Bitter melons

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.

Corn earworm is the same species as tomato fruitworm and cotton bollworm — a polyphagous caterpillar that bores into ears of corn through fresh silks, into tomato and pepper fruit, into lettuce heads, and into bean and pea pods. In sweet corn, losses can reach 50%. The species migrates north annually from southern overwintering grounds; in much of the northern US, it does not survive the winter when temperatures drop below 30°F.

Triggers: Overwinters as pupa in top 2-4 inches of soil where winter temps permit. North of I-70 (Illinois IPM): does not reliably overwinter — populations arrive via migration mid-July through September. Females prefer fresh corn silks for egg-laying; older silks rejected.

Risk fades when: In the North (above roughly 40 degrees latitude) the insect does not overwinter, so populations arrive only by summer migration and are wiped out by fall frost, resetting each year; in the South it overwinters as a soil pupa and pressure lasts longer (UMN VegEdge, Wisconsin Hort, Cornell IPM).

Cucumber beetles are the most damaging early-season pests of cucumbers, melons, and squash, both by direct feeding on seedlings and as the vectors of bacterial wilt (Erwinia tracheiphila). They emerge in spring when air temperatures reach the mid-50s, locate host plants by volatiles, and produce an aggregation pheromone that brings more beetles to small plants — exactly when those plants are most vulnerable.

Triggers: Adults overwinter in leaf litter and field margins; emerge when temps reach 54-62°F (UMD). Striped cucumber beetle specializes on cucurbits; spotted (southern corn rootworm) is a generalist. One generation in north; 2-3 in south.

On Bitter Melon: Cucurbit-feeding beetles damage seedlings and can vector bacterial wilt.

Prevention: Use row cover until bloom and inspect young plants often.

Risk fades when: Risk is highest at the seedling stage (cotyledon to 5-leaf), when a single beetle can transmit bacterial wilt; once plants establish past the 5-leaf stage they tolerate feeding and become far less susceptible to wilt, so the critical window closes as plants mature (UMD Extension, UConn IPM, Cornell).

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.

Cucurbits — squash, cucumber, melon, watermelon, pumpkin — almost all require insect pollination to set fruit. The flowers are open for only 4-7 hours in the morning, and each female flower needs multiple bee visits during that window for the ovary to receive enough pollen across all three lobes of the stigma. A watermelon female flower needs about 1,000 pollen grains distributed across its stigma to produce a uniformly shaped fruit. When bees are scarce, when insecticides have been sprayed during morning flowering hours, or when temperatures suppress bee activity, fruit set fails or fruit develops misshapen — bottlenecked at the stem end (cucumber), lopsided or pinched (squash), small or triangular (melon). Honey bees, bumble bees, and native squash bees all contribute. Some cucumber varieties are parthenocarpic and don't need pollination; for everything else, the answer is more pollinator activity.

Triggers: Per U Kentucky/UMD/UNebraska Extension: cucurbit flowers open dawn-noon. Insufficient bee visits during that window cause poor pollination. Common contributors: pesticide use during bloom hours, rainy mornings preventing bee flight, excessive nitrogen producing leaf canopy that hides flowers, very high temps reducing both pollen viability AND bee activity. Watermelon needs ~1,000 pollen grains distributed across the female stigma's 3 lobes for uniform fruit shape.

On Bitter Melon: Cool, rainy, or very hot weather reduces bee activity and flower fertility.

Prevention: Hand-pollinate early flowers if needed and plant pollinator-friendly flowers nearby.

Risk fades when: Already-formed misshapen fruit won't reshape; pick them so the plant directs energy to better-pollinated fruit. For next season: plant pollinator-attracting flowers nearby, avoid spraying during morning bloom hours, consider commercial bumble bee colonies for large plantings.

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).

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