Red Pontiac Potato

A virus spread by the beet leafhopper (Circulifer tenellus), mainly a problem in the western US — California, Arizona, Colorado, Idaho, New Mexico, Utah, and Washington. It has a broad host range, infecting beets, tomatoes, peppers, beans, and cucurbits. Infected plants develop curled, thickened leaves with purple-tinged veins on the undersides, stunted growth, and fruit that ripens prematurely. The leafhopper transmits the virus while migrating: it lands and briefly probes plants, and a single feeding of just a few seconds is enough to infect.

Where it lives: Western US, from the Great Plains to the Pacific

Triggers: Driven by leafhopper migration, not weather directly. The bugs overwinter in foothill weeds and head for gardens in late spring once the wild vegetation dries up. Hot, dry years push more of them into populated areas. Symptoms show up 7-14 days after a single leafhopper visit — and a single bite is all it takes.

Risk fades when: Migration peaks in late spring; once the main wave passes, transmission risk drops sharply. The virus doesn't hide in soil or plant debris between seasons, so risk resets each year.

A soil-borne fungus (Sclerotium rolfsii) that attacks plant stems right at the soil line during hot weather. It hits over 500 different plant species. Two telltale signs to look for: white fan-shaped fungal growth on the lower stem, mulch, and soil surface, and tan-brown spherical sclerotia (they look like mustard seeds) on infected tissue. Most active during sustained heat with humid conditions.

Triggers: Optimal at 86°F (30°C) soil and air temperature with humid conditions. Inactive below 70°F. Most damaging during sustained mid- to late-summer heat waves. It extends further north in warmer-than-normal seasons.

Risk fades when: Sustained cooler weather — highs below 80°F and overnight lows below 70°F for 5+ days — reduces fungal activity. The sclerotia (resting bodies) persist in soil for years, so resolution is seasonal, not curative.

The Colorado potato beetle is the most serious insect pest of potatoes, and it also feeds on eggplant, tomato, tomatillo and pepper. The striped adults and the humpbacked red larvae chew the leaves, often feeding in groups, and can strip plants badly enough to cut the harvest or kill the plant. Adults spend the winter in the soil and become active in spring at about the time potatoes come up. Most areas get two generations a year. The beetle is resistant to over 50 insecticide ingredients, so hand-picking and other non-spray steps matter most.

Where it lives: Most of the US except Alaska, California, Hawaii and Nevada

Triggers: Adults spend the winter 5 to 10 inches deep in the soil of gardens, fields and field edges. They become active in spring at about the time potatoes come up, feed briefly, then lay clusters of yellow-orange eggs under the leaves. Eggs hatch in about a week. Larvae finish growing in about 10 days when temperatures average in the mid-80s F, but take over a month near 60 F. Full-grown larvae drop and pupate in the soil. Egg to adult can take as little as 21 days. Beetles cannot move very far until temperatures reach 70 F. Two generations a year in Minnesota, Pennsylvania and Maryland; two and sometimes a partial third in Kentucky.

Risk fades when: In Maryland adults come out of the soil in late April to early May. In central Pennsylvania overwintered adults come out in early May and lay eggs from early to late June, earlier in warmer parts of the state. Adults that emerge in late July and early August usually start a second generation; those that emerge after mid-August feed for a few days and then dig in for the winter. By midsummer eggs, larvae and adults can all be present at once (UMN Extension; Penn State Extension; University of Maryland Extension).

A seedling killer caused by several different fungi working together. It hits vegetables, flowers, herbs, microgreens, and cover-crop seedlings the same way — seeds rot before they emerge, or young seedlings collapse right at the soil line. Wet seed-starting mix and poor airflow in seedling trays are the classic conditions.

Triggers: Wet soil or starting mix, poor drainage, seedlings packed too tightly, contaminated trays or media, and stagnant air all favor damping-off.

Risk fades when: Drying the soil surface and improving airflow slows new spread. Collapsed seedlings don't recover, but the rest of the tray can be saved.

The most destructive disease of tomatoes and potatoes — it can kill mature plants within days once it gets going. Spreads explosively in cool, wet weather, which is why outbreaks tend to hit suddenly after a stretch of rainy nights.

Triggers: Infection takes only about 10 hours when humidity stays above 90% and the mean temperature is 60-78°F. As a daily proxy: a wet day with the mean temp in that range puts you in the danger zone.

What it looks like: Small, pale to dark green, water-soaked spots on potato leaves, irregular in shape with a yellowish ring, that spread fast and turn brown to purplish black. In humid weather white growth shows at the edge of the spots, mostly on the leaf underside. Stems get brown to black lesions, and tubers get a firm brown rot starting at the skin.

Disease or overwatering? Water-soaked potato leaf spots with a yellow ring that blacken fast, with white growth at their edges underneath, point to late blight. Potato yellowing from wet soil has no spots and no white growth.

Will it recover? Blighted potato foliage does not recover. When leaves are blighted and tubers are at risk, let the vines die completely for 2 to 3 weeks before digging. Next time, plant certified seed tubers, get rid of cull piles and volunteer potatoes, and give foliage room to dry each day.

Risk fades when: Spores survive roughly 5 hours at 80% humidity. Three consecutive dry days with highs above 75°F will likely break immediate spore viability, though damaged tissue stays damaged.

