Dutch elm disease moves from diseased to healthy trees by which mechanisms?

• A. Elm bark beetle feeding only
• B. Elm bark beetle feeding and through natural root grafts
• C. Wind-dispersed fungal spores only
• D. Soil movement of pathogens only

Answer: (B)

Dutch elm disease spreads between trees mainly through two pathways: elm bark beetles carrying fungal spores as they feed on and move through trees, inoculating healthy trees in the process; and natural root grafts that connect neighboring elm roots, allowing the fungus to move directly from an infected tree to a healthy one. So, beetle-mediated transmission plus root graft spread together account for how the disease moves. Wind-dispersed spores or soil movement aren’t the primary routes between trees, which is why those options don’t capture the real mechanism.