Mark Mazzola, Shashika S. Hewavitharana, and Sarah L. Strauss
United States Department of Agriculture–Agricultural Research Service (USDA-ARS) Tree Fruit Research Lab, 1104 N. Western Ave., Wenatchee, WA 98801.
USDA-ARS Publication #311425
Phytopathology, April 2015, Volume 105, Number 4 Pages 460-469 View Full Article

Interpretive Summary: Soil-borne disease management without chemical fumigants remains a major challenge for many crop production systems including tree fruits. Soil fumigation has been the primary means used for the control apple replant disease but impending modifications to existing regulations are likely to increase the cost of soil fumigation and/or restrict use of this disease control measure on certain sites. Brassicaceae seed meal amendments have been effective in controlling apple replant disease but require post-plant application of the fungicide mefenoxam. Therefore, a composite seed meal formulation was devised which included both yellow mustard (Brassica juncea) and white mustard (Sinapis alba) and examined for disease control in organic orchard systems. The composite seed meal formulation was as effective as soil fumigation in controlling replant disease and resulted in tree growth and yield that was equivalent to or greater than that attained in fumigated soils. The pre-plant seed meal amendment suppressed recolonization of orchard soils by multiple pathogens including parasitic lesion nematodes and root rot organisms such as Pythium spp. In contrast, fumigated soils were rapidly recolonized by both pathogens and trees were infested by these organisms resulting in lower yields than that attained in seed meal treated soil. Long-term suppression of plant pathogens in response to pre-plant seed meal soil amendment was associated with the persistence of a distinctive microbial community associated with apple roots, and included microorganisms known to parasitize nematodes and Pythium spp. In contrast, the rapid re-colonization of fumigated soil by root pathogens was associated with the reversion of the soil microbial community to one that was indistinguishable from that of the no-treatment control. The difference in pathogen re-establishment corresponding with enhanced tree growth and yield in seed meal amended soils demonstrates that the beneficial effects of Brassicaceae SM amendment be persistent leading to enhanced orchard economic viability. These findings also demonstrate that pathogens such as lesion nematode and Pythium spp. not only suppress plant growth in newly established orchards but that they also have the ability to suppress productivity and economic vitality of established orchards.