2nd September, 2024

Hi All,

Well, here I am ensconced on the Lynn Peninsular at Pwllheli, an area I love for its 'Welshness'. and the fact that Pwllheli beach is shingle / sand so it doesn't get mobbed like others up the coast at Abersoch and the like. A beautiful evening last night chasing Bass and Garfish on the fly has been replaced by a 20mm rain day. Well it is North Wales and technically summer has now ended.

An interesting fact I found out yesterday talking to the local fisherman on the beach is that Bonito (a type of Tuna) are turning up in the catch reports off the Lynn Peninsular in North Wales. Something that would of been unheard of 10 years ago. Is this a sign of warming sea temperatures ? (Note - I would post a picture but all my gallery comes up with is a picture of a tin of Tuna 🙄)

Watching the forecast for Wales last night, the forecaster gave a outlook for the next two days and then followed it with that immortal line "Still a lot of uncertainty in the forecast from mid-week onwards". Midweek ?, that's 2 days ago and you can't give a firm forecast ? Eh ?

So, I took a look at Meteoblue's forecast for this week and above I have highlighted the predictability of the forecast which they kindly publish in their Meteogram.

As you can see it is pretty low with a rating of 31% for Wednesday's forecast. In essence this means there's only a 1/3 chance of the forecast being correct and a 2/3 chance it will be incorrect. Normally I would associate this type of % with a forecast 7 days away, not 2 and I sit here wondering why the weather has suddenly become so unforecastable.

Obviously the conspiracy theorists will point to the global manipulation of the weather by 'Cloud Seeding' because there's a proportion of the population that like to think that way. I even saw it mentioned as the reason there's less Mullet around on our coasts this year rather than the more likely reason that there's more gill netting and commercial trawling ! Geez Louise.

On a serious note though, the 'forecastability' of our weather has taken a turn for the worst and I intend to do some digging to find out why. Why does it matter ? I mean we moan when the forecast is incorrect and we moan when it is correct if the type of weather correctly forecast doesn't meet our aspirations :)

Well, consider disease modelling.....

This relies on an equation for a specific disease digesting weather forecast data for the next 7-10 days and converting that into a disease occurrence / severity probability.

What chance of this being anywhere near correct when the 'forecastability ' has such a low probability of being correct ?

Well fortunately disease modelling doesn't rely on all the parameters associated with a forecast and in my work in a previous life, the parameters it does rely on are more reliable in their forecasting accuracy, but there's still a question mark in my mind when we have this short-term unpredictability in our weather. It just highlights the challenges behind successful disease probability forecasting now and in the future. This isn't a criticism of disease modelling, more a realisation that it is only as good as the accuracy of the forecast and end-users should acknowledge that limitation.

With the low predictability of the forecast in mind and the fact I am on holiday, I won't spend too much time on the forecasting side of things aside from telling you how I think it looks this week and next....

This week's weather conundrum

The difficulty I think the forecasters are having is predicting the behaviour of these 'teardrop low pressure systems' as I call them. Consider the GFS output for Thursday and you can see the low pressure sitting across Scandinavia. This low pressure teardrop 'broke off' from a main area of low pressure at the start of this week as you'll see in the animated GIF below for this week. Forecasting whether it or the Atlantic high pressure will dominate our weather going forward is where the issue lies....

GFS prediction - Sept 2nd - Sept 9th, 2024 courtesy of Tropicaltidbits.com

If I take it on its merit we look to have a mid-week dip in temperatures this week, particularly over western and north western UK as the wind swings more northerly, before recovering at the end of the week. Rainfall will never be far away but for this week I think it'll be heavier in the west / north west with the exception of Ireland where I think the dip in temperatures will be less marked due to their closer proximity to the high pressure system. Less rain for Ireland as well once Monday / Tuesday is out the way with just at present, a question mark for Friday. rainfall-wise. If the low pressure develops as indicated in the animated GIF, then we shall see a higher likelihood of rainfall across the east and cooler as well from the end of the week / weekend as those northerly winds return.

GFS prediction - Sept 9th - Sept 16th, 2024 courtesy of Tropicaltidbits.com

With my previous comments in mind, looking ahead to next week's weather would seem slightly foolhardy but here goes....

