Crosne tubers

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. Infected plants get curled, thickened leaves with purple veins on the underside, stunted growth, and fruit that ripens way too early. Tomatoes aren't a leafhopper's preferred meal, but the bugs will land and "taste-test" plants while migrating. A single bite takes seconds and can transmit the virus.

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: U Maine

A carrot-family Alternaria disease that causes dark brown to black lesions with yellow halos on carrot foliage and petioles. Severe disease shrivels the tops and stunts root sizing — bad enough infections make harvest a wash. It mainly affects carrots but can spread to closely related Apiaceae like parsnip and parsley.

Triggers: Leaf wetness from rain, dew, or overhead irrigation combined with wind or water dispersal favors infection. Older leaves are the most susceptible.

Risk fades when: Leaf wetness is required for new infections to start and spread. Multiple days of dry foliage lowers immediate 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

Root crops are biennials that flower in their second year after winter cold. When spring-sown plants experience cold exposure (below 50°F for 1-4 weeks) during their juvenile stage, then warm days arrive, they may interpret the cold as 'winter complete' and flower in their first season instead of bulking up roots. Beets, chard, spinach, lettuce, radishes, and turnips have short vernalization requirements (1-4 weeks) — direct-seeded crops are most at risk. Carrots, parsnips, storage onions, and fall-set shallots need much longer vernalization periods, so they bolt less from spring cold. Once the plant commits to flowering, root development slows or stops.

Triggers: Per MSU Extension: beets, chard, spinach, radishes, turnips have 1-4 week vernalization requirements — direct-seeded plantings most at risk in cold springs. Carrots, parsnips, storage onions need 8-12 weeks vernalization, so rarely bolt from spring cold. Plants must have reached juvenile size threshold to be vernalization-susceptible.

Risk fades when: Once bolted, root development stops. Harvest whatever's usable. Fall sowings rarely bolt because plants don't reach vernalization-susceptible size before harvest.

*Cercospora beticola* causes circular gray-tan spots ringed with dark purple-brown margins on the leaves. It primarily affects beets and Swiss chard, with documented cross-infection to Malabar spinach (*Basella alba*) and New Zealand spinach (*Tetragonia tetragonoides*) — even though those are different botanical families, peer-reviewed cross-inoculation studies confirmed the same pathogen. Favored by warm, humid conditions with leaf wetness.

Triggers: High humidity, extended leaf wetness, rain or splash, and infected debris or seed all favor infection and spore production.

Risk fades when: Dry leaves reduce both new infections and the spore production that fuels spread.

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

Forked roots happen when a taproot hits a physical obstruction — a stone, hard clod, buried debris, compacted subsoil — and splits into two or more roots going around it. Common in heavy clay or rocky soils, especially when the bed wasn't loosened to a full 12-inch depth before sowing. Other contributors: excessive nitrogen fertilization pushes hairy lateral roots, transplanting damages the taproot, crowded sowing forces neighboring roots to twist together, and root-knot nematodes mimic the symptom. Prevention is in bed prep — loosen soil to 12 inches, remove stones larger than 1/2 inch, avoid fresh manure or heavy nitrogen the season of planting. Direct-seed rather than transplant. Thin seedlings to proper spacing.

Triggers: Per UMN/UIllinois Extension: forking from physical obstructions in soil (stones, clods, debris), compacted subsoil, excessive nitrogen, or crowded plantings. Root-knot nematode infestation causes a similar symptom but with visible galls.

Risk fades when: Affected roots are still edible but cosmetic. For next planting: deep-loosen bed 12 inches, sieve out stones, work in compost (not fresh manure or high-N), direct-seed not transplant, thin to proper spacing.