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Black spot is caused by the fungus Diplocarpon rosae - and it only affects roses.The fungus produces spores. These overwinter on infected plant debris. Black spot develops when plant surfaces are moist, humidity is high and temperatures are in the upper 60s to lower 70s (F degrees) for several days. These combination of factors often begin in early June (earlier in some regions).
Black spot is spread when water splashes the fungus from infected debris onto the lower leaves of the plant. As new generations develop, it works its way to higher leaves (new spores develop in 11 days).
June is the time to start watching for small black spots. Several fungicide sprays and dusts are available, but need to be applied every seven to 10 days all summer (twice a week during wet, humid periods).
A more practical approach may be to select resistant rose varieties (such those from Jackson & Perkins).
Tips to help prevent Blackspot on your roses.