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.
From an ornamental point of view, it also makes shrubs and hedges look sickly.
- Rose shrubs in particular are often infected.
- Hedge shrubs that may catch leaf spot include photinia, hawthorn, cotoneaster, firethorn and amelanchier, among others.
- Tall trees can show signs of black spot as well, such as maple, oak and the uncanny strawberry tree.
Treating black spot disease
- Spray 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
- 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.
- 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:
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.