What is Dutch Elm Disease primarily spread by?

Dutch Elm Disease is primarily spread by elm bark beetles, which are key vectors in the transmission of the pathogenic fungus responsible for the disease. When these beetles feed on the sap of infected elm trees, they pick up fungal spores on their mouthparts and body. As they move to healthy trees to feed, they inadvertently deposit the spores into the plant's vascular system, leading to infection. This biological process significantly contributes to the rapid spread of the disease within and between elm populations.

The other options do not play a direct role in the disease's transmission. Wind may disperse fungal spores over short distances, but it is not the primary means of spread. Water does not contribute to the disease's transmission in any significant manner. While human activity can influence the spread by facilitating the movement of infected materials, it is the elm bark beetles that are the most direct and effective means of transmitting the pathogen itself.