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Potato Blight (Phytophthora infestans) attacks weeks earlier than usual!


05 June 2014

I'm sorry to say that there have been confirmed cases of potato blight, Phytophthora infestans, up and down the country! Usually we say that you should look out for potato blight as it has the potential to devastate your potato crops, and any time from late June onwards I start to get particularly nervous , waiting for those dark brown patches to develop on the foliage and stems of affected plants, killing them off section by section.  One day it looks as if you are going to have a fabulous crop, the next you are left with virtually nothing.   Like so many things in gardening it is largely down to the weather : the blight fungus can be spread both on air currents and by rain splash and for infection to take place the fungus requires at least two consecutive 24-hour periods where the temperature does not drop below 10C (50F) and where the relative humidity is 89% or more.  This year we have had plenty of weather like this already, and blight is here a good three or four weeks earlier than usual.

 

With potatoes I find that  it really pays to grow some of the more resistant varieties such as Romano, Pentland Crown, Cara, Estima, Maris Peer and Kondor.  Having said that, I still love to grow some of my blight-prone favourites including Foremost but unless blight strikes unusually early I still get a good crop even if the plants are infected – prompt removal of all the affected haulms  prevents the spores from being washed down on to the tubers, so these remain unaffected. All the Sarpo potatoes show very good resistance, but I have to say I find them a bit too much of an acquired taste!

 

Tomatoes grown in a greenhouse or polytunnels are unlikely to be attacked but I do like to grow a few outdoor varieties too.  A few years ago I planted up a huge area with tomatoes, it was a lovely warm, fertile, sunny spot.  The plants grew well, good sized trusses of fruits formed.  Then blight struck.  The foliage blackened, blotches formed on the stems, the plants keeled over both because they were weakened and because of the heavy weight of the crop.  The fruits themselves had those tell-tale gingery brown patches on them. I had envisaged huge crops of tomatoes, for eating, cooking and freezing.  Instead I spent many hours bagging up the lot of them and then dragging over a dozen heavy bin-bags down to the wheelie-bin.  Normally the bin men do not take ‘extra’ rubbish but luckily for me they are obliged to do so after a bank holiday, so my timing was perfect!  You should never risk leaving even the tiniest bit of infected plant around the garden, and neither should you ever contemplate composting it – the infection will simply build up in the compost and then be spread around when you use the compost.

 

If the pathogen attacks your potatoes it will make a similar mess of the stems and foliage and if you are unlucky enough to find that the tubers are infected  they will  show dark patches on their skin often with a gingery brown discolouration beneath.  This is a perfectly odour-free rot and one you may not realise is there until the potatoes are in store when all of a sudden there is the most breath-takingly revolting smell, because secondary soft-rotting bacteria have colonised the affected tubers.

 

Solutions :

*Listen to weather forecasts regularly  and check online weathe forecasts as these show humidity and temperatures and pin-point those high-risk periods.

 

Spray the foliage of potatoes with a suitable fungicide such as a copper-based product.  This needs to be done as soon as the foliage starts to meet between the rows and should be done before the symptoms appear.

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Earth up potatoes regularly and deeply to reduce the risk of the tubers themselves becoming infected.

 

Cut off the haulms of potatoes to reduce the risk of infection spreading to the tubers, or remove the entire top growth of outdoor tomatoes. Doing this should prevent the spores washing down when it next rains and getting into the soil to attack the tubers. Harvest what you can as soon as possible, and definitely before it rains

 

At the end of the season remove and bin or burn the remains of all tomato or potato plants and ensure every last potato tuber is removed from the soil.

 

Grow tomatoes in a greenhouse and choose blight resistant potato varieties.

 

 



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