Potatoes

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Insect Feeding Damage

Small pits or tiny holes on leaf surfaces


Cause

Flea beetles (various species).

Treatment

Fruit is rarely damaged. Treatment is usually not required. Control by eliminating weeds near field edges, and eliminating overwintering sites by removing crop residue.

Pale leaf veins, curling crinkling, or puckering leaves, which turn brown and are often stunted


Cause

The potato leafhopper (Empoasca fabae), a small (1/8" long) light green insect.

Treatment

The potato leafhopper does not overwinter in Minnesota, but rather is carried north by wind currents each summer. Effective treatments include pesticides like Orthene, Sevin, and dimethoate. For additional protection avoid planting potatoes near alfalfa, which is another preferred food source for this insect.

Leaf skeletonization or chewing damage on leaf edges in late spring or early summer


Cause

The Colorado potato beetle (Lepitinotarsa decemilineata). Both adult beetles and larva feed on leaves. The larva is more destructive than adult beetles.

Adult beetles are about 3/8 of an inch long with tan heads and conspicuous black and yellow-white striped bodies. Larvae are 1/8 to 1/2 inch long with soft, reddish-tan bodies. On either side of the body larva have 2 noticeable rows of black spots.

Yellowish-orange egg masses may also be noticeable on the undersides of leaves in clumps of 20 to 40 eggs.

Adult beetles emerge from underground in June feeding on the upper leaves of young plants and laying eggs. In parts of southern Minnesota a 2nd generation may occur. After eggs hatch larvae emerge and begin feeding. These immature beetles overwinter underground where they pupate before emerging as adults in the spring.

Feeding damage often begins at leaf edges progressing inward resulting in complete defoliation in badly infested fields. Often times leaf blades are consumed while the leaf veins are left intact creating a skeletonized leaf.

Treatment

Control of the Colorado potato beetle is difficult. This insect develops rapid and frequent resistance to pesticides rendering them ineffective. Pyrethrins and Rotenone formulations are sometimes effective, as is Sevin, although in many areas resistance to Sevin is widespread. In addition Bt formulations (varieties San Diego and Tenebrionis) are commercially available for control of this insect.

Row covers may offer some physical protection against infestation. Other control practices involve frequent crop rotations--avoiding consecutive plantings of tomato, potato, and eggplant year after year.

Colorado potato beetle populations of more than 1 beetle for every 2 plants should be considered a significant action threshold and control action should be taken.

Yellow leaves with a shiny or sticky leaf surface


Cause

Aphids and whiteflies (various species). These small soft-bodied insects are less than 1/8 inch long and appear in a variety of colors. Using piercing-sucking mouth parts, aphids and whiteflies remove plant sap and often spread disease in the process. Ants are often drawn to their sticky excrement.

Treatment

Treatment is usually not required as many natural predators such as lady beetles exist. However in widespread infestations (more than 50% of all leaves in field infested), action may be necessary. Low toxicity insecticidal soaps and horticultural oils are highly effective.

Spots, Lesions, Discoloration on Leaves or Tubers

Irregular-shaped black, brown, or purple lesions


Cause

Late blight (Phytophthora infestans), a fungus-like organism often with white mold on the undersides of leaves.

Treatment

No resistant varieties are available. Rotate crops annually, and remove crop debris at the end of each season. Approved fungicides include chlorothalonil and copper formulations.

Concentric ring-shaped spots


Cause

Early blight (Alternaria solani), a fungus.

Treatment

Avoid over-watering, wounding or mechanical damage to growing tubers. Remove crop debris and rotate crops annually. Eliminate alternate hosts for the disease especially weeds like nightshade. Approved fungicides include chlorothalonil and copper formulations.

Rapid wilting


Cause

Verticillium wilt, a fungal disease. In early stages wilting begins on lower leaves. As the disease progresses, the entire plant wilts and dies. Dark streaks can be seen in the vascular tissue of cut stems.

Treatment

Rotate crops annually. Do not plant potatoes in the same field where tomato or eggplant was grown the previous season. Approved fungicides may be available, contact your local Extension agent for current recommendations. Resistant varieties are available, including 'Century Russet,' 'Ranger Russet,' and 'Targhee.'

Black, decaying stems


Cause

Black leg (Erwinia carotover), a bacterial disease. Decay starts at the bottom of the plant and moves upward--often stunting growth. Infection is caused by mechanical damage to wet plants, or by excessive irrigation.

Treatment

Remove infected plants by hand. Avoid working around wet plants. Approved antibiotics are available for extreme cases, however they should be used with caution.

Corky, woody, or bark-like raised or sunken lesions


Cause

Common scab (Streptomyces scabies). Lesions begin as raised areas, then become deep and sunken during wet weather.

Treatment

Rotate crops annually, removing crop debris at the end of each season. Do not follow other root crops. Use disease-free seed. Resistant cultivars include 'Superior' and 'Norland.' If the soil pH is greater than 5.2 acidify soil.

Brown lesions with white mold


Cause

Fusarium dry rot.

Treatment

Use disease-free seed potatos, and seed potatos treated with fungicide. Avoid mechanical damage to tubers.

Soft, rotting tubers


Cause

Black leg (Erwinia carotover), a bacterial disease. Decay starts at the bottom of the plant and moves upward--often stunting growth. Infection is caused by mechanical damage to wet plants, or by excessive irrigation.

Treatment

Remove infected plants by hand. Avoid working around wet plants. Approved antibiotics are available for extreme cases, however they should be used with caution.

Purple or black lesions


Cause

Early blight (Alternaria solani), a fungus.

Treatment

Avoid over-watering, wounding or mechanical damage to growing tubers. Remove crop debris and rotate crops annually. Eliminate alternate hosts for the disease especially weeds like nightshade. Approved fungicides include chlorothalonil and copper formulations.

Brown discoloration within the tuber


Cause

Verticillium wilt, a fungal disease. In early stages wilting begins on lower leaves. As the disease progresses, the entire plant wilts and dies. Dark streaks can be seen in the vascular tissue of cut stems.

Treatment

Rotate crops annually. Do not plant potatoes in the same field where tomato or eggplant was grown the previous season. Approved fungicides may be available, contact your local Extension agent for current recommendations. Resistant varieties are available, including 'Century Russet,' 'Ranger Russet,' and 'Targhee.'