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  • Writer's pictureMark Hunt

May 18th - Quite a growth and disease flush this week...

Hi All,

A mini blog this week as time is short.

I was a tad frustrated to see most of the U.K & Ireland bathed in cloud cover at the weekend, denying us a view of the Blood Red Moon. My friend out on his remote ranch in the wilds of Colorado though had no such problems. So jealous Bigtop....

The cloud cover was unexpected for me because the GFS had showed high pressure asserting itself last weekend and this week and so little chance of cloud and rain.

What actually happened though is that the synopsis shifted eastwards a little and that allowed a nearby Atlantic low pressure to introduce some humid air into the weather picture and we then picked up some pretty hefty showers from the continent as a result. As I have explained before, summer rainfall (and particularly rain from the continent) is a fickle beast to predict and either appears when you don't expect it to or doesn't when you do. Last weekend and the beginning of this week has been the former.

A pleasant surprise for many though because we are on the dry side and need that rain, especially for the garden. Mine has perked up significantly though still no flowers from my extremely fussy Glaucidium palmatum - Japanese Wood Poppies (bah humbug !)

I was right about the heat though as Heathrow recorded 27.5°C yesterday, the hottest day of the year so far.

So how are we fixed for the remainder of this week after some unexpected rain ?

Well today sees rain across the north west of Scotland and a significant rain front pushing into the south west of Ireland this morning. This will slowly move east and bring rain to all areas of Ireland through the second half of the day, clearing the west as it does so. For England and Wales, another warm and bright start to the day but cloud will build from the south and west later and that humidity will trigger off some showers later, some of them thundery. These will likely develop over the French coast and then cross The Channel later this afternoon / evening to affect the southern half of the U.K. We experienced this on Monday and you can see from the lightning archive graphic below the areas that were affected by lightning. These strikes were carrying over from Sunday evening but I'd expect anytime from 4 p.m. onwards to see a repeat. Temperature-wise, high teens to low twenties I expect and down on Tuesday's peak.

So a potentially wet night tonight across some areas of the U.K and possibly The South East and East Anglia seeing that rain hang around till dawn on Thursday. So Thursday looks a sunshine and cloud sort of day, cooler than of later but still pleasant temperatures in the high teens for most with light westerly winds vectoring initially from the north. A dry outlook for Thursday with maybe just the risk of showers across the north west of Scotland where the cloud cover will be thicker.

Closing out the week on Friday and that Atlantic low pressure sitting off Ireland will push another rain front into the west of Ireland early on Friday morning. This will be joined by rain across The South West. Both of these rain fronts will vector north and east across Ireland into Scotland and across the southern half of the U.K during the course of Friday morning. Later the rain will consolidate and become confined to the west coasts of Wales, England and Scotland before once again pushing east across northern England for the second half of Friday. A brisker wind on Friday and that'll drop temperatures down into the mid to high teens, so a little cooler than of late.

The outlook for the weekend is largely dry but on Saturday a weak band of showers will cross Ireland and we will see some rain across the west and north west of Scotland. A sunshine and cloud type weekend with a declining westerly wind, spinning round to southerly on Sunday. This change in the wind direction may vector some showers across The Channel from France later on Sunday. Temperature-wise, high teens on Saturday but with the dropping wind, I expect temperatures to pick up into the low twenties for Sunday. This extra heat may well trigger those showers later in the day.

All GFS output courtesy of

Weather Outlook

So an interesting GFS image projected for the start of next week because the jet stream position has nudged south a tad and that means cooler, more unsettled air will come into play for a time next week bringing some further rainfall. Now opinions are divided between the GFS and ECMWF models as to the exact position of the low pressure and so that means in my books there's a degree of uncertainty about just how the weather will shape out next week. I see the first part of next week as being unsettled, with a cooler north west wind direction keeping temperatures in the mid-teens sort of territory. Some rain around on Monday and Tuesday, mainly I think across the west and north west but probably across the southern half of the U.K as well, though here it'll be mainly showers. As for the second part of next week, that's where projections differ markedly, GFS has high pressure interjecting and pushing the rain from The Atlantic more north and west, whereas ECMWF has it affecting the southern half of the U.K more, as the low tracks more southerly. Time will tell on this one. My hunch is the former.

Agronomic Notes

When we get this heady mix of temperature and rainfall we know we will also have associated high humidity and that means perfect conditions for not only grass growth but also disease. There are a raft of diseases that like nothing more than high temperatures, a wet plant leaf and humidity.

So first let's look at the temperature and humidity stats using data from a Davis Vantage Pro 2 down at Sevenoaks.

I've focussed on the south of the U.K because that's where we saw the hottest temperatures this time around.

