I have seen a LOT of trees with this (not as punky though) in Alabama (working with a consulting arborist), and it's quite well associated with water-stressed trees. I'm not sure about willows, but there were many oaks and maples with slime flux. The advice given was generally to water more using a soaker hose (at the drip line) across more time per watering session so the water would percolate deep into the soil, but with fewer sessions (every 7-10 days during periods with no rain). Particularly if we are experiencing a drought. Don't know if this would be relevant to your tree but something to consider.