Follow these links to Michigan State University's articles on Root Pruning.
Also a study from 2011
"NEW" Disc type root pruner.
30" Tall discs slice easily into heavy soil.
Thin, rolling discs require much less power to pull through heavy soil.
Optional cylinders allow 30 inches of adjustment on each side.
• Heavy duty construction.
• Adjustable to row width.
• Adjustable skid plate.
• Tool stand for safe and easy hook-up.
• Slide cylinder for single or double row models.
• Slows growth of over vigorous trees.
• Increased firmness and storage life of fruit.
• Gives better color to red varieties.
• Greatly decreases dropping of fruit at harvest.