Brassica seed meal soil amendments transform the rhizosphere microbiome and improve apple production through resistance to pathogen reinfestation.

Brassicaceae seed meal (SM) formulations were compared with preplant 1,3-dichloropropene/chloropicrin (Telone-C17) soil fumigation for the ability to control apple replant disease and to suppress pathogen or parasite reinfestation of organic orchard soils at two sites in Washington State. Preplant soil fumigation and an SM formulation consisting of either Brassica juncea-Sinapis alba or B. juncea-B. napus each provided similar levels of disease control during the initial growing season. Although tree growth was similar in fumigated and SM-amended soil during the initial growing season, tree performance in terms of growth and yield was commonly superior in B. juncea-S. alba SM-amended soil relative to that in fumigated soil at the end of four growing seasons. SM-amended soils were resistant to reinfestation by Pratylenchus penetrans and Pythium spp. relative to fumigated soils and corresponded with enhanced tree performance. Phytotoxic symptoms were observed in response to SM amendment at one of two orchard sites, were dependent upon season of application, and occurred in an SM formulation-specific manner. After 2 years, the rhizosphere microbiome in fumigated soils had reverted to one that was indistinguishable from the no-treatment control. In contrast, rhizosphere soils from the SM treatment possessed unique bacterial and fungal profiles, including specific microbial elements previously associated with suppression of plant-pathogenic fungi, oomycetes, and nematodes. Overall diversity of the microbiome was reduced in the SM treatment rhizosphere, suggesting that enhanced "biodiversity" was not instrumental in achieving system resistance or pathogen suppression.