At the first sign of blackspot or mildew, it is recommended
gardeners spray their roses once a week for 3 weeks with a solution
of 1 tablespoon baking soda and 1 teaspoon of vegetable oil (or
1/4 teaspoon of horticultural oil,
from the garden store) to 1
gallon of water. Also, remove infected leaves, and especially clean
up the leaves that fall to the ground, as the cycle starts when the spores
are on the ground, and are splashed up onto the plant
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