Apple scab and pear scab
Apple scab is a fungal disease that causes brown patches on apple tree leaves and on the skin of fruits. Pear scab is very similar.
Other rose-family trees such as crab apples can also be affected.
The fungus species that causes the problem is called Venturia. Brown marks appear from spring onwards and badly affected leaves may fall early. Young branches may also be affected by cracking.
Most damage to fruit is superficial; bad scarring may lead to apples and pears being prone to rot. The problem is most serious for commercial growers as customers prefer unblemished produce.
Where does it come from?
Venturia spores are airborne and spread in water; there is no way to completely protect a tree. Spores also overwinter on fallen leaves and other material, being released in spring, so clearing up debris, affected fruit and twigs may lessen infection. Nevertheless, spores can travel a long way on the wind, so unless your trees are completely isolated they may encounter Venturia. Fortunately it isn't usually a serious problem.
How to tackle scab
Well nurtured young trees are best able to stand up to pathogens. Make sure your tree is planted according to best practice, properly staked and kept well watered during establishment.
Some apple varieties are more resistant to scab than others, including: Beauty of Bath, Blenheim Orange, Charles Ross, Discovery, Egremont Russet, Scrumptious, Tom Putt and Winston. Beurre Hardy pear is also resistant.
There are no suitable fungicides approved for domestic use that can be used on fruit for consumption, but there are some that can be used on purely ornamental trees.
Clearing fallen leaves and fruit can reduce the amount of spores in the area, but don't pull off leaves that are still attached. Don't add infected material to your home compost.
Other species affected
Hawthorn (Crataegus), rowan (Sorbus) and fire thorn (Pyracantha) may also be affected by types of Venturia.