Oak apples are galls on oaks made by the gall wasp. Spider mites, cankers, and galls are a problem with some trees. The rust fungus "Gymnosporangium cornutum" produces leaf galls. This applies particularly to some insect and mite plant galls. Many thrips form galls on plants when feeding or laying their eggs. Mites form scale-like galls on twigs, retarding growth and reducing the fruit crop. Mite galls commonly form on the foliage. They cause powdery scabs and galls and disrupt growth. The female kermes scale causes galls to grow on kermes oak. In some regions, the twigs commonly bear galls. A gall midge, "Contarinia negundinis" joins and enlarges the galls of "Aceria negundi". The plant galls mostly develop directly after the female insect lays the eggs. Bacteria cause plant diseases including leaf spot and crown galls. This fungus is typically diagnosed by these large black galls at the site of infection. The fungus forms galls on all above-ground parts of corn species.