The best cure for black spot is GROW BLACK SPOT RESISTANT ROSES! Black spot resistant roses are coming out from the rose breeders more and more each year, from: bushs like the Knock Out series, ground covers like the Drift series, floribundas like Poseidon, to even hybrid teas like Pinkerbelle. That's your best bet to grow roses having the most pristine foliage without having to spray them.

May I respectfully ask you to examine the underside of your leaves for spider mites.

I do not see strong evidence of a black spot infestation in your sample leaf. Black spot, it may be, but usually several black, roundish, fuzzy-edged spots of varying sizes are present on a rose's leaf, often turning the leaf a bright yellow before it drops off. There are several strains of black spot, though, each with its own visual appearance. The most common black spot strain's symptom is as I just described. An individual spot being up to about 3/8" diameter, and adjacent spots do grow into each other making for a much larger 'community,' spot to 3/4" across, but usually not circular in its shape, but irregular.

If it is a fungus, looks more like Cercospora leaf spot because the spots are not black, but tan/gray in color, particularly at their centers. However, the difference between the two, cercospora leaf spot and black spot, is a difference without much distinction. Both are harmful rose fungi. Athough in my garden, black spot is more deadly. BS can kill a bush at the worst, and at the least, weaken it until it performs very poorly. Cercospora is a little less deadly.

Moses