Plants affected
This is the most significant and widely known disease of apples and pears. Apple scab (Venturia inaequalis) also attacks ornamental plants in the rose family such as Cotoneaster, Sorbus (rowans) and Crataegus (hawthorns). Infections can develop both on fruit and leaves and some ornamental Malus (crab apples) can be particularly seriously affected. The pear scab pathogen (Venturia pirina) has an almost identical biology, but attacks only pears.
Symptoms
Initial infections spread to form velvety, dark green patches on the leaves, which then liberate a different type of spore with slightly different infection requirements.
The fungus dies out in older lesions, leaving brown, dead patches of tissue. On fruit, infection causes scarred, scabby lesions on the skin that do not expand at the same rate as uninfected tissue during ripening; this causes misshapen, often cracked fruit.
Such cracks render fruit susceptible to rots, such as brown rot or Penicillium infections. Infections occurring close to harvest may only become apparent as dark, sunken spots on fruit in storage. Infected leaves fall prematurely and carry the fungus through to the next season. Pear scab can also infect young, green shoots; apple scab less so.
Cause
The disease is spread by rain-splash and wind throughout the tree and on to neighbouring trees.
The fungus overwinters in the fallen leaves. In the spring spores are released into the atmosphere and carried by wind into the trees. The disease spreads most rapidly when there are frequent showers in spring and early summer.
Spore discharge and infection occur under precise conditions of temperature and humidity, known as ‘Mills periods’. Commercial growers are warned when such conditions are prevalent, to assist with precise spraying.
Although unsightly, this disease affects only the skin of the fruit, and they are still edible. The damage to foliage is more serious, causing leaf fall and reduced tree vigour and fruit crops in future years.
Non-chemical control
Clearing up and destroying or composting fallen leaves helps reduce scab, but since the spores are airborne this never eliminates infection. Prune out infected shoots during the winter. Rake up and destroy fallen leaves to reduce the carry-over of the disease from one season to the next.
Choose apple cultivars resistant or partially resistant to scab, such as: 'Ashmead's Kernel', 'Belle de Boskoop', 'Cheddar Cross', 'Discovery', 'Edward VII', 'Ellison's Orange', 'Emneth Early', 'Fortune', 'Gavin', 'Golden Noble', 'Grenadier', 'Howgate Wonder', 'King of the Pippins', 'Lane's Prince Albert', 'Lord Lambourne', 'Melba', 'Miller's Seedling', 'Monarch', 'Sunset', 'Winston'. Sadly ‘Cox’s Orange Pippin’ is especially susceptible.
When planting avoid damp, stagnant sites which favour the development of the disease.
Chemical control
Spray at regular intervals during the growing season with mancozeb (Bio Dithane 945), or myclobutanil (Bio Systhane Fungus Fighter). The number of applications depends on the severity of attack - in some seasons it is necessary to continue spraying until late July.