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  • Andrew Turnbull

Disease Modelling & Microbes

The UK is blessed with a relatively benign climate, with no extremes of heat, cold, rain, ice or snow etc, but things could be changing. The higher average temperatures and increasingly heavy rainfall events during the summer season can increase disease outbreaks on turfgrass surfaces, such as Dollar Spot, caused by the fungus Sclerotinia homoeocarpa (recent studies attribute the disease to fungi from the Rutstroemia genus). Dollar Spot is increasingly being seen on fine turf surfaces in the UK such as golf courses and bowling greens and Figures 1-3 show the damage it can cause.

Fig. 1 Dollar Sport on a bowling green Fig. 2 Fungal hyphae of Dollar Spot Fig. 3 Damage to grass leaves

One way to deal with disease outbreaks is to be reactive i.e., see disease & spray a pesticide. An alternative is to predict disease outbreaks using a probability model and apply controls just before disease is predicted to show itself. One such model is the Smith-Kerns Dollar Spot Prediction Model. The Smith-Kerns Dollar Spot Prediction Model integrates the daily averages for temperature and humidity and calculates the probability of Dollar Sport breaking out on turfgrass surfaces. Intrigued, I accessed the weather data for my local area, (Tavistock in Devon, UK) and kept a running model up to the 31st July 2023. The results are shown in Fig4.

Fig. 4 – Smith-Kerns Output for Dollar Sport (Sclerotinia homoeocarpa) at Tavistock, UK, from 5th April to 31st July 2023

The red line on the chart is 20% probability that, once the red line is crossed by the % probability, often results in visible expression of this fungal disease. If the turf manager is keeping weather records or has an irrigation system linked to a weather station, the average temperature and humidity figures can be entered into a spread sheet that calculates the % probability of Dollar Spot disease and a chart can be linked to the data and monitored for the danger of disease outbreak.

It is the mission of South West Agronomy to help turfgrass managers to stop the reliance on chemical controls, and the use of microbial inoculants on turfgrass surfaces is increasing being accepted. Applying SigmaBio 2-3 days before the 20% probability line is reached can have a dramatic effect on the grass plant. Donald Thornburgh, Director of Agronomic Programmes at Sigma AgriScience, is a former Golf Course Superintendent and used SigmaBio in his disease control programmes. Don says,

“In my trials I used this model to narrow down the need for applications regarding dollar spot fungicides. As I got better at deferring when my spray threshold was going to be needed, I realized that 3-5 days prior to this I could get my biological application out. This turned into an average that if I was spraying on a 21 day rotation generally speaking the microbial population was always in the colonization phase when it is translocating maximum nutrients from the soil to the plant, this regulates the positive/ negative charge balance in the thatch layer and sulfur in the soil is more available, Nitrogen uptake from the profile is maximized, plant growth is focused on tillering and root improvement not shoots and overall the dollar spot doesn’t have a leg up on the balance of the disease triangle.

For a fee of £150 + vat, South West Agronomy will send a Smith-Kerns chart for your locality, as in Fig. 4. Alternatively, I will include a spreadsheet and chart with a purchase of 1 x 2.5 US gallon (9.46L) of SigmaBio.

Contact me for more information on 07900 692052, or email

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