Pat, I'd be really cautious with wood ashes unless your soil is low pH because it also has a "liming" effect. A soil test for pH and nutrient levels would be the best bet first, then you'd know if you actually need more potassium (or anything else) and whether you can safely use wood ashes if adjustment is necessary. According to Marschner once your soil goes above the optimum amount of potassium for plant growth you aren't likely to see a further increase in disease resistance.
