Apple trees may grow poorly when planted in non-sterilised soil. This poor growth is most frequent when apple orchards are replanted but may also occur when apple is planted in soils which have not previously grown apple.
- Affected trees have a reduced root system that results in poor growth and cropping, particularly during the early years after planting.
- The root system is reduced mainly because of the effects of several Pythium species.
- Apple variety/rootstock combinations vary in their susceptibility e.g., Cox, Golden Delicious or James Grieve on M.9 rootstock are more likely to suffer replant problems.
- More vigorous variety/rootstock combinations such as Bramley on MM.106 are much less likely to be affected.
- Previously it was possible to test potential new orchard sites for replant disease, but the test is no longer provided as a service.
- Soil fumigation pre-planting can reduce the effects of replant disease. However, the most effective fumigant is chloropicrin. This could only be applied by contractor and is no longer approved for use in the UK.
- Treatments other than soil fumigants include placing alternative substrates in the planting hole or using trickle irrigation, or soil mulches or a combination of these.
- Replanting in the areas that were the alleyways of the previous orchard offers an alternative approach, which may reduce the replant problem.
In 2018, AHDB funded Xiangming Xu and his colleagues at NIAB EMR to undertake a review of the current knolwedge and best practice in managing apple replant disease.