The Casual Gardener: Don't let your spuds be blighted

Potato blight poses a perennial threat to crops but the hobby grower can take steps to avoid succumbing to an outbreak

Ireland has a long-standing association with potatoes – and potato blight

UNUSUALLY, I have no potato crop this year. Space constraints and a reluctance to plant the same kind of crop in the same earth two years running means I had to sacrifice this veg patch favourite. If I did have a crop of spuds, I like to think I'd already be harvesting them, having adhered to the rule-of-thumb guidance that recommends planting first earlies on St Patrick's Day and harvesting from the Twelfth onwards.

I opt to grow first earlies because they are less susceptible to Phytophthora infestans, AKA potato blight – or more specifically, late blight. Unlike blight-resistant potato varieties, most of which originated in the former Eastern Bloc and were specially bred so as not to become infected with pathogens, first earlies are grown earlier in the year – hence the name – at a time when conditions aren't conducive to the spread of the airborne blight fungus.

The hardier the potato variety, the earlier it can be planted; however, the later planted varieties produce a better yield. You will get fewer tubers from an early variety like Rocket, which tend to be going over by the time blight spores spring into life in July, and more from main crop varieties like Pink Fir Apple, which can take more than 20 weeks to come to maturity, though will store better than their early counterparts.

According to the Agri Food and Biosciences Institute (AFBI) in Belfast, where the bulk of the north's plant pathogen research is carried out, there have been no reports of late blight so far this season. They are hoping that like 2015, when the north had only six confirmed outbreaks, that this year will be see extremely low disease incidence. Although 2015 was a good year for growers as far as blight was concerned and therefore latent sources of late blight are likely to have been low, a wet and warm winter created ideal weather conditions under which the disease prevails.

"This year there has been a very warm start to the summer, but this has been coupled with dry weather, which is not conducive to late blight," according to Afbi. "However, in the past number of days, warm, wet conditions have arisen. This has led to an earlier warning compared with last year, when conditions were more consistently cooler and drier early in the season."

Afbi release blight warnings for the benefit of potato farmers which are based on recent weather conditions. Ideal conditions are formally known among the potato fraternity as a ‘Smith Period'. Named after LP Smith, the former head of the Met Office agricultural branch in Britain who in the 1950s developed blight prediction models, a Smith Period is defined as at least two consecutive days in which the minimum temperature is 10C or above and on each day the relative humidity is greater than 90 per cent for at least 11 hours.

Notably, Afbi's latest circular to growers highlights how several different strains of blight exist in the north, each with differing characteristics and levels of resistance to fungicide.

What are known as "older A1 types" are the most common strains, but Afbi has also identified an "aggressive A2 mating-type genotype known as Blue 13", which is resistant to phenylamide fungicides. Less common is Pink 6, a genotype that favours warmer weather and is frequently found in Britain but rarely in Ireland.

Those growers who wish to keep abreast of the threat to their potato crop from blight can avail of 'Blightwatch', a service from the Met Office which provides up-to-date, localised warnings about the likelihood of an outbreak. You can sign up for free at


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