Key Takeaways

• Research conducted at the University of the West of England studied the effects of foliar-applied phosphite on the growth and quality of cool-season turfgrass and the fate of phosphite in the plant and soil.
• Researchers determined that phosphite was rapidly taken up, translocated and accumulated in leaf and root tissues. However, foliar-applied phosphite does not supply any phosphorus (P) fertilization since there is no conversion to plant-available phosphate in cool-season turfgrass plants.
• Results showed that sequential foliar applications need to be made at regular intervals to maintain phosphite in the leaf at a level necessary for effective disease suppression.
• Sequential applications of phosphite over two years led to significant increases in rootzone P levels. This could have undesirable implications depending on the specific situation.
• In P-sufficient rootzones, foliar-applied phosphite increased the growth and quality of treated turfgrass. In P-deficient rootzones, foliar-applied phosphite had a detrimental effect on growth and quality.
   

Phosphite Use in Turfgrass

Research Study: Uptake, Translocation and Accumulation of Foliar-Applied Phosphite

To assess how foliar-applied phosphite was taken up, translocated and accumulated in turfgrass tissues, researchers initiated a two-year study to evaluate the fate of phosphite within cool-season turfgrass plants.

Materials and Methods

Phosphite was derived using lab-grade phosphorous acid and reagent-grade potassium hydroxide. The phosphite solution was then applied to creeping bentgrass and annual bluegrass in February 2011 and July 2012 at a rate of 1.15 ounces per 1,000 square feet of actual PO₃^3-. Analysis of tissue content for phosphite and phosphate was carried out over a six-week period using High Performance Ion Chromatography (HPIC).

Results

The results showed that foliar-applied phosphite was rapidly taken up into the leaf tissues and was detectable in the roots within hours, confirming phloem mobility and suggesting phosphite has the rare ability to both move up a plant’s xylem and down its phloem, giving it the quality referred to as symplastic ambimobility. Results from February 2011 (Figure 1) show that in creeping bentgrass and annual bluegrass, maximum leaf accumulations were attained within one week after treatment (WAT) with 3,332 ppm in creeping bentgrass and 4,395 ppm in annual bluegrass. Leaf accumulations of applied phosphite progressively declined and by six weeks had reduced to 14.9% and 19.6% of the maximum levels in creeping bentgrass and annual bluegrass respectively.

Results from the July 2012 study (data not shown) were similar to February 2011 with regard to rapid uptake and translocation rates, with maximum leaf accumulations at 1 WAT. However, there was a more rapid decline in leaf amounts during this period of higher turfgrass growth rates. In creeping bentgrass, phosphite amounts in leaf tissues at 6 WAT in 2012 had decreased to 7.6% of the maximum accumulation, compared to 14.9% during the 2011 study. Similarly, in annual bluegrass, accumulations in leaf tissues had dropped to 6.3% of the maximum accumulation in 2012, compared to 19.6% in 2011.

Demonstration of symplastic ambimobility – i.e., that the foliar-applied phosphite translocated and was detected in the roots of treated turfgrasses – was an important outcome of this study. Although the maximum root accumulations, 479 ppm in creeping bentgrass (February 2011) and 457 ppm in annual bluegrass (July 2012) were much less than in the leaf tissues, it remains a significant result as no other compound used for pathogen suppression in turfgrasses demonstrates symplastic ambimobility and this was the first time that this mobility of phosphite had been reported in turfgrass.

Implications for Turf Managers

These results are of particular significance to many golf course superintendents who utilize phosphite on a two- to three-week interval as part of their maintenance programs. Many apply phosphite prior to and during periods of high disease pressure, as phosphite treatments have been shown to suppress disease incidence and stimulate defense responses (Dempsey et al., 2012; Mattox et al., 2020; Dempsey et al., 2022). The results show that sequential foliar applications need to be made at regular intervals to maintain phosphite in the leaf at a level necessary for effective disease suppression. Favorable growing conditions, like those observed in the 2012 study, can cause a significant decline in the amount of phosphite in leaf tissue. Additionally, grass species impacts persistence of phosphite within the plant. Both these factors should be taken into consideration when planning spray intervals.

