The gray powder on the surface of the plums is a wild yeast that can be used to make sourdough starter. To use wild plums to create a sourdough follow these steps:
On day one combine one cup whole wheat flour with ½ cup cool, non-chlorinated water, and four to six undamaged, gray-dusted wild plums in a bowl and gently stir together. Cover with a towel and let sit somewhere warm and undisturbed.
After 24 hours discard half the mixture but leave the plums in the retained portion. Add one cup unbleached flour and ½ cup non-chlorinated water. Gently mix everything. Cover with a towel, let sit somewhere warm for 24 hours.
By now (day three) you may see some bubbling in your starter and hopefully it has a somewhat fruity smell. If the starter is bubbling, it's time to remove the plums. You will also have to start “feeding” it twice a day: For each feeding scoop up a heaping ¼ cup of the starter and combine it with one cup unbleached flour and ½ cup non-chlorinated water about every 12 hours. Unused starter should be shared or discarded.
Keep repeating step three for three to seven days until it almost doubles in size between feedings and has a nice, tangy aroma. Now your starter is ready and you can start making sourdough breads!
Move unused sourdough starter to the fridge, discarding half and feeding it one cup flour and ½ cup water once a day. If you don't remove some starter every day it will overrun your container and also likely become too acidic, killing itself. (Humans aren't the only creatures that take over paradise and end up killing ourselves with our waste. Yeast does this, too.)