Garden: Tomato Blight & Smith Periods

To spray or not to spray?

Well, ideally, not to spray. A chemical spray costs money and even if not actually bad for the environment is unlikely to be beneficial beyond hopefully achieving what it was designed to do. I'm thinking of the fungicide Bordeaux mixture (copper sulphate and slaked lime) versus tomato blight, of course. 

Blight can run through tomatoes and potatoes (as well as aubergines) and, if you're very unlucky, destroy the plants within a few days of the first signs. Fruit becomes distorted, marked brown and decays and the plants themselves simply collapse. Once you see the first signs it is probably too late!

Looking healthy this morning.

So when to spray, then?

Blight requires warmth and humidity to take hold so a hot and dry summer is ideal for avoiding the disease, except, here at least, hot means regular thunderstorms pushing up the humidity. Plants are considered to be at risk following 2 consecutive days when the temperature sits above 10 degrees and the relative humidity is above 90% for 11 hours of that day, called a Smith Period.

Recording snow depths. Not!

We have a weather station (see side panel for current conditions) to record our specific weather plus I use a number of weather forecasting websites - Meteo France, Meteo65, The Weather Channel and Meteo Bleu - to get some idea of what to expect. They never all agree; a downside of living in the foothills of the mountains is that sometimes we get mountain weather and sometimes that of the plains. It is fascinating when the boundary between the two zones sits right overhead!

As I write, the temperature is a rather chilly 19.6 degrees, after a week or so in the low to mid 30s, and the relative humidity is back down to 80% after an evening of thunderstorms and rain, but the average for the last 24 hours has been a scary 92%. With rain forecast on and off all tonight, throughout tomorrow and into the following night, and with the temperature unlikely to fall below 10 degrees I think I'll be spraying on Thursday.

Whatever I do, the blight will get us in the end. The challenge is to get a good crop of tomatoes first.

More info on blight HERE and on calculating Smith Periods HERE