Here are some tips for keeping black spot from infecting your roses:
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When planting, give roses plenty of space between plants for good air circulation.
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Don't use overhead irrigation; use a soaker hose or drip system, or water from the bottom.
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Prune infected leaves. Clean fallen leaves off ground to keep them from spreading any diseases.
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Keep interior of plant pruned well so it doesn't get too dense.
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Mulching helps contain disease by providing a barrier between disease spores from old, fallen leaves and new growth. Research from Oregon State University shows that baking soda/horticultural oil spray is not effective against black spot.
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If you decide to spray, try Rose Defense, which is neem oil.
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Next step is fungicidal soap or sulfur spray every seven to 10 days.
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After that, try fungicide spray if necessary. All sprays are more effective if continued on a regular seven-to-10-day schedule as long as weather is wet. Rotate fungicides to keep from breeding resistance.
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If a rose is a black spot magnet, though, the best thing may be to yank and replace it with a healthier cultivar. The Portland Rose Society offers a list of
in our area.
-- Homes & Gardens staff
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