Look at what my cherry tree is doing:
Naturally I googled "red nodules on cherry leaves" and I found out what it is, courtesy of "Ant Man" (page down in the comments):
"Cherries generally have glands on their petioles. They secret nectar that attracts ants. Studies on various plants that have similar glands (like passionflower vines) indicate that the ants help the plant by killing and otherwise interfering with insects that would otherwise eat the plant. Sort of like the plant paying "protection" money to the ants.
Also, as another poster indicates, ants often farm aphids as well for honeydew. The cherry tree's strategy is to provide nectar directly to the ants, removing the "incentive" for ants to farm aphids, This way the cherry tree avoids the damage by the aphids that ants would otherwise protect."
It's working! I think that is so cool.