If you have a bad case of black spot, you'll probably need to spray the plants with the sodium bicarbonate mixture once a week for a few weeks, in addition to having treated the soil around the plants with cornmeal. Once the weather gets warmer and dryer, it will work itself out. With all the wet weather we've been having, its no surprise that black spot is a problem for you right now. Can I ask what types/varieties of roses you're growing? There are many varieties, especially modern tea hybrids, that are very suceptible to the disease. It may sound like a drastic suggestion, but if you've continually had problems with the disease, it may be time to get rid of these roses, and plant varieties that are more tolerant. Antique varieties are much more disesase tolerant and easy to take care of.
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