How do I tell whether the spots on my plant leaves are bacterial or fungal?
A yellow halo around a leaf spot is a useful — though not definitive — indicator of bacterial infection, while angular spots bounded by leaf veins and a concentric ring pattern suggest different fungal diseases, and the circumstances of spread help distinguish them.
Bacterial leaf spots tend to be water-soaked and translucent when young, turning brown or black as they age. A yellow halo — a zone of yellowing surrounding the central dead spot — is common with bacterial infections, caused by toxins the bacteria produce diffusing into surrounding tissue. The spots are often irregular in shape. Angular, straight-edged spots that are clearly bounded by the veins of the leaf (because bacteria spread in the spaces between veins and are blocked by vein tissue) are strongly suggestive of a bacterial cause. Common bacterial leaf spot diseases include bacterial speck and speck on tomato, angular leaf spot on cucumber, and halo blight on beans.
Fungal leaf spots have a different pattern. They tend to be circular or roughly round rather than angular, and many develop a concentric ring appearance as they expand — the outer zones representing successive waves of fungal growth. Early blight on tomato is a classic example: dark brown spots with concentric rings and a yellow halo, but the spots themselves are usually more circular and develop the characteristic bull's-eye pattern as they enlarge. Septoria leaf spot on tomato produces tiny spots with gray centers and dark borders, which is distinct enough to ID from description. Fungal spots often have visible fungal structures (tiny black dots, gray fuzz, or powdery coating) under magnification.
The conditions under which spots spread help confirm the diagnosis. Bacterial diseases spread primarily through water splash — overhead irrigation, rain, and handling wet plants. They often flare after rainy periods and spread rapidly across a planting. Fungal diseases also spread in wet conditions but can also move by wind-dispersed spores, and they may progress more slowly in patterns that follow prevailing wind direction across a bed. Both benefit from similar management: remove affected tissue, avoid working with plants when wet, improve airflow, and apply copper-based sprays (which work against both bacteria and some fungi) at early stages.
Precise identification matters when you're choosing a product. Copper hydroxide or copper octanoate products work reasonably well against bacterial diseases. Fungal diseases respond better to appropriate fungicides — sulfur for powdery and downy mildew, chlorothalonil-based products for blights and septoria (where allowed), and biofungicides like Bacillus subtilis products. A county extension plant diagnostic lab can often ID a disease from a submitted leaf sample for a small fee, which is worth doing if you're uncertain and the problem is recurring year to year.
- AphidSoft, clustered insects on new growth causing curled leaves and sticky honeydew.
- Black RotV-shaped yellow lesions at brassica leaf margins with blackened veins inside — a bacterial disease that moves through the vascular system.
- Brown Marmorated Stink BugSunken, corky dimples on fruit and pods caused by a mottled brown shield bug feeding through the skin.
- Cabbage MaggotBrassica transplants wilting and dying as white maggots tunnel through roots at or below the soil line.
- Imported CabbagewormRagged holes in brassica leaves with pale green caterpillars and green frass nearby.
- What are the black spots on my tomato leaves?Black or dark-brown spots on tomato leaves are usually early blight or Septoria leaf spot — two common fungal diseases with different spot patterns that are managed the same way.
- Why do my bean leaves have rust-colored spots?Rust-colored or orange-brown pustules on bean leaves are bean rust, a fungal disease that spreads in warm, humid conditions and is manageable but rarely fully eliminated once established.
- Why do my squash, cucumber, or melon leaves have a white powdery coating on them?White powdery coating on cucurbit leaves is powdery mildew — a fungal disease that spreads in warm, dry conditions with moderate humidity and typically appears in mid to late season.
- Why are my cucumber leaves puckered, crinkled, or distorted?Puckered or crinkled cucumber leaves usually mean a mosaic virus (spread by aphids) or severe aphid feeding itself — check the undersides of leaves for aphid colonies before concluding it's a virus.