Early blight and late blight, two serious diseases of potato, are widely distributed. Both are found everywhere potatoes are grown. The terms “early” and “late” refer to the relative time of their appearance in the field, although both diseases can occur at the same time.
Early blight of potato is caused by the fungus, Alternaria solani, which can cause disease in potato, tomato, other members of the potato family, and some mustards. This disease, also known as target spot, rarely affects young, vigorously growing plants. It is found on older leaves first. Early blight is favored by warm temperatures and high humidity.
Symptoms. Spots begin as small, dark, dry, papery flecks, which grow to become brown-black, circular-to-oval areas. The spots are often bordered by veins that make them angular. The spots usually have a target appearance, caused by concentric rings of raised and depressed dead tissue. A yellowish or greenish-yellow ring is often seen bordering the growing spots. As the spots become very large, they often cause the entire leaf to become yellow and die. This is especially true on the lower leaves, where spots usually occur first and can be very abundant. The dead leaves do not usually fall off. Dark brown to black spots can occur on stems.
Tubers are affected, as well, with dark, circular to irregular spots. The edges of the spots are often raised and purple to dark metallic gray in color. When the tuber is sliced open, the flesh under the spots is usually brown, dry, and leathery or corky in texture. As the disease advances, the potato flesh often becomes water soaked and yellow to greenish yellow. Early blight spots are less likely to become rotted by secondary organisms than the other tuber rots.
Prevention. Varieties resistant to this disease are available. In general, late maturing varieties are more resistant than the earlier maturing varieties. Keep plants healthy; stressed plants are more predisposed to early blight. Avoid overhead irrigation. Do not dig tubers until they are fully mature in order to prevent damage. Do not use a field for potatoes that was used for potatoes or tomatoes the previous year. Keep this year’s field at least 225 to 450 yards away from last year’s field. Surround the field with wheat to keep wind-blown spores from entering. Use adequate nitrogen levels and low phosphorus levels to reduce disease severity. See current recommendations for chemical control measures.
Late blight of potato is a serious disease caused by Phytophthora infestans. It affects potato, tomato and, occasionally, eggplant and other members of the potato family. Late blight is the worst potato disease. It was first reported in the 1830s in Europe and in the US. It is famous for being the cause of the 1840s Irish Potato Famine, when a million people starved and a million and a half people emigrated. Late blight continued to be a devastating problem until the 1880s when the first fungicide was discovered. In recent years, it has reemerged as a problem. It is favored by cool, moist weather and can kill plants within two weeks if conditions are right.
Symptoms. Leaf spots begin as small, pale to dark green, irregularly shaped spots. The spots often have pale green to yellow rings surrounding them. The spots are not bordered by veins but can grow across them. In cool, moist weather, the spots grow rapidly into large brown to purplish black areas. The disease may kill entire leaflets or grow down the petioles and into the stem, killing the plant above it. When the weather is moist, a white fungal growth appears on the edges of the dead areas, usually on the undersides of the leaves. In the field, plants often give off a distinctive fetid or decaying odor.
On susceptible potato varieties, the tubers can become infected. Small to large, slightly depressed areas of brown to purplish skin can be seen on the outside of the tuber. When the tuber is cut open, there is a tan-brown, dry, granular rot, which extends ½” to ¾” into the tuber. The border of this area is indistinct. If potatoes are stored under warm or humid conditions, the rot will continue to progress. Often secondary rot organisms set in and completely destroy the tubers.
Disease Identification. White, fluffy fungal growth is present on the bottoms of leaves in moist weather. Leaf spots are not bordered by veins.
Prevention. Use disease-free seed potatoes. Keep cull/compost piles away from potato growing areas. Destroy any volunteer potato plants. Keep tubers covered with soil throughout the season to prevent tuber infection. Remove infected tubers before storing to prevent the spread of disease in storage. Kill vines completely before harvest to avoid inoculation of the tubers during harvest. Resistant varieties are available, although some fungicides must still be applied to resistant cultivars. See current recommendations for chemical control measures.
This information was developed for conditions in the Northeast. Use in other geographical areas may be inappropriate.
The information in this document is for educational purposes only. The recommendations contained are based on the best available knowledge at the time of publication. Any reference to commercial products, trade or brand names is for information only, and no endorsement or approval is intended. The Cooperative Extension System does not guarantee or warrant the standard of any product referenced or imply approval of the product to the exclusion of others which also may be available. The University of Connecticut, Cooperative Extension System, College of Agriculture and Natural Resources is an equal opportunity program provider and employer.
Early blight is a common foliar disease that affects potato crops worldwide This disease can significantly reduce yield and tuber quality if not properly managed. As a potato grower, it is critical to understand what early blight is, how to identify it, and strategies for effectively managing this problematic disease.
What Causes Early Blight of Potato?
