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There is one method that some people use, that is certainly worth considering. Choose an area you want to improve and water it either with garlic granules or garlic crushed and steeped in the water. Do this in mild weather and repeat at intervals over a couple of years. The theory is that the chemicals from the garlic trigger the sclerotia to "germinate" expecting to latch onto a new allium, only to find there is nothing actually present to infect. So the natural cycle of reinfection gets interrupted. You can get cheapish garlic granules from equestrian suppliers. If all goes well the levels of sclerotia are considerably reduced allowing a much healthier crop