Unless you have good airflow i wouldnt even spray the leaves. You complete the disease triangle for downy mildew. The high humidity with high temps and moisture on the leaves, and low air flow it is mildew heaven. If you have good airflow the mildew cannot live there because it is an unsuitable environment, if that is the case spray away. i dont think time of day matters, plants are able to deal with rain, fog, dew, at any point in the day, they are adapted to it. However spraying in the morning may help reduce the chance of downey mildew.
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What I say comes from my knowledge of growing horticultural plants. I know nothing about growing marijuana nor have I ever propagated it.
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