Research Study: Conversion of Phosphite to Phosphate in Turfgrass Tissues

Turf managers and golf course superintendents have often expressed interest in understanding the fate of P from phosphite applications and whether any conversion to plant-available phosphate takes place. Determining phosphate levels following phosphite treatment formed part of this study since this question about conversion of phosphite to phosphate in the plant is often raised, with numerous commercial suppliers also claiming phosphite as a P source following conversion of phosphite to plant-available phosphate.

Materials and Methods

The same method outlined in the research study section above was used to assess how much, if any, foliar-applied phosphite was converted to phosphate in two cool-season turf species, creeping bentgrass and annual bluegrass.

Results

The results were conclusive. In creeping bentgrass and annual bluegrass, during both assessment periods, there was no significant difference in the level of leaf phosphate following treatment by phosphite (data not shown). Foliar applications of phosphite made to cool-season turf as part of a plant health and protection program will not result in any phosphorus fertilization by way of conversion to phosphate.

Implications for Turf Managers

The study concluded that foliar-applied phosphite does not convert into phosphate within creeping bentgrass or annual bluegrass plants. Therefore, if testing or visual cues indicate a P deficiency, golf course superintendents should rely on fertilizers containing phosphate, not phosphite, to meet the plant’s nutrition needs. If turfgrass quality improves following foliar applications of phosphite it is likely due to overall plant health factors or disease suppression.

Research Study: Rootzone Nutrient Analyses Following Sequential Phosphite Treatments

Another question often posed by turfgrass managers regarding the fate of foliar-applied phosphite is if any amount of P, in any form, ends up in the soil. Knowing the impact foliar-applied phosphite has on rootzone P levels would aid golf course superintendents and other turf managers in making fertilization decisions.

Materials and Methods

To evaluate the effect of long-term, sequential phosphite treatments on turfgrass rootzone P levels, phosphite and phosphate were applied at 1.15 ounces per 1,000 square feet of actual PO₃^3- and PO₄^3- at monthly intervals to greenhouse samples of creeping bentgrass and annual bluegrass from July 2012 to July 2014.

Results

Rootzone nutrient levels prior to the start of the two-year treatment program and at the conclusion of the study are shown in Table 1. Sequential foliar applications of either phosphite or phosphate significantly increased soil P levels in the rootzones of both turfgrass species compared to pre-treatment levels.

Over the two-year period, P levels in phosphite-treated rootzones increased significantly greater than phosphate-treated rootzones. In creeping bentgrass rootzones, P levels following phosphite treatments increased from 37 ppm to 51 ppm, but only increased to 40 ppm following phosphate treatments. Similarly, in the annual bluegrass rootzones, P levels increased from 37 ppm to 57 ppm following phosphite treatments but only increased to 44 ppm after phosphate treatments.

Implications for Turf Managers

Soil type, organic matter, microbial activity and other factors likely play a role in the amount of P accumulation in the rootzone. The significantly higher level of rootzone P accumulation following phosphite treatment could be due to it being locked into the rhizosphere by soil microorganisms, since oxidation of phosphite to phosphate in soil relies on microbial activity, which is a slow process. More research is needed on the mechanism by which this occurs. The movement of P out of a turfgrass system is not desirable due to its impact on water bodies. Phosphorus in the rootzone following phosphate treatment would be less persistent and more easily leached, keeping in mind the cation exchange capacity status of the studied rootzones was extremely low. Whatever the reason, the steady increase of soil P levels following sequential phosphite treatments may pose a problem for turfgrass management. Higher levels of soil P are often correlated with increased populations of annual bluegrass – a species that is dominant in many golf greens but is still widely regarded as an undesirable weed. Turf managers should monitor P levels in their regular soil tests if they are making foliar phosphite applications to be sure that an undesirable accumulation does not develop.

Research Study: Effects of Phosphite on Turfgrass Growth

The major aim of this final aspect of the research study was to assess the properties of phosphite as a source of P nutrition for turfgrass and its effects on growth and rootzone P deficiency responses. As stated earlier, the disease suppression and plant health properties of foliar-applied phosphite are well researched, but little is known regarding its effect on turfgrass growth and development.

