Plant Diseases in the Garden

What it looks like

A dusting of white-to-gray powder on the tops of leaves — squash, cucumbers, beans, phlox, and roses are the usual hosts. It starts as small round spots and spreads until whole leaves look flour-dusted, then yellow and dry from the edges in.

How it spreads

Wind-carried spores that, unlike most garden fungi, germinate on dry leaves in humid air — which is why it shows up in late summer’s warm days and cool, dewy nights even without rain. Crowded, shaded plantings hold the humidity it needs.

The move

Space and prune for airflow, water at the soil in the morning so leaves dry fast, and plant mildew-resistant varieties where you can — that is the single biggest lever. At first sign, pull the worst leaves; horticultural oil or a potassium-bicarbonate spray slows it when you catch it early.

What it looks like

Angular yellow patches on the tops of leaves — boxed in by the leaf veins — with a gray-purple fuzz on the undersides in damp weather. Common on cucurbits, basil, grapes, and lettuce. Unlike powdery mildew, it needs the leaf actually wet.

How it spreads

Water-loving spores that thrive in cool, wet, humid spells and move fast on splashing rain and overhead watering. It can blow in on regional weather fronts, arriving in a garden that had shown nothing the week before.

The move

Water at the base in the morning, space for airflow, and grow resistant varieties — basil and cucurbit breeders have made real progress here. Remove affected leaves early; copper-based options help in prolonged wet stretches if you catch it fast.

What it looks like

Dark brown spots with concentric rings — like a small target or bullseye — on the oldest, lowest tomato and potato leaves first, each spot ringed by a yellow halo. Leaves brown and drop from the ground up.

How it spreads

A fungus that overwinters in soil and old plant debris, then splashes up onto the low leaves with rain and irrigation. Warm, humid weather and stressed plants speed it along.

The move

Mulch to stop soil from splashing onto leaves, water at the base, stake and prune the lowest leaves for airflow, and rotate where you plant tomatoes and potatoes each year. Remove affected leaves as they appear — it is manageable and rarely fatal.

What it looks like

Fast-moving dark, greasy-looking blotches on tomato and potato leaves and stems, often with a faint white fuzz on the leaf underside in wet weather, and firm brown patches on the fruit. This is the disease behind the Irish potato famine — it can take a plant down in days.

How it spreads

Spores ride wind and splashing water for long distances and explode in cool, wet spells. It overwinters in living potato tissue, so cull piles and volunteer potatoes are the usual reservoirs.

The move

This is the one to act on fast. Start with certified disease-free seed potatoes and resistant tomato varieties, give plants full sun and airflow, and stake for quick drying. If it appears, pull and bag affected plants the same day — do not compost them — to protect your neighbors’ gardens too.

What it looks like

Many small round spots with dark borders and pale gray-tan centers, often peppered with tiny black dots (the fungus’s fruiting bodies), on the lower tomato leaves. It works upward and yellows leaves fast, but it does not spot the fruit — the tell that separates it from early blight.

How it spreads

Overwinters in debris and on weeds in the nightshade family, then splashes up in warm, wet weather — the classic humid-summer tomato defoliator.

The move

Same playbook as early blight: mulch, base-watering, airflow, rotation, and prompt removal of spotted leaves. Clean up every scrap of tomato debris at season’s end — the spores overwinter in it.

What it looks like

Round black spots with feathery, fringed edges on rose leaves, the leaf around them yellowing until it drops. A badly hit rose can defoliate by midsummer.

How it spreads

A fungus that needs leaves wet for several hours to infect, so it flares in humid, rainy, or heavily dewed weather and spreads by splashing water.

The move

Grow disease-resistant roses — many modern shrub and landscape roses shrug it off — then give them sun and space, water at the base in the morning, and rake up fallen leaves, where the spores overwinter. Prune for an open center so air moves through.

What it looks like

Raised orange, rusty-brown, or yellow pustules on leaf undersides — on beans, hollyhocks, snapdragons, roses, and many others — that rub off as colored powder, with yellow flecks showing on the leaf tops above them.

How it spreads

Wind-carried spores that germinate in humid weather and prolonged leaf wetness. Many rusts are host-specific, so the bean rust will not jump to your roses.

The move

Space for airflow, water at the base, and remove infected leaves promptly. Grow resistant varieties where they are offered and clean up debris in fall; sulfur applied early helps on ornamentals.

What it looks like

Dark, sunken spots on fruit (beans, tomatoes, cucurbits, peppers) and irregular dark blotches along leaf veins; on ripe fruit the spots ooze pink spore masses in wet weather. On shade trees it shows as brown leaf blotches and early leaf drop.

How it spreads

Overwinters in seed and debris and spreads by splashing water in warm, wet conditions. Working among wet plants carries it from plant to plant.

The move

Use clean, certified seed, rotate crops, mulch, and stay out of the beds while foliage is wet. Remove infected fruit and debris. Airflow and base-watering are the through-line here, as with the other wet-weather fungi.

