Answer: In part the answer to your question depends on which pests they were, but in my experience insects tend to stay with the plants while diseases more often stay with the soil. Whiteflies in particular can be very hard to control and they are worst in hot dry weather, as are spider mites. This type of weather is also very hard on astilbes in general so it may have been a combination of factors -- especially when you consider the difficulty in keeping containers well watered during hot dry weather and the fact that astilbe loves water. You might try replacing the top few inches of soil and spritzing the pots and surrounding (non plant) surfaces with a bleach and water solution just to be on the safe side. The fresh soil will be better for the new plants, anyway. Let's hope the new plants fare better!
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