How does Dutch elm disease primarily infect the stems?

The primary mode of infection for Dutch elm disease is through elm bark beetles, which act as vectors for the pathogen, specifically the fungus Ophiostoma ulmi. When the beetles feed on the inner bark of infected elm trees, they introduce fungal spores into the tree's vascular system. This process allows the fungus to spread rapidly, blocking the flow of nutrients and water, leading to the characteristic symptoms of the disease.

In contrast, means such as air pollution or waterborne transmission do not adequately explain how the disease spreads, as the pathogen does not thrive in water or air but rather relies on the beetles to bridge the gap between infected and healthy trees. Root contact can also facilitate some diseases, but it is not the primary method for Dutch elm disease, as the fungus is specifically vectored by the beetles rather than spreading through roots.