Black spot disease, diagnose and treat it naturally

Black spot on leaves

Black spot disease is a plant sickness where black or brown spots appear on leaves.

It impacts many trees, but usually doesn’t cause critical danger.

Black spot disease is a fungus that mostly attacks fruit trees like the cherry tree, walnut, grapevine, raspberry, blackcurrant, strawberry or red currant, certain shrubs like hydrangea and also vegetables such as cucumber, bean, pea, or tomato.

Black spot, also called tar spot, is a fungus that appears on leaves like this infected maple leaf.It attacks foliage, but doesn’t directly imperil the tree or plant’s survival. It does, however, degrade the harvest in orchards and veggie patches.

From an ornamental point of view, it also makes shrubs and hedges look sickly.

Treating black spot disease

Preventive care

  • How to treat against black spot on leavesSpray Bordeaux mixture (or other copper-based treatments) at the end of winter or at the very beginning of spring.
  • Another option is a natural fungicide prepared from fermented horsetail tea or fermented nettle tea.
  • Always avoid treating whenever the temperature is below freezing because the product will lose all or part of its effectiveness.
  • If neighboring trees in your garden or in the vicinity show signs of black spot on their leaves, rake them up and destroy them either by burning them or composting them.

→ Comes in handy: a pump sprayer

Curative care

  • From the moment the disease has appeared, eliminate or pick out diseased leaves as you notice them, and burn them to avoid contagion.
  • Once the tree has lost its leaves in fall, treat it with a solution prepared from Bordeaux mixture.

Life cycle of black spot disease fungus

Each variety of trees is usually attacked by only a few select varieties of fungus and vice-versa: each variety of fungus is associated to only one or few tree species, usually.

Biology and lifecycle of black spotHowever, the life cycle of black spot fungus is always very similar:

  • spores that have overwintered are released in the wind, either from leaves on the ground or from crevices in the tree’s bark and branches.
  • Carried by wind or water, they stick to leaves and start growing.
  • In fall, they’re protected from the cold either because the tree bark they’re nestled in keeps them warm, or because the fallen leaves form an insulating barrier of light mulch. This is when new spores develop.

As a general recommendation, whenever you notice leaves infected with black spot, rake them up and destroy them. Composting works well because bacteria and heat break the spores down before they get a chance to spread, but burning is even more effective.

Specific types of black spot disease

Here are a few examples of black spot disease in more detail:

Tar spots spreading on a green maple tree leaf.
Black maple leaf spot

Strawberry tree (Arbutus unedo) leaf infected by black spot disease (leaf spot fungus)
Strawberry tree leaf spot

A cluster of rose bush leaves with conjoined black spot marks.
Rose leaf Marssonina

Black leaf spot on a young photinia leaf
Photinia leaf spot

Smart tip about black spot disease

Over winter, prune infected trees so that they may regain vigor come spring. Burn old branches and spray Bordeaux mixture.

Diligently disinfect your pruning tools before and after, so that you don’t spread the disease to other plants.

Images: 123RF: matunka, Sergei Finko, CC BY-SA 3.0: John Vangelis; of the © Canadian Natural Resources, Canadian Forestry Service, Laurentian Forestry Center: Al Funk; Pixabay: Michaela, Petra Karrasch; Public Domain: Scot Nelson