Researchers studied a weak strain of gray mold fungus found in Spanish vineyards to understand why it cannot infect plants when exposed to light. Using genetic analysis, they discovered that the weakness is caused by a mutation in a single gene called bcltf1, which normally helps the fungus sense light and decide when to grow or reproduce. By restoring this gene in mutant strains, scientists confirmed its importance for fungal virulence and light responses, providing insights that could eventually help develop better disease control strategies.