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  • Writer's pictureMark Hunt

August 30th

Well that's August just about done and dusted and for many they'll be glad to see the back of it. Traditionally the start of September tends to be associated with stable high pressure systems before the jet stream drops south and opens the door for autumn Atlantic low pressure systems. Not this one though as a period of prolonged rain could be on the horizon for the start of the month.

This would be entirely typical as I'm off to the lovely (and very Welsh) town of Pwllheli for a late summer break and another (most likely) futile opportunity at throwing a fly into the sea and gaze into the horizon (probably). Must practice by 'Diolch' pronunciation. (Welsh for thank you).....

Out on the mountain bike last week, I had a brutal reminder of how prolonged this summer's drought has been when I cycled over one of my local brooks. Langton brook is a feeder stream to the River Welland and a home to Wild Brown Trout. Not any longer as the riffles and gravel runs are now bone dry and stagnant pools are all that remain. The image on the bottom left shows the brook in winter flood (I had to carry my bike over on that occasion and wade through up to my waist, a tad chilly it was too !!)

Thankfully last week saw some welcome rain for central and eastern areas although some eastern locations still missed out. The rain pushed up country and then slowly drifted east with some torrential downpours in places as this radar image from shows.....

Some places in Herts and Essex received over 60 mm in one day with peak rainfall rates exceeding 100mm per hour (see below). That's tropical downpour territory.

So looking forward now and as hinted above, what has the coming week got in store for us ????

General Weather Situation - w/c 29th August, 2022

Looking at the animated GIF's from for the next 7 days, we can see how we are settled for the majority of this week with high pressure in charge but the change begins on Friday as two low pressure systems form a 'trough pattern' in the jet stream. I call them a trough as its the opposite of a peak pattern when warm / hot air pushes upwards into the U.K & Ireland.

Now when this happens the low pressure systems tend to stay in situ for a number of days rather than pushing across the U.K & Ireland over the course of 12-18 hours. This means 'trough patterns' tend to be associated with slow-moving weather systems and high rainfall totals.

Yet again and as predicted last week, it is the formation of a BOB low (Bay of Biscay) low pressure system that 'seeds' the conditions for change by keeping high pressure and warm air supressed in Southern Europe and allowing an Atlantic low to push in...

It is clear to me that the formation and behaviour of Bay of Biscay low pressure systems are a key feature in our weather and particularly rainfall for the southern half of the U.K and Ireland.

So Monday to Friday will see warm days with temperatures pushing into the mid-twenties for England and Wales by Thursday, with Scotland down at 20°C highs and Ireland in the mid-teens. Wales is warm because the predominant wind direction will continue to be easterly and so that means eastern coasts will feel a little on the chillier side.

As we reach Friday, we see that low pressure system push down from the north west and that'll introduce rain into the north west / west of Ireland and Scotland on Friday morning before pushing eastwards later on Friday into Wales and then overnight into Saturday for central areas. Saturday's rain won't I think mean too much for these areas but Sunday sees a much heavier band of rain push across Wales and into England and Scotland having crossed Ireland on Saturday night / Sunday morning.

Weather Outlook - w/c 5th September, 2022

The animated GIF's for next week show how the low pressure systems stay locked in place for the majority of the week so it'll be case of bands of rain pushing through with some sunshine in-between. One of the things it won't be is particularly cool with temperatures dropping down to high teens / 20°C for England and Wales and high teens for Scotland and Ireland. This combination of warmth and humidity does have a negative connotation though and spells trouble in terms of turfgrass disease formation which I will cover shortly.

Agronomic Notes

As I mentioned above, the prospect of cooler, wetter and more humid weather, though welcome on the whole, will bring an increased threat of disease activity.

The Smith Kerns Dollar Spot probability model is a useful tool for showing not only disease pressure for this pathogen but also it gives an indicator to some of the other humidity affected diseases like Red Thread, Microdochium nivale, Anthracnose and the like.

The above graph shows the plotted mean daily humidity vs. the Smith Kerns Dollar Spot probability during this August for a location near Guildford. This location had rainfall on the 15th / 16th (42 mm) and 25th / 26th (37 mm) which its associated higher humidity. You can clearly see the humidity spikes and how the Smith Kerns probability has increased to just under 45% in the last few days before dropping back slightly. That level has been high enough to trigger Microdochium activity and also we have seen some rapid colonisation by Dollar Spot.

