Spots on Plant Leaves — How to Diagnose

Spots on plant leaves are usually fungal, sometimes bacterial, and occasionally just sunscald or hard-water mineral residue. The color, texture, and pattern matter more than the size — and they’re usually enough to identify the disease without a lab test. This guide covers the common spot types in PNW gardens.

First check: try to wipe a white spot off

If white spots wipe off with a finger, it’s powdery mildew. If they don’t wipe and look wet/greasy, it’s bacterial. If brown spots have rings, it’s a specific fungal disease. The patterns are recognizable once named.

Then ask: does the spot have a halo? A bullseye? Is it concentrated on one part of the plant?

Likely causes

Powdery mildew (white)

Pattern: White, flour-dusted patches on leaf surfaces. Wipes off with a finger. Common on squash, zucchini, cucumber, beans, zinnias. Nearly guaranteed in PNW humidity by August.

Confirm: White coating that comes off when touched. Often starts on lower or shaded leaves and spreads.

Fix:

• Improve air circulation — prune crowded interior leaves
• Water at soil level only — never overhead
• Remove affected leaves early before it spreads widely
• Milk spray — 1 part milk to 9 parts water, sprayed weekly. Surprisingly effective as a preventive
• Sulfur or potassium bicarbonate sprays for advanced infections
• Accept some loss — powdery mildew on PNW squash is essentially inevitable. It reduces production but doesn’t kill plants.

Septoria leaf spot (small brown circles with yellow halos)

Pattern: Small brown circles with yellow halos and a tan center. Spreads from the lowest leaves upward. Common on tomatoes, almost guaranteed in PNW.

Confirm: Small spots (under 1/4 inch) with distinct yellow halo. Multiple spots per leaf; spreads up the plant over weeks.

Fix:

• Remove affected leaves immediately — don’t compost, bag and trash
• Mulch heavily — prevents soil splash, the main vector
• Water at soil level only
• Improve air circulation — prune lower branches
• Copper sprays — slow spread but won’t cure existing damage

Early blight (bullseye rings)

Pattern: Brown spots with concentric bullseye rings, often surrounded by yellowing. Common on tomatoes and potatoes. Spreads from bottom of plant upward.

Confirm: The bullseye pattern is distinctive — concentric circles within each spot.

Fix: Same as septoria. Remove affected leaves, mulch, soil-level watering, copper for severe cases. Crop rotation matters — don’t plant tomatoes or potatoes in the same bed for 3+ years if you’ve had blight.

Late blight (large irregular dark patches)

Pattern: Large, irregular dark patches on leaves, often dark brown to black with a fuzzy or oily appearance. Spreads quickly — can defoliate a plant in days. Tomatoes and potatoes are vulnerable. The most serious garden disease.

Confirm: Rapid spread (days, not weeks). Damp gray fuzz on leaf undersides. Dark patches turn fruit black quickly.

Fix:

• Remove and destroy entire affected plants — late blight spreads on the wind and can wipe out a neighborhood’s tomato crop
• Don’t compost infected material — bag and trash
• Don’t replant tomatoes or potatoes in the same bed for 3+ years
• Resistant varieties — Mountain Magic, Defiant, Plum Regal for tomatoes

PNW late blight is most common in cool, wet summers. Some years it doesn’t show up; others it’s everywhere.

Bacterial spot

Pattern: Small dark spots that look greasy or water-soaked, often with yellow halos. Doesn’t wipe off. Spreads fast in warm wet weather.

Confirm: Wet/greasy appearance distinguishes from fungal spots. Spots may eventually become angular (limited by leaf veins).

Fix:

• Remove affected leaves
• Don’t water from above
• Copper sprays can slow spread
• Crop rotation — bacterial spot persists in soil
• Sanitize tools between plants

Sunscald (whitish papery patches on fruit)

Pattern: Whitish, papery patches on fruits exposed to direct sun, especially after pruning that removed the leaves shading them. Common on tomatoes, peppers.

Confirm: Damage is on fruit, not leaves. Always on the sun-facing side. Often follows aggressive pruning.

Fix: Don’t prune leaves that shade fruit. Use shade cloth during heat waves above 90°F if fruit is exposed. Damaged fruit is still edible — cut away the affected spot.

Water spots (small white dots)

Pattern: Small white dots from mineral residue when hard water dries on leaves. Cosmetic only.

Confirm: Water spots appear where droplets dried. No tissue damage underneath.

Fix: Cosmetic; harmless. Switch to soil-level watering to prevent.

Plant-specific notes

• Tomato — septoria leaf spot is nearly universal in PNW; early and late blight occur in some years. Watch the lower leaves first.
• Squash, Cucumber — powdery mildew is essentially guaranteed by mid-summer.
• Zinnia (or any flower) — powdery mildew is the dominant disease on most PNW annuals.
• Blueberry — leaf spot diseases occur but rarely serious; check soil pH first if leaves look bad (more likely a chlorosis problem).
• Pepper — bacterial spot can be a significant problem in wet years.

When to worry vs. when not to

Don’t worry: A few spots on lower leaves of a healthy plant; powdery mildew on squash in August (annoying but not killing); minor sunscald on a few fruits.

Do worry: Late blight (dark spreading patches), spots covering more than a third of leaves, spots progressing rapidly, or any spot disease on a young plant.

Related

• Diseases — full disease taxonomy and prevention
• Environmental stress — sunscald lives here
• The diagnosis guide — full diagnostic framework