Weeding the Web

How to combat tomato blight

My very own sketch of Bob Flowerdew's blight protection shelter

So, talking about Blightwatch as we were, supposing there is a blight warning? What can you do? 

 

The Potato Council offers a pdf of Leisure Growers’ Advice on how to avoid blight, which includes photographs to aid identification. In their words, “The most effective control for the spread of infection is warm, dry and sunny weather.”

But, in the absence of that, they suggest repeated applications of copper-containing fungicides, such as Dithane 945 and Bordeaux Mixture.

How to control Potato Blight is one of the Top Ten subjects that come up again and again on Gardeners’ Question Time.

Matthew Biggs suggests various tactics, most of which you should have implemented before the dreaded warning. These include avoiding sheltered sites (presumably because it reduces the possibility of a hot and humid patch with its very own Smith periods), planting wider apart, and looking for early signs on neighbours’ crops and spraying with Bordeaux mixture before the blight reaches yours.

Blightwatch - so you know what's coming

I recently received my second Blightwatch Alert of the year. If you grow tomatoes and potatoes and haven’t already signed up to this service, you should as, once you have, you’ll get warnings of Smith periods, which are perfect times for blight to flourish.

For anyone who doesn’t know, a full Smith period occurs when the temperature during at least 2 consecutive days is 10°C or more, and relative humidity is greater than 90% for at least 11 hours each day.

Back in August 2007, I wrote about Blightwatch and Smith periods for Organic Gardening magazine (sadly defunct) and spoke to Steve Gerrish, Information Resources Manager at the British Potato Council. He said, “If there’s a spore on a leaf at the beginning of a Smith Period, then it will have germinated by the end, but it’s all risks and probabilities. A blight spore requires free water on a leaf in order to germinate. The Smith period is an estimate of a high-risk period.” It’s also why you should always water tomatoes at the base, so the leaves don’t harbour any “free water”.

So, even if you receive a warning, the occasional Smith Period isn’t a reason to panic, as it won’t automatically result in blight ravaging your plants. However, if periods occur every 7 to 10 days, that’s serious, as blight takes 7-10 days to develop and sporulate (excellent word, must repeat it)…sporulate again, so it thrives in such conditions.

Blightwatch also alerts you to Near Misses, when conditions are considered almost perfect. These need to be noted too, as your plants may be situated in a spot that, with the help of such climatic conditions, nurtures its own personal Smith periods.

So, forewarned is forearmed! But forearmed with what?

Something for the next blogpost, I think.