Upland Cress

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).

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

Birds cause significant damage to ripening fruit (berries, grapes, figs, cherries), to newly-planted seeds, and to seedlings. Damage peaks during fruit ripening on berries and during seed-head ripening on sunflowers. Robins feed low on bushes; starlings feed in upper canopy; species and feeding patterns vary widely. Even partial damage opens entry points for diseases and insects.

Triggers: Damage seasonal — peak during fruit ripening (June-August in north). Dry years intensify damage as birds use fruit for water. Proximity to forest edges or harvested grain fields amplifies pressure.

Risk fades when: Risk is tied to each crop's ripening window - berries in June through August, corn at silking, sunflower at seed-head fill - so pressure fades once the vulnerable fruit or grain is harvested; netting or row cover during that specific window is the main protection (Oregon State Extension, EM 9286).

Bolting is a plant deciding, earlier than the gardener wants, that it is time to reproduce. The stalk shoots up, leaves turn coarse and bitter, and no amount of cutting the stalk off reverses it. Two separate cues drive it, and confusing them leads to the wrong fix. The first is vernalization. MSU Extension explains that most winter-hardy biennial and perennial crops need a certain number of hours at cool temperatures, after reaching a certain size, before they will flower - and that the period before a crop becomes responsive to those cool temperatures is its juvenile period. Some biennials have no juvenile period at all and can even vernalize as seed. So for spring-sown biennials such as Florence fennel, a string of cool days or nights can precondition the plant to flower sooner than desired. The second cue is the opposite: heat and lengthening days. Cool-season greens sown in spring race into flower as summer arrives, which is what happens to mache and upland cress. Both routes end the same way. The plant switches from making leaves to making seed, and the harvest is over. Note that heat and day length lengthen together, so the two are easily conflated.

Triggers: MSU Extension: in addition to daylength requirements, most winter-hardy biennial and perennial crops need a certain number of hours at cool temperatures, after they have grown to a certain size, before they will flower - a process called vernalization. The time, or leaf stage, before a crop becomes responsive to vernalizing temperatures is its juvenile period; not all vegetable biennials have a juvenile period, and some can vernalize as seed. For some spring-planted biennial crops, a string of cool days or nights can precondition them to flower sooner than desired, and this unwanted premature flowering is called bolting. Brussels sprouts, cabbage, collards and kohlrabi need longer to vernalize and are less threatened by early spring cool temperatures. University of Delaware Extension: bolting in cabbage, collards and kale, and buttoning in cauliflower, can occur if early-planted crops are subjected to low temperatures between 35 and 50°F for ten or more continuous days. NOTE ON EVIDENCE: the cool-season salad crops here - mache and upland cress - bolt in response to heat and lengthening days rather than to vernalization, and no species-specific extension source was retrieved for either. Angelica archangelica is monocarpic: it flowers once and dies, so its bolting ends the plant rather than merely the harvest. Florence fennel is susceptible from both directions - a cold check early or heat and root disturbance later. Basil and brassicas have their own dedicated entries.

On Upland Cress: Warm temperatures and lengthening days push flowering.

Prevention: Grow in spring or fall, harvest often, and provide afternoon shade as temperatures rise.

Risk fades when: Bolting cannot be reversed. Cutting off the flower stalk does not restore leaf quality, because the plant has already switched. Harvest whatever is still usable, promptly, and move on. Everything useful is preventative. Sow in succession every two to three weeks so a young planting is always coming on. Time crops to mature in cool weather - spring plantings finished before midsummer, or autumn plantings as days shorten. Avoid checking growth: transplant carefully without disturbing roots, keep moisture even, and do not let plants sit dry. For spring-sown biennials such as Florence fennel, avoid transplanting too early into cold ground, since a run of cool nights after the juvenile period can vernalize the plant and trigger bolting weeks later. Choose varieties described as bolt-resistant or long-standing. And let a bolted plant flower if you can spare it - the umbels of fennel, lovage and angelica are excellent for beneficial insects.

A cool-weather foliar disease most damaging on seedlings and young plants. It shows as faint yellow spots on the upper leaf surface with a white to gray fuzzy growth on the underside. On heads and curds it can cause dark speckling that downgrades quality. It is driven by cool temperatures, leaf wetness, and high humidity.

Triggers: Favored by cool temperatures and prolonged leaf wetness or high humidity. Most serious on seedlings, where it can kill young plants; on older plants it spots leaves and can speckle heads.

On Upland Cress: Cool humid leaf wetness and poor airflow.

Prevention: Thin properly, water early, and rotate brassicas

Risk fades when: Warm, dry weather suppresses it. Three dry days with highs above 75°F exits the active window; spotted leaves stay damaged.

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.

On Upland Cress: Adult beetles feed on tender brassica leaves.

Prevention: Use row cover and maintain steady growth.

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

Japanese beetle adults feed gregariously on the foliage and flowers of more than 300 host species, skeletonizing leaves by chewing tissue between the veins. Larvae (white C-shaped grubs) damage turf and root systems. Adults are active during the warmest part of sunny summer days, when populations can rapidly defoliate roses, grapes, and ornamentals. Do not use Japanese beetle traps near plants — the lures attract more beetles than they catch.

Triggers: One generation per year. Overwinters as larva in soil. Adults emerge late June-early July (peak July). Adults active during warmest part of sunny days. Females lay 40-60 eggs in turf 3 inches deep.

Risk fades when: One generation per year: adult emergence peaks around early August and tapers to the last beetle by about September 1, so above-ground feeding pressure ends in early fall as the adults die off and grubs move underground (UMass Extension, UMN Extension).