Scab diseases of trees and shrubs are caused by a group of closely related fungi, mainly of the genera Venturia, Fusicladium and Fusicladosporium. They are favoured by wet weather in spring and summer. Infection can cause sooty, dark spots on the leaves, premature leaf loss, aborted blossoms and shrivelled or scabbed fruit.

Apple scab is one of the most important diseases of apple trees and their fruit. Other hosts commonly affected by scab diseases include loquat, olive (on which the disease is also called peacock spot), pear, poplar, pyracantha, Sorbus and willow. On olive, scab causes a characteristic ringspot symptom on the leaves.

N.B. The scab diseases of potato tubers (common scab and powdery scab) are caused by completely different organisms.

You may see the following symptoms:

• Dark, olive-green, purple or black lesions on the leaves, often irregular in shape and sometimes becoming sooty as large numbers of spores are produced
• Leaf yellowing and premature defoliation
• Dieback of heavily infected shoots
• Blackening and abortion of flower buds
• Brown to black ‘scabby’ lesions on fruit, sometimes leading to cracking or shrivelling
• Olive scab initially causes purple to dark brown ringspots (with a green centre) on the leaves, followed by yellowing and defoliation

The RHS believes that avoiding pests, diseases and weeds by good practice in cultivation methods, cultivar selection, garden hygiene and encouraging or introducing natural enemies, should be the first line of control. If chemical controls are used, they should be used only in a minimal and highly targeted manner.

Non-chemical control

• Do not allow fallen, scab-affected leaves to remain at the bottom of the plant over winter – collect them up and dispose of them
• Prune out affected shoots
• General pruning to open up the canopy will lead to better air circulation and more rapid drying of the foliage after rainfall
• Resistant cultivars are available for some plants. Resistant apple, pear and pyracantha cultivars are listed in the separate profiles on apple and pear scab and pyracantha scab

Fungicides

The RHS recommends that you don't use fungicides. Fungicides (including organic types) may reduce biodiversity, impact soil health and have wider adverse environmental effects. If you do intend to use a fungicide, please read the information given in the link and download below to ensure that use, storage and disposal of the product is done in a responsible and legally compliant manner.
  
The products listed in the ‘Fungicides for gardeners’ document below are legally available for use by home gardeners in the UK. This information is provided to avoid misuse of legal products and the use of unauthorised and untested products, which potentially has more serious consequences for the environment and wildlife than when products are used legally. Homemade products are not recommended as they are unregulated and usually untested.

There are no fungicides available to home gardeners for the control of scab diseases on edible crops.

Download

Fungicides for gardeners (Adobe Acrobat pdf document outlining fungicides available to gardeners)

Link

Chemicals: storing and disposing safely

Scab diseases tend to have fairly limited host ranges. For example, the species of scab affecting willow (Venturia saliciperda) is different from that attacking olive (Venturia oleaginea). The fungus Venturia inaequalis has several host-specific strains known as 'formae speciales', which affect the genera Cotoneaster, Malus, Pyracantha and Sorbus. This means that, for example, the apple scab fungus cannot infect pyracantha and vice versa. However, in a few cases cross-infection can occur; for example, pyracantha scab (Venturia inaequalis f.sp. pyracanthae) can affect loquat and medlar, in addition to pyracantha.

Most scab fungi overwinter on fallen leaves or as small cankers and lesions produced on twigs and branches. Spores produced from the fallen leaves are dispersed by air currents, whereas those forming on twigs and subsequently on leaves, flowers and fruit are spread mainly by rain-splash.

Prolonged surface wetness is required for germination and infection by either spore type, and scab diseases are therefore more severe in wet weather.