Weather Outlook

Well next week starts settled particularly in the southern half of the U.K & Ireland. Pleasant temperatures initially but the influence of a cooler northerly airstream will begin to make its presence felt across Scotland and the north of England, where it'll become winder, cooler and more unsettled from the start of the week. That cooler, more unsettled air will push southwards to all areas by mid-week, with the wind swinging from westerly to more northerly / north easterly.

So I'd expect rain for the north and north west of the U.K & Ireland at the start of next week breaking down to a more cooler showery outlook by mid-week for all areas. Now as we approach the end of next week, high pressure looks to exert its influence again so the weather settles down, warms up, a sort of Indian-summer type feel.

Agronomic Notes

This week I wanted to talk about Anthracnose as I had a good number of queries last week concerning this disease and I believe there's plenty about. Now we have some pretty straight-forward criteria for Anthracnose as a disease. Firstly, we need some successive days of high temperature to germinate the spores. This is held to be 25°C, but I think we should qualify this because often surface organic matter can heat up to a higher temperature than the air temperature, so I tend to think anything over 23°C air temperature is fair game. Besides this I am sure spore germination will follow a normal distribution pattern rather than a threshold temperature below which no spores germinate.

Maybe something like this ?

Once we get those 3 days of successive high temperature, we need the next piece of the disease initiation puzzle to fall into place and that is prolonged leaf wetness to allow the fungal mycelium to grow from the spore into the leaf. Now Anthracnose is a real crawler of a fungus in terms of speed, unlike Microdochium and Dollar Spot, it prefers life in the slow lane and so needs a high humidity / prolonged leaf wetness to develop onto and into the leaf. So we might have the temperature trigger but unless it is followed by high humidity it is unlikely that the fungus will develop. Of course when the temperature is high we tend to be irrigating (out of necessity!) and at night, so we kind of make things easy for it in some respects.

Below I have charted out the maximum air temperature (in °C) and mean relative humidity (in %) at our default location this summer. I have highlighted in red the periods when we met the temperature part of the Anthracnose spore initiation criteria. Now what would really be useful would be to show the severity and duration of leaf wetness as measured by our old friend, the Davis leaf moisture sensor (above) but that's for another day and another disease model 😉

So I think from May 1st to the present day at our default Thame, U.K location, we have had 5 potential Anthracnose triggers temperature-wise in summer 2024. If you look at the mean humidity graph (in blue) immediately after the periods in red, you can see peaks in the mean humidity > 80%. Now although the mean humidity is 80%, the maximum humidity is well over 90% for all of these trigger periods so there is a likelihood that all of the 5 triggers could have seen not only spore germination but fungal development.

If you saw early signs of Anthracnose (before July) then it is likely this was a result of the May trigger. If I was a betting man I think that the main Anthracnose trigger points for this summer was the 28th July - August 2nd, followed by August 9th - August 13th. Give it 14 days maybe thereafter to begin to manifest itself as symptomatic on leaf tissue as foliar blight (dependent on plant stress levels) and there you have it.

So the bottom line here is that you need to monitor your maximum air temperature AND your relative humidity (and in an ideal world your duration / severity of leaf wetness via a leaf moisture sensor) to clearly identify a trigger period. Once identified then it's up to you whether you take on board the cultural management IPM processes clearly identified by Bruce Clarke et al @ Rutgers university and / or apply a fungicide.

On this subject I would say that if you want to break the cycle between successive annual Anthracnose outbreaks, spore production and then the same the next year, an effectively-timed application of an effective fungicide is a necessity. Now I will probably be criticised for the next statement, but I don't think that is a Strobilurin.

Why ?

Well I think there's a very good chance of resistance to this fungicide family existing out there in the field just because of the amount of time they have been around for and the fact that they are single-site effective (and therefore it is easier for resistance to develop). This is also borne out by market feedback. My other contention is that we are in the fortunate position to have some more effective chemistries available to us and it is these that I would turn to first. Just my ten cents worth and happy to be proved wrong here if someone wants to throw some resistance testing data my way for Strobilurins and Anthracnose.

So choose your products correctly, identify your timings for application and hopefully you'll manage to break the cycle going forward.

There's a lot more I could say here but time and tide wait for no man (literally), so I need to get on the fly tying vice and rustle up some Garfish and Bass specials before this evening 😊

All the best.

Mark Hunt