Answer: Bougainvilleas can get powdery mildew if there is not enough air circulation and light. Keeping the foliage too damp can also cause it. There's no registered spray for powdery mildew on bougainvillea so I can only suggest that you prune back the affected plant parts. Then you'll need to adjust the growing conditions.
They must have at least six hours of direct sun daily to bloom. If there is less than four hours of direct sun, they may lose their leaves. Direct sun means they must cast their own sharply defined shadow. If they are not getting enough sunshine, supplement with artificial light. They need warm temperatures and adequate air circulation. The humidity should be low and the soil should be allowed to dry out slightly between waterings.
If the problem is spreading to other plants in the room it indicates they are overcrowded, too wet, and the humdity is too high. Give each plant plenty of elbow room and turn on a fan to help circulate the air around the plants. You can use a small table fan (I have one that is only 6 inches in diameter; I turn it on low and it does a great job with my houseplant collection.)
Hope this information helps!
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