Materials and Methods

Foliar treatments of phosphite and phosphate at 1.15 ounces per 1,000 square feet as PO₃^3- and PO₄^3- and potassium chloride (KCl) as a control were applied to perennial ryegrass (Lolium perenne L.) and annual bluegrass biweekly over a six-month period. Two soil P levels were used in the study, P-sufficient (P greater than 38 ppm), and P-deficient (P less than 6 ppm). Treatment effect on shoot growth was determined by the cumulative dry leaf weights. Crowns and roots were collected at the end of the trial and weighed for dry mass determination and calculation of root to shoot ratios.

Results

The results shown in Figure 2 illustrate that for turf plants growing in a phosphorus-sufficient rootzone (P greater than 35 ppm), the phosphite treatment significantly increased dry weights in leaf, crown and root tissues, compared with phosphate- and KCl-treated plants (perennial ryegrass data not shown). Phosphite treatments can inhibit pathogenic soil microorganisms which have a deleterious effect on turfgrass development. Phosphite has proven efficacy in reducing these organisms allowing for the healthier development of turfgrass and may account for the increase in dry weights. The enhanced growth may also be caused by a growth-regulatory or phytohormonal factor affecting sugar metabolism, stimulation of the shikimic acid pathway, or internal hormonal and chemical changes.

Conversely, the results determined that phosphite treatment to both perennial ryegrass and annual bluegrass growing in phosphorus-deficient rootzones (P less than 6 ppm) significantly reduced dry weights in leaf, crown and root tissues compared with phosphate- and KCl-treated plants (Figure 2, perennial ryegrass data not shown). This is likely due to phosphite suppressing many of the plant’s responses to limited P, such as enhanced root growth and increased root to shoot ratios. Phosphite also competes with phosphate for uptake via the same plant transport system, which leads to an even further reduction in usable P in the soil.

Implications for Turf Managers

If foliar-applied phosphite is a regular part of superintendents’ agronomic programs, it’s important to maintain plant-available P amounts in the rootzone at sufficient levels. Foliar phosphite applications do not increase the amount of usable P in the soil and can enhance the negative effects of a P-deficient rootzone on turf plants, so applications of phosphate fertilizer are still required when soil tests or visual cues indicate a P deficiency.

Conclusion

Foliar phosphite applications are a valuable tool for golf course superintendents to manage disease throughout the year and promote plant health during periods of stress. This research showed phosphite is rapidly taken up and translocated by turfgrass, and that sequential applications at label rates can maintain beneficial levels within the plant. However, foliar-applied phosphite does not supply any P fertilization since there is no conversion to phosphate in cool-season turfgrass plants. Long-term, sequential phosphite treatments can also lead to cumulative increases in soil P levels. The mechanism by which this occurs and how much of this soil P becomes plant-available requires further research, but turf managers regularly applying phosphite should monitor soil P since increased levels favor annual bluegrass and can have other undesirable consequences. Finally, use caution when applying phosphite if P is deficient in the rootzone as foliar-applied phosphite decreases the growth and quality of cool-season turfgrass in this situation. Turf managers should keep the results of this research in mind to maximize the benefits of foliar-applied phosphite on turf health and limit any negative impact on turf quality.

This article is based on research published in the Journal of Plant Nutrition. You can find additional information, including all tables and figures here.

References

Cook, J., Landschoot, P. J., & Schlossberg, M. J. (2006). Phosphonate products for disease control and putting green quality. Golf Course Management, April, 93-96.

Dempsey, J.J., Wilson, I., Spencer-Phillips, P.T.N., & Arnold, D. (2012). Suppression of Microdochium nivale by potassium phosphite in cool-season turfgrasses. Acta Agriculturae Scandinavica, Section B – Soil & Plant Science, 62, (Suppl. 1), 70-78.

Dempsey, J. J., Wilson, I., Spencer-Phillips, P.T.N., & Arnold, D. L. (2022). Phosphite-mediated enhancement of defence responses in Agrostis stolonifera and Poa annua infected by Microdochium nivale. Plant Pathology, 71, 1486-1495. doi.org/10.1111/ppa.13584

Mattox, C.M., Kowalewski, A.R., McDonald, B.W., Lambrinos, J.G., & Pscheidt, J.W. (2020). Combinations of rolling, mineral oil, sulfur, and phosphorous acid affect Microdochium patch severity. Agronomy Journal, 112, 3383-3395.