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Fight against Blight

Fight against Blight

Growing potatoes commercially is not for the faint hearted, they are a high input crop to grow with costs in many cases over €2,500 per acre. One of the biggest costs in the crop is disease control, particularly potato blight.Crops specialist Shay Phelan has some advice

There are many similarities between the current Covid19 pandemic and the Famine of the 1840’s. Both were caused by a largely unknown microscopic organism, both caused massive social and economic upheaval in this country and sadly both have cost many lives. While we have vaccines and fungicides to deal with both, the number of vaccines we have is increasing while the number of fungicides is decreasing. Blight is not as important as Covid19, but it is one of the most important parasitic fungi in the world and it costs growers around the world billions of euro every year.

The battle against blight has already started 

Our annual battle against potato blight (Phytophthora Infestans) is already under way, whether you are a commercial, organic or hobby grower “growing a few drills for the house”. We are all familiar with the regular blight warnings issued by Met Eireann throughout the summer such is the importance of the disease. These warnings indicate that a period of weather which favours the spread of the disease is on the way. Growers are advised to apply a suitable fungicide to protect the crop. 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. The warnings shouldn’t be ignored by casual or hobby growers because not only do they put your own few drills at risk they can put neighbouring crops at risk as well.

Integrated Pest Management (IPM)

Blight knows no boundaries and can develop not just in a commercial crop, it can also infect single plants grown in gardens or patios, volunteers in other crops or dumps i.e. where plants grow from waste potatoes or even skins. If any of these become infected with blight, they can then spread very quickly into the commercial crops which can in turn destroy these crops. We hear many advisors talking about Integrated Pest Management (IPM) in regard to controlling weeds, disease and pests. Put simply this means using both chemical and non-chemical measures to control the particular pest in question and one simple IPM measure when it comes to controlling 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 County 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 the infection continuously through a hole in the hedge in a V-shaped pattern across the nearby commercial crop. Part of this crop, subsequently, had to be destroyed in an effort to save the rest.

Another IPM tool used by organic growers, is to use resistant varieties such as Sarpo Mira, although it has an acquired taste which many people don’t necessarily like. Many organic growers grow varieties such as Setanta, which have a high resistance rating for blight but are not resistant, and use copper sulphate solution “Bordeaux Mixture”, which is an approved fungicide under the organic scheme, to control the problem if needed.

Last year for Mancozeb

By far the most famous fungicide used in controlling blight over the years was Mancozeb and most who grew up on farms would be familiar with the can of Dithane 945 used for the potatoes. In many cases it was used almost exclusively throughout the season. However this is the last season that Mancozeb can be used on potatoes as it’s registration for use expired earlier this year. While its use commercially has declined in the last number of years, due to growers alternating chemistry, it is still widely used, with the majority of crops be they commercial or not receiving at least one application of a product containing Mancozeb.

New strain of blight in Ireland

In the last few years products containing the active ingredient Fluazinam e.g. Shirlan, Volley etc. had become popular among growers as it had good activity on tuber blight. Similar to Mancozeb many small growers used this product throughout the season as it was available in small quantities which were suitable to smaller growers. Like many other organisms, blight is continually evolving which results in different strains of blight developing which have different characteristics, including their resistance to fungicides. A strain of blight called 37 A2 has developed across Europe over the last number of years, which is largely resistant to fluazinam, this strain has spread to most countries across the continent and Dr. Steven Kildea in Oak Park has confirmed its presence here in Ireland in 2020. We know that we have mixtures of strains in the population however if you crop contains the 37 A2 strain then we can predict that control from Fluazinam products will be poor. Therefore the advice would be to only use Fluazinan once during the season and mix a partner product with it to reduce the risk.

Again just like controlling the spread of Covid19, 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.

The Teagasc Tillage Crop Specialists are regular contributors to Teagasc Daily and issue an article on a topic of interest to Tillage farmers every Thursday here on Teagasc Daily