Apple replant disease is a continuing problem for growers in Washington. Ongoing studies have identified cultural and biologically active methods that provide partial control of the pathogen complex that incites apple replant disease in Washington.

Mark Mazzola, a plant pathologist with USDA’s Tree Fruit Research Laboratory in Wenatchee, Washington, conducted studies that have identified soil amendments effective in controlling certain elements of the causal pathogen complex and have demonstrated the ability to enhance growth of apple on replant sites.

When applied independently, these control practices have improved vegetative growth and enhanced yield of apple (Gala/M.26) in orchard trials; however, these individual treatments did not achieve the level of disease control and crop yields realized in response to pre-plant soil fumigation. “It is apparent that the synthesis of multiple control methods has significant potential to achieve levels of disease control and tree growth and yield that is comparable to that obtained through pre-plant soil fumigation,” says Mazzola.

Subsequent field trials indicate that Brassica napus seed meal was applied as a soil amendment in the fall prior to planting in conjunction with a post-planting application of the fungicide. Ridomil (metalaxyl) has significant potential as an alternative to pre-plant soil fumigation for the control of apple replant disease. The post-plant fungicide application is required due to the increased activity of Pythium spp. in response to this soil amendment. The Malus germplasm has been shown to vary in tolerance to the biological complex that incites replant disease, according to a 2000 paper by D.K. Isutsa and I.A. Merwin. The current study has identified tolerance within commercially available rootstocks to specific elements of the causal disease complex. These findings indicate that host tolerance will be an important element to consider during the design of a systems approach to the management of apple replant disease.

Treatments were established in 2000 and 2001, and trials were planted to Gala/M26 and Golden Delicious/M7 in the spring of 2002 at the Wenatchee Valley College-Airport (WVC) and Columbia View (CV) Orchards, respectively. Treatments included application of B. napus seed meal in the fall prior to planting in conjunction with solarization, wheat cropping in varying sequence, or a Ridomil soil drench at 2 weeks post-planting. The target pathogen complex at the CV orchard includes Cylindrocarpon destructans, Phytophthora cactorum and Ph. syringae, Pythium heterothallicum and Py. intermedium, and Rhizoctonia solani AG-6. Causal agents at the WVC site include C. destructans, Ph. cactorum, Py. sylvaticum, R. solani AG-5, and Pratylenchus penetrans, a nematode.

Tolerance of commercially available apple rootstocks toward R. solani AG-5, and P. penetrans was assessed in greenhouse assays. Relative growth of rootstocks in replant orchard soils was assessed in field trials at the CV orchard.

Mazzola found that B. napus seed meal amendments, in conjunction with a Ridomil soil drench and Telone-C17 soil fumigation, were effective in controlling the pathogen complex that was targeted at the WVC and CV orchards. Over the initial 3-year life span of the CV orchard, growth of Gala/M26 planted in seed meal/Ridomil-treated soil has been equivalent to that of trees established in fumigated soil. In addition, initial yields from trees receiving this alternative treatment has exceeded those obtained in response to soil fumigation (table 1). Wheat cropping followed by seed meal amendment and solarization also enhanced tree growth relative to the control, but both were inferior to soil fumigation. At the WVC orchard, 16-month growth of Golden Delicious/M7 has been enhanced relative to the non-treated control by all alternative treatments evaluated, with the exception of soil solarization. However, alternative treatments have provided a growth response that is inferior to that obtained with pre-plant soil fumigation.

“I certainly think this regimen is feasible for growers,” says Mazzola. “The cost is about $100 to $150 per ton, applied at 3.8 tons per acre. B. napus seed meal is also a good source of nitrogen, at about 6%, which can reduce fertilizer inputs.”

Apple rootstocks exhibited considerable difference in tolerance toward the fungal pathogen R. solani AG-5 (table 2). Among the rootstocks evaluated, M.7 and Bud.9 were consistently the most susceptible, regardless of the soil in which assays were conducted, and tree death was commonly observed. In contrast, Seedling and Geneva16 rootstocks exhibited the greatest tolerance to this pathogen. Geneva16 has also demonstrated significant tolerance/resistance to the lesion nematode, while M.9, MM106, and MM111 supported significantly higher populations of this plant parasitic nematode.

Follow-up trials established in 2004 are investigating the incorporation of host tolerance/resistance as an additional element within the context of the integrated disease management system. As B. napus seed meal may function through the activity of specific microorganisms that colonize apple roots, selection of a specific rootstock may have multiple influences on the success of such a disease control strategy.

Last Updated: June 24, 2005