Efficacy of Brassicaceous Seed Meal Formulations for the Control of Apple Replant Disease in Conventional and Organic Production Systems

Preplant soil fumigation has been the standard treatment for the control of perennial crop replant diseases, including apple replant disease (8,10). Although significant effort has been invested in the development of alternatives to soil fumigation for control of replant diseases (5,21– 23,26–28,30,31), there continues to be a lack of effective and economically feasible nonfumigant control strategies. Confusion regarding the etiology of replant diseases has been a significant impediment in the design of alternative control methods, and this uncertainty can be attributed to the diversity of components in an orchard ecosystem that can ultimately depress tree growth in new plantings established on old orchard sites (26). However, despite the potential of multiple factors to influence replant disease severity, repeated reports demonstrating dramatic improvement in tree growth and fruit yield in response to soil pasteurization (9,14) or fumigation (8,10,19,23) prior to replanting old orchard sites offer evidence that replant disease is primarily a biological phenomenon rather than the direct result of abiotic factors. The etiology of apple replant disease is commonly characterized as a pathogen complex varying from site to site, even among orchards within close proximity (26). In contrast, studies conducted in Washington State indicated that the pathogen complex was somewhat consistent among orchard sites and that the primary agents included fungi/oomycetes of the genera Rhizoctonia, Cylindrocarpon, and Pythium, along with Phytophthora cactorum, at times acting in concert with the lesion nematode, Pratylenchus penetrans (14,19). Subsequent studies have decisively supported original findings documenting a causal pathogen complex (19). Regardless of the apparent regularity in pathogen composition, the development of targeted approaches for the management of a disease syndrome incited by such a diverse biological consortium seems improbable; however, progress has been attained in achieving this goal in conventional production systems. For instance, on sites lacking significant lesion nematode numbers, preplant Brassica napus seed meal soil amendment used in conjunction with a postplant application of mefenoxam provided effective disease control and enhanced fruit yields, at times to levels similar to those attained in response to soil fumigation (19). Mefenoxam application is a necessary component of this management strategy due to the stimulatory effect of the amendment on resident populations of Pythium spp. (16,19). In the instance where lesion nematode was a significant component of the pathogen complex, the seed meal–mefenoxam treatment enhanced tree growth relative to the nontreated control, but was inferior to preplant soil fumigation primarily due to ineffective suppression of Pratylenchus penetrans (19). The need for a postplant mefenoxam soil drench diminishes or negates the utility of B. napus seed meal amendment as a disease control option in organic production systems. In addition, widespread adoption of such a disease control strategy will require further demonstration of consistent efficacy, particularly across soil systems where the lesion nematode contributes to disease development. Brassicaceous seed meal soil amendments provide disease control through various mechanisms, both biological and chemical, which vary with both the seed meal source and the target pathogen (6,16). Although products resulting from the hydrolysis of 3-butenyl glucosinolate, the dominant glucosinolate found in B. napus seed meal utilized in the previous studies, are not capable of suppressing Pythium spp. (12), alternative seed meals exist which do not elicit a stimulatory effect on Pythium populations resident to orchard soils (17). Individual and composite seed meal formulations were evaluated in greenhouse and growth chamber assays for the capacity to suppress the diverse components of the pathogen complex inciting apple replant disease (16). In the current study, brassicaceous seed meals possessing dissimilar mechanisms of biological activity were evaluated in field trials with or without a postplant mefenoxam soil drench to assess their potential to serve as an effective nonfumigant alternative for the management of apple replant disease. Based ABSTRACT Mazzola, M., and Brown, J. 2010. Efficacy of brassicaceous seed meal formulations for the control of apple replant disease in conventional and organic production systems. Plant Dis. 94:835-842.