Copper sprays against downy mildew have limited protection because rainfall is limited to what amount?

• A. 20 mm of rain
• B. 50 mm of rain
• C. 100 mm of rain
• D. 5 mm of rain

Answer: (A)

Copper sprays act by leaving a protective copper residue on the leaf surface, which blocks infection by downy mildew. This residue is a surface film, so it’s easily washed away by rain. Once rainfall reaches about 20 mm, the protective film is largely removed, and the spray loses most of its effectiveness. That’s why copper sprays have limited protection in wet conditions—the residual protection only persists when rain is minimal. After rain of this magnitude, reapplication is usually needed to restore protection.