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symptoms The first evidence of the disease generally is witting or flagging in one or more oflhe upper branches. Leaves on affected branches turn dull green to yellow and curl, then become dry and brittle, and turn brown. Some trees die several weeks after becoming infected; others wilt slowly and survive for a year or longer. Systemic infection (fungus present in large sections ofthetree) may resutt in wilting, as well as dead and dying shoots along the infected limbs. Peeling bark from witted branches reveals lightto dark brown streaks or solid blue to gray discoloration oflhe wood beneath the bark (ftgure 1). In cross section this appears as a brown discontinuous ring in the outer sapwood of the wilting, dead, and dying branches. Although other fungus diseases and wounds can cause similar discoloration, when Dutch elm disease is prevalent, then such discoloration is sufficient evidence of Dutch elm disease for sanitation measures to be initiated immediately. ~ :,,:,~, J ".. y" Figure 1_ This cross section and stripped elm branch show the discoloration beneath the bark which can mean Dutch elm disease. When positive identification ofthe disease is required, diseased portions showing vascular discolora- tion can be laboratory tested for the presence of the Dutch elm disease fungus. Samples should be about 1/2-inch diameter, 5-10 inches iong, and must be from the branch which is witting (the fungus cannot be isolated from dead, dried branches). Samples may be sent to: Cause Dutch Elm Disease Laboratory 90 West Plato Boulevard Division of Plant Protection SI. Paul, MN 55107-2094 The fungus Ceratocystis ulmi invades and grows in the water conducting vesselS of elms, induc- ing the host tree to produce tylosis (growths) and gums which, together with the fungus, plug the vessels, preventing water uptake. This causes the tree to wilt and die. 2