Answer: It's not a good idea to mix fungicides and insecticides together. There might be a chemical reaction between the compounds, or the combination could be phytotoxic (react with the sun to burn leaf surfaces). Fungicides can be used to help prevent black spot of roses if sprayed during the dormant season prior to leaf emergence. If fungal diseases develop during the season you can usually stop the spread of the disease by picking off the leaves that develop spots. Insecticidal soaps are effective against soft-bodied insects such as aphids, but these insects are usually found on the tender new growth, not on older, diseased leaves.
So, the bottom line is: To avoid potential problems, use either if necessary, but not at the same time, and not mixed into a homemade brew.
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