A root-centric view of root-microbe interactions in apple replant disease

View the project final report

Commodities

• Fruits: apples

Practices

• Soil Management: soil microbiology

One of the greatest challenges for apple growers in the Northeast is the sustainable and economically viable management of apple replant disease (ARD). Due to the lack of suitable orchard sites, existing sites are continuously replanted with apple which leads to a build-up of soil-borne fungal pathogens. These fungi target the fine, absorptive roots needed for water and nutrient uptake, which in young trees, can lead to root death, stunted tree growth and potentially tree mortality. The current ARD management strategies in the Northeast include a two-year crop rotation program and pre-plant biofumigation which have had variable success.
Sustainable and economically viable management strategies are needed to manage ARD. Growing healthy roots and protecting roots from ARD pathogens by encouraging beneficial fungi may be a solution. However, our understanding of root-microbe interactions is limited making manipulations exceedingly difficult.
This study will investigate the effects of nitrogen (N) source (mineral N, organic N, water) and location at an apple replant site (herbicide strip previously planted in apple contrasted with row middle previously in grass) on root growth and root interactions with arbuscular mycorrhizal fungi (AMF) and ARD pathogens. Roots will be accessed using root boxes with clear-acetate windows. Root proliferation, root growth rate, duration of growth and colonization of AMF and ARD pathogens will be quantified. We predict that the production of healthy roots will occur by either enhanced root proliferation to overcome ARD pathogen infection or enhanced production of fast-growing roots that are colonized by AMF and inhibit pathogen infection.

Objective 1) Determine the effect of nitrogen (N) source and location at an apple replant site on root proliferation in young “Golden Delicious” apple trees on M.26 rootstock. Expected outcome: Trees with localized addition of the readily available N source, urea, will have more root proliferation than trees with the slow-release organic N addition and the control (water) treatment. Trees grown in the grass row middle where perennial grasses were previously grown will have more root proliferation than trees grown in the herbicide strip where mature apple trees were previously grown.

Objective 2) Determine the effect of nitrogen (N) source on growth rate, duration of growth and colonization of individual apple roots by beneficial fungi and pathogenic fungi implicated in apple replant disease (ARD). Expected outcome: Compared to the organic N and control treatments, trees with localized urea addition will have more fast-growing roots that grow for a longer duration. Colonization by both arbuscular mycorrhizal fungi (AMF) and non-mycorrhizal fungi (NMF) will be moderate. Trees with the slow-release organic N addition will have fewer fast-growing roots with greater AMF colonization and minimal NMF infection. Trees with the control treatment will have more slow-growing roots and higher NMF infection.

Objective 3) Determine the effects of location at an apple replant site on growth rate, duration of growth and colonization by AMF and NMF of individual roots. Expected outcome: Trees planted in the herbicide strip where soil-microbial communities developed in association with apple trees will have more slow-growing roots with higher NMF infection and more putative soil pathogens. Trees grown in the grass row middle where apple trees had little influence on development of soil-microbial communities will have more fast-growing roots, higher AMF colonization and fewer putative pathogens.

Objective 4) Determine the effect of root growth rate on root interactions with AMF and NMF. Expected outcome: Fast-growing roots will be rapidly colonized by AMF compared to slow-growing roots which will be uncolonized or colonized by NMF. The presence of AMF in roots will minimize the likelihood of NMF infection in young absorptive roots. These trends are expected across location and nutrient treatment.

Objective 5) Prepare results for academic and extension audiences at academic meetings and multi-state extension events such as Penn State field days and the Mid-Atlantic Fruit and Vegetable Convention. Results will also be disseminated through written extension materials supported by Penn State as described in the outreach plan.