Potato blight treatment Ireland: fungicides, timing and prevention

Bring Teagasc’s disease guidance, the 2026 programme and Met Éireann’s forecast to your agronomist. Before you talk, write down the field identifier, variety, crop purpose (ware, seed or processing), planting date and the sprays already applied with dates, so the advice fits your crop rather than a generic one.

Fungicide for potato blight, early blight and common scab: three different diseases

Teagasc’s potato fungicide report for 2026 has separate parts for potato fungicides, Alternaria solani (early blight) and the 2026 blight programme; it was updated on 8 September 2026.

Common scab is caused by a soil organism, does not affect yield, and can greatly reduce tuber quality and affect grade. Its prevention depends on a combination of practices: avoid planting scabby seed tubers, a 3 to 4 year rotation is helpful, and soil moisture is kept high for 4 to 6 weeks from tuber initiation with small amounts of water, 12 to 15 mm.

A search for “best fungicide for potatoes” leaves the target open. Decide first what you are managing: late blight, early blight or scab. If you cannot tell which it is, identify the disease before choosing any product.

Blight on potato plants: stem blight comes first

Early infections often appear just on the stem; this is known as stem blight. Later in the season the leaves get attacked, and if the disease is not controlled at this stage spores get washed into the soil and infect the tubers.

So check stems as well as leaves. Photograph the stem and the whole plant even when the first thing you notice is damaged foliage, mark the row or field sector where you first saw it, and include an unaffected neighbouring plant for comparison.

Potato blight disease: early blight and late blight are different

Despite its name, early blight is a different disease. Alternaria solani produces dark brown spots with concentric zones on the leaves, often bounded by a leaf vein, and occurs more often late in the season. The fungus persists on debris in the soil. Late blight is caused by Phytophthora infestans.

Do not pick a product because an online picture looks similar. Note whether the spots are on older leaves, headlands or a wet patch, and have the diagnosis confirmed before you decide on treatment.

Blight on potato leaves: how the attack develops

The airborne spores of late blight can travel long distances. Blight is encouraged by warm, muggy weather and rainfall; the optimum temperature for the disease is 18-22°C with an optimum humidity of 95%. Blight normally appears during July and August but can occur much earlier, especially in early potato growing areas such as Cork and Wexford.

To get a useful diagnosis, send your adviser a small, labelled set of photographs:

• the field reference, variety and observation date;
• a whole-plant image showing leaves and stem together;
• close views of both leaf surfaces;
• an overview showing whether affected plants form isolated patches or a wider pattern;
• the products and dates of sprays already applied.

Label each photograph with the field, plant part and date. Ask whether a specimen or laboratory examination is needed and how to collect it, and keep the sample labelled with the field location rather than sending an unlabelled bag of leaves.

Ask for the conclusion in words you can act on: which location it covers and whether another visit or sample is needed.

Late blight in potatoes: where the infection comes from

The pathogen survives the winter in blighted tubers. These may remain in the soil after the previous crop, or in dumps where potatoes have been discarded after grading. Planting blighted tubers is also a potential source of the disease. Home-saved tubers from an infected crop may also develop the disease when planted.

Blight can spread from single plants grown in gardens or patios, volunteers in other crops, dumped potatoes or even discarded skins. To prevent potato blight from these sources, pick all tubers at harvest, do not allow volunteer potato plants to grow the following year, destroy all potato dumps by applying glyphosate or diquat based products, and sow only blight free tubers. Pulling or destroying potato plants growing where they are not supposed to be removes sources of infection.

Walk the holding and note where volunteers, grading waste and dumps are, and record this source clean-up separately from your sprays so an unfinished task stays visible.

Blight timing and warnings

Blight is commonly recorded for the first time each season in unsprayed maincrop potatoes around mid to late July. It spreads when temperatures rise above 10°C and relative humidity above 95%, which is typical of a mild, damp Irish summer.

Met Éireann issues a blight warning when a spell of mild and humid weather is forecast to last long enough for blight spores to develop and spread to nearby plants. The warning gives the areas affected and the duration of the blight spell. It is based on the Bourke Rules: a lead-in of 12 continuous hours with T>10°C and RH>90% and rainfall around the end of the lead-in (±3 hrs), or 16 continuous hours with T>10°C and RH>90% and no rainfall; Effective Blight Hours then accumulate while T>10°C and RH>90% continue, and a warning is issued if 10 to more than 12 Effective Blight Hours are expected.

