Q. I found these fuzzy things on my oak leaves. What are they and should I be concerned? Oak leaf gall caused by  tiny wasp A. These fall under the general category of plant galls. Galls are swellings of plant tissue and located on the roots, stems, trunk, leaves and flowers. Galls can be caused by insects, diseases, nematodes or mites. This particular gall is on the leaf of oak, and my guess it is ‘Heritage’ live oak. These leaf galls are very common to oak and caused by a tiny wasp. The purpose of the gall is to protect the developing youngster until it is mature and exits the gall to find a mate. Later on in the season you will see a small exit hole and the gall will probably turn brown or the leaf may turn brown and drop from the tree if the infestation is quite severe. Ash flower gall caused by eriophyid mites feeding on flower buds The basic lifecycle is for the female to lay an egg inside the leaf or on a leaf surface. The egg hatches and the very tiny larva eats the plant tissue inside the leaf. It also releases chemicals that control the growth of the leaf around the area where it is feeding. This disturbed area grows in response to the feeding and causes the leaf to grow into a tumor-like growth totally surrounding the youngster. Inside this gall the youngster can continue to feed protected from predators and the environment. Eventually the youngster grows up into an adult and exits the gall in pursuit of a mate. Once the mate is found, the life cycle repeats itself. In some cases a large number of these wasps can exit numerous galls and build a large population that can cause leaf drop. These leaf galls normally don’t present a problem and I would just ignore them. However if they do create a problem you might consider applying a tree systemic insecticide to the soil surrounding the tree near the source of irrigation water. I would apply it as soon as you see leaf galls developing in the spring. A picture of this same oak gall appears at this website by Armstrong with a short description along with a bunch of other galls. http://waynesword.palomar.edu/pljuly99.htm#insects You can learn more about plant galls by visiting this website located at Brandeis University. http://www.bio.brandeis.edu/fieldbio/Plant_Galls/samplepage.html Leaf drop could also occur from excessive shade and a lack of water or applying water too often. You should not be watering daily. All it takes is one or two missed irrigations and you could have leaf drop from a lack of water during hot, dry weather. Watering too often would cause root rots and leaf drop and a sudden death of the tree during hot weather. Applying a heavy dose of nitrogen fertilizer close to the trunk could also cause leaf drop and possible tree damage.