On the graph above I've charted air temperature vs. humidity and you'll see 4 red circles and 4 blue ones. These mark night time temperatures and humidity peaks. We can see over the last 4 days, the humidity and temperature has ramped up and especially during the night. So the stats below show the following for the last 4 days using midnight as a benchmark point ;

Date Humidity at 00:00 Air temperature at 00:00

15th May 94% 10.0 °C

16th May 98% 13.4 °C

17th May 98% 12.1 °C

18th May 97% 16.2 °C

Bearing in mind we know Microdochium nivale hits peak growth rate at 15°C, you can see why the combination of high night time temperatures and humidity levels close to saturation point would lead to a disease outbreak. So that's why you may be seeing plenty of things come out of the woodwork on your surfaces over the last few days.

A near-saturated atmosphere also means extended periods of plant leaf wetness as we can see from the graph below. This data is from a Davis Leaf Moisture Sensor (image above) which uses a number of electrode graduations on a plate to estimate leaf moisture wetness. It reports these back on a scale of 0-15, where 0 = dry, >8 = wet and 15 = a totally saturated leaf surface.

Here is the data from the same location for leaf wetness ;

The red dots indicate the key period with extended leaf wetness from 23:00 on 14/5 all the way through to 07:30 on 16/5. That's over 30 hours of a wet plant leaf and means even if you take the dew off in the morning, it is likely to re-form. Now granted some of this leaf wetness is caused by rainfall, but the fact remains that the combination of a wet leaf, high humidity and high air temperature is a calling card for disease development so be on your guard.

The flipside to this coin is that grass growth is also at a peak with the following Daily Growth Potential figures from the same site ;

Date Daily Growth Potential

15th May 0.76

16th May 0.92

17th May 0.98

18th May 0.96 (projected)

So the grass is growing at nearly optimum rate (1.0 = optimum) and that means hopefully that we are growing the disease out as fast as it is developing.

Of course if you've PGR'd (which is likely given the conditions, then you will tip the balance in favour of disease development, how much is dependent on rate.

hat said it is like most things in life, all about balance.

Holding back the growth to keep good surfaces and prevent the 'post 4 p.m. comments' about whether the greens have been cut that day is definitely a key objective at this time of year so I'm not suggesting that I wouldn't be applying a PGR because I would for sure. May is often the 'boom month' for growth and you need that recovery and infill on surfaces, so it's often the time when a PGR application provides the most worth. Not just on fine turf but also holding back semi and rough growth and difficult to maintain areas like steep tee banks and bunker faces.

It is just holding back the growth will make the disease more likely to progress down the plant towards the crown (because the leaf isn't extending upwards as fast due to the PGR). Foliar diseases like Cortisium (Red Thread) and Clarireedia homeocarpa (Dollar Spot to you and me) are encouraged to a certain extent by PGR applications but you also have to take into account that a healthier grass plant (from a PGR application) is less likely to succumb to disease activity, so like I said earlier, it is a balancing act between growth and retardation.

This is where your experiences on PGR applications on your site and the resulting retardation in growth and appearance or non-appearance of disease come into their own.

On the subject of Dollar Spot and The Smith Kerns Model, I think the situation is a bit ambiguous. This disease prediction model relies on a 5-day average of temperature and humidity in order to churn out a probability of Dollar Spot development. Well if you take the average 24-hour humidity figures and put them into the equation, the probability currently is around 15-20%. If you put the night time maximum humidity figures in, this climbs to 45-55%. The problem is that the use of a 5-day average smooths out these humidity peaks and I think sometimes under-predicts disease development to a certain extent.

High humidity isn't just about foliar diseases and the combination of high temperature and rainfall that we have experienced over the last few days will kick into action some of the other diseases we see. Fairy Ring, Superficial Fairy Ring and possibly a bit of Waitea Patch if you experience some of those thundery downpours because the latter loves moisture. They aren't the worst diseases by a long shot and are more aesthetically-displeasing than they are in terms of affecting a surface and ball roll, but they are likely nonetheless in our current climatic conditions.

Ok that's it for this week, enjoy the merry-go-round we are on currently weather-wise and let's see whether the GFS or the ECMWF prediction is the most accurate for next week or more precisely, the second part of next week.

All the best..

Mark Hunt

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May 18, 2022

We could chat about your Glaucidium issues mark, they are ,if happy , usually pretty reliable and trouble free. Shade ,full or part , rich but well drained soil, Your photo shows other shade lovers in good form, are you perhaps , like lots of groundsmen, an irrigator? They love drainage and only have average water requirements. Something to consider maybe ? Always happy to give you the benefit of my years of knowledge

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