Early blight of potato is caused by the fungus Alternaria solani. This pathogen can overwinter in plant debris left in fields after harvest in the soil, on seed tubers and on solanaceous weeds. The primary inoculum in the spring comes from spores produced on the overwintering fungus. These spores are spread by wind, rain splash, and insects onto potato foliage.
Once the spores land on leaves, they germinate under warm, humid conditions. The fungus penetrates the leaf tissue directly through the cuticle or through natural openings like stomata. The symptoms of early blight become visible 5-7 days after infection.
How to Identify Early Blight of Potato
Being able to accurately identify early blight is the first step toward effective management. Here are the key symptoms to look for:
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Small dark brown or black lesions first appear on older, lower leaves.
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Lesions enlarge and are restricted by leaf veins, causing them to be angular or irregularly shaped.
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The most distinctive symptom is the concentric rings within the lesions, creating a bull’s eye or target spot pattern.
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Lesions may coalesce, covering large areas of the leaf.
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Leaves with heavy infection turn yellow, wither, and die.
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Stem lesions are elongated, black lesions on stems and petioles.
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Tuber symptoms appear after harvest as dark, sunken lesions with raised borders.
Conditions Favoring Early Blight Development
Several environmental factors influence the development and spread of early blight disease:
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Warm temperatures – The optimum temperature for infection is around 68°F.
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High humidity and prolonged leaf wetness – Free moisture is required for spore germination and penetration. Overhead irrigation and frequent rainfall promote disease spread.
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Age of plants – Older, senescing foliage is more susceptible than young growth.
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Potato variety – Earlier maturing varieties tend to be more susceptible.
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Stressed plants – Factors like poor fertility, drought, insect damage, etc. increase susceptibility.
Impacts of Early Blight Infection
If early blight is left unchecked, impacts on potato yield and quality can be significant:
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Premature defoliation due to extensive leaf lesioning reduces photosynthetic capacity.
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Smaller, fewer tubers are produced due to impaired photosynthesis.
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In storage, tuber lesions enlarge and result in shriveling.
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Fresh market quality is reduced by poor appearance of infected tubers.
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Processing quality suffers from decay and blemishes on tubers.
Integrated Management of Early Blight
An integrated approach utilizing cultural practices and targeted fungicide applications provides the most effective control of early blight.
Cultural Control Methods
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Use clean seed tubers free of the early blight pathogen. Avoid saving your own seed tubers.
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Rotate potato fields to a non-host crop like corn or soybean for 2-3 years to reduce pathogen levels.
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Eliminate cull piles and volunteer potatoes, which harbor the fungus.
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Control solanaceous weeds like nightshade which can be alternate hosts.
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Plant later maturing, longer season potato varieties which are less susceptible.
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Maintain proper fertility and irrigation to avoid plant stress.
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Dig vines promptly after harvest to speed decomposition of foliage.
Fungicide Applications
Foliar fungicides are the most effective means of protecting against early blight. These provide recommendations for fungicide use:
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Begin scouting fields regularly at row closure for early symptoms.
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Base spray programs on an early blight forecasting system using P-days or disease severity values.
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Initiate fungicide applications at 300 P-days or when early symptoms are found.
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Apply protective fungicides like mancozeb, chlorothalonil or fluopyram on a 7-10 day schedule.
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Alternate different mode-of-action fungicides to minimize resistance development.
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Increase spray frequency during periods of rapid disease development.
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Cover undersides of leaves for best results.
Early blight is a challenging but manageable disease in potato production. Careful monitoring to detect initial symptoms, avoidance of stress, and properly-timed fungicide programs provide the best opportunity to minimize yield losses and protect potato quality threatened by this disease. Integrating cultural practices and fungicides that utilize multiple effective modes of action is key to success.
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Blight: 5 Ways to Control Potato Blight (Late Blight)
FAQ
How do you treat early blight in potatoes?
Early blight in potatoes can be managed through a combination of cultural practices and, when necessary, fungicide applications. Key strategies include using disease-free seed, ensuring good air circulation around plants, avoiding overhead irrigation, and promptly removing infected plant material.
What are the first signs of blight on potatoes?
The first sign of potato blight will be small, dark brown marks on the leaves. These will quickly increase in size and number. In many cases the stems of the plants will also have some brown marks on them.
What is the treatment for early blight?
Remove infected leaves during the growing season and remove all infected plant parts at the end of the season. Apply a synthetic fungicide or an organic fungicide (fixed copper) according to label directions, early in the season, when symptoms appear to slow the spread of the disease.
What are the symptoms of early blight?
Early blight is primarily a disease of stressed or senescing plants. Symptoms appear first on the oldest foliage. Affected leaves develop circular to angular dark brown lesions 0.12 to 0.16 inch (3–4 mm) in diameter. Concentric rings often form in lesions to produce characteristic target-board effect.