Annual ryegrass cultivation did not improve apple root biomass relative to control treatments in greenhouse trials using soil collected from two orchards with apple replant disease. In contrast, wheat cultivation improved apple root biomass relative to the control, but the magnitude of the impact was dependent on the wheat variety and conditions under which the varieties were selected. When averaged across the four entries representing each breeding category, cultivation of wheat varieties from the advanced organic breeding program significantly increased apple root biomass relative to historic varieties. In contrast, differences in apple root biomass following cultivation of varieties selected under high-input conditions did not differ from historic varieties. Cultivation of perennial wheat also increased apple root biomass relative to the control, annual ryegrass and historic wheat varieties, however, there was wide variability in the performance among the four perennial wheat entries tested. One perennial wheat accession (P-0006), increased apple root biomass equal to the pasteurized treatment, while the other perennial accessions did not positively impact apple seedling health.
Improvements in apple seedling health following wheat cultivation were correlated with modification of the soil microbial community. The top entries representing organic (Onas/Madsen) and high-input (cv. Penewawa) breeding programs, a perennial wheat variety under development (P-0006), and a wheat ancestor (T. intermedium cv. Rush), all increased populations of beneficial microbial species and decreased soil-borne pathogens belonging to the complex that is widely recognized to incite apple replant disease.