Black spot disease, diagnosis and treatment

Black spot, also called tar spot, is a fungus that appears on leaves like this infected maple leaf.

Black spot disease is diagnosed when black or brown spots appear on leaves.

It impacts many trees, but doesn’t usually present any critical danger to them.

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

It attacks foliage, but doesn’t directly imperil the tree or plant’s survival. It does, however, degrade the harvest.

From an ornamental point of view, it also makes shrubs and hedges look somewhat sickly. Hedge shrubs that may fall victim to black spot or leaf spot include photinia, hawthorn, cotoneaster, firethorn and amelanchier, among others. Rose shrubs in particular are often infected.

Tall trees may show signs of black spot as well, such as maple and the uncanny strawberry tree.

Treating black spot disease

Preventive care

  • Spray Bordeaux mixture type treatment 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.

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.

However, 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.

Varieties and 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 the branches and spray Bordeaux mixture.

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