Answer: The rust on barberry is caused by the fungus Puccinia graminis, which also affects wheat, but not roses or apples. The bacterial leaf spot on barberry is caused by Pseudomonas berberidis. Black spot on roses is caused by the fungus Diplocarpon rosae. Rust on apple tree leaves is Gymnosporangium and an alternate host for this fungi is the common cedar tree. I can find no reference to the transfer of rusts and leaf spots on barberry to any other plant. Blackspot is a common malady of roses. You can help prevent the development of the disease by pruning to increase air circulation around and within the plant, and by picking off any leaves as soon as you see spots beginning to develop. Try, also, to keep the leaves dry when you water the plants - to avoid splashing any disease-causing organisms from leaf to leaf. There are some fungicides on the market that may help protect the leaves of your plants if you have a serious problem with black spot or rust. As with all chemicals, carefully read the label and apply according to directions.
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