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Explainer: When and why are blight warnings issued?

And what should potato-growers do in response? takes a look…

YOU MAY HAVE noticed Met Éireann’s blight warnings as part of its forecasts, particularly over the summer.

But when and why are the warnings issued – and what should potato-growers do in response?

When are blight warnings issued?

Forecaster Pat Clarke from Met Éireann told that this is the time of year when conditions are ripe for the spread of potato blight, which kills plant tissues, and that the service has an agricultural meteorologist on staff who keeps an eye on developments that could affect farming.

An alert is issued when conditions are ripe for the spread of a blight infestation and do not necessarily indicate that crops are having serious problems at that point in time.

The ‘ideal’ conditions for blight involve a particular temperature and humidity. “In general, you’d expect something like a 12-hour period with temperatures of 10 degrees or high and relative humidity greater than 90 per cent,” Clarke told

“You couldn’t have too much wind either over that period because it would affect the moisture deposits on the leaves. But if you have the ideal conditions for about 12 hours – blight needs about 12 hours to take a hold – then the blight process kicks in and you get what we call ‘effective blight hours’.”

The conditions for blight are more likely to be met at this time of the year, considering the necessary minimum temperatures involved, and Met Éireann says it usually issues blight warnings from about mid-May onwards.

So… what should farmers do once the alert has been issued?

In the mid-19th century when the Great Famine took place in Ireland, potato strains were more susceptible to blight. Nowadays, there are more resistant strains of potato available to farmers – as well as a selection of spraying options to fight the disease.

Clarke says that the blight warnings are issued when meteorologists see the type of weather coming in that is conducive to the spread of the disease, although sometimes the period of humid weather doesn’t last long enough for blight to occur.

Farmers are one of Ireland’s main users of Met Éireann forecasts and the main point of issuing a blight warning is to give farmers a chance to react to the danger.

However, weather conditions may also hamper efforts to tackle the disease. ”There’s no point in having a warning though if conditions aren’t right for spraying,” says Clarke. “If it’s too windy, for example.”

Grow It Yourself, an organisation which encourages people to grow their own food, told that its members have found homemade garlic sprays very effective in fighting both blight and insects. It is made by boiling garlic in water then spraying that water onto the plant and its leaves.

GIY also says that open spaces allowing the wind to break up the moisture around the plants can help in tackling the conditions conducive to blight – although spacious planting might not be an option in smaller allotments or gardens.

In terms of resistant strains, Grow It Yourself recommends planting a Hungarian strain called ‘Sarpo Mira’ which is naturally resistant to blight.

According to Teagasc, a GM potato line using gene from a wild potato strain found in Central and South American has been developed which is more resistant to late blight because those wild potatoes have learned to deal with the disease.

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