Towards the Development of Sustainable Alternatives for the Co

Due to its uncertain etiology, the control of apple replant disease has primarily been dependent upon the use of biologically broad-spectrum pre-plant soil sterilants such as methyl bromide. Formulation of alternative measures that approach the levels of disease control achieved with soil fumigants has awaited a clearer understanding of the biotic and abiotic elements which contribute to the disease. As an initial step in the development of biologically sustainable disease control alternatives, a systematic examination of the etiology of apple replant disease was conducted in Washington state (Mazzola 1997, 1998a). In these studies, it was demonstrated that a fungal complex, composed of species belonging to the genera Cylindrocarpon, Phytophthora, Pythium, and Rhizoctonia, is the dominant cause of apple replant disease in Washington. Surprisingly, composition of the pathogen complex was consistent across the study sites, though the relative dominance of individual components varied among orchards. It was also shown that, in contrast to previous studies conducted in other geographic regions, the lesion nematode (Pratylenchus spp.) generally is present in low populations and has a limited or no role in disease development. Based on results from these and other studies (Mazzola 1998b), biological, cultural and narrow-spectrum chemical controls were selected which target various components of the causal fungal complex and which operate via differing mechanisms. However, the intent of these studies is to formulate a systems approach to the management of replant disease, thus an emphasis has been placed on utilizing the biological and physical potential of the growing system to suppress disease development. In preliminary greenhouse trials, the biological agents Pseudomonas fluorescens 4A8 and Pseudomonas putida 2C8, and the fungicides metalaxyl, difenconazole, and fludioxinil were evaluated for the ability to enhance growth of apple in replant soils from the WVC and GC orchards located in Wenatchee and Manson, WA, respectively. In addition, due to the presence of significant populations of Pratylenchus penetrans, the nematicide fenamiphos was evaluated in GC orchard soil. With the exception of fludioxinil, all treatments significantly enhanced growth of apple in WVC orchard soil, but effective fungicide treatments performed significantly better than the biological agents tested (Table 1). At the GC orchard, all treatments that selectively suppressed Rhizoctonia spp., including strain 2C8, fludioxinil and difenconazole, significantly improved the growth of apple. Efficacy, or lack thereof, correlated well with specific activity of the control measure and the relative importance of elements of the pathogen complex in the two soils. For example, fludioxinil enhanced growth of apple in GC soil, where R. solani AG 5 dominated, but not WVC soil where all components of the complex contributed to disease development. Application of fludioxinil to WVC soil resulted in an increase of nearly 100% and 50%, respectively, in the recovery of Pythium and Cylindrocarpon spp. from the roots of apple.