Growth on leaves
This looks like the hackberry nipple gall, which is caused by insects. Please see this Colorado State Univ. Cooperative Extension fact sheet for more information on galls. You'll see a picture of the hackberry nipple gall on the first page. http://www.ext.colostate.edu/pubs/insect/05557.html
Generally, no treatment is recommended as populations fluctuate. Also, gall makers rarely harm trees and shrubs.