Apple Replant Disease (ARD) Evolution and Rootstock Interaction (ARDERI)

Replant disease is a widely recognized problem in apples and other rosaceous species, such as cherries, plums, strawberries and roses. Apple replant disease (ARD) is the problem of new apple trees failing to thrive when planted in soil where apples have previously grown. Historically, many causes have been proposed to explain this disorder such as accumulation of pesticides and the release of volatile toxins from decomposing roots. However, current scientific consensus focuses on a mix of specialist root-rotting pathogenic microorganisms, comprised of fungus-like "water moulds" and true fungi (degrading root tissues) and microscopic eelworms (providing easy points of infection when they feed). Surviving affected trees typically fruit later in life and, over their lifetime, produce less fruit of reduced quality. These compound effects result in a loss of profitability of up to 50% in commercial orchards. Modern production systems involve much higher tree density (up to 30x greater) and faster replacement cycles exacerbating the ARD problem. Furthermore, extreme environmental events such as flooding often lead to tree death requiring gaps to be replanted in young orchards. ARD is an even more significant problem for the nurseries that raise the trees, affecting their production of rootstocks (root system in a commercial tree) as well as the health of the finished tree, after the fruit-bearing variety (scion) is grafted. Commercial rootstock varieties are selected mainly for their effects on tree vigour and precocity. They also affect orchard performance on particular soil types and resistance to pests and diseases including ARD but the underlying reasons for differences in rootstock susceptibility to replant and the legacy effect of different rootstock genotypes on the subsequent severity of ARD are poorly understood.
On-going changes to agri-chemical registration procedures and environmental pollution regulations continue to reduce the options for effective chemical control traditionally used to manage ARD. This means that growers will be reliant on more costly management options including sterilization of soil with steam (currently uneconomical), rotation with stone fruit trees or other unrelated crops or encouraging beneficial soil microbes by adding various compost-like conditioners to soil. A clear grasp on the organisms involved in ARD and their common pathogenic mechanisms is essential to develop financially-viable management strategies. Rootstock choice will become an increasingly important component of an integrated approach to ARD management; thus better understanding of the interactions between rootstocks and the microbe populations across different soil types in relation to ARD development is urgently needed.
East Malling Research (EMR) has a worldwide reputation for the development and characterisation of apple rootstocks. In collaboration with UK, Dutch and Belgian tree nurseries, apple breeders, and UK apple growers, we propose to provide the apple growing industry with detailed information about the interactions between rootstocks, soil and microorganisms that will enable development of new management strategies for ARD and direct future rootstock breeding programmes for improved tolerance to ARD. The main scientific discoveries will be 1) relationship of the relative abundance of ARD components with soil properties, microbial population and rootstocks, 2) Elucidation of rootstock - soil biota interactions and their effects on root and shoot architecture and growth in terms of ARD development, and 3) clarification of the legacy effect of different rootstock genotypes on ARD. This will benefit one of the UK's major horticultural industries by providing new opportunities for disease management to increase yield efficiency and reduce on-farm food wastage. As a result, the UK consumers will continue to enjoy the health benefits conferred by one of our most popular home-produced fruits.

In this project the interactions between apple rootstocks and the soil organisms that cause apple replant disease (ARD) will be investigated using the latest techniques in culture-independent molecular biology, especially the role played by rhizosphere microbiota recruited by individual rootstock genotypes. The overall aim is to understand the relationship of the ARD complex with rootstock genotypes, soil microbiota, and soil physiochemical properties. This understanding will improve ARD management through (1) better selection of rootstocks for a given site, (2) better estimation of ARD risk for a given site, and (3) improved selection efficiency for ARD tolerance in rootstock breeding. The research activities have been organized into three work packages (WPs), each addressing a separate objective. The three objectives are: 1) to determine the relationship of relative abundance of ARD members with other factors, 2) to elucidate the interactions among individual ARD members in ARD development, and 3) to investigate the effect of rootstock succession on ARD severity. In WP1 next generation sequencing will be used to identify the microorganisms and the nematodes. Relative abundance of ARD members will be related to the physiochemical characteristics of soil and rootstock genotypes. For WP2 root system responses such as growth rate and branching to various components of the ARD complex will be determined to ascertain the nature of interaction among ARD components and to identify resistant genotypes. WP3 will investigate the effect of the succession of rootstock genotype on the ARD in relation to rhizosphere microbiota. Industry partners will provide required orchards and nursery sites for sampling and trialling, and will also provide necessary assistance in conducting trials at these sites. The inclusion of the apple growing industry from the outset will ensure that our science is not only relevant to the needs of end-users, but also that our res

Apples are the most important tree fruit grown in the UK but our annual crop of dessert and cider apples (~203,000 tonnes, worth ~£1.2M) meets only 31% of national demand. The shortfall is met with imports from New Zealand, South Africa, US and France to ensure year-round supply making the UK the third biggest importer of apples worldwide. After a long decline, orchard acreage is recovering in the UK (9% increase in dessert apples since 2008) as growers seek to improve production efficiency and maximize profits by adopting new varieties and high density planting systems. The spectacular growth in cider consumption (worth £3,000M p.a. retail) has also led to new plantings of cider varieties. This project would provide new information on the biology of ARD and the interactions of its components with different rootstock genotype to (1) inform site-specific rootstock selection facilitating the replacement of established apple orchard sites (prone to apple replant disease) with more efficient high-density plantations, (2) assist in the development of improved management strategies, and (3) better selection criteria in rootstock breeding. ARD-tolerant rootstock genotypes would enable high yielding scions to perform at their best avoiding ARD-related crop losses.

UK growers heavily rely on EU propagators for new trees (~ 95% of rootstocks planted in the UK have been produced in the continent); consequently representatives of the continental nursery trade are an integral part of the industry contribution to the project. Shortening nursery rotations for apple rootstock production, thus reducing the constant need for fresh land, will increase the efficiency and the sustainability of the (rootstock) nursery industry in the UK and Europe-wide improving their ability to respond to increased demand for high-quality trees.

The overall benefit to our fruit industry will be an increase in the competitiveness of UK production of both dessert and cider apples. In addition to growers and nurserymen, the results of ARDERI will also feed to the supporting consultancy and advisory industries worldwide and, nationally, to policy makers, government agencies and regulators such as the Plant Health Propagation Scheme enforced by the Plant Health and Seeds Inspectorate. The involvement of key industry members will ensure that EMR scientist never lose sight of the needs of the end users and remain aware of the barriers to translation from the outset. ARDERI's impact is not restricted to the scientific advancement of commercial apple production and propagation, an improved understanding of ARD would also benefit the rejuvenation of traditional apple orchards with heritage varieties of apples. Traditional orchards are a UK Biodiversity Action Plan Priority Habitat with significant landscape, cultural and social value. Community Orchards under local stewardship develop social capital by providing a venue and focus for inter-generational shared activities, such as 'Apple Day' celebrations of apple diversity on 21 October, which have become an integral part of the calendar in rural and urban communities alike.