Apples are the easiest tree fruit to grow, thriving in a wide range of soils and growing conditions, and will crop for many years. However, apple trees may suffer from several common diseases, though most can be dealt with by a little regular care and attention. Where at all possible, prevention is the best way to tackle apple tree diseases, including good soil preparation, planting, and aftercare, as – unlike pest problems – it’s not possible to ‘cure’ disease, only to prevent or to limit its spread.
Apart from the diseases of apple trees described here, be aware that there are other causes for damaged or disfigured fruit. These include pests such as codling moth and apple sawfly, lack of water that reduces uptake of nutrients from the soil, extremes of drought and rainfall that cause fruit to split, and frost damage.
Apple canker is a fungal disease that affect woody parts of the tree, and occasionally the fruit. It’s more prevalent in wetter parts of the country. Canker affects pear trees as well as apples.
How to identify apple canker
Flaking, cracked, or sunken patches develop on the bark, sometimes with raised or swollen bark around them. The canker can eventually spread to girdle a shoot or branch, which then dies beyond the affected point. White pustules may develop around the canker in summer. Occasionally apple canker causes fruit to rot and fall. Canker is most likely to develop on young growth, particularly when too much fertilizer has been applied that results in soft growth.
How to treat apple canker
Prune out affected shoots when winter pruning, cutting well back beyond the affected part. Take care to apply only the correct amount of fertilizer. Apple tree canker is more liable to attack trees growing on ground that is prone to water-logging, or on acid soils or where there is poor air movement caused by close planting or high hedges. Many varieties have good natural resistance to this disease and should be chosen where growing conditions are less than ideal.
Apple scab affects leaves and fruits of apple trees, causing damage and premature falling. This disease is more likely to occur in areas of high rainfall and during wet summers. Although known as apple scab, this disease also affects other trees including cotoneaster and sorbus.
How to identify apple scab
Spots that are black or brown in colour develop on the leaves and fruits, from spring onwards. Affected leaves may fall early. On fruit, the spots may join up to form lesions which can split, allowing entry to other infections.
How to treat apple scab
Apple scab disease spores overwinter on the fallen leaves and then infect new growth the following spring. Gather up the leaves by raking or mowing as soon as they fall in autumn, which interrupts the life cycle of the disease. Dispose of in garden waste collections, rather than composting or using to make leaf mould. As with apple scab, there are many varieties that have good resistance to apple scab disease.
Brown rot is a disease of the fruit that enters the apples through holes made by birds or insects such as wasps.
How to identify brown rot
Brown marks quickly spread to infect the whole fruit that then either falls or remains hanging on the tree where it becomes covered with creamy white pustules. Brown rot is likely to spread to other apples in contact with infected fruit, either when hanging on the tree or when fruit is stored.
How to treat brown rot
Inspect trees and the ground beneath regularly, picking and disposing of infected fruit, which removes the source of infection. Dispose of in your garden waste collection – do not compost or keep in your garden.
Powdery mildew disease appears as a white coating on the leaves and may also affect blossom. It is important to control the disease as affected growth may then develop other infections. This disease affects a wide range of plants.
How to identify powdery mildew
Powdery mildew is clearly identifiable by a white coating on the upper surface of the leaves, which can then be reduced in size, become distorted, and fall early.
How to treat powdery mildew
This fungal disease overwinters on the tree to re-infect growth the following year, so break the cycle of infection by removing diseased material. Prune out affected shoots in summer and gather up and remove fallen leaves in autumn. Good airflow discourages mildew, so bear this in mind when winter pruning and thin out dense growth. Powdery mildew infections are worse when the soil is dry, so give an occasional thorough watering when there are long periods without rain. There are growth invigorators and chemical fungicides available to combat powdery mildew but sprays are hard to apply on a tree scale.
Preventing apple tree diseases
Grow resistant varieties
A number of varieties have good natural resistance to disease. Popular varieties include ‘Grenadier’, ‘Katy’, ‘Lane’s Prince Albert’, ‘Lord Derby’, ‘Sunset’ and ‘Newton Wonder’. A good retailer, online supplier or specialist nurseries should be able to supply more detailed information on the full range of resistant varieties.
Prune each winter to remove any dead, diseased, or damaged growth, as wounds allow disease to enter. Thin out the canopy of branches to encourage good air movement and to avoid stems crossing and rubbing on each other, which creates wounds.
Choose a good site. While apples tolerate a wide range of conditions, their ideal is sun, shelter from strong winds, and a fertile well-drained soil with a pH of between 6 and 7. Avoid low-lying sites where cold air gathers (known as a frost pocket) and where the soil could become waterlogged. Never plant on ground where an apple has grown previously.
Prepare soil thoroughly. An apple tree should live and crop well for many years, so never skimp on giving it a good start in life. If the soil needs improving, dig over an area a couple of metres square and incorporate well-rotted manure or garden compost. Clear weeds, especially every bit of root of perennial ones.
Plant in autumn, so the tree has months to become established before bursting into growth in spring. When planting at other times, it’s vital to keep the tree watered for the whole of its first growing season. Be careful to plant at the same depth as the tree was growing previously and secure using a stake and tie. Water in, then finish by covering the soil with a mulch such as chipped bark or a mulch mat.