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Managing Replant Disease in Apple Orchards

One of the more outstanding issues around crop culture of apples is the relative inability of growers to exercise a predictable measure of control over the success of replanting nursery stock into old orchards rows when renovating. These problems can vary with differing soil types.

We asked Dr. Ian Merwin of Cornell University to address this problem in the Apple Program at our recent Ontario Fruit and Vegetable Convention (OFVC) at Brock University. Ian has worked on this problem for decades and has tried pretty much everything conceivable in controlled experiments to find out how to beat Replant Disease.

In his talk at the OFVC, Ian recalled his work with pot bioassays of nursery stock planted into treated soils. The use of pasteurization of orchard soils resulted in 2x the growth for most replants into pot culture. However, pasteurization is a costly process and it sometimes results in a complete physical collapse of the soil. By contrast, in a greenhouse situation, pasteurization works fine where soils are amended with various types of stabilizing materials blended in.

The commercial fumigants Ian used in his research experiments did not work as consistently as he would have liked. He also added compost to the soil without getting any consistent result. Dr. Merwin emphasized that irrigated soils produced better results regardless of how the soil was treated ahead of time.

Other problems Dr. Merwin encountered with management of the young trees were things like leafhoppers taking the vigour out of the trees, tree quality itself, soil drainage, meadow voles and fireblight disease. Oddly enough, it appeared to be that the orchard management problems of getting trees off to a good start were a far worse problem than the soil borne pathogens responsible for replant disease.

Ian has also tried several of the approaches that could be defined largely as Biological Controls. This would include growing nematode resistant cover crops and plough downs of various types of covers containing biologically active ingredients. The problem was in making these treatments work consistently in the presence of such things like herbicide residues. The other problem with the biological control approach was getting hold of the specialized equipment needed to handle these methods of biological control.

After testing a variety of hypotheses and conducting numerous large scale trials, Dr. Merwin looked at the Geneva rootstocks. He focused on CG 30, CG 6210, and CG 16. To be noted is the fact that M9, B9 and CG series rootstocks are resistant to the Phytopthora fungus pathogen, one of hundreds of soil inhabiting fungi.

Ian found that CG 30 and CG 6210 appeared to overcome the replant problem. Trees planted into the old row or into the grass lane did equally well. With further lab work he found that the roots of these rootstocks did not support the same amount pathogen growth as did other commonly used rootstocks. Ultimately, clones of resistant rootstocks like the CG series would only selectively allow for the growth of specific fungi when comparing to M9. He observed that tree root function and longevity of the CG series rootstocks was 2-3x better than other commonly used rootstocks.