Potato Diseases

The most common diseases of potatoes are early blight, scab, and late blight.

Early Blight

Early blight causes leaf spots and tuber blight. The disease can occur over a wide range of climatic conditions and can be very destructive if left uncontrolled, often resulting  in complete defoliation of plants. Despite the name, it rarely develops early, instead usually appearing on mature foliage. Early blight is caused by the fungus Alternaria solani, which survives in infected leaf or stem tissues on or in the soil. This fungus is universally present in fields where the crops have been grown. It can also be carried in the tubers.

Symptoms: The first symptoms usually appear on older leaves and consist of small, irregular, dark brown to black, dead spots ranging in size from a pinpoint to 1/2 inch in diameter. As the spots enlarge, concentric rings may form which gives the lesion a characteristic "target-spot" or "bull's eye" appearance. There is often a narrow, yellow halo around each spot and lesions are usually bordered by veins. When spots are numerous, they may grow together, causing infected leaves to turn yellow and die. Usually the oldest leaves become infected first and they dry up and drop from the plant as the disease progresses up the main stem.

Surface lesions on tubers results in that they appear a little darker than adjacent healthy skin. Lesions are usually slightly sunken, circular or irregular, and vary in size up to 3/4 inch in diameter. There is usually a well defined and sometimes slightly raised margin between healthy and diseased tissue. Internally, the tissue shows a brown to black corky, dry rot, usually not more than 1/4 to 3/8 inch deep. Deep cracks may form in older lesions.

Management

• Use a crop rotation that includes potatoes only every third or fourth year to allow infested plant debris to decompose in the soil. Rotations with small grains, corn or legumes are preferable.

• Use tillage practices such as fall plowing that bury all plant refuse.

• Select cultivars that have a lower susceptibility to early blight.

• Use appropriate measures to control weeds.

• Maintain fertility at optimal levels--nitrogen and phosphorus deficiency can increase susceptibility to early blight.

• Time applications of overhead irrigation to allow plants to dry before nightfall.

Although the above measures are important to minimize infection, it is usually necessary to apply fungicide sprays to fully protect plants from early blight. Fungicide applications should be initiated when plants begin to flower.

Adapted from the Ohio State University Extension, 2000

Scab

Scab is a disease of potato tubers that is caused by a group of filamentous bacteria called actinomycetes. It results in lowered tuber quality due to scab-like surface lesions. There are no above-ground symptoms. Two forms of scab occur: common scab occurs in all production areas and is most severe in soils with a pH above 5.5; acid scab, a less common form, is important in acidic soils (below pH 5.5).

Symptoms: Scab symptoms are quite variable. Usually roughly circular, raised, tan to brown, corky lesions of varying size develop randomly across tuber surfaces. Russet scab occurs as a rather superficial layer of corky tissues covering large areas of the tuber surface. Pitted scab occurs where lesions develop up to 1/2 inch deep; these deep lesions are dark brown to black, and the tissues underneath are often straw-colored and somewhat translucent. More than one of these lesion types may be present on a single tuber. Small brown, water-soaked, circular lesions are visible on tubers within a few weeks after infection. Mature tubers with a well-developed skin are no longer susceptible, but existing lesions will continue to expand as tubers enlarge, increasing disease severity throughout the growing season. Scab is most severe when tubers develop under warm, dry soil conditions. Coarse-textured soils that dry out quickly are therefore more conducive to scab than are fine-textured soils.

Management: Control in the vegetable garden for the most part involves correcting the conditions most favorable for scab production.

• Keeping soil pH at or below 5.2; sulfur can be applied to the soil to lower the pH and make it more acidic. Acid-forming nitrogen fertilizers, such as ammonium sulfate and diammonium phosphate, are more effective in reducing scab than ammonium nitrate, while calcium and potassium nitrate can raise soil pH and favor scab development. Fresh barnyard manure applied to the soil can create a scab problem and the pathogen may persist in the soil for many years afterward.

• Follow a crop rotation schedule with rye, alfalfa or soybeans when scab is a problem. Plant at least three years of nonsusceptible crops between potato crops.

• Keep soil moist during early tuber development (for about 2 weeks after the plants emerge from the soil) but avoid overwatering.

Plant certified seed potatoes of the russet-skinned varieties, which are more resistant to scab: Nooksack is highly resistant; Russet Burbank, Netted Gem and Norgold have moderate resistance; , Red Norland, a red variety, is also moderately resistant.

Adapted from the University of Vermont Extension and the Ohio State University Extension, 2000

Late Blight

Late blight is one of the most devastating diseases of potato in the northeast. It was responsible for the devastating  Irish potato famine of the 1840s. If left unmanaged, this disease can result in complete destruction of potato crops. It is caused by the fungus Phytophthora infestans which, unlike most pathogenic fungi, cannot survive in soil or dead plant debris. For an epidemic to begin, the fungus must survive the winter in potato tubers, be reintroduced on seed potatoes, or be blown in as live spores during rainstorms. Disease development is favored by cool, moist weather. Nights of 50oF weather and days around 70oF, accompanied by rain, fog or heavy dew are ideal.

Symptoms: Late blight appears on potato leaves as pale green, water-soaked spots, often beginning at leaf tips or edges. The circular or irregular leaf lesions are often surrounded by a pale, yellowish-green border that merges with healthy tissue. Lesions enlarge rapidly and turn dark brown to purplish-black. During periods of high humidity and leaf wetness, a cottony, white mold growth is usually visible on lower leaf surfaces at the edges of lesions. In dry weather, infected leaf tissues quickly dry up and the white mold growth disappears. Infected areas on stems appear brown to black and entire vines may be killed in a short time when moist weather persists.

Late blight appears on potato tubers as a shallow, coppery-brown dry rot that spreads irregularly from the surface through the outer tissue. On tuber surfaces, lesions appear brown, dry, and sunken, while infected tissues immediately beneath the skin appear granular and tan to copper-brown. When tubers are stored under cool, dry conditions, lesion development is retarded and, upon prolonged storage, lesions may become slightly sunken and desiccated. Secondary bacteria and fungi frequently enter late-blight lesions, usually resulting in a slimy breakdown of entire tubers.

Management

• Infected cull potatoes are a major source of spores of the late blight fungus and must be disposed of properly – DO NOT COMPOST OR MAKE CULL PILES. Cull potatoes should be spread on fields not intended for potato production the following year so that they will freeze and be destroyed during the winter. If this is not possible, they must be destroyed in by complete chopping, burial, burning or feeding to livestock.

• Plant only certified seed potatoes.

• Seed sources should be selected very carefully to avoid bringing in late blight on seed potatoes, especially new strains of the fungus. Look for the characteristic coppery-brown discoloration of the potato flesh under the skin of seed tubers.

• Volunteer potatoes can be a significant source of spores of the late blight fungus. All volunteers should be destroyed as quickly as possible by herbicides, chopping, or cultivation.

• Growers should scout fields regularly to look for late blight. Special attention should be paid to early-planted fields where the disease is likely to develop first.

• Use of a good protectant fungicide program is necessary to fully protect any crop of potatoes. Verify that potato vines have been completely dead for 2-3 weeks prior to harvest. Fungicide applications should be continued until vines are dead.

Adapted from the Ohio State University Extension, 1999