Phosphite - What’s all the fuzz about?
John Dempsey, Course Superintendent at the Curragh Golf Club in County Kildare, has been conducting his own trials and, in this article, explains the method and the results to date
So, what is phosphite, and what exactly can it do for turfgrasses?
Phosphorous is a major plant nutrient - the P in NPK - which, in plants, is taken up and used in the form of phosphate (PO4). Phosphite is very similar chemically to phosphate -its chemical formulation is PO3 - only one molecule of oxygen difference. However, where phosphate is vital for numerous metabolic processes in the plant, phosphite, because of its molecular shape, cannot be utilised by plants as a source of P nutrition and, importantly, should not be applied to plants which are deficient in P.

Since 2005, potassium phosphite products have been marketed in the UK and Ireland as biostimulants, or even defence activators, and are promoted as a means to reduce Microdochium nivale, the fungus which causes fusarium patch.
Chemical plant protectants are an integral part of IPM and, whilst the efficacy and safety of these products is not disputed, their use sometimes gives rise to a number of contentious issues:
- adverse public opinion due to the perception of high frequency of use
- associated costs of chemicals
- possible inhibition of non-target beneficial microorganisms
- development of fungicide-resistant populations
- possible legislative restrictions
This makes new or alternative means to reduce susceptibility to M. nivale a desirable target for turfgrass research.

The research has two straightforward questions to answer:
- Does phosphite suppress Microdochium nivale in turfgrasses?
- By what means does this occur?
To answer these questions, we have been carrying out extensive field trials and numerous laboratory procedures, which have produced interesting and significant results.
Field trials

Treatments applied:
1: PK Plus 3:7:18 (Grigg Bros, 14% KH2PO3) applied at 20 l/ha-1
2: PK Plus (20 l/ha-1) + Ultraplex Biostimulant (Grigg Bros, 5-0-3 applied at 10 l/ha-1)
3: Chipco Green, (Bayer, 255 g/l iprodione) applied at 20 l/ha-1)
4: Chipco Green (20 l/ha-1) + PK Plus (20 l/ha-1)
5: NPK control (3:7:18 to match nutritional input of PK Plus) applied at 20 l/ha-1
6: Untreated control
Assessments
The plots are assessed each month for M. nivale occurrence and rated on a scale of 0-100. Turf quality (which excludes the impact of disease within each plot) is also measured and assessed visually and marked on a scale of 1 - 10.
Field trial results - disease incidence

The treatment which was most effective at reducing M. nivale incidence was the combination of iprodione (Chipco Green) and potassium phosphite. These fully inhibited M. nivale incidence on the majority of plots, this total inhibition indicating either differing modes of suppression or a possible synergistic effect.
Turfgrass quality

By the end of the trial periods, the quality and density of all phosphite treated plots were far superior to all other treatment regimes.
What has been concluded from these trials is that sequential applications of potassium phosphite significantly reduced Microdochium nivale incidence and led to an enhancement in turfgrass quality.
Means of suppression

- Direct, which means phosphite acts as a fungicide and kills or stops the growth of the pathogen
- Indirect, by stimulating the turfgrass plants natural defence mechanisms.
To test whether phosphite acted directly on the pathogen, we carried out a range of studies. This involved propagating M. nivale samples taken from infected golf greens. This was then used for a range of laboratory studies which assessed phosphites effect on Microdochium mycelial growth. The fungal mycelium was grown on petri dishes which had been amended with various concentrations of phosphite. We then compared growth to mycelium grown on phosphate and unamended controls.
Phosphite effects on mycelial growth

What has been learned to date with this section of the research is that phosphite, when in contact with M. nivale, directly inhibits the mycelial growth and conidial germination and causes disruption of hyphal morphology. In the plant, this means phosphite slows the growth of the pathogen, causes release of stress metabolites, thereby allowing for increased time for the plant to initiate defence responses.
What happens when phosphite is applied to turfgrass?

We did this by applying the phosphite in spray, collecting leaf, crown and root samples over a period, and then analysing them using a laboratory procedure which separated and quantified the individual ions of phosphite and phosphate.

The research shows that phosphite is rapidly assimilated by turfgrass and translocates throughout the entire plant. The chart above shows phosphite accumulations in turfgrass over six weeks. Importantly, at forty-eight hours post application, amounts were almost 5000ppm, proving rapid assimilation. Amounts declined to 2561ppm after two weeks, indicating a three to four week application cycle would maintain levels consistently around 4000ppm. The analysis

A longer term study is showing that metabolic rate affects the amount of time phosphite persists in the leaf tissues, with a trend for increases in accumulations in meristematic tissues.
Our analyses of phosphite treated turfgrass tissues determined rapid in planta accumulations, symplastic mobility and no in planta conversion to phosphate.
Further research

Our current research is also looking at the possibility that phosphite can reduce M. nivale infection through enhancement of the turfgrasses natural defence processes.
Does Phosphite enhance the defence responses in infected turfgrass?
Obviously, we first need to understand the Microdochium infection process and turfgrass responses before we can determine if these responses are stimulated by phosphite treatment. We are presently studying this using pot samples and infected greens, and employing a range of microscopy procedures.
It will be a further two to three years before the study can be fully published and then, hopefully, will give a valuable insight into the question "So what is phosphite and what exactly can it do for turfgrasses?"
In summary, what can be concluded from the results of this research to date is that routine and sequential applications of phosphite, as part of an integrated disease management programme, whilst not reducing M. nivale to an acceptable level, will significantly reduce the incidence and severity of the disease. Furthermore, the addition of phosphite to a fungicide can significantly enhance suppression of M. nivale.
Phosphite treatment also gives rise to significantly improved visual quality and greater density than the untreated controls.
Chipco Green and Bullet Liquid Phosphite are available from the Pitchcare Store
This article was originally published in Greenside magazine, the official publication of the GCSAI.