What it looks like

A fuzzy gray-brown mold on flowers, fruit, and soft stems — strawberries, tomatoes, lettuce, and many ornamentals — usually starting on spent blooms or damaged tissue and spreading into healthy parts, which turn soft and rot.

How it spreads

Spores are everywhere; the fungus colonizes dead and damaged tissue first, then moves into healthy tissue. It thrives in cool, damp, still, crowded conditions — greenhouses and dense plantings especially.

The move

Airflow is the fix: space plants, prune for circulation, and remove spent flowers, ripe fruit, and dead leaves promptly. Water at the base, harvest ripe fruit quickly, and avoid bruising. Under cover, ventilate to drop the humidity.

What it looks like

A plant that wilts in the heat of the day and recovers at night, then stops recovering — often one side or one branch first, with the lower leaves yellowing. Slice a stem near the base and you will see brown streaking in the tissue just under the skin. Common on tomatoes, eggplant, potatoes, and many others.

How it spreads

Soil-borne fungi that enter through the roots and clog the vessels that conduct water. They persist in soil for years and travel on tools, water, and infected transplants.

The move

There is no cure once a plant is infected, so the win is prevention. Plant resistant varieties, rotate away from the nightshade family for several years, keep plants vigorous, and pull and destroy infected plants. Raised beds with clean soil sidestep an infested plot entirely.

What it looks like

Plants that yellow, stunt, and wilt for no obvious above-ground reason; pull one and the roots are brown, soft, and mushy instead of firm and white, sometimes with the outer layer sliding off. Seedlings and plants in soggy ground go first.

How it spreads

Water-mold pathogens that thrive in saturated, poorly drained soil and spread in standing water and runoff. Overwatering and a buried hardpan are the usual setup.

The move

This is a drainage problem first. Improve drainage, ease off watering, plant in raised beds or mounds where the soil stays wet, and do not plant into cold, waterlogged ground. Choose rot-tolerant rootstocks for trees and shrubs.

What it looks like

Seedlings that sprout fine, then topple — a thin, water-soaked, pinched spot at the soil line collapses the stem overnight. Or seeds that never emerge at all. A whole tray can go in a day or two.

How it spreads

A group of soil fungi and water molds that flourish in cold, wet, overcrowded seed-starting conditions with poor airflow.

The move

Start seeds in clean containers with fresh sterile mix, do not overwater, give good light and gentle air movement, and do not sow too thickly. Bottom-water and let the surface dry between waterings. Warmth speeds germination past the vulnerable stage.

What it looks like

Small dark spots — sometimes ringed by a yellow halo — scattered across tomato and pepper leaves and fruit; heavy infections give leaves a scorched, tattered look and blemish the fruit. Speck runs cool-and-wet, spot runs warm-and-wet, but the garden moves are the same.

How it spreads

Bacteria carried on seed and transplants and splashed plant to plant by rain and overhead watering; they enter through pores and wounds. Warm, wet, humid weather drives them.

The move

Start with clean, certified seed and healthy transplants, water at the base, stay out of wet foliage, rotate, and remove debris. Copper sprays give partial protection in wet spells. There is no cure mid-season — prevention and sanitation carry it.

What it looks like

On cucumbers and melons: a vine that wilts and collapses seemingly overnight, often starting with one runner, with no spots or rot to explain it. The test: cut a wilted stem, press the two cut ends together and slowly pull them apart — a fine sticky thread strings between them.

How it spreads

Carried by cucumber beetles — the bacteria overwinter inside the beetles and are introduced when they feed. This is a pest-driven disease, which is the key to controlling it.

The move

Control the cucumber beetles and you control the wilt: row covers over young plants (lift them at flowering for pollination), prompt beetle management, and resistant varieties where available. Pull and destroy wilted plants so they do not feed more beetles.

What it looks like

Leaves mottled in light-and-dark green or yellow, often puckered, curled, or narrowed to a fern-like shape; plants stunt and fruit comes in spotted or misshapen. Tomatoes, cucurbits, peppers, and beans are common hosts. It does not wash off or spread as discrete spots — the whole plant is affected.

How it spreads

Mostly by sap — on hands, tools, and especially aphids and other sucking insects that carry it plant to plant. Some viruses ride on seed. Once a plant has it, it has it for life.

The move

There is no cure, so this is about limiting spread: pull and destroy infected plants, control aphids, wash hands and tools between plants, do not handle plants when wet, and grow resistant varieties. Keep the aphids down and you keep the virus down.

What it looks like

A sunken, leathery, dark brown-to-black patch on the bottom (the blossom end) of tomatoes, peppers, squash, and melons — right where the flower was. It looks like a disease and gets treated like one, but no pathogen is involved.

How it spreads

It does not spread — it is physiological. The fruit could not get enough calcium at a critical moment, almost always because water was uneven (a dry spell, then a soak) rather than because the soil is short on calcium.

The move

Steady, even moisture is the whole fix: mulch to hold soil moisture, water deeply and consistently rather than in feast-or-famine cycles, and go easy on high-nitrogen fertilizer. Most soils already carry plenty of calcium — a soil test confirms it. Early fruit often shows it; later fruit usually comes in clean once watering evens out.