This image was sent to me from a Poa / Bent bowling green (thanks Pete) and it is affecting both grass species, so that's Poa annua var. reptans (Perennial Poa biotype as it is known) and Browntop Bent.

Now Dollar Spot is a disease on the increase across Europe and I feel its increasing incidence is linked to climate change. In the past on the continent, Dollar Spot was mainly an 'off-green' disease affecting fairways, tees, approaches and the like. Nowadays on the continent and especially in Scandinavia, it is an aggressive greens disease as well. It shares this feature with the same species of pathogen in the U.S. I say the same species but probably not the same genetic background and this is where it gets complicated.

Recently scientists changed the latin name of Dollar Spot to Clarireedia homeocarpa (from Sclerotinia homeocarpa, they love messing about with latin names you know).

In turn it seems that there are at least five species in that fungal genus (Clarireedia) that can cause dollar spot disease. At least two species of Clarireedia have been documented in Scandinavia. These are Clarireedia jacksonii (also present in the States and Canada) and an unnamed species of Clarireedia. I expect the same to be true in the U.K & Ireland, i.e. that we have at least two distinct species of Clarireedia causing this disease.

Recently STERF (Scandinavian Turfgrass & Environmental Research Foundation) and NIBIO (Norwegian Institute of Bioeconomy Research) carried out some in vitro screening work using Dollar Spot isolates from the U.S and Europe (including 2 from the U.K) and investigated their pathogenicity on and the resistance of different grass species. I am indebted to Kate Entwistle (as per usual) for sending me this poster abstract from the recent International Turfgrass Research Conference in beautiful Copenhagen. It makes very interesting reading.

I would just point out in the case of Clarireedia and Poa annua resistance that this research used a Poa annua cultivar called 'Two Putt' for their Poa annua resistance work. Two Putt is a Seed Research of Oregon cultivar and so I think (and please someone correct me if I'm wrong) that it has its genetic / geographical links in the U.S rather than Europe. So what I'm saying here is just because Poa annua does shows poor resistance to Clarireedia in this work, it doesn't mean all Poa annua will be affected the same because of the genetic variability across the pathogen and Poa annua.

Interestingly, if you look at the resistance of grass species vs. Clarireedia sp. isolated from the UK and Norway (NO), you can see distinct differences for Slender and Strong Creeping Red Fescue, Perennial Ryegrass and Kentucky Bluegrass dependent on the geographical origin of the isolate. These grass species show lower resistance (highlighted) to the Clarireedia species isolated from the U.K vs. the one isolated from Norway and that makes me think the two species are genetically as well as geographically different.

It is also probably why we tend to see more Dollar Spot on areas that feature Fescue / Rye across the U.K & Ireland, more than greens. (though it is a problem on golf and bowling greens particularly along the south coast and south west of England).

So where it occurs seems to be dependent on the specific Dollar Spot isolate and its geographical origin.

It is great work and you can read the excellent paper here

Kate also sent me a link to a paper produced by the same organisation that showed the results of in vitro sensitivity work on Clarireedia, Fusarium and Microdochium species to the range of fungicides registered in Scandinavia. It may surprise some of you to know they have a wider choice of actives in Denmark, Sweden and Norway than we do in the U.K and to a certain extent, Ireland as well :)

For your info, a fungal pathogen is said to be 'sensitive' to a fungicide active if that particular A.I is effective against it. It is termed 'insensitive' if either the A.I doesn't work on the pathogen because it isn't in the control spectrum or the pathogen has developed resistance.

You could read this paper here

Looking ahead to the projected change in weather at the end of the week and the associated increase in humidity from late Friday onwards (and likely continuing through next week), I expect to see a lot more disease activity than we have of late. Specifically Dollar Spot on sites that have been affected in the past but also Microdochium nivale, Red Thread, Anthracnose Foliar Blight and probably a lot of etiolated growth on collars and approaches as well. I'm expecting plant leaf wetness levels to elevate and this will be trigger I think.

With this weeks more settled weather, it might present a good opportunity to get a control spray down in either pesticidal or non-pesticidal form dependent on disease and area applied of course. Remember that with Dollar Spot (ass with Red Thread) an effective strategy is to grow it out using a nitrogen & iron spray for instance on outfield areas.

On the plus side, the rain and warm temperatures will push grass growth along nicely, so recovery from recent aeration and / or disease activity should be pretty quick.👍

Now it looks to me like next Monday could be a wet one in North Wales so I'll probably be back on the blog again next week rather than skipping one :)

All the best.

Mark Hunt

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