Black spot disease, diagnose and treat it naturally

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Fringed, feathery-edged blotches ringed with a yellow halo spread across the leaves, which drop early and leave stems bare. Black spot, caused on roses by a single fungus (Diplocarpon rosae), attacks foliage without threatening survival, though it spoils harvests and makes hedges look sickly; rake up and destroy every fallen leaf, where the spores overwinter, and the problem fades.

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.

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.

• 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

Preventive care

• 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

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.

Which treatment, and when to use it

Black spot is fought on two fronts: stopping the spores before they settle, and removing infected material once the spots have appeared. Here is how each option in this article fits into the year.

Whichever product you choose, never treat while the temperature is below freezing.

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.

Specific types of black spot disease

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

Black maple leaf spot	Strawberry tree leaf spot
Rose leaf Marssonina	Photinia leaf spot

• Rose black spot is recognised by its fringed, feathery-edged blotches surrounded by a yellow halo, and infected leaves drop early, leaving bare stems.
• Thin a photinia hedge to let air move through it, and avoid drenching the soft red new growth, which is where the spots appear first.
• The strawberry tree keeps its leaves year-round, so pick off spotted ones by hand instead of waiting for leaf fall to clear them.