Biology and management of apple replant disease
Commercial orchard production is based on the combination of two genotypes in each tree planted; the rootstock providing the collar and root system and the scion that bears the fruit. Apple (Malus x domestica Borkh.) is the most important tree fruit in the UK, reaching near 16K hectares in the last decade, after years of decline. Growth has led to a steep increase in the area planted every year thus demand for trees has doubled since the 1990’s.
Poor establishment of apple trees grown in soils previously planted with apple trees is a world-wide problem that affects the production of both rootstocks and fruit. Symptoms of Apple replant disease (ARD) are manifested within three months of replanting and include uneven growth, stunting and shortened internodes aboveground and discoloured roots, root tip necrosis and reduced root biomass belowground. This effect is particularly pernicious in propagation nurseries where land needs to be rotated very frequently. Surviving trees exhibit greater tolerance of ARD and their growth rates are the same as those of unaffected trees in the second year, but initial fruit production is delayed by 2-3 years and yields are depressed by up to ~60% for the tree’s commercial life-span. There is emerging consensus that the causal agents are site-specific complexes of nematode pests (e.g. Pratylenchus penetrans) and necrotrophic microbial pathogens including Phytophthora spp., Pythium spp., Cylindrocarpon spp. and Rhizoctonia spp.
With financial support from BBSRC and industrial partners, we are carrying out research studies to understand the nature of ARD complex and how the differing responses of rootstock genotypes can modulate the effects of ARD. The research work is primarily carried out by three co-project leaders: Nikki Harrison, Feli Fernandez and Emma Tilston.