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Fungal leaf spot

Also known as Leaf spot, Septoria, Alternaria, Cercospora

DiseaseHarmful
Septoria leaf spot on a tomato leaf Photo: MerielGJones · CC BY-SA 4.0

In short

Fungal leaf spots are small dead patches caused by many different fungi that infect leaves when they stay wet. Most are cosmetic at first but can defoliate a plant if left unchecked. Keeping leaves dry and removing infected material are the core controls.

How it looks

Brown, black, tan or reddish spots, often with a darker margin and sometimes a yellowish halo; some show concentric rings like a target. Spots can merge until much of the leaf dies.

Signs

Spotting on leaves, yellowing around spots, leaf drop and thinning foliage in bad cases.

Life cycle

Fungi survive in infected leaves and fallen debris. Spores are spread by wind, splashing rain or irrigation water, contaminated tools and infected plants, and need a period of leaf wetness to infect.

What favours it

Prolonged wet and humid conditions, overhead watering, misting, crowded plants and fallen leaves left under plants.

Prevention

  • Water the mix rather than the leaves, and water early so foliage dries.
  • Space plants to improve air circulation.
  • Remove fallen leaves and debris.
  • Choose resistant cultivars where available.

Treatment

Start with the first step and move down only if the problem persists.

  1. Remove and dispose of spotted leaves.
  2. Stop misting and avoid splashing water on foliage.
  3. After removing infected material, protectant products such as copper soap or Bacillus-based biofungicides, or conventional fungicide groups, can reduce new infections. Follow the product label.

Pros and cons

Cons

  • Heavy infections cause leaf loss and weaken the plant.
  • Spores spread easily by water splash to nearby plants.

Look-alikes

Bacterial leaf spot
Bacterial spots are often angular, water-soaked and limited by veins, with a yellow halo and no fungal dots.
Anthracnose
Anthracnose usually forms larger, irregular blotches or tip dieback.
Black spot of roses
On roses, black spots with feathery edges and yellowing leaves are black spot, a specific fungal leaf spot.
Edema
Edema forms corky, raised bumps on leaf undersides from excess water uptake, not dead spots with margins.

Affected plants

A very wide range of houseplants, vegetables (for example Septoria leaf spot of tomato), shrubs and trees.

Sources

  1. Houseplant Diseases & Disorders — Clemson HGIC
  2. Leaf spot — Wikipedia

Written in our own words from these sources. Check a product label before using any treatment.