Just found this super helpful FAQ on dealing with late blight of tomatoes/potatoes, and thought I would share.
Of particular use to me was question #26, which says you can plant anything other than tomatoes or potatoes in the same soil/mix immediately after blight. So I don't need to sterilize/solarize/freeze my blighted mix before starting my fall spinach.
"26. What crops can be grown immediately after tomatoes or potatoes are lost to late blight?
The only crops you wouldn't want to plant are tomatoes or potatoes, which typically it will be too late for anyway. Since the pathogen has a limited host range and cannot survive as a saprophyte on dead plant tissue, it does not matter what is planted immediately after crops lost to late blight. Crops most likely to be successful planted in mid to late summer are those that quickly produce something edible and grow well in cool, fall weather. Spinach, lettuce and other leafy greens are good options. Where harvesting of individual leaves can be done, these crops can begin yielding produce quite quickly. Peas also do well under cool temperatures, but might not produce many pods before a killing frost."