How to identify the major leaf-spot diseases

Leaf spots -- discrete, localized discolored areas on leaf tissue -- are caused by dozens of different fungal and bacterial pathogens. They vary in color, shape, size, and distribution, and the differences between them carry management implications. Not all leaf spots require treatment. Many are.

—- title: "How to identify the major leaf-spot diseases" slug: how-to-identify-leaf-spots hub: problems category: "Identification guide" description: "Identify common fungal leaf-spot diseases by spot shape, color, and halo pattern. Covers Cercospora, Septoria, Entomosporium, and black spot on rose — and when treatment is warranted." date: 2026-06-10 updated: 2026-06-10 author: "Thomas A." reading_time: 8 —-

Leaf spots — discrete, localized discolored areas on leaf tissue — are caused by dozens of different fungal and bacterial pathogens. They vary in color, shape, size, and distribution, and the differences between them carry management implications. Not all leaf spots require treatment. Many are cosmetic and do not threaten plant health. Others — black spot on rose, Entomosporium on photinia — can defoliate plants repeatedly and cause significant decline.

Septoria leaf spot

Hosts: Tomato, hydrangea, many vegetables and ornamentals.

Appearance: Per Penn State Extension, Septoria on tomato produces small (0.1–0.25 inch), round spots with a dark brown to black border and a lighter tan-gray center. Tiny black specks (pycnidia — spore-producing structures) are visible in the center of spots under a hand lens. Spots coalesce and leaves yellow and drop.

On hydrangea: Per Penn State Extension, Septoria hydrangeae produces similar spots — circular, with a brown border and lighter center, often with a yellow halo.

Spread: Water splash from soil or infected lower leaves spreads conidia (spores). Per Penn State Extension, infection starts on the lower leaves first, progressing upward.

Management: Remove and dispose of infected leaves. Per Penn State Extension, fungicides containing chlorothalonil or copper are preventive — they prevent new infections but do not cure existing spots. Apply before symptoms appear in problem years.

Cercospora leaf spot

Hosts: Many, including beets, Swiss chard, lilac, coneflower, and crape myrtle.

Appearance: Per NC State Extension, Cercospora spots are round to irregular, tan to pale brown with a reddish-purple to brown border. On crape myrtle, spots are grayish-white with a reddish-purple halo, causing premature defoliation. On beets and chard, spots are circular, tan with a red-purple border and a lighter gray center.

Gray sporulation: Under humid conditions, the center of Cercospora spots produces gray to olive-gray fuzzy sporulation visible under a hand lens — the conidiophores of the fungus.

Management: Per NC State Extension, avoiding overhead irrigation reduces spread. Copper-based fungicides or chlorothalonil provide suppression.

Black spot of rose (Diplocarpon rosae)

Hosts: Rose (Rosa spp.), particularly hybrid teas and many modern roses.

Appearance: Per NC State Extension, black spot produces distinctive circular spots with fringed or feathery margins (not smooth-edged) — this ragged edge is diagnostic. Spots are dark black to black-brown, 0.1–0.5 inch. A yellow halo surrounds each spot. Leaves yellow and drop rapidly.

Severity: Black spot can completely defoliate a susceptible rose in wet summers. Per NC State Extension, repeated defoliation weakens roses over multiple seasons. Preventive fungicide programs are required for highly susceptible cultivars.

Management: Per NC State Extension, planting disease-resistant roses (Knock Out series, many shrub roses) is the most practical long-term solution. For susceptible roses, weekly preventive fungicide applications (myclobutanil, trifloxystrobin, or copper) beginning in spring are required in wet years. Remove fallen leaves — they are the primary overwintering inoculum source.

Entomosporium leaf spot (Entomosporium spp.)

Hosts: Photinia, loquat, Indian hawthorn, and pear (Pyrus spp.).

Appearance: Per Clemson HGIC, spots are small, red to purple-red, circular, 0.06–0.12 inch, often with a white to pale center. Under magnification, the center shows a tiny dark fruiting body (acervulus) with radiating spore-bearing structures that gave the fungus its name (from the insect-like spore shape).

Severity: Per Clemson HGIC, Entomosporium is the primary reason photinia (Photinia × fraseri, "red tip photinia") has declined in favor in southern landscapes. Repeated severe infection causes complete defoliation and eventual plant death after multiple seasons. Indian hawthorn is similarly susceptible.

Management: Per Clemson HGIC, the most effective management is replacing susceptible photinia with disease-resistant species. For existing plants, fungicide applications (myclobutanil) beginning at leaf emergence in spring and continuing every 10–14 days during wet weather reduce severity.

Angular leaf spot (bacterial, Pseudomonas and Xanthomonas)

Hosts: Cucumber, squash, pepper, and other vegetables.

Appearance: Per Penn State Extension, bacterial angular leaf spots are angular rather than round — the spots are bounded by leaf veins, giving them a boxy, irregular polygonal shape. The spots are water-soaked initially, turning yellow-brown, and often appear translucent when held to light.

Distinguishing from fungal: The angular shape bounded by veins is the most reliable separator from the round, regular spots of fungal leaf diseases. Per Penn State Extension, bacterial spots also tend to dry out in the center and fall out, creating a "shot-hole" appearance in older infections.

Leaf spot comparison table

When to treat vs. when to tolerate

Per Penn State Extension, treatment is generally warranted when:

• Defoliation is occurring before normal autumn leaf drop
• The plant is young, newly transplanted, or otherwise stressed
• The same disease recurs severely every year on a high-value plant

Treatment is generally not warranted when:

• Spots are cosmetic only (a few spots per leaf, plant remains vigorous)
• Infection occurs in September or October on deciduous plants
• The plant is not high-value or has good tolerance

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Frequently asked questions

How do I tell fungal leaf spots from insect damage on leaves? Per Penn State Extension, insect damage (chewing insects, thrips, mites) tends to be asymmetric and not circular. Fungal leaf spots are typically circular to oval with defined margins. Insect damage often tears through the leaf; fungal spots have intact (if discolored) tissue. The slime trail test and visual search under magnification confirms insects vs. fungal disease.

Can I prevent black spot on rose with baking soda? Per NC State Extension, baking soda (sodium bicarbonate) has mild antifungal properties but is not effective as a primary management tool for black spot. It can raise leaf surface pH slightly, reducing spore germination, but does not provide reliable season-long protection. It is useful only as a supplement to a proper fungicide program.

My coneflowers have gray spots on the leaves every August. Is this Septoria? Likely Septoria or a related leaf spot — coneflower (Echinacea spp.) is a common host. Per Penn State Extension, late-season leaf spots on coneflower are common and generally cosmetic by late summer. Treatment is rarely needed.

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Sources:

1. Penn State Extension — Septoria leaf spot
2. NC State Extension — Black spot on rose
3. Clemson HGIC — Entomosporium leaf spot
4. NC State Extension — Cercospora
5. Penn State Extension — Angular leaf spot