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Potato - Diseases, Pests and Problems

Basic Information

Problem: Flea beetles
Affected Area: Leaf

Description: Adult insects are 1/16" long, hard shelled, shiny, dark-colored beetles that jump when disturbed. The larvae are slender, whitish, and cylindrical and may feed on plants for 3 to 4 weeks before pupating. Adults chew tiny holes ?pinholes? in leaves. Two generations of adults may be produced yearly. may cause pimple-like swellings on the skin of the tuber with brown feeding tunnels 1/32" in diameter and up to 1/2" deep. Damage may be significant on potatoes. In severe infestations, small plants may be killed and larger plants stunted. Flea beetles are also carriers of viruses.

Control: Keep debris removed. Spray or dust plant with Sevin, diazinon or Thiodan. Use Sevin sparingly as it kills beneficial insects. Rotate location of planting from year to year. * Pesticide use and recommendations for various areas are constantly changing. Check with your County agent for current recommendations.

Problem: Aphids
Affected Area: Leaf and Stem

Description: Small Insects found on new stems and the underside of the leaf. Usually green. They suck fluids from the plant leaving a honey dew substance behind. Leaves turn pale yellow. Aphids can also spread viruses.

Control: Insecticidal soaps or a strong stream of water. Ladybug beetles are natural predators. * Thiodan or Diazinon ? more than one application may be required. A layer of aluminum foil under plants reflects light to underside of leaves and may deter aphids. * Pesticide use and recommendations for various areas are constantly changing. Check with your County agent for current recommendations.

Problem: Wire Worms
Affected Area: Roots and Tuber

Description: Larvae of click beetles. Shiny, slender, brown, hard-shelled, soil dwelling worms ranging from 3/8" to 1 1/2" long. May be soft-bodied, white with yellowish brown heads and tails. Wire worms may remain in the larval stage for 2 to 5 years moving up and down in the soil depending on the temperature and moisture. Adults feed little, but larvae feed on potatoes and underground roots. They bore into tubers allowing rot to enter doing their worst damage when they bore straight into the tuber.

Control: * Diazinon granules worked into the soil (read label directions) gives season-long protection. Only needed when damage is at critical levels. * Pesticide use and recommendations for various areas are constantly changing. Check with your County agent for current recommendations.

Problem: Colorado Potato Beetle
Affected Area: Leaf

Description: Adults are 1/2" long, yellow with black stripes. Larvae are plump, hump-backed, copper colored grubs up to 3/5" long. Both feed on leaves and may defoliate potatoes. Eggs are bright yellow-orange and laid in clusters on the underside of leaves.

Control: Hand pick adults and egg masses if possible. * Can use Sevin (sparingly to avoid spider-mite build-up) or Thiodan . * Pesticide use and recommendations for various areas are constantly changing. Check with your County agent for current recommendations.

Problem: Die-back
Affected Area: Entire plant

Description: Vines whither and tops partially die. Plant may die.

Control: Increase fertilization. Irrigate early in the day, but avoid excessive water.

Problem: Early Blight
Affected Area: Tuber

Description: Dark bulls-eye target spot of dead tissue on leaves. Can lead to rot in stored tubers. Tuber spots which occur less often, are sunken, brown or black spots with underlying brown, corky, dry rot. One of the most common potato diseases in the Intermountain West.

Control: Plant resistant varieties. Rotate location. Sprinkle irrigation can enhance its spread. Remove and destroy infected vines at harvest. * Can use fungicides like Daconil or Maneb at 14-day intervals. Complete coverage including lower leaves is essential for control. * Pesticide use and recommendations for various areas are constantly changing. Check with your County agent for current recommendations.

Problem: Common Scab
Affected Area: Tuber

Description: Yield is not influenced and potatoes are edible, but may be severely blemished. Characterized as ?looking dirty? but dirt will not wash off. Very common in alkaline soils of the west. Soils with high calcium levels as in the Great Basin favor the disease as will dry soils early in the season.

Control: No direct control. Rotate crop and use certified seed. Most critical time for careful irrigation is from 2 to 7 weeks after emergency. Russet varieties are generally less affected than smooth skin varieties.

Problem: Aphids
Affected Area: Affects the ability of the plant to resist viruses

Description: Several potato virus diseases are transmitted by aphids, and identification of the vector involved is often necessary. Aphid species differ not only in morphology and ability to transmit potato viruses but also in form, life cycle, and behavior, depending on the environment (temperature, relative humidity, photoperiod, and host plant condition to which the aphid or its mother is exposed. Where winter conditions are severe, most potato-infesting aphids overwinter as sexually produced eggs laid on the rough bark of a woody host or on the crown and leaves of an herbaceous biennial or perennial plant. In mild climates, sexual aphids and eggs do not occur, and parthenogenetic, viviparous females are produced throughout the year.

