# Manage potato late blight and plan resistant varieties, seed and late-season crops URL: https://leafwrit.com/us/potato-late-blight/ Reviewed: 2026-10-29 ## Potato blight: recognize late blight and plan the next action **Potato late blight causes large dark-brown leaf blotches, firm stem lesions and tuber damage.** The cause is *Phytophthora infestans*, a water mold. Leaf blotches have a green-gray edge and are not confined by major veins. White sporulation develops at lesion margins, principally underneath leaves in humid conditions. ### Conditions and scouting Late blight develops readily in **60–70°F and damp conditions**. Under cool, wet conditions, the pathogen can infect and produce thousands of spores per lesion in fewer than five days. Scout before dew dries, concentrating on shaded areas and places with poor air circulation. Minnesota growers should contact the university’s Fruit and Vegetable team about suspected infection because neighboring crops may be at risk. ### Sources of infection Seed tubers, cull piles and volunteers can carry inoculum. Plant certified seed, control volunteer potatoes and destroy cull piles before the growing season. Cultivar resistance varies with pathogen lineage. Even resistant potatoes can show symptoms under highly favorable conditions. ### Preventing blight in potatoes and blight control: diagnosis and fungicide timing Send living symptomatic plants to Minnesota’s Plant Disease Clinic. Follow its [packaging instructions](https://pdc.umn.edu/submit-sample). Apply a labeled fungicide effective against water molds before infection when disease-favorable conditions occur. Rotate fungicide groups or use suitable mixtures to reduce resistance risk. For Minnesota farm production, consult the current Midwest Vegetable Production Guide for the crop-specific program. ### Blight on potatoes: confirm it before you choose a product Before treating, confirm the disease from the symptoms and a tested sample, then read the directions of the exact potato product against your crop stage and harvest plan. ## Best blight resistant potatoes: choosing resistant potato varieties and seed for crop planning Choose seed using evidence for the disease and the local pathogen population, together with your market and maturity needs. Elba, Kennebec, Sebago, Allegany and Chieftain are late-blight-resistant options for home gardens in Maine. For commercial planting, ask your local adviser how those varieties perform. ### Look at foliage and tubers separately Defender, released in 2004 for frozen processing, has resistance to foliar and tuber infection in cultivar evaluations. This white-skinned processing potato also has weaknesses including common scab, greening, internal necrosis and blackspot. ### Read the disease name before selecting a variety Write down which disease a supplier’s resistance claim covers, and ask for locally relevant commercial performance before ordering. For early blight, Maine’s home-garden options include Elba, Ranger Russet and Lamoka. ### Pathogen lineage can change the interpretation Newer *Phytophthora infestans* clonal lineages have infected Satina, Defender and Jacqueline Lee in cultivar evaluations. These varieties had previously shown resistance. When you compare late-blight performance, ask about the pathogen lineage as well as the cultivar. ### Use a compact comparison table before ordering Compare disease resistance, processing quality, maturity and the offered seed lot before you order. | Variety | Market | Maturity | Late-blight evidence | Other weaknesses | Seed lot | | --- | --- | --- | --- | --- | --- | | Defender | Frozen processing | Confirm fit for your local season | Historical foliar and tuber resistance; later tested lineages were virulent | Blackspot, common scab, greening and internal-necrosis susceptibility | Get the actual offered lot and its health information | ### Keep the rest of the late-blight plan Combine a resistant cultivar with field monitoring, healthy planting material and the regional crop-protection program. Resistance is one component of integrated late-blight management. ## Pesticides used in potato farming: identify the pest and read the label first Identify the potato pest or disease and crop stage before choosing a pesticide. Use the [potato management guide](https://ipm.ucanr.edu/agriculture/potato/) to distinguish insect, disease, nematode and weed problems throughout the season. Open the relevant problem page before you compare products. ### Review possible late blight separately If you suspect late blight, record water-soaked leaf spots, white sporulation, stem lesions and tuber symptoms separately before you submit a sample. Late blight is common in coastal environments and in the southern San Joaquin Valley in California. Under conducive conditions, lesions expand rapidly and become brown to purplish black as tissue is killed. In districts commonly subject to late-blight outbreaks, preventive fungicide applications are advised when environmental conditions favor the disease. ### White mold White mold affects lower foliage and stems. Look for hard, dark sclerotia in affected stems. The pathogen persists in soil, and prolonged moisture favors disease. ### Blight free potatoes and blight free seed potatoes Blight free potatoes are not a variety class: choose late-blight-resistant varieties such as Elba, Kennebec, Sebago, Allegany and Chieftain for Maine home gardens, and inspect the seed's history and condition, as described in the late-season planning section below. ## Late-season potatoes (late maincrop and late cropping potatoes): variety and planting-window planning Choose a late-season potato variety that fits the available growing season and intended market. Late maturity and late planting are different choices. Later varieties take longer to develop; Russet Burbank is a long-season, indeterminate type. ### Choose maturity for the market Storage conditions differ for seed, tablestock and processed products. ### Make the planting-window question specific The time to tuberization depends on planting date, variety, temperature, seed-piece quality and nitrogen fertilization. A days-to-maturity figure copied without those conditions will not answer your planting question. ### Inspect seed history and condition Ask for the lot identity and seed-production information before you accept delivery, and keep any storage and handling advice that comes with the seed. ### Follow development in the field Indeterminate varieties such as Russet Burbank continue growing vines after flowering. In your crop notes, record emergence, tuber initiation, bulking, vine decline and harvest as separate stages. ### Check the label before any late-season treatment Before any late crop-protection operation, check the current [product label](/us/read-pesticide-label/) and your state registration, and identify the exact product, permitted use and harvest restrictions from its directions. Labeling defines the method, amount, frequency and site of pesticide use. ### Separate chronological age from physiological condition Chronological seed age is the time since harvest; physiological age reflects internal biochemical aging. Precutting ages seed physiologically, as do growing-season stress and warm storage. Bruised seed tubers are physiologically older than unbruised ones. Check those factors before preparing seed for a late planting. ### Follow vines and tubers with separate observations Late nitrogen can delay vine senescence and encourage regrowth, so a vigorous canopy close to harvest does not mean the variety failed. Photograph the canopy as it develops and keep notes on any tuber sampling: what you inspected, where and when. Record tuber number, size and visible condition separately from vine height or greenness, since green vines alone do not show tuber development. If the canopy stays vigorous near planned harvest, review nutrient applications, irrigation, weather and variety behavior together. ### Assess maturity and handling before harvesting for storage Distinguish skinning, blackspot, shatter bruise and pressure bruise when inspecting harvested potatoes. Note where damage first appears during lifting, transfer and handling, so you can trace the stage responsible. ### Prepare storage for the intended market Before harvest, check the available store, its capacity, ventilation and monitoring. Tell your storage adviser whether the crop is for seed, fresh sale, chips or other processing, and set storage conditions for that market. ## Next step For suspect disease, follow the [Plant Disease Clinic’s sampling instructions](https://pdc.umn.edu/submit-sample) and contact the university’s Fruit and Vegetable team promptly.