There are many similarities between the Covid-19 pandemic and the Famine of the 1840s.

Both were caused by a largely unknown microscopic organism, both caused massive social and economic upheaval in this country and both have cost many lives.

We have vaccines and fungicides to deal with each.

However, while the number of vaccines is increasing, the number of fungicides is decreasing.

While blight is clearly not as serious as Covid-19, it is still one of the most important parasitic fungi in the world and costs growers around the world billions of euro every year.

Growing potatoes commercially is not for the faint-hearted: they are a high-input crop, with costs for commercial growers coming in many cases at over €2,500 per acre.

One of the biggest costs is disease control, particularly potato blight (Phytophthora infestans), and our annual battle is already under way, whether you are a commercial, organic or hobby grower “growing few drills for the house”.

Met Éireann issues regular blight warnings each summer, indicating that a period of weather which favours the spread of the disease is on the way; growers are advised to apply a suitable fungicide.

 

Most commercial growers are using fungicide programmes on a weekly basis to keep ahead of the fungus as it can be very difficult to ‘cure’ if you get a bad infection.

So these warnings shouldn’t be ignored by casual growers, because they don’t just put your own few drills at risk — they can put neighbouring crops at risk as well.

Blight knows no boundaries. It can infect single plants grown in gardens or patios, and volunteers in other crops or dumps, where plants grow from waste potatoes or even skins.

If any of these become infected, blight can spread very quickly into commercial crops, potentially destroying them.

Advisors often talk about Integrated Pest Management (IPM) in regard to controlling weeds, disease and pests — using both chemical and non-chemical measures to control the pest in question.

One simple IPM measure to combat blight is to pull or destroy potato plants growing where they are not supposed to be. This can remove an initial source of infection and reduce the overall risk to the national crop.

A number of years ago I visited a field in North Co Dublin where a farmer had a particularly bad infection which he was struggling to control despite using a good fungicide programme.

After some investigation we discovered the initial source of infection came from a garden grower next door, who was trying to grow their crop ‘organically’ and didn’t use any fungicides.

These plants developed blight, which spread through a hole in the hedge in a V-shaped pattern across the nearby commercial crop. Part of this crop had to be destroyed.

Another IPM tool used by organic growers is to use resistant varieties such as Sarpo Mira, although it has an acquired taste.

Many organic growers grow varieties such as Setanta, which have a high resistance rating for blight but are not resistant; they use copper sulphate solution Bordeaux Mixture — an approved fungicide under the organic scheme — to control the problem if needed.

The best-known fungicide used in controlling blight over the years was Mancozeb; the can of Dithane 945 was often used almost exclusively throughout the season.

However, this is the last season that Mancozeb can be used on potatoes as its registration expired earlier this year.

While its use commercially has declined in recent years, due to growers alternating chemistry, it was still widely used.

In the last few years products containing the active ingredient Fluazinam — Shirlan, Volley etc — has become popular as it had good activity on tuber blight.  However, blight is continually evolving; different strains develop, with different characteristics, including their resistance to fungicides.

A strain called 37 A2 has developed across Europe which is largely resistant to Fluazinam; Dr Steven Kildea in Oak Park confirmed its presence in Ireland in 2020.

If you crop contains the 37 A2 strain, control from Fluazinam products will be poor.

So the advice is to only use Fluazinam once during the season and mix a partner product with it to reduce the risk.

As with controlling the spread of Covid-19, we all have a role to play when it comes to stopping the spread of potato blight.

Hopefully in the future neither will be as big a problem as they are today.

Shay Phelan is a Teagasc crops and potato specialist, based at Oak Park, Co Carlow