Blight on potato plants: what potato blight disease looks like (picture guide)
Inspect leaves for pale brown or black, water-soaked patches. White growth can develop at the boundary between diseased and healthy tissue. Check stems and tubers as well as foliage: late blight can affect all of them. Compare these symptoms with any picture of blight on potatoes before you decide which disease is in the crop.
Leaves, stems and tubers each show their own picture. Blight on leaves is a rapidly spreading, watery rot: leaves collapse, shrivel and turn brown, and stems can develop lesions. On the lesion edge, light green tissue can show where the pathogen is spreading, and in wet or very humid weather whitish growth may be visible on the underside of the leaf. Affected tubers have a reddish-brown decay below the skin, firm at first and soon a soft rot; peel a tuber partly to see it in the flesh.
Wind disperses spores from leaf and stem lesions. Heavy rainfall can wash them down to developing tubers.
Early blight or late blight: how to tell them apart
Early blight is Alternaria, a different disease from late blight, and the lesion on the leaf shows which one is in the crop. Alternaria lesions often form a target spot of concentric rings. They start as very small black or brown spots on the lower leaves, usually a few weeks after emergence, and merge as the leaf tissue dies.
Alternaria spores cannot be seen on the leaf, and there is no milky ring of sporulation around the lesion on the underside. A late blight lesion is pale brown or black and water-soaked, so a target-spotted lower leaf and a water-soaked patch call for different decisions.
Early blight is also mistaken for other problems: its symptoms are often confused with magnesium deficiency and manganese toxicity, and with Verticillium wilt.
Alternaria survives in the soil and on crop debris, so the lower leaves pick up spores first. Wet and dry spells favour it, and irrigated crops are ideal for it. Left uncontrolled it causes heavy leaf loss and yield reductions of up to 30%.
The late blight programme comes first. Effective control of late blight remains the most important factor in the fungicide strategy, early blight included. Remove volunteers and alternative hosts such as nightshade, keep rotations long, and avoid crop stress through sound fertiliser and irrigation practice.
Prevent potato blight by removing infection sources
Control groundkeepers and outgrade piles, and start with good-quality seed. Clean machinery, boxes and yards that handle potatoes. Discarded tubers, infected planting material and surviving volunteer plants can carry infection into a new crop.
Match protection to canopy growth
The protection objective changes through the season:
- At emergence, antisporulant activity helps address infection associated with the soil surface.
- During rapid canopy growth, systemic or translaminar activity helps protect expanding foliage.
- At stable canopy, strong protectant activity remains important during periods of high blight pressure.
- During senescence and haulm destruction, tuber protection becomes a priority. Continue protection until living green haulm has gone.
The selected product's label sets the crop authorisation, dose, application count, timing and harvest restrictions.
Best fungicide for potato blight: choose potato late blight fungicides by mode of action
Partner effective fungicides with different modes of action and alternate those modes between successive sprays. A change of trade name may still leave the same mode of action in consecutive applications.
Host resistance and fungicide resistance describe different relationships. A resistant potato variety restricts disease development in the plant. A resistant pathogen population has reduced sensitivity to a fungicide.
Keep your spray records in one place. Free to start.
Resistance-management groups include phenylamides, uncouplers of oxidative phosphorylation, carboxylic acid amides and oxysterol-binding protein inhibitors. Changes in the sampled blight population affect which groups remain useful.
Bayer potato blight fungicide: reading a product page
A Bayer potato blight fungicide is read like any other: by its label. Infinito is a protectant and systemic fungicide for late blight in potatoes. It contains fluopicolide and propamocarb hydrochloride (MAPP 16335); the maximum individual dose is 1.6 litres of product per hectare, the maximum total is 6.4 litres per hectare per crop, and the latest application is 7 days before harvest. These figures belong to that product, so a different product from the same company has different figures; take them from the label of the product you hold, and record the product, dose and date in the spray record.
Forecast risk and inspect the field
Use weather-based forecasting together with local crop observations. A forecast identifies conditions favourable to infection; an inspection establishes what is happening in the crop.
Good application coverage contributes to control. Angled nozzles, appropriate water volume and lower forward speed can improve coverage within the product and equipment instructions.
Fight Against Blight: sampling and strain tracking
Fight Against Blight analyses the samples that blight scouts submit and genotypes the strains present. Genotyping identifies genetically distinguishable subdivisions of the pathogen and helps track their distribution. It contributes to resistance surveillance and subsequent programme planning.
Blight resistant potatoes: RHS advice and Sarpo Mira
Athlete, Alouette, Carolus and the Sarpo cultivars (for example Sarpo Axona and Sarpo Mira) are currently thought to show good resistance. They are not completely immune and can still develop blight in prolonged favourable conditions. Resistance listed for a cultivar does not give its maturity group: for a blight resistant second early potato, ask the seed supplier for the maturity group of each cultivar on that list. Early varieties are less likely to be affected by blight, and blight is less prevalent in dry summers.
Sarpo Mira is a large, pink-skinned maincrop potato. Its late-blight resistance is strong, although limited disease can develop. New pathogen populations can overcome resistance, so inspect resistant varieties throughout the season.
Foliage resistance concerns the canopy; tuber resistance concerns the harvested crop. Spores can move from infected leaves towards tubers, making both traits relevant to variety selection.
Sarpo Mira lacks normal natural senescence, so the grower decides when to stop growth and harvest instead of waiting for haulm dieback.
Sarpo Mira grows very tall and smothers weed competition. Left to grow, it develops very large tubers with very high dry matter, which makes a floury potato.
Sarpo Axona is a pink-skinned maincrop; Sarpo Kifli is a waxy early maincrop.
How the pathogen survives and changes
Phytophthora infestans reproduces asexually through sporangia and zoospores. Rapid repeated cycles support epidemics dominated by clonal lineages. Sexual reproduction between compatible mating types produces oospores, which can survive in soil for several years without potatoes and generate new genotypes.
Fungicide resistance can emerge rapidly or develop through progressively reduced sensitivity. Within some genotypes, individual strains differ in their response to CAA and OSBPI chemistry. Genotyping and sensitivity testing therefore provide complementary information when an outbreak is investigated.
Next step
See the IPM plan, the HSE register guide and the tank-mix checklist. For organic potato-blight control see the organic crop-protection guide.