Dutch Elm Disease. The tracks of elm bark beetles (family Scolytidae) on the trunk of a tree infected by Dutch Elm Disease. The bark-boring beetle is a vector of the fungus Ophiostoma ulmi, which is the causative agent of the disease. The sticky spores of the fungus adhere to inseminated female beetles and are deposited under the tree's bark when they lay their eggs. Once the spores germinate under the bark, infection spreads rapidly, but is confined to the water-conducting vessels of the tree. The symptoms of the disease are yellowing and browning of leaf foliage followed by wilting, defoliation and finally tree death.