Which statement describes cross resistance?

• A. Two Or More Herbicides Share The Same Site Of Action
• B. Two Herbicides Have Different Sites Of Action
• C. A Single Herbicide Is Effective Against All Weeds
• D. Resistance Is Present Only In One Weed Species

Answer: (A)

Cross-resistance happens when a weed population that has become resistant to one herbicide also shows resistance to other herbicides that act on the same target or share the same resistance mechanism. The key idea is that the resistance mechanism affects a single site or process, so multiple herbicides that rely on that same site become less effective.

Since two herbicides share the same site of action, a single resistance mechanism can reduce their efficacy, which is exactly what cross-resistance describes. If herbicides act at different sites of action, resistance to one does not automatically give resistance to the other unless the weed has developed multiple-resistance traits. The other statements don’t describe this situation: one herbicide effective on all weeds isn’t about resistance, and resistance limited to one weed species doesn’t capture the idea of resistance extending across herbicides with a shared target.