Red Burgundy Onions

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.

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.

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 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.

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).

An allium-specific rust on garlic, onion, leek, shallot, and chives. Produces orange to reddish pustules on older leaves first. Favored by cool, humid weather.

Triggers: Optimal infection is near 59°F with saturated humidity for several hours. The disease stays active in cool 57-75°F weather with dew or high humidity.

Risk fades when: Rust thrives in cool saturated air. Dry foliage or heat above the favorable range lowers new infection pressure.

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.

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

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.

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

Microscopic soil-dwelling roundworms that burrow into plant roots and cause swollen knots (galls). Above ground, the plant looks stunted, yellowed, and wilted even with plenty of water. They attack over 2,000 plant species, so almost nothing is safe. They're most active in warm soil (70-85°F) and do more damage in sandy soils, where they move easily. Once a bed has them, populations stick around for years.

Triggers: Soil temperatures of 70-85°F are ideal for them; below 60°F they go dormant. Sandy soils make it easy for them to move and reproduce, while heavy clay slows them down considerably. In warm soil, a full generation completes in about 27 days.

Risk fades when: Activity drops sharply once soil cools below 60°F. Damage stops accumulating for the season, but the population stays in the soil and returns when warmth does.

Slugs and snails are nocturnal mollusks that chew irregular holes in leaves and clip off succulent seedlings. They leave characteristic silvery slime trails. Hermaphroditic and prolific, brown garden snails lay around 80 eggs per month for up to six clutches per year.

Triggers: Active at night and early morning in damp conditions. Coastal CA and southeast — active year-round. Spring rains and dense ground cover (mulch, debris, weeds) create harborage.

Risk fades when: Hot, dry summer weather drives slugs and snails into inactivity, so risk drops sharply once conditions turn warm and dry; populations are highest in cool, wet spring and fall, and in mild-winter coastal regions they can stay active year-round (UMN Extension, Penn State, UC IPM).

Golf-ball onions are almost always a day-length problem, not a fertility problem. Onions stop making leaves and start making a bulb when daylight hits a variety-specific threshold: short-day types begin bulbing at 10 to 12 hours, intermediate-day at 12 to 14, long-day at 14 or more. And here is the part that decides everything - Iowa State Extension puts it plainly: bulb size is largely determined by the number and size of the leaves at bulb initiation. Every leaf becomes a ring. Once bulbing starts, no new leaves form, and the onion can only fatten the rings it already has. So a short-day onion planted in the north hits its 10-hour trigger in early spring while the plant is still small, bulbs immediately, and Nebraska Extension calls the result exactly what it is - disappointingly small bulbs. Run it the other way and a long-day onion in the far south may never reach 14 hours and never bulb at all. Crowding does the same thing by another route: plants that never build big tops cannot build big bulbs. Match the variety to your latitude, plant as early as the soil can be worked, and give the tops every week of growth you can before the days get long.

Triggers: Iowa State Extension: short-day cultivars begin forming bulbs at 10 to 12 hours of daylight, intermediate-day at 12 to 14 hours, and long-day cultivars at 14 or more; bulb size is largely determined by the number and size of the leaves at bulb initiation. Nebraska Extension: short-day onions grown in northern regions start bulb development too soon, producing disappointingly small bulbs. UMN Extension: sweet or mild onions are short-day types and will generally develop small bulbs in Minnesota; overcast skies and cool temperatures during the season delay bulb formation. Crowding limits top growth and therefore bulb size. Note that bunching onions (Allium fistulosum) do not form true bulbs at all, and this disorder does not apply to them.

On Red Burgundy: Wrong day-length adaptation or crowding reduces bulb size.

Prevention: Match day-length type to region and space correctly.

Risk fades when: Nothing can be done once bulbing has started - no new leaves form, so no new rings. The bulb will only fatten the rings it already has. Fix it at the next sowing: choose long-day varieties north of roughly the 36th parallel and short-day varieties south of it, intermediate-day in between. Plant as early as the soil can be worked, use transplants rather than direct seed in short-season areas to buy leaf-growth time, and thin to 3 to 4 inches. Small bulbs are perfectly good eating - they just will not size up this year.