Blight on the landscape...

Been a while, but Ms Monkey is back! Now, let's have a little chat about Blight. All this hot sunny drought (this is sarcasm) means that there is a real risk of this pathogen affecting one's potatoes and outside tomatoes.

What is Blight?

Blight is caused by the fungus Phytophera infestans. It can attack all members of the Solanaceae family. This includes potatoes and tomatoes and is prevalent in warm, wet conditions. In other words; the English summer....summer being a relative term and is basically the slightly warmer wet bit in between the colder wet bits of spring and autumn.

Here's the technical bit... blight is an incredibly expensive problem to commercial producers (we all remember the Irish potato famine....well, we don't as it was between 1845-1852 and none of us are that old, but go and have a look on Wikipedia if you didn't do it at school...or were asleep or doodling on your exercise book) and so blight forecasting is highly important. Blight is forecast based on  'Smith Periods' (named for the meteorologist who developed the system). A Smith Period is a 48 hour period in which the minimum temperature is 10oC and the relative humidity exceeds 90% for at least 11 hours in the first 24 and at least 11 hours in the second 24 hours of that period. Clear? Basically, if it's warm and wet, then there is a threat of blight. Ordinary UK gardeners can access blight warnings using the Blightwatch service: http://www.blightwatch.co.uk/content/bw-Home.asp Just register your postcode and Bob's your uncle! If you are not from the UK....lucky you! We are having the most miserable summer since records began...and we Brits love our records....along with whinging about the weather!

How do I know if I have blight?

Good news, you are human and therefore haven't got it....but if bits are starting to rot off, you might want to visit your GP. Now, recognising blight varies slightly between potatoes and tomatoes.

Tomatoes

Courtesy of Royal Horticultural Society

Initial symptoms are rapidly spreading, wet brown rot of the leaves, occasionally white fungus can be seen at the leaf edges.

Splitting of the stems may occur, along with browning.

Brown patches will appear on unripened fruits and ripe fruits will quickly decay.












Potatoes

Courtesy of Science Photo Library

Brown patches on leaves, with a spreading yellow discolouration.

Splitting and browning stems.

Brown marks on tubers (potatoes) which, when cut into, are a mass of foul, smelling black goo (technical term....or maybe not).

Treating Blight

The best method is prevention, if you are British....move somewhere with less water; like the rain forest or the middle of the Atlantic Ocean (Ms Monkey is getting a little annoyed by the lack of fair weather....does it show?)! It is far rarer in indoor production, although a potato plant in your living room is not particularly attractive...although it would be a talking point.

If the attack is in its early stages, remove affected leaves, stem and fruit and burn. Best not to compost, the pathogen 'should' be killed in a hot compost heap, nor should it survive the winter, if it's cold enough...but that's too many should's and if's for me. If you are unable to burn then bury it, in an area (of your own garden) that is unlikely to be disturbed or that you are unlikely to plant Solanaceae species in.

If you have it within your potatoes, check very carefully that you are not storing any infected tubers with your healthy ones. I would not advise you to wash your spuds prior to storage, as healthy potatoes will store for longer if uncleaned. Just discard any with large black spots or soft areas.

If you catch the attack early enough...can't say exactly what is meant by this as I go for the slash and burn method and hope for the best...then a spray of 'Bordeaux Mixture' can be applied. No...this doesn't mean that you pour a bottle of France's finest into a spray can and give the greenfly the treat of their life! This is a fungicidal remedy, which is cleared for all you organic gardeners, however, it contains copper sulphate (remember that bright blue stuff you used to make crystals from in chemistry sets in the old days...yeah, that) which is OK for consumption in small amounts, indeed we need traces of it in our diet, but it's worth noting that it will be banned from next February in the EU...up to you if you want to use it!

Courtesy of Dave Barry's Blog. Lovely!

On a final note, you may be surprised how many plants that are in the Solanaceae or Nightshade family: Nicotiana, Tobacco (ornamental as well as the content of roll ups); Capsicum, chillies and peppers; Physalis, tomatillo and cape gooseberry; Datura; Brugmansia; Mandragora....and many more

All of these could harbour the disease, so you may buy an ornamental and introduce the problem to your edibles. It should also be noted that you are less likely to have problems with indoor grown species, where the humidity is lower (unless you live in Ms Monkey's house of course....you could grow mushrooms on some of my walls!)