Source for the signs above: UC IPM Pest Management Guidelines: Potato, Late Blight

The Delia root-maggot complex includes the cabbage maggot (D. radicum) on brassicas and root crops, the seedcorn maggot (D. platura) on bean/pea/corn seedlings, and several other species. Larvae tunnel into roots, basal stems, and seeds, killing seedlings outright or creating tunnels that ruin root crops for market. A 2021 Oregon industry survey found 100% of root crop growers reported cabbage maggot damage; 44% with 10-25% yield loss.

Where it lives: Throughout the United States, including Alaska and Hawaii

Triggers: Overwinter as pupae in soil/crop residue. Adults emerge early spring (300-600 GDD base 40°F after Jan 1 in PNW). Cool moist soils favor egg survival; soil >95°F in top 2-3 inches kills eggs. Multiple generations per year. Seedcorn maggot attracted to decaying organic matter — high risk after fresh-incorporated cover crop.

Risk fades when: Cool, wet weather favors root maggots and damage is worst in early spring, dropping to its lowest in midsummer when soil temperatures above about 95 F kill the eggs; a late-summer flight can still injure fall root crops, so risk returns as weather cools (UMaine Extension, Wisconsin Hort, UNH Extension).

Hornworms are the largest caterpillars commonly found in vegetable gardens — up to 4 inches long, green, with a distinctive horn on the rear. Two large caterpillars can defoliate a tomato plant rapidly. Adults are large sphinx/hawk moths that hover like hummingbirds at evening flowers. The two species are virtually identical in damage and management; tobacco hornworm is more common in the south, tomato hornworm in the north.

Where it lives: Across the US: tobacco hornworm mostly in the South, tomato hornworm mostly in the North

Triggers: Overwinter as pupae in soil. Adults emerge mid-May to June. Females lay 1-5 eggs per plant visit on leaf undersides; up to 2,000 total. 2-3 generations in NC, 2-4 elsewhere. Larvae feed 3 weeks through 5-6 instars; bulk of feeding in last instars.

Risk fades when: Feeding ends by early fall when mature larvae drop from the plant to pupate and overwinter in the soil, so pressure resets each season; fall tilling of the bed destroys up to 90 percent of overwintering pupae and lowers next year's numbers (UMN Extension, Wisconsin Hort, UMass Extension).

Aphids are soft-bodied sap-sucking insects that cluster on tender new growth. Most established plants tolerate moderate populations and will outgrow damage on their own, but aphids are the most important plant virus vectors in the garden, transmitting more than 100 plant viruses including potato leafroll, cucumber mosaic, and turnip mosaic. Honeydew excreted while feeding supports sooty mold growth and attracts ants that protect aphids from natural enemies.

Where it lives: Throughout the United States

Triggers: Optimal development at ~75°F (green peach aphid) per UC IPM Floriculture; melon aphid develops fastest above 75°F. Many species heat-intolerant above 90°F and crash in mid-summer. Soft new growth and over-fertilization with high N favor population buildup. Females give live birth parthenogenetically most of growing season — one generation in ~1 week under optimal conditions.

Risk fades when: Per UC IPM and Clemson HGIC, populations crash in mid-summer heat (>90°F) for many species, return in cooler conditions

A common late-season tomato disease that shows up as distinctive bull's-eye spots starting on the lowest leaves and creeping upward. Plants rarely die from it, but yield drops as more leaves get infected. The same conditions, crops, and treatments apply to septoria leaf spot — both look like leaf spots on tomatoes during warm, wet weather and respond to the same management.

Triggers: Develops from 59°F upward and is worst at 82-86°F, its optimum (UMN). The spores need either standing water on the leaf or 90%+ humidity for 5-10 hours straight to germinate.

What it looks like: Starts on the oldest potato leaves as round to angular dark brown spots about 1/8 inch across, often with rings inside that make a target pattern. Badly infected leaves yellow and drop. Infected tubers show a brown, corky dry rot.

Disease or overwatering? Dark brown spots with target rings on the oldest potato leaves point to early blight. Yellowing from wet soil has no spots or rings. Early blight mostly hits stressed or aging plants, so check the lowest leaves first.

Will it recover? Spotted potato leaves yellow and drop and do not recover. Tubers can pick up the fungus as they are dug through the soil. Next time, keep plants growing well with proper feeding and watering, grow later-maturing varieties, and clear potato debris, where the fungus overwinters.

Risk fades when: Three consecutive dry days break the moisture cycle the spores need to keep spreading. Existing leaf damage stays, but new infections stop.

Source for the signs above: UC IPM Pest Management Guidelines: Potato, Early Blight

Flea beetles are small (1/16-inch) shiny beetles that jump like fleas when disturbed. They chew small round 'shothole' or 'pinhole' damage in leaves and can destroy emerging cotyledons of broccoli or eggplant in 24 hours. Most species are host-family specific — crucifer flea beetle on brassicas, tuber flea beetle on potatoes, eggplant flea beetle on solanaceous crops.

Where it lives: Across the US

Triggers: Overwinter as adults in leaf litter and field margins. Active at mid- to late-spring temperatures. Warm winter → higher next-spring populations (NC State). Hot dry conditions amplify damage on stressed seedlings.

Risk fades when: UMN: 1-2 generations in Minnesota, populations crash after mid-June