Answer: I wish there were an easy solution to black spot. I am guessing you sprayed lime sulfur on your roses after pruning, which would have protected them for a bit, but to really get the upper hand on the disease, you will need to spray at 7-14 day intervals throughout the growing season. As is true with most fungi, the fungus that causes black spot requires free water for infection to occur. The spores must be wet for at least 7 hours before they can germinate. A temperature of 65oF is best for spore germination and the disease develops most rapidly at about 75oF. Temperatures of 85oF and above inhibit the spread of the disease.
The spore producing structures (acervuli) form within two weeks of the initial infection. These structures release spores which are blown or splashed or otherwise carried to new tissues initiating new infections. The fungus survives the winter in fallen leaves and at infection sites on the canes.
As you can see, wet spring weather is perfect for the spores to develop and infect your plants. Picking off infected leaves and pruning out infected canes will not only reduce the disease but also open your rose plants up to better air circulation. Try to keep water off the foliage when you water and plan on spraying with a fungicide every 7-14 days to keep your roses as black spot free as possible.
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