Sweet Marjoram

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.

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.

On Sweet Marjoram: Tender tips can attract aphids in sheltered beds or indoor starts.

Prevention: Pinch back infested tips and wash off colonies early.

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 mint-family powdery mildew that produces white to gray powdery growth on mints, sages, oregano, marjoram, catnip, catmint, anise-hyssop, and shiso. Not the cucurbit or legume model — these are mint-family-specific pathogens. They favor humid canopies, crowding, and poor air movement.

Triggers: High humidity, dense herb canopies, poor airflow, shade, and dry leaf surfaces with humid air all favor infection. Rain isn't required and may even suppress spores temporarily.

Risk fades when: Pressure eases when conditions move outside the warm, humid-canopy zone the pathogen prefers.

Premature dieback covers a range of end-of-season behaviors. Annual herbs (basil, cilantro, dill) decline naturally as days shorten and temperatures cool — this is normal once flowering has happened. Tender perennials in cold climates may die back to the ground when frost arrives. But premature dieback before its time can signal: cold snap below the plant's tolerance, root disease (Fusarium, Verticillium, Pythium), pest pressure (root-feeding larvae), or simply the plant having reached the natural end of its cycle. Annual basil typically dies after first frost; if basil dies in mid-summer, that's a sign to investigate. Tender perennial herbs in zone 5-6 die back annually and regrow from roots — that's normal. Distinguishing 'natural cycle complete' from 'plant has a problem' is about timing and visible symptoms.

Triggers: Most herb dieback is natural end-of-season. Mid-season dieback usually indicates root disease, pest damage, or severe drought stress. Investigate visible signs before assuming normal cycle.

Risk fades when: Natural dieback of annual herbs at season end is expected. Mid-season dieback warrants investigation: dig up the plant, check roots for rot/galls, examine stems for borer damage.

Poor drainage and overwatering suffocate roots and collapse the crown.

On Sweet Marjoram: Poor drainage and overwatering suffocate roots and collapse the crown.

Prevention: Use fast-draining soil, let the surface dry slightly between waterings, and avoid waterlogged pots.

Scales are immobile sap-sucking insects under protective waxy or hardened covers. Soft scales produce sticky honeydew that supports sooty mold; armored scales cause direct dieback. They are primarily pests of woody plants and perennial herbs — fruit trees, berries, citrus, bay, rosemary — rather than annual vegetables. Females are sedentary as adults; the mobile 'crawler' stage that hatches in late spring/early summer is the target for control.

Triggers: Most species: one generation/year in north; multiple in south. Lecanium overwinters as 2nd-instar nymphs on twigs; crawlers emerge June-July. San Jose scale overwinters as 2nd instar on bark; crawlers May-June. Stressed plants more susceptible; ant attendance protects scales by deterring parasitic wasps.

Risk fades when: Penn State, UMN

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: UC IPM

Spider mites are tiny arachnids (1/50 inch) that feed on the undersides of leaves, producing characteristic silver-yellow stippling. Heavy populations produce visible webbing that interferes with pesticide coverage. They thrive in hot dry weather and drought-stressed plants. The two-spotted spider mite feeds on more than 180 cultivated plant species.

Triggers: Hot dry conditions; >90°F lifecycle <2 weeks. Drought stress amplifies. Broad-spectrum sprays (carbaryl, pyrethroids) trigger outbreaks by killing predators. Wisconsin Ext: 'as little as a month without significant rain during the growing season can favor a mite outbreak.'

Risk fades when: UMN Extension

Thrips are tiny (1/16 inch) slender insects with fringed wings that puncture and rasp leaf surfaces, leaving silver stippling with black frass dots. The biggest concern is virus vectoring: western flower thrips is the principal vector of tomato spotted wilt virus (TSWV) and impatiens necrotic spot virus (INSV), which affect more than 600 plant species. Greenhouse and high tunnel infestations can be devastating.

Triggers: Hot dry weather; greenhouse/high tunnel environments. Female lays eggs inside leaf tissue. 2 larval stages feed; 2 non-feeding pupal stages in soil/litter. Lifecycle 10-21 days. Many overlapping generations. Bridge crops (spring wheat, peach, strawberry per NC State) build populations before vegetable hosts available.

Risk fades when: Wisconsin Hort, NC State