Drying, brown, and withered pistils can be a sign of flower maturity.
When a cannabis flower's pistils are 50-75% brown, it's usually time to harvest.
But don't be tricked!
Spray applications and dry conditions can mimic this look on plants that are not yet ready to harvest.
Potassium bicarbonate is a common organic fungicide that can be effective in controlling powdery mildew. But if applied during the flowering cycle, it will dry all of the pistils and turn them brown. To the inexperienced eye, this can give a crop the appearance of maturity.
Chronically dry or hot conditions can also cause pistils to wither.
Fortunately, after a spray application or dry event, healthy plants will continue producing new white pistils until the crop is truly ready for harvest.