Biological mediation of apple replant disease in organic apple orchards


Investigator: Lori Hoagland, Washington State University, Pullman, WA
Project location: Eastern Washington state

Apple replant disease, caused by a complex of soil-borne pathogens, is a major impediment to the widespread adoption of organic orchard production systems. Currently the only effective treatment for control is pre-plant fumigation. Because of the long-lasting severity and negative economic impact of this disease, organic orchardists will remove their land from organic certification in order to treat the soil with chemical fumigants, and then recertify after three years. This destructive practice not only reduces the integrity of organic production systems, but it also results in a dramatically altered soil microbial community with a disruption in function of beneficial as well as pathogenic soil organisms.

Wheat has been found to enhance resident soil organisms antagonistic to the soil-borne pathogens responsible for apple replant disease and improve apple health in a cultivar specific manner. However, there are currently no efforts being made to actively select for this trait in wheat or to develop a wheat-apple system to help control these pathogens. The conditions under which cultivars are selected may impact their ability to enhance populations of beneficial soil microbial communities.    

Additionally, because perennial wheat has much deeper and more extensive root systems than annual wheat, it will likely have a better capacity to modulate the resident microbial community in a manner that leads to a disease suppressive state in the field. Our unique proposal brings together researchers from a wide variety of disciplines with organic orchard managers to develop a biological, systems approach to mediate apple replant disease and improve the health and productivity of organically produced orchard trees.

Project objectives are:

  1. Determine the ability of annual and perennial wheat cultivars to selectively support resident microbial antagonists of apple root pathogens and enhance apple seedling health;
  2. Determine whether selection conditions and introgression of genes from wild relatives impact the ability of wheat cultivars to facilitate beneficial plant-microbial interactions;
  3. Determine the ability of an annual or perennial wheat-apple system to mediate apple replant disease and enhance the health of newly established organic apple orchards.

Similar pathogen complexes affect various crop plants, thus findings from these studies are likely to be of value across multiple systems.