Eurasian water milfoil (Myriophyllum spicatum L.) declined significantly in Chesapeake Bay from 1965 to 1967. This decline generally exceeded 95%, and was associated with several pathologic conditions, previously named Northeast disease and Lake Venice disease. We believe that these pathologic conditions have been responsible for the decline. No study of Lake Venice condition has yet been undertaken, but this paper presents evidence that Northeast condition is an infectious disease. Northeast disease symptoms can be transmitted in the laboratory by a bacteriologically sterile filtrate passed through a 0.2 μ filter. This indicates a virus, virus-like particle, or a toxin as the etiologic agent. The primary pathogen is probably a virus, and the evidence for this will be presented in a later paper. Gram-negative bacilli obtained from diseased milfoil probably represent secondary infections.
In September 1967, diseased milfoil showed a resurgence of growth and successfully flowered in several areas of the Bay. This may represent the development of genetic resistance to the pathogen, or it may be a natural cyclic pattern with reduced virulence of the pathogen. In November 1967, however, milfoil in Middle River collapsed to a greater extent than expected from normal winter kill, and that remaining was in very poor condition.