suppress Microdochium patch. However, some areas in the cool-humid region are also affected by pesticide restrictions or loss of effective chemistries – specifically northern Europe and the United Kingdom, as well as some areas of Canada and the U.S. Limited flexibility in product options or application can make it a challenge to manage Microdochium patch.

Building a Microdochium Patch Suppression Program

Traditional Fungicides: Where fungicides are readily available, there are ample choices of products that can effectively suppress Microdochium patch. University extension publications have information for choosing effective products and for best practices to reduce the risk of fungicide resistance.

Nontraditional Fungicides: The remainder of this article will focus on USGA-funded research at Oregon State University investigating the use of phosphorous acid (commonly referred to as phosphite), iron sulfate heptahydrate, elemental sulfur, and the mineral oil Civitas Turf Defense to suppress Microdochium patch on annual bluegrass putting greens. This research has primarily focused on using combinations of these different products in the absence of traditional fungicides, although one study did apply fungicides over the treatments with no negative effects.

Phosphorous acid (a great tank-mix partner for Microdochium patch suppression): Phosphorous acid applied at 0.075 lbs. per 1,000 square feet every two weeks suppresses Microdochium patch in the Oregon studies, although plots frequently have over 5% of their area affected by disease. Data from western Oregon has consistently shown that phosphorous acid added as a tank-mix partner has improved Microdochium patch suppression when applied in combination with iron sulfate heptahydrate, Civitas Turf Defense, or elemental sulfur (Mattox et al., 2020a). It is important to note that phosphorous acid has been shown to worsen phosphorus nutrition deficiencies when applied to soils that are low in phosphorus. Therefore, it is recommended to remediate phosphorus deficiencies prior to applying phosphorous acid. Data from Oregon State University trials suggest that soil phosphorus levels do increase slowly over time following frequent phosphorous acid applications.

Iron sulfate heptahydrate (suppresses Microdochium patch, but use with caution): Iron sulfate applications suppress Microdochium patch on annual bluegrass putting greens at rates of 0.25 to 2.0 lbs. of iron sulfate heptahydrate per 1,000 square feet every two weeks (Mattox et al., 2016). Higher rates suppress Microdochium patch more consistently than lower rates, however, more thinning of the annual bluegrass sward developed from these applications. Research exploring carrier volumes found that turfgrass thinning can be reduced when using carrier volumes of 7.5 or 10 gallons per 1,000 square feet compared to lower carrier volumes, although unacceptable turfgrass thinning still occurred (Mattox et al., 2019a). Other research exploring less-frequent applications concluded that consistent Microdochium patch suppression was not achieved with application intervals longer than two weeks (Mattox et al, 2019b).

One method of reducing annual bluegrass thinning is to tank-mix lower rates of iron sulfate with phosphorous acid. Rates as low as 0.25 lbs. and 0.50 lbs. per 1,000 square feet of iron sulfate heptahydrate in a tank mix with 0.075 lbs. of phosphorous acid per 1,000 square feet applied every two weeks suppresses Microdochium patch as well as the 1.0 lbs. rate of iron sulfate heptahydrate applied alone every two weeks. Most of this research has been conducted on annual bluegrass putting greens, however, some research on annual bluegrass fairways suggests that similar control results are likely with less risk of thinning to the higher-cut turfgrass.

A recent trial suggests that iron sulfate suppresses Microdochium patch more than similar rates of chelated iron, although the addition of phosphorous acid as a tank-mix to the iron sulfate and chelated iron treatments leads to equivalent levels of Microdochium patch suppression. Treatments that include iron sulfate cause a moderate to severe blackening of turfgrass leaves and more thinning than the chelated iron. This blackening must be aesthetically acceptable for iron sulfate to be a viable option in your disease control program.

Elemental sulfur (pH effects, summer anthracnose, and turfgrass color remain a concern): Elemental sulfur has been shown to suppress Microdochium patch on annual bluegrass, but not to levels considered acceptable for golf course putting greens. Tank-mixes of 0.25 lbs. of elemental sulfur and 0.075 lbs. of phosphorous acid per 1,000 square feet every two weeks have increased the level of suppression compared to either product alone (Mattox et al., 2020a). This tank-mix may provide acceptable disease suppression for areas on the golf course that can tolerate some disease. While sulfur is a macronutrient and can sometimes improve turfgrass health – especially when sulfur levels are low, which is common on sand-based putting greens in the Pacific Northwest – it can also decrease the pH of the soil. Studies at Oregon State University have shown that sulfur applications can also increase the risk of anthracnose in the summer on annual bluegrass putting greens (McDonald et al., 2018). If anthracnose is a concern, it is currently recommended to apply no more than 1.5 lbs. of elemental sulfur per 1,000 square feet per year on annual bluegrass putting greens. In an ongoing long-term trial where 4 lbs. of elemental sulfur per 1,000 square feet per year has been applied in the same location for three consecutive years, plots receiving elemental sulfur have sometimes exhibited unacceptable turfgrass color. Research is continuing to investigate this phenomenon.

