If you can reach most or many of the lower limbs, spread a tarp under the tree and shake the limb. Also, you can throw a rope over limbs shortly out of reach and pull on both ends of the rope simultaneously to shake the limb. Pick up the 4 corners of the tarp sending acorns to a pile in the middle of the tarp for easy pickup. This way you'll have fresh acorns right off the tree that have not touched the ground for possible contamination.