Potato blight, the weather, and Dew Points

Potato blight, the weather, and Dew Points

For the last year and a bit I have been recording the weather for various reasons but have many plans for how this data can be used.

A blog post by Deano Martin about Potato blight and compost tea not only raised a few interesting questions but brought to my attention that the weather has a large part to play with blight. Although I was well aware that blight is caused by warm damp conditions I knew little else and his blog post lead to some reading up upon the subject as well as much thought as every good blog post should induce.

I hadn't realised that there are some predefined conditions that would indicate a good chance of blight. Over the years two ways of predicting when blight may start have been developed.

The first one was called the Beaumont Period and is defined as:

Within a 48 hour period 46 hours must have been above 10 degrees C and humidity must have been over 75%

The other, and now preferred method, is called the Smith Period and is defined as:

Any 2 consecutive days where the minimum temperature was 10 degrees C or above and on each day humidity was 90% or above for 11 hours

Since I have been recording the weather details every 10 minutes I have plenty of data in which to work out previous periods when blight may have started but also predict when the next period will be. Working out what to do when you can predict when blight may start is another thing entirely.

A few hours of programming and I had knocked up a way of calculating both Smith and Beaumont methods and am able to compare both but also look back over last year and this year to see how often favourable conditions exist for Potato Blight (or Tomato blight for that matter). This should now enable me to predict, or should the word be forecast, when Blight may start. 

Running through the data I see that the conditions were met on the following days:

Possible blight days (Smith Period): 24th,25th and 25th,26th and 26th,27th of August 2013

Possible blight days (Beaumont Period): 10th,11th of September 2013

Possible blight days (Beaumont Period): 11th,12th and 21st,22nd of October 2013

Possible blight days (Beaumont Period): 27th,28th and 28th,29th of May 2014 

Possible blight days (Smith Period): 27th,28th and 28th,29th of May 2014

Possible blight days (Beaumont Period): 10th,11th of July 2014 

For a start we can see that the Smith Period occurs less often and is considered more accurate. The Smith Period has been used since the 1970's. (My data only goes back to May 2013 and is up to July 11th 2014).

What we can see, which seems to tally with the fact I didn't hear about people having blight as a problem last year, is that Blight was late and many people would have dug up their potatoes around then, leaving just the main crop to suffer, but this year it looks like we have had or might have a more early blight starting the end of May and with July being wet and humid we may well see a few more days which are favourable for blight, the Beaumont period has already been met so clearly conditions have come close.

Obviously these dates are only for my exact location.

Interestingly I have one or two plants which are turning yellow, and started looking poor around the end of May so I am wondering if it is Blight, although it also looks like a mineral deficiency. I'll need to look closer. I had put this down to them being planted directly into young manure and we have had a lot of rain and that bed got rather water logged several times. The bed that I have used for potatoes this year was hastily prepared by simply piling manure onto the ground to a depth of about 18 inches. Allowing the manure to compost in situ, ready to be dug over for an Autumn crop. I simply took advantage of the bed to see if it would grow potatoes in fresh manure, they started growing, so I planted more hoping for a bonus crop when otherwise I didn't have room for potatoes this year.

Knowing that humidity is key to when Blight may start, observing the weather in detail would be in keeping with Permaculture's principle of Observe and Interact. In Permaculture you are supposed to Observe 90% of the time and Act / Interact 10% and although I have been very critical of Permaculture, or to be more precise the way it is taught and the information provided within the Permaculture community, I won't go into this now, I do totally agree that you need to make lots of good observations, although I tend to observe and awful lot but also act much more than perhaps is considered good.

Having an enquiring mind and wanting to clarify observations and also get accurate data I started to wonder how accurate humidity readings taken from a weather station can be when applied to ground, or in this case, Potato canopy level, humidity.

The question is does a humidity reading of 90% at weather station height, also correspond to ground level? I can't take a reading at ground level easily because my weather station bits are up a pole on top of the greenhouse, 3.5 to 4 metres high. This got me thinking about Dew on the grass. 

The Dew Point

The Dew Point, which is the point that water will appear on surfaces, the ground or grass for example, is where the air can not hold any more water and will give up water and transfer it to an object or thing. When Relative Humidity reaches 100% this will happen. The Dew Point is the Temperature when Humidity is 100%

When we see Dew on the ground we know that the Humidity is 100% so, at that time, the weather station should show 100% humidity. It doesn't. At least mine doesn't. I know my readings are accurate to within a sensible margin and there is no reason why they wouldn't be so straight away I can tell something about this observation. Clearly the humidity is slightly higher at ground level, not at all surprising since there is a different micro climate in and around the grass compared to 4 metres above.

Temperature and Humidity are closely related. If the humidity is higher at grass level, as evidenced by the fact we have dew when the humidity reading I take 4 metres up suggests 90%, then the temperature is lower at grass level than we observe at weather station height. We can see that the grass level temperature is between 1 and 2 degrees C lower.

For what it is worth the Dew Point Formula is:

Dew Point Temp in Celsius = Temp - ((100 - Relative Humidity) / 5 )

If I have a reading of 15 Deg C and 90% humidity at 4 metres high but there is due on the ground the Dew Point would be 13 Deg C

This therefore raises the question, does the Smith Period of 2 days above 10 Degrees and 11 hours 90% humidity on each day take into account that readings are taken from above ground normally or should I be taking my readings from ground level? ie, does the Smith Period not mind that the micro climate within the Potato Canopy is different to the place where you measure weather? I'm guessing not.

I'm guessing not because the way the Met Office gives out Smith Period warnings will necessarily be based upon weather stations and not little sensors within potato fields.

If we therefore take into account that Blight may start when temperatures are below 10 Deg C or Humidity levels are higher than 90% at the plant level would we be able to prevent it easier or predict it better?

The other thing about taking readings from above the ground is that it takes no account of the wind. Air flow or wind, will be less around the plants than higher up, so another question to answer would be does blight happen always on these Smith Periods or is wind playing a big part?

Recording the weather in detail, observing, will allow me to look back at the conditions and answer some of these questions but first I need Blight....or perhaps not! Perhaps for me these questions are better left unanswered.