Mineral oil (suppresses Microdochium patch, although repeated applications in the winter has led to turfgrass thinning): The mineral oil Civitas Turf Defense suppresses Microdochium patch when applied at 8.5 ounces (oz.) per 1,000 square feet every two weeks, and suppression is improved when it is tank-mixed with 0.075 lbs. of phosphorous acid per 1,000 square feet (Mattox et al., 2020a). Research suggests that Civitas Turf Defense primes the defense systems in the plant (Cortes-Barco et al., 2010); therefore, it is encouraged to apply mineral oils before symptoms occur. Research at Oregon State University suggests that application frequency of mineral oil should be reduced or alternated with other products when the turfgrass is not actively growing because repeated applications throughout the winter has led to unacceptable thinning of the turfgrass sward. Research suggests that mineral oils may block the stomates, resulting in a reduction in gas exchange which could be the cause of thinning (Kreuser and Rossi, 2014).

Putting it all together: In western Oregon, the most consistent method of suppressing Microdochium patch on annual bluegrass putting greens and maintaining acceptable turfgrass quality using nontraditional fungicides has been to start in September with an application of 8.5 oz. of Civitas Turf Defense tank-mixed with 0.075 lbs. of phosphorous acid per 1,000 square feet (Table 1). This application is followed two weeks later with a tank-mix of 0.25 lbs. of sulfur and 0.075 lbs. of phosphorous acid per 1,000 square feet (Mattox et al, 2020b). These tank-mixes are rotated every two weeks throughout the fall through early spring. In many trials in western Oregon, the level of suppression was complete – i.e., no disease on any plots compared to the control – although in some years as much as 2% disease coverage has been observed compared to a control plot with over 40% Microdochium patch coverage. With this rotation, usually 16 or 17 applications are made each year equating to eight applications of the sulfur and phosphorous acid tank mix totaling 2.0 lbs. of sulfur per 1,000 square feet per year. This is higher than the maximum recommended annual amount of 1.5 lbs. of elemental sulfur per 1,000 square feet per year. An attempt at extending the application interval from two to three weeks with this rotation did result in disease suppression, although the severity of Microdochium patch was as high as 4% in one year of the trial compared to 50% severity in the nontreated control (Mattox et al., 2020b).

Final Thoughts

There are many ways of incorporating phosphorous acid, iron sulfate heptahydrate, elemental sulfur, and mineral oil into a Microdochium patch management program. In areas where traditional fungicides are permitted, these products may serve a role in reducing disease pressure and mitigating resistance issues. Possible options include using these products at lower-pressure times of the year such as early to midfall or late spring coupled with fungicides applied during high-pressure periods. Another option might be to tank-mix phosphorous acid or sulfur with your fungicides to reduce the disease pressure and increase the effectiveness of your fungicides. A third option might be to reduce the fungal spores and inoculum in early and midfall with two traditional fungicide applications 30 days apart and then switch to these alternative products in December and January when the temperatures are colder and the disease pressure is lower, after which you can make another fungicide application. Where fungicides are severely restricted or outright forbidden, some of these tools may mean the difference between a minor amount of disease or widespread damage because of Microdochium patch.

References

Cortes-Barco, A., T. Hsiang, and P. Goodwin. 2010. Induced systemic response against three foliar diseases of Agrostis stolonifera by (2R,3R)-butanediol or an isoparaffin mixture. Annals of Applied Biology. 157:179-189.

Dwyer, P., B. Horvath, A. Kravchenko, and J. Vargas. 2017. Predicting Microdochium patch on creeping bentgrass. Crop Science. 51:139-144.

Kowalewski, A., B. McDonald, C. Mattox, and E. Braithwaite. 2018. Effects of winter nitrogen, phosphate and potassium rates on Microdochium patch. 6th European Turf Society Conference Proceedings. 6:36.

Kreuser, W.C., and F.S. Rossi. 2014. The horticulture spray oil, Civitas, causes chronic phytotoxicity on cool-season golf turf. Hort Science. 49:1217-1224.

Mattox, C., A. Kowalewski, B. McDonald, J. Lambrinos, B. Daviscourt, and J. Pscheidt. 2016. Nitrogen and iron sulfate affect Microdochium patch severity and turf quality on annual bluegrass putting greens. Crop Science. July/August. 57(Supplement 1):S293-S300.

Mattox, C., A. Kowalewski, and B. McDonald. 2019a. The effects of iron sulfate heptahydrate water carrier volumes on Microdochium patch suppression and turfgrass quality. Agronomy Journal. doi:10.1002/agj2.20133

Mattox, C., A. Kowalewski, and B. McDonald. 2019b. The influence of iron sulfate application interval on the suppression of Microdochium patch on an annual bluegrass research green in western Oregon. Crop, Forage and Turfgrass Management. doi:10.2134/cftm2019.05.0041

Mattox, C., A. Kowalewski, B. McDonald, J. Lambrinos, and J. Pscheidt. 2020a. Combinations of rolling, mineral oil, sulfur and phosphorous acid affect Microdochium patch severity. Agronomy Journal. doi: 10.1002/agj2.20191

Mattox, C., M. Dumelle, B. McDonald, M. Gould, C. Olsen, E. Braithwaite, and A. Kowalewski. 2020b. Suppression of Microdochium patch using rotations of mineral oil, sulfur, and phosphorous acid. Agronomy Journal. doi: 10.1002/agj2.20557

McDonald, B., C. Mattox, M. Gould, and A. Kowalewski. 2018. Effects of sulfur and calcium source on pH, anthracnose severity, and Microdochium patch management on annual bluegrass in western Oregon. Crop Forage and Turfgrass Management. 4:1-3.