Intumescence injury,also known as edema or oedema, is a nonpathogenic physiological disorder that can cause blister- or callus-like tumor growths to develop on the surface of leaves, or petioles,and stems of plants. Problematic intumescence injury has been observed in many different plant species. Researchers from the School of Plant Sciences at The University of Arizona published a study in the June 2016 issue of HortScience in which they report on recommended light treatments to mitigate the disorder for tomato plants grown in UV light-deficient environments.
Source: phys.org