One popular argument states that washing pots supposedly reduces the chances of damping off disease. But researchers now know that to be untrue.
The conditions that cause damping off originate not from dirty pots, but from poor growing practices like spacing seedlings too close together, overwatering, watering the leaves, and leaving plants in non-ventilated greenhouses or under plastic humidity domes and cloches.
While pests and diseases can certainly transfer from an old plant to a new one, the chances of that actually happening are slim.
There may be the rare occasion that a dirty pot harbors root aphids lurking in leftover soil or spider mites hibernating under the rim, or perhaps a plant you tried to grow in the pot was plagued with allium rust or clubroot.
If any of that were the case, and serious enough to warrant consideration at all, don’t bother with washing and disinfecting that pot. It’s far more economical to throw it out and use a brand-new pot with brand-new soil instead.