Control: NA

Problem: Blackleg
Affected Area: Affects the entire plant.

Description: Blackleg affects stems and may produce soft rot in tubers. Stems, petioles, and leaves may also become infected through wounds such as petiole scars, hail, or wind damage. Tubers produced by infected plants may show symptoms ranging from slight vascular discoloration at the stolon end to soft rot of the entire tuber. Soft rotted tissues are wet, cream to tan, with a soft, slightly granular consistency. The disease is easily cultured and produces deep pits, or craters, on selective media that contain polypectate and forms acid from maltose. Bacteria may move for some distance in the soil water and contaminate developing daughter tubers of adjacent plants. Erwinia cells released into the soil survive for varying periods of time, depending upon soil temperature and, to a lesser extent, soil moisture. Tubers produced under warm (23-25 ?C or higher, dry conditions are less likely to be contaminated because the pathogens are less likely to survive, and they spread through the soil for shorter distances than when the soil is cool and moist. Cool wet soils at planting time followed by high temperatures after plant emergence favor postemergence blackleg expression and higher soil temperatures favor seed piece decay and preemergence death of shoots. Invasion of seed pieces by Fusarium species may predispose tissues to soft rot and favor blackleg development. Anaerobic conditions resulting from poor aeration, flooding of soil, or the presence of a water film on tubers after washing favor disease development.

Control: NA

Problem: Common Scab
Affected Area: Affects the tubers.

Description: This disease is a major production problem that affects grade quality and has only a small effect on total yield or storing ability. Tuber lesions are usually circular, 5-8 mm in diameter, but they may be irregular in shape and larger when infections group together. Brown to tan stem and stolon lesions originate at lenticels as elongate lens-shaped lesions or at other natural wounds as approximately circular lesions. The organism causing scab occurs on the fleshy roots of other plants such as beets (red and sugar, radish, rutabaga, turnip, carrot, and parsnip. Actively growing tubers are infected through young lenticels and probably also through stomata of the epidermis before the periderm differentiates. Scab lesions may be deep or shallow. Continuous crops of potatoes generally increase severity of scab.

Control: Maintaining adequate soil moisture during tuber set and enlargement is critical in controlling the extent of scab. Chemical treatment of soil depends on the proper incorporation of chemicals into the soil and should be coordinated with other prevention methods.

Problem: Early Blight
Affected Area: Affects the leaves and tubers.

Description: This disease is found worldwide wherever potatoes are grown. Initial infection is most frequent on lower, older leaves. Tuber lesions are dark, sunken, circular to irregular in shape, and often surrounded by a raised border of purplish to gun metal color. Depending upon the location, the disease persists in crop debris, soil, infected tubers, or other solanaceous hosts. Immature tuber surfaces are easily infected, whereas those of mature tubers are much more resistant. Most rapid progress of the disease occurs during periods of alternating wet and dry weather. Field resistance to foliage infection is associated with plant maturity. The fungus is pathogenic on tomato and other solanaceous crops and has been reported on other genera such as Brassica species.

Control: NA

Problem: Fusarium Dry Rots
Affected Area: tubers.

Description: Fusarium dry rots are found on potatoes worldwide. This disease affects tubers in storage and planted seed tubers. After about one month of storage, tuber lesions at wounds are visible as small brown areas. Internal dry areas are shades of brown from fawn to dark chocolate, with the advancing margin faint for lighter shades and distinct for darker shades. Whole-tuber seed becomes infected through wounds during storage or preparation for planting. In the field, the shriveling of infected seed tubers and pitting of infected pieces may not be evident. Fusarium can survive for several years in field soil, but the primary inoculum is generally borne on seed tuber surfaces. Tubers are tolerant to infection when harvested. Wound healing can reduce infection. Dry rot develops most rapidly in high relative humidity and at 15-20 ?C. If the soil temperature and moisture are suitable for rapid sprout growth and emergence, seed tuber or piece decay after planting may be of little consequence.

Control: NA

Problem: Late Blight
Affected Area: Affects the tubers.

Description: Late blight is probably the single most important disease of potatoes worldwide. Leaf lesions are highly variable, depending on temperature, moisture, light intensity, and host cultivar. In the field, plants severely affected with late blight give off a distinctive odor. Positive identification of late blight requires confirmation of sporangia and sporangiophores either on lesions in the field under moist conditions or on leaf or tuber lesions incubated in a moist chamber. On susceptible cultivars, exteriors of infected tubers show irregular, small to large, slightly depressed areas of brown to purplish skin. In tropical areas where the crop is grown all year, overwintering of the disease is not an important consideration. Tubers on cull piles frequently sprout and form dense masses of succulent tissue that are easily infected by spores from diseased tubers. Tubers, particularly those inadequately covered by soil, may be infected in the field by spores that have been washed from infected leaves into the soil by rain or irrigation. Field infection is most successful under cool, moist conditions.

