What to do now
Mildest step first.
- Note which leaves are affected, and turn several over; leafhopper nymphs are pale green and scuttle sideways.
- Water evenly through dry spells; stressed plants take leafhopper damage harder.
- Pick off leaves with target-ring spots, and water at the base instead of overhead.
- Dig up plants with stiff, rolled, leathery leaves, tubers and all, and plant certified seed potatoes next year.
Not sure this is it? Check other potato problems
Which leaves, and how?
Where and how the leaves change is the quickest clue. Tips and edges that turn yellow, then brown, and curl, starting on older leaves in early summer, point to potato leafhoppers. Leaves that roll upward and feel stiff or leathery, starting at the bottom of the plant, point to potato leafroll virus. Round brown spots with rings inside them are early blight. One or a few stems that yellow and wilt while the rest of the plant looks fine suggest verticillium wilt. A whole planting yellowing late in the season, after flowering, is usually the vines finishing up; the University of Illinois Extension says mature potatoes are dug after the vines die.
Potato leafhoppers
Potato leafhoppers are about 1/8 inch long, wedge-shaped and bright green, and the wingless nymphs run sideways on leaf undersides. Virginia Tech says they don't survive the winter there and arrive each year on warm winds from the Gulf states. The University of Wisconsin vegetable entomology program says the first signs of feeding are paling veins and curling leaves, followed by triangular yellowing or browning of the leaf tip called hopperburn, usually on older leaves first, and that yield loss often starts before the damage is obvious. Virginia Tech adds that damage is often worse in hot, dry weather and can be mistaken for nutrient deficiency or herbicide injury. Its home-garden advice is to pick and destroy infested leaves, which can hold eggs, clear weedy hosts nearby, and let lacewings, lady beetles, minute pirate bugs and spiders work. Wisconsin notes that healthy, vigorously growing plants withstand the damage better than stressed ones.
Leafroll and other viruses
Potato leafroll virus is carried in seed tubers and spread by aphids. Washington State University's HortSense says plants grown from infected seed pieces show a distinctive upward rolling of the leaf margins, with leaves that feel brittle or leathery, starting at the base and moving up, on plants that are slightly stunted and yellowish. The University of Minnesota Extension adds that the rolled leaves turn yellow on white potatoes and reddish on red ones. Tubers can look normal outside but show a brown net of fine lines inside. There is no cure; WSU advises removing and destroying infected plants, tubers included, and controlling aphids. For a mottled mosaic of light and dark green, sometimes with crinkled leaves, Minnesota points to another virus, potato virus Y.
Spots and wilting stems
Early blight makes round brown spots with target-like rings. Minnesota says that when many spots grow together, the leaves turn yellow, and that the disease is more common in warm, humid weather. Pick off spotted leaves, and water at the soil rather than overhead; the University of Maryland Extension says overhead watering can spread fungal diseases. Late blight makes large olive to brown blotches that aren't limited by leaf veins, and in cool, wet weather it can brown and shrivel leaves and stems quickly. If the leaves on one or a few stems turn yellow, then brown and wilt while the stems stay upright, suspect verticillium wilt; Minnesota says a stem cut near the soil line may show brown streaks right under the skin.
Next season
Most potato viruses start with the seed. Illinois and Maryland both recommend certified, disease-free seed potatoes, and Maryland warns that viruses and diseases often show up the year after you save your own tubers. Illinois cautions that grocery store potatoes haven't been inspected and certified, so the risk of bringing in disease is much higher. Keep the plants evenly watered, and check leaf undersides for leafhoppers from late spring on; Wisconsin notes that numbers can seem to jump overnight when nearby alfalfa fields are cut and the insects move out.
Read the whole label first: it must list your crop, and its directions (amount, timing, gear, bees, days before harvest) are the law. If this page and the label differ, follow the label.
Growing it again? When to plant potato for your ZIP code
- Early BlightDark spots with concentric rings on lower leaves that yellow and drop, working up the plant.
- Late BlightLarge water-soaked leaf lesions with white fuzzy growth beneath; plants can collapse within days.
- Verticillium WiltLower leaves yellow with V-shaped browning; wilting in midday heat with partial evening recovery.
- AphidSoft, clustered insects on new growth causing curled leaves and sticky honeydew.
- My potato plants flowered — why are there no potatoes?Flowering doesn't mean potatoes are ready. At bloom, tubers are often only new-potato size, and they keep growing until the vines die. If you find few, look at soil heat near 80°F, too much nitrogen, dry spells and hilling.
- How early can I plant potatoes?Plant seed potatoes about 2 to 4 weeks before your last frost date, once the soil is at least 45°F and dry enough to work. Potato leaves are hurt by frost, but if early shoots are frozen, the plant usually sends up new ones within two weeks.