If you’ve ever pulled out an old apple tree and planted a new one in the same place, you’ve probably discovered what is called “replant disease”, where the organisms that were attacking the resilient mature tree wreck havoc on the hapless young tree. Commercial orchards typically fumigate before re-planting, but the methyl bromide they used is being phased out.
An article in Good Fruit http://www.goodfruit.com/new-replant-disease-treatment/ advocates using mustard seed meal tilled in during the fall before planting in the spring for pathogen control. Because I have a small lot, when I pull out a non-performing tree (and shred it in sight of the other trees as a warning to them), I usually have to re-plant in the same hole.
Mustard seed meal is a bit hard to find here in the desert, but I was able to mail order some from Farm Fuel Inc., a biodiesel company on the California central coast that sells mustard seed meal for fertilizer as a by-product of their manufacturing process. I’m going to try incorporating some what they call “Pescadero Gold” (pelletized mustard seed meal) into the soil this summer for fall planting to see if it helps any.
I’ve been following that a bit in Good Fruit Grower. I wonder how much, how deep, and how far from the old hole? Commercially they probably treat the entire orchard area. Still it doesn’t seem like it would have much effect deeper than it was incorporated. Tree roots go many times deeper than a rototiller.
I wonder, though, how deep the feeder roots go? The anchor roots probably would not be affected, I would think? I know citrus feeder roots are pretty shallow. And, the dwarfing and semi-dwarfing stone and pome fruit rootstocks tend to be pretty shallow I think (hence the warning with some of those rootstocks that staking may be necessary). This is very, very interesting information, Kevin, thanks for sharing. I will look forward to more info about this. I wonder if this would be beneficial for other types of fruit trees beyond apples.
The feeder roots go as deep as any of the roots. Every root has feeder roots at the tips. What varies with depth is the density of feeder roots. Those will be most dense where ever the tree is extracting water. Since rain and irrigation is usually applied at the surface, feeder roots are usually most dense in that region. But a tree surviving off a deep water table could have most of it’s feeder roots in the moist capillary zone just above the water table.
With regards to fumigation and replant disease it is helpful to the tree if even only part of the root zone is favorable, say the upper 12 inches. That helps the tree get established. After it gets a decent root system it’s better able to survive attack by nematodes and fungi.
Commercial farms plow it in, 3-4 tons per acre. Methyl bromide injections didn’t go that deep, my guess is 18" at the most. The main benefit claimed with mustard is that is has residual effect, while the methyl bromide quickly dispersed. It was interesting that letting the old orchard ground lie fallow for even several years had no effect.