Control: Systems for forecasting late blight and for timing fungicide applications rely on records of temperature and rainfall or temperature and relative humidity and predict the probability of late blight development, assuming the presence of inoculum. Late blight often severely affects tomatoes and occasionally affects eggplant and many other members of the Solanaceae. Resistance is recognized as specific resistance and general resistance.

Problem: Nematodes
Affected Area: roots and tubers.

Description: Nematodes pathogenic to potatoes occur in all climates and cause serious crop losses, but much of this damage is unrecognized or attributed to other causes. Confining nematode populations to areas where they already exist by restricting movement of infected seed tubers and plants may be the most effective way of preventing loss of productive land.

Control: NA

Problem: Pink Rot
Affected Area: Affects the tubers.

Description: Wilting, which is sometimes the initial symptom, may occur at any time but generally occurs late in the season. Tubers generally become infected through diseased stolons, but some infections appear to occur at buds or lenticels. The disease is soilborne and endemic in many soils. Disease develops in soils approaching saturation from poor drainage or excessive precipitation or irrigation. Although it has been reported as a pathogen only on potatoes and tulips, it has been recovered from the roots of 17 nonsolanaceous plants, including wheat and rye.

Control: NA

Problem: Psyllid Yellows
Affected Area: Affects the leaves.

Description: This disorder results from insect feeding, and no infectious microorganism is involved. On vines, young leaves, which are often red or purple, become erect and have cupped basal portions. Border parenchyma surrounding the phloem is first affected, and later tissue breakdown extends laterally, causing phloem drying. Few if any tubers are set on plants attacked in early development. Abnormally small tubers sprout without a dormant period. The disorder is not tuber-transmitted. The disease results from toxic substances introduced during feeding of nymphs of the tomato or potato psyllid also known as the jumping plant louse. Similarity between psyllid yellows, the mycoplasma disorder haywire, leaf rolls of various types, and Rhizoctonia has caused some confusion in diagnosis.

Control: NA

Problem: Rhizoctonia Canker
Affected Area: Affects the tubers.

Description: Rhizoctonia canker, commonly called black scurf, is present in all potato growing areas. Black or dark brown sclerotia develop on surfaces of mature tubers. Plants are most severely damaged in the spring shortly after planting and killing of underground sprouts delays emergence, especially in cold, wet soils. Reddish brown lesions on stolons cause stolon pruning or tuber malformation. The perfect stage of the pathogen occurs on stems just above the soil line as a whitish gray mat on which basidiospores are formed, giving the surface a powdery appearance. A type of tuber malformation, incompletely understood and not directly linked to Rhizoctonia infection, is frequent when Rhizoctonia is severe on tubers. The mycelium is generally tan to dark brown and hyphae are rather large (generally 8-10 ?m in diameter. Rhizoctonia produces a growth-regulating toxin that may be partially responsible for tuber malformation. The pathogen overwinters as sclerotia on tubers, in soil, or as mycelium on plant debris in the soil. Rhizoctonia populations may increase in soils where little or no rotation is practiced. R. solani is a pathogen of numerous crops and weed hosts throughout the world.

Control: NA

Problem: Ring Rot
Affected Area: tubers

Description: Plant symptoms begin with wilting of leaves and stems after midseason. This disease derives its name from the characteristic internal breakdown in the vascular ring of an infected tuber cross-sectioned at the stem end. The organism overwinters primarily in infected tubers, either those in storage or those that survive the winter in the field. Conditions for dissemination of the pathogen are most favorable in the spring when infected seed tubers are warmed before planting, thus increasing bacterial activity.

Control: Disease-resistant cultivars have been developed, but immunity is as yet unknown.

Problem: White Mold
Affected Area: Affects the stem and tubers near the ground.

Description: White mold is a cool temperature disease occurring in the Andes and in temperate zones nearly everywhere that potatoes are grown. Water-soaked lesions covered by a cottony mycelial mat and sclerotia are most frequent on the main stem at the soil line or on lateral branches in contact with the soil. Tubers near the soil surface may be infected, beginning with small depressed areas sometimes located near the eyes and having a distinct demarcation between affected and healthy tissues. Soilborne sclerotia near the surface germinate, forming either an apothecium or, if enough moisture and organic matter are available, a mycelial mat. Cool temperatures (16-22 ?C and high relative humidity (95-100% favor disease development. S. sclerotiorum has a wide host range, attacking many dicotyledonous crops and weeds.

Control: NA



Green that forms in potatoes exposed to the sun is caused by the formation of solenoids, which are mildly poisonous. Remove all green areas from potatoes when peeling.