Apple replant disease

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.
• It is now generally accepted by scientists that apple replant disease (ARD) is a disease-complex primarily caused by microbial pathogens.
• The oomycete pathogens Pythium and Phytophthora along with the pathogenic fungi Cylindrocarpon, Rhizoctonia and Fusarium are all thought to contribute to ARD across the world.
• With the recent increase in apple fruit wall production systems on old orchard ground, there has been a corresponding increase in apple replant disease which has significantly reduced the financial returns in the early years after establishment.
• 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.

Recent research has assessed three different management strategies to combat ARD:

1. Using a rootstock different from the previous one
2. Replanting trees in the alley instead of the original tree stations
3. Individual and combined use of arbuscular mycorrhizal fungi (AMF), biopesticides and plant growth promoting rhizobacteria (PGPR) products

Rootstocks

• Work on rootstocks demonstrated that trees planted in the same stations as the previous orchard using the same rootstock or one with closely related parentage, displayed more severe ARD symptoms.

Alley planting

• Replanting trees in the alley of a previous orchard significantly improved tree establishment and growth in the first six years compared to planting in the previous station.
• Recorded tree girth expansion was consistently higher for trees planted in the alley. 
• Overall, the impact of changing the planting location on ARD severity was more profound than rotating rootstocks.

Microbial soil amendments

• In work to assess the use of soil amendments, the combined use of AMF (Diverspora sp.) and biopesticides (one fungal and one bacterial), when applied to the soil at planting, led to nearly 22% increase in annual tree girth expansion compared to the unamended control. 
• The individual products did not interact with each other. 
• Amending with PGPR at planting time had no significant effect.
• In one season only, both AMF and biocontrol amendments led to increased fruit numbers and marketable yields.

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.