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  • Writer's pictureMark Hunt

September 5th, 2023

Hi All,

Just a mini-blog this week because I am on my hols up on the beautiful Llyn peninsular in North Wales and I need to get my legs working and some quality caffeine into my system. This time last week I forecast some nice weather for the coming week but not the magnitude of the heat which has ramped up since I sat at my laptop.

The reason as shown above is the behaviour of the Bay of Biscay (BOB) low pressure which instead of acting as a block to a southerly airstream has dropped south and west and instead served to funnel heat up from southern Europe and Africa as you can see from the edited GFS output above, courtesy of This weather pattern is what is known as a heat plume and it is precisely the same pattern that brought us record temperatures in July and August last year.

This heat plume will no doubt have some agronomic consequences which I'll touch upon later.

So this week we are stuck in this pattern till the early part of next week when the high will begin to dissipate and slink off south east into southern Europe. Now this doesn't mean that we will be dry with non-stop heat across the U.K & Ireland in the meantime because that BOB will funnel moist air into this pattern and that means some rain showers and particularly thunderstorms.

Forecasting where a thunderstorm is likely to occur is like looking for an honest man in parliament or maybe a water company that runs its business ethically ? It doesn't happen.

Bearing in mind you just need a seeding updraft as the precursor for a thunderstorm. This can come from heat radiated from cars in a car park or hot concrete for example and you can see the difficulty. Years ago I was out cycling in beautiful Leicestershire / Rutland when I saw a column of straw radiating upwards as an updraft was forming in a newly-harvested wheat field. I was fascinated and stopped to watch it. At the time I didn't realise its significance, but half an hour later as I got soaked to the skin, the penny dropped !

As I type there is a band of heavy storms pushing off The Irish Sea into Wexford and Cork with the beginning of that rain band over North Devon. During this week we can expect more storms and localised downpours feeding up across the west / south west coast of Ireland and England, Wales during Wednesday night / Thursday morning and again overnight into Saturday morning. Just don't expect me to forecast them accurately.

By Sunday we see more and more rainfall incursions as the high begins to lose its influence but these will be more north and westerly-orientated because of the position of the low pressure with heavier rain over Ireland and Scotland on Sunday. By Monday / Tuesday, these showers are more likely to affect central and eastern parts and we lose those heat extremes temperature-wise.

As we get to mid-week, net week, an Atlantic low pressure begins to influence our weather with stronger winds and rain across Ireland and the west of the U.K. As we progress through mid-week, the wind will swing round to the north west and this will both drop the temperatures significantly and push the rain further south.

Later next week we have a temporary high pressure insert itself before a deep Atlantic low pressure lines up behind it and so the rodeo ride of this years weather continues.

Below is the animated GIF for temperature, pressure and rainfall for the next 14 days....

Agronomic Notes

If you follow social media over the last few weeks, you would have seen plenty of images of disease whether that be Dollar Spot, Anthracnose Foliar Blight, Take All, Red Thread and Plant Parasitic Nematode. The rub of a cooler, wetter summer is higher levels of humidity than we would normally see in the summer and that means longer periods of plant leaf wetness and more dew. As I have talked about last week, we have seen some pretty significant dew events of late and even this week, mist, fog and Haar are prevalent.

Humidity drives disease, I couldn't put it much simpler.

When we look over the last two months we can illustrate this by using the Smith Kerns Dollar Spot Probability model. Now I realise I talk about this model a lot and just accept people know about it, so if you want to read about the basis for this model and how it is used, have a read here.

Now in the U.S, they use a guideline level of 20% to prompt application of a fungicide to prevent Dollar Spot occurrence.

Over here we aren't in that mindset, we have a different attitude in many instances to disease formation (particular off green) and of course, we don't have the arsenal of fungicides that they do in The States that both allow effective disease control and (just as importantly) rotation of chemical active ingredients to prevent the formation of resistance.

How I use the Smith Kerns Dollar Spot Probability Model (I'll call it Smith Kerns for the sake of my typing) is to match up disease occurrence with the model levels. So I know for example if I approach 40% and have a history of Dollar Spot on a facility then the potential for an outbreak is high. Some sites / locations it might be 25%, some it might be 50% and some may not see Dollar Spot at all as a disease. The bottom line is you need to establish your own baseline from monitoring, observation and then matching that up with data.

The Smith Kerns model isn't just used for Dollar Spot of course, it also works for Red Thread, Microdochium nivale and I think Leaf Spot in certain situations. It doesn't work entirely for Anthracnose as the initial trigger for spore germination is temperature and then humidity plays its part in terms of fungal development. So it kind of works more on the second stage of disease development for Anthracnose but you need the precursor of temperature for spore germination and then plant stress for further disease development as it switches into the Necrotrophic phase. (plant damaging phase)

One thing we should be aware of with Dollar Spot for example is this disease is on the move. In most (but not all) instances currently, Dollar Spot is a disease of tees, approaches and fairways with a lower incidence on greens, but this is changing with more and more movement onto fine turf. You only have to look across The North Sea to Scandinavia to see how rapidly this disease develops. 10 years ago it was hardly an issue, now it is one of their most-damaging turfgrass diseases.

You would have to point the finger at a changing climate and particularly I think to the warmer, more humid air patterns that appear more and more frequently.

I charted out the last 2 months Smith Kerns levels from 3 locations around and about and you can see some very clear peaks culminating with the highest in the 3rd week of August. That's the one driving the symptoms you are probably seeing now.

Of course before I sign off this blog, chuck on the walking shoes and head off for another lovely walk with the obligatory Flat White and cake stop, I should mention the heat this week.

With daytime temperatures in the high twenties, tipping into the low thirties in places and night time temperatures in the high teens / low twenties, this combination is high enough to induce plant stress. If you track Growth Potential you'll see how it begins to drop away by 20-30% to reflect the fact that temperatures are above-optimum for cool season grass growth, albeit temporarily.

The one saving grace is that wind speed levels are low so we shouldn't see sky-high E.T levels even though the temperature is high so that makes irrigation management less of a handful. If I look at some of my Prodata Reports output I see E.T levels are in the 3.5 - 3.9 mm per day sort of territory, that's high but nothing like the 5.5-6.5 mm per day we saw in June for example.

It probably goes without saying that this isn't a week to increase plant stress even further if it can be avoided because you may end up seeing more plant parasitic nematode activity / symptoms and definitely more Anthracnose foliar blight. Bide your time (if possible) till we lose the temperature in the early part of next week. Of course if it is in the diary and unavoidable, then so be it.

Right that is me done, normal weather service will be resumed next week and in the meantime, all the best.

Mark Hunt

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