what to use on roses with persistant black spot. using insecticidal oil now
I should have added that if there is any question whether the disease is black spot or another leaf disease, take a sample to your county extension office. Find your county's office here: https://pickyourown.org/countyextensionagentoffices.htm
Black spot on roses typifies the saying "an ounce of prevention is worth a pound of cure". It is easier to prevent black spot than treat it. Preventive spraying begins soon after plants leaf out. Spraying continues every 7-10 days especially during the critical spring period. (Black spot is less active in hot weather.) Don't water your plants with lawn sprinklers or overhead sprays. Every fall, pick up all fallen leaves so the disease doesn't start all over in spring. Fungus spores easily live through winter on leaves, even in cold climate areas. Horticultural oil does not treat fungal infections. Switch to a product that says on the label details (research online) that it is for black spot. Follow instructions obsessively! Prune the rose vigorously in early spring to maintain an open, airy branch structure. Air and sunlight need to penetrate the entire bush to reduce the likelihood of disease. Two articles follow. The first one covers treating black spot including fungicide choices. The second one is from LA agCenter - your extension service. It covers all things rose related.