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Phytophthora infestans

Many of you have probably looked at the title and said, 'What is that?' Well, as soon as I tell you, you will probably know.

It means potato blight! As a horticulturist, I know the Latin names better than the common ones. This is mainly down to being taught this version when training, as I had to use it in my job as a horticultural advisor. Okay, many people were more familiar with the common names, which is why I had to learn both. This was an awful lot to learn and remember!

I had a tape I listened to while stuck in the usual traffic to and from work, with Latin names of plants, pests and diseases. It gave a brief description on what each was and how to say them. There was I going Phhhyyyytopphorraa using the lip movement as indicated by the tape, when I looked across at the car next to me and found I was getting some very funny looks!

Anyway, potato blight. This year I have had very bad luck with my first early potatoes. About a couple of months ago, they were struck by the horrid blight. The seconds and main crop look very healthy on the top, which is why I am convinced the blight was in the first early tubers when I bought them.

Home guard was the name of the variety I thought I would try this year. My granddad always said good things about them and as they were supposed to be more resistant to blight, I thought that I would give them a try. When I bought them around March time, I can always remember the shoots being more forward than the other varieties but they looked okay. I put them in the shed to chit and when it was time to plant, I noticed they were a little soft.

I presumed they would be okay but about a couple of months ago, the foliage started dying back which is when we decided to dig them up. They had definitely got potato blight and if left, rotting would possibly occur. This morning (Saturday 18th July), we dug them up and discovered we had a few potatoes, but very small in size.

It doesn't matter about the size, though, as I am in favour of the usual saying 'Size does not matter!' They went very well with a salad tonight!

So, what is potato blight and how does it occur?

Potato blight is a spore organism, which can travel from miles away if the conditions are right.

It is a fungal disease which is carried by wind and rain. In order for them to survive, they need a high humidity with an air temperature of a minimum 10°c (50°f).

Many of these spores are reproduced from the previous year's potatoes or plants within the same family, which may have had the blight. It is important to discard all foliage and rotten material, preferably by burning. Many people put them in their compost bin or on a pile down the garden, which can cause the spores to re-grow the following year, infecting your prize vegetables once again.

The blight will start on the foliage and is distinguished by patches on the leaves which start off a yellowish colour, turning to black. Also, there is usually a white patch on the leaves, which is more noticeable as the foliage starts to die. See the following picture.

Within a couple of weeks, the whole plant will completely die back and collapse. See the following picture.

It is important to cut the foliage off to ground level as soon as you notice any sign of the blight. This will ensure that it does not spread down the stem and into the tubers itself, which will rot the potatoes.

The potatoes will probably be small, like mine, but should be edible if they are not stored for too long. Always ensure that you dig every single one out as it could start up again next year! Some commercial growers do not plant potatoes in the same field for five years if potato blight has been active. However, this is not possible for a lot of gardeners because of limited space. If we take steps to destroy the foliage and try to get every potato out of the ground, then it all helps to reduce the risk of further problems in the next growing season.

If the potatoes are not lifted as soon as the foliage starts to die back, then you will get rotting potatoes.

If you earth them up about three times as the foliage emerges from the ground, this will help to reduce early onset of potato blight.

Another precaution is to apply either Bordeaux mixture (copper oxychloride) or a spray such as mancozeb (Dithane) as prevention before the blight appears.

Some blight resistant varieties are Cara, Orla, Valor, Remarka and Homeguard but there are many more. There are some that were bred in Hungary, which are supposed to be totally resistant. I do believe they are quite expensive to buy, and are not readily available at the moment, like the normal varieties. However, it may be worth trying if you can find a supplier!

I must not forget to mention the potato famine caused by potato blight in the late 1840s. This would make an article on its own, so I will just give you the following link which makes very interesting reading.

Phytophthora infestans can infect not just potatoes, but all relatives of the potato family (Solanaceae). There are approximately 2,500 plants within this group, which are all susceptible to this particular type of problem.

Tomatoes are in this group and the fruit will be destroyed if caught by the blight. It is less likely to happen if they are grown under glass, but the spores could still drift into the greenhouse and survive if the conditions are right.

Deadly nightshade is also part of the same family but don't worry - your spuds are perfectly safe provided you don't eat them when green!

Well, that is all from me this time. I will leave you with a smile and some recipes.

Nigel's Gardening Hints and Tips




23.07.09 Front Page

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