Age-old potato blight battle

Fiann Ó Nualláin says prevention is key when fighting to keep spoiling spores at bay

I am a big fan of the seanfhocail — there is just so much wisdom in those old Irish proverbs. I have often dropped them into my writing and talks and I apply them in my personal life constantly.

Cards on table — I have a book coming out at the end of August about learning mindfulness and resilience through the proverbs, so they have been even more to the forefront of my consciousness for months now.

But even if I wasn’t recently immersed, when it comes to blight I automatically think Is fearr réchonn ná iarchonn — “foresight is better than hindsight”.

That’s the key to blight — forward thinking and vigilance. Better to prepare for war —if I can paraphrase a Roman proverb — to have some peace of mind. Generally from this weekend on, we productive gardeners are on the watchout for potato blight and tomato blight.

Listening intently to the weather forecast or frantic with an eye on the crop; fretting and anticipating when to spray with organic solutions if we haven’t planted blight resistant crops, we are on a war footing.

Although routinely called ‘Potato blight’, Phytophera infestans can infect other members of the Solanaceae family. Frequently tomatoes, but also aubergines etc.

While heavy dews in late summer is the ideal breeding ground, blight can strike in any humid week from May on, it is an airborne, soil-held and moisture and heat-triggered fungal disease.

The greatest risk of spore outbreak occurs when the temperature/climate is in a Smith Period — this is a warm period (minimum 10C) for two consecutive days with accompanying high humidity levels exceeding 90%. That’s the type of warm and wet we’ve had in some places recently.

The first signs are dark brown blotches occurring near the leaf tips that soon wilt or curl, white mould may be present on the underside of the blotches. Necrosis of stems soon follows.

The extent of damage ispotentially the total loss of crop. With potatoes, the necrosis and collapse of the haulm is soon echoed underground with a reddish brown, foul-smelling rot in the tubers. The damaged tomato foliage is speedily followed by rotting fruits.

Daniel MacDonald (1820-1853) did the only known painting of the Great Famine, which was titled ‘An Irish Peasant Family Discovering the Blight of their Store’. It was exhibited in London in 1847, and is now in the National Folklore Collection, UCD (bequested by Cecil Woodham Smith, in gratitude for the college’s support when she was writing Great Hunger: Ireland 1845-49). Pic: National Folklore Collection, UCD

The best way to inhibit outbreak – because you can’t change the weather is to practice crop rotation and grow resistant varieties. Some of the more tasty, blight-resistant potatoes include Colleen, Orla, Kerr’s Pink, Record, Golden Wonder, Setanta, Blue Danube, Sarpo Miras and Sarpa Axona

You can plant early and lift early to be up and out before blight strikes - not always practical and some years, Smith periods are possible in May. Some allotmenteers only grow early and second early potato crops to avoid late blight. In Ireland the main threat lies with main crop potatoes — those that are left late to lift.

How you plant is important — space well (at good tuber distance), so that the growing tops have room between each other for airflow. Do adhere to regular earthing up (bringing soil up around the stems) this is not just a technique to increase yield, but one which will protect the tubers should blight attack.

How you clean up post-harvest is important too — blight cannot survive in soil, but can over-winter in living plant tissue including tubers not extracted, and badly composted, diseased material. Always fork over two weeks after your harvest to remove these ‘volunteer potatoes’.

Traditionally, preventative copper based fungicides have been employed from May onwards, but there are issues with sustained exposure to copper with human health and also with soil toxicity and diminished earthworm populations.

The thing is that it is no good spraying after you spot blight — it only works as a preventative. Some of the old treatments such as bordeux mixture are cautioned against, or even withdrawn from sale in some places. There are some bio- fungicidal products from the fermentation of bacteria that work well, but they are not always in regular supply. So with all that up in the air what do you do if blight strikes?

Well if and when blight has hit your spuds, all is not lost, you can still get a harvest, you just have to use technique; firstly remove the haulm (top growth) and dispose of it in a manner that will not contaminate soil or compost heap (traditionally bin it or burn it) . Now do nothing else for at least two weeks, if you lift potatoes now you are lifting them to the surface where the blight spores are, they are safe underground, shielded.

The greatest risk of spore outbreak occurs when the temperature/climate is in a Smith Period — this is a warm period (minimum 10C) for two consecutive days with accompanying high humidity levels exceeding 90%.

Over the next 14 days the surface spores should become inactive. When then you do lift, wait for a hot dry day and lift as early in the day as you can, as this gives the potatoes more time in the sun to harden skins for better storage, a best practice event anyway, but when it comes to rescuing a blighted crop, you want them well dried and store ready.

I have a mesh frame that I use to dry root crops and even to desiccate weeds prior to addition to the compost heap, a chicken wire device will suffice, wash the lifted potatoes and allow to dry for a day in sun before storing, the mesh just allows better air circulation and you don’t have to be moving the potatoes around to dry all sides.

When it comes to tomato blight – outdoor varieties are subject to the laws of nature, but with indoor varieties you are the law. So overhead irrigation, poor ventilation and other simple fixes can limit the chances of outbreak. I would add that the control of insects as vectors in the spread of fungus spores is vital too. it is not so easy to save the fruits.

Otherwise, all the same preventions and treatments apply to tomatoes as to potatoes.


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