A warning should ideally indicate a time for suitable spraying conditions 48 hours before the blight spell. Optimal spraying conditions are winds Force 3 or less (under 11 knots), with little or no rain and trafficable soil. Warnings issued in the first two weeks of May include the specification “for early sown crops”. Most commercial growers use weekly fungicide programmes to keep ahead of the fungus.

Treat a warning as weather risk, not as a diagnosis of infection in your field. Save the warning’s issue time, area and period alongside your field notes, and check separately whether the ground is trafficable and the product’s conditions fit the available window.

Potato blight what to do: prevention, then an affected canopy

Prevention is key because an outbreak cannot be eradicated once it is in the crop. Planting earlies or second earlies gives a good chance of harvesting before the blight season gets into full swing.

If blight gets a hold in the crop and the tubers are of sufficient size, cut down and remove the tops to prevent the spores being washed down by the rain to infect the tubers. Do not dig the crop for a minimum of two weeks, to allow spores on the soil surface to die off.

Before you ask what “blight treatment” means for the job, decide which case you are in: protecting a crop that is still clean, investigating suspect symptoms, or managing a confirmed affected area. Keep the dates of the first sighting and of the work done since, in order. Also ask who will revisit the field and how they will report back, using the same field reference.

Late blight fungicide: the Teagasc resistance programme

The Teagasc blight programme alternates and mixes products, which reduces the risk of resistant strains such as EU43 and 37A2 dominating in the Irish population.

Give your agronomist the full sequence of products already applied, with names and dates, and ask how the next spray fits the programme. Check a new brand’s active substances rather than assuming a change of brand is a change of chemistry. Note which programme edition you used, so that if the advice changes mid-season you know which later sprays it affects.

Fluazinam products (for example Shirlan or Volley) became popular in Ireland, mainly for late-season use because of good activity on tuber blight. In 2021 Teagasc reported that strain 37 A2 had been confirmed in Ireland in 2020, and that where a crop contains it, control from fluazinam products can be predicted to be poor; the advice was to use fluazinam only once during the season and mix a partner product with it, for example cymoxanil.

Best fungicide for potato blight: where the Irish product list is published

The list of products in the Teagasc 2026 potato fungicide report is hidden behind a ConnectEd login. Authorised products in Ireland are searched in the DAFM products database, which can be searched by product name, active substance or function/crop. The pesticide register guide explains that database.

Before you compare products, find the Irish listing for the candidate formulation, not a label found through an international search. Then prepare these points for your adviser:

Check the label for the crop, timing, application limits, intervals and water protections, and keep the product document with the recommendation. Give the contractor the exact product name, not just an active-substance abbreviation.

Dithane for potato blight

Dithane 945 was the well-known mancozeb product for potatoes, in many cases used throughout the season. Mancozeb’s registration for use on potatoes expired in 2021, and the 2021 season was the last in which it could be used.

If you still have an old Dithane pack in store, do not spray it on potatoes; note the pack details and see the waste-disposal guide. Keep disposal, purchase and spray records separate, so material taken out of store is not recorded as sprayed.

Tubers: lifting and storage

A healthy-looking canopy does not tell you the condition of the tubers. Plan lifting, grading and storage from your actual crop and spray history. If you submit a tuber sample, describe whether the defect is on the skin or inside, when you noticed it, and whether the tubers came straight from lifting, a graded load or store. Follow the diagnostic service’s instructions on how many to send. Keep a buyer’s defect description with the consignment reference, and record a commercial rejection separately from a laboratory diagnosis.

Treating blight on potatoes and Bayer fungicides for potato blight

Treating blight on potatoes starts with prevention and the Teagasc fungicide programme. A search for a Bayer potato blight fungicide is a product search: find the product name in the register and read its label for potatoes.

Next step

Send labelled stem and leaf photographs with your spray history to the agronomist, and compare the foliage and tuber resistance of potato varieties.