How can apple scab be recognised?
Apple scab is a fungal disease that can produce olive-green to dark lesions on leaves and fruit. Management involves understanding spring infection conditions and reducing sources of disease, as well as sorting affected fruit.
Interpreting spots correctly
1. Appearance on leaves and fruit Young leaves may initially develop olive-coloured, velvety lesions that darken with time. Fruit can develop dark corky spots, distortion and sometimes cracking. Not every black mark is scab: insect damage, hail scars and other fruit diseases can produce superficially similar symptoms. Inspect both leaf surfaces and compare photographs taken during early fruit development. A single harvested apple does not provide enough evidence for a certain diagnosis. The relationship between leaf lesions, fruit lesions and the season’s weather is particularly useful. 2. Spring leaf-wetness periods For Venturia inaequalis, the interaction between spring temperature and leaf wetness is important. Infected leaves remaining from the previous season can provide inoculum for the next. How long leaves remain wet after rain may explain risk better than the rainfall total alone. Symptoms also take time to become visible after infection. Assuming that a lesion noticed today must have come from yesterday’s shower can lead to incorrect conclusions about when protection was needed or whether a previous intervention worked. 3. Understanding cultivar resistance Apple cultivars differ in their susceptibility to scab. Choosing a locally suitable cultivar with known resistance can reduce maintenance requirements in a home garden. Resistance to one disease does not protect against every apple disease, however. Rootstock influences tree size and growth, but does not replace the scion cultivar’s disease characteristics. Record both cultivar and rootstock when buying a tree. Current local experience is often more useful than a broad catalogue description, especially where disease behaviour differs between growing regions.
A prevention and monitoring plan
4. Managing fallen leaves Collecting affected leaves, or appropriately shredding them to encourage decomposition, can reduce the initial source of infection. Methods suitable for a small garden may differ from those used in a larger orchard, where local production recommendations should guide decisions. Do not move diseased leaves beneath healthy new trees as mulch. Leaf management alone cannot eliminate every infection, particularly in wet climates or near other sources. The goal is to reduce disease pressure rather than attempting to make an outdoor orchard sterile. 5. Helping the canopy dry Crowded canopies can restrict air movement and delay leaf drying. Appropriate training and moderate pruning improve light distribution and make inspection easier. Where possible, direct irrigation to the root zone without leaving foliage wet for long periods. Avoid unnecessary dense shoot growth driven by excessive nitrogen. Nevertheless, severe pruning solely to open the canopy can expose fruit to sunburn. Consider tree age, cultivar vigour and local summer heat when deciding how much growth to remove and when to do it. 6. Base treatment timing on local risk An arbitrary application during winter dormancy is not equivalent to protecting susceptible spring growth. Develop a risk plan using orchard history, cultivar susceptibility and weather information. If chemical protection is needed, use only locally authorised products for that use, following current technical advice and label requirements. Existing corky lesions will not disappear after treatment. Record the development of new spots, premature leaf loss and final fruit losses so that next season’s plan can be adjusted using actual orchard evidence.