In discussion with Glenn Kirby, Research Director
A course manager asked a useful question this week, “Is it going to get hot enough to put a pause on dollar spot?”
For UK conditions, the answer is almost certainly no.
In most UK situations, hot weather does not push us beyond the dollar spot window. More often, it moves us deeper into a favourable temperature range. The deciding factors then become humidity, leaf wetness, duration, and the condition of the plant.
Heat alone is unlikely to stop activity
The Smith-Kerns dollar spot model is the most useful piece of work here. It is based on a five-day rolling average of air temperature and relative humidity. That is a very important detail.
The model is not reacting to one hot afternoon or a single forecast maximum. It is looking at the average conditions over several days. That makes it much more useful for disease pressure, because dollar spot development is driven by sustained patterns rather than isolated weather events.
The model includes temperature limits too, with activity constrained below roughly 10°C and above roughly 35°C, based on five-day average temperature. Those limits reflect the point at which conditions become too cool or too hot for meaningful dollar spot development (when the work was done in the USA)
The upper limit can sound relevant during a heatwave, but it is rarely realistic in the UK.
A forecast high of 30–35°C is not the same as an average temperature of 30–35°C. To approach a 35°C daily average, the night would also need to remain exceptionally warm. To reach that as a five-day rolling average, those extreme conditions would need to repeat for several days.
Even during the UK’s most extreme heat events, that is a very high bar. The UK’s highest recorded daytime temp is 40.3c and overnight its around 28c. So even a record-breaking day and night does not hit that 35c threshold. We almost certainly won’t see it as a five-day average.
So, when UK temperatures climb, the better assumption is not, “This may be too hot for dollar spot.”
It is more likely, “These are suitable temperatures. What is happening with humidity and leaf wetness?”
Humidity is usually the better question
Once temperature is suitable, the risk picture depends heavily on moisture in the canopy.
A hot, dry spell with low relative humidity, rapid drying and little overnight leaf wetness is very different from a hot, humid spell with warm nights, heavy dew and a canopy that stays wet for hours.
Both can feel like “hot weather” to us. They are not the same disease environment.
If humidity drops away and the leaf dries quickly, pressure may ease even when temperatures remain high. The plant may still be under heat and moisture stress, but the pathogen is not getting the same sustained wet, humid conditions.
If heat is combined with high humidity, warm nights and prolonged leaf wetness, pressure can remain high or build quickly. That is especially true where dollar spot is already active or where the usual weak areas are beginning to show.
This is why maximum temperature on its own can be misleading. A hot day followed by a dry evening and a clean, quick-drying surface is one situation. A hot day followed by a warm, humid night and heavy dew is another entirely.
Duration changes the decision
A brief spike in temperature or humidity deserves attention, but it is not the same as a sustained pressure period.
This is where the five-day rolling average becomes useful in practice. It helps separate a short-lived weather event from a pattern that can drive disease. One or two uncomfortable days may mean closer monitoring, especially on known problem areas. Several days of suitable temperatures, high humidity, warm nights and repeated leaf wetness is a different level of concern.
The question for turf managers is not simply, “How hot will it get?”
It is, “How long are we staying in the pressure window?”
That distinction matters. If conditions are only briefly favourable, the response might be observation, dew removal, irrigation discipline and checking vulnerable areas. If the pattern is sustained, especially where the plant is already under stress, the risk is more likely to become visible disease.
Warm nights are particularly important. Daytime highs may fluctuate, but if nights stay warm and humid, the disease environment can remain consistent even when the daytime forecast looks variable.
Plant condition determines how visible the pressure becomes
Weather creates the opportunity. Plant condition often decides how much damage we actually see.
Hot weather increases stress on the sward. Moisture demand rises, dry patch becomes more obvious, weaker areas thin faster, and existing issues with nutrition, rootzone performance, thatch, airflow or mowing stress tend to show up.
That matters because dollar spot is rarely just a weather problem. It becomes more damaging when suitable weather arrives at the same time as a vulnerable plant.
A dense, healthy sward with even soil moisture, balanced nutrition, good airflow and limited overnight leaf wetness is in a much stronger position than turf that is already dry, hungry, thin or sitting wet through the night.
So the practical management question becomes, “Have we reduced plant stress enough give the plant a chance in suitable weather?”
That brings the focus back to the basics: moisture uniformity, irrigation timing, nutrition, dew control, mowing stress, airflow and the areas where dollar spot normally appears first.
The real message
In UK conditions, hot weather is unlikely to become hot enough to pause dollar spot on temperature alone.
The Smith-Kerns model includes an upper temperature cut-off of around 35°C, but that refers to a five-day average temperature, not a single daytime maximum. UK heat events may produce very high afternoon temperatures, but they are unlikely to deliver sustained five-day averages near that upper limit.
So we should not wait for heat to stop the disease.
The better questions are:
Are temperatures sitting in a suitable range?
Is humidity staying high enough to keep the canopy wet or slow to dry?
Are warm nights and dew extending the infection window?
Is this a brief spike or a sustained pressure period?
Is the plant strong enough to tolerate the pressure?
If the weather is hot and dry, with humidity falling away and little overnight leaf wetness, dollar spot pressure may ease.
If the weather is hot, humid, wet overnight and settled for several days, the risk is very different, particularly where turf is already stressed.
For most UK heat events, the issue is not that conditions are becoming too hot for dollar spot. The issue is whether suitable heat is being paired with enough humidity, leaf wetness and plant stress to let the disease develop.
