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Scab Disease

Scab Disease, Scab

A close up of a hand holding an apple with apple scab
Holding Apple with scab by Margalob (CC BY-SA 4.0)
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Various fungal species cause scab disease in plants. Each species tends to be specific to its host (the plant infected), however, all-cause similar symptoms in plants. Some of the most common Scab diseases are Apple and Pear Scab. The main symptoms involve dark lesions on the leaves, stems and fruits. The lesions of fruits crust over, forming a black scab, which ultimately turns sooty when the spores are produced. Scab diseases rarely kill plants, and the damage is mostly aesthetic. However, harmful infections can reduce the quality of crops, and overall growth may be stunted. Trees may weaken and be susceptible to infections by other pathogens.

Identification

Dark irregular-shaped lesions which can be olive-green, purple or black. Lesions develop a sooty substance when spores are produced. Leaves turn yellow and fall from plants prematurely. Flower buds blacken and fall early. Fruits develop lesions which scab, dry, then crack open.

Growth factors

Spores are produced during wet weather, in most cases.

Symptoms

Greenish-brown blotches or blisters on leaves.
Premature leaf fall.
Blistered swellings on twigs
Dark spots or cracked and corky blisters on fruit.

Biological treatment

Removing fallen leaves from diseased plants can interrupt the disease cycle. Pruning out infected branches and burning them.

Chemical treatment

In foliage outbreaks, spraying with an approved fungicide once a fortnight from when the buds first break until the blossoms drop. If the wood is also infected, then the spraying will need to continue until harvest. Rigorous spraying can eliminate the apple scab by the third growing season. Be mindful when using fungicides, as they can be toxic. Always take care to read the label of the bottle.

Lifecycle

The disease-causing fungi lie dormant during winter, either on plant debris or as small cankers and lesions on trees. When the spores are produced, they are carried to new hosts in the wind. Scab can also be spread during heavy rainfall. Both require prolonged wet surfaces to successfully infect a new host.

Prevention

General hygiene around plants, e.g. cleaning gardening tools regularly and removing the dead material from the base of the tree. Try to take good care of the tree; Scab disease is often a sign of neglect. Regular pruning to encourage sufficient air circulation in the tree, allowing surfaces to dry quicker.

Affected plants

Apple

Malus spp.

Pear

Pyrus spp.

Poplar Tree

Populus spp.

Willow

Salix spp.

Firethorn

Pyracantha spp.

A close up of the green fruit of an olive tree lit by a low sun.

Olive

Olea spp.

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