Whoa! Don't scrub your canes with a wire brush, certainly not green canes. Damaged bark provides entry to fungi. Could she have suggested instead scrubbing the bud union to promote basal breaks (new shoots from the graft)? I don't know whether this works or whether it is folklore.

Whether SulPoMag is beneficial or harmful depends on what nutrients are already in the soil. To find out, you need a soil test, or at least apply the material to only half your roses and see if there is any difference two months later. The other things are fine.

I doubt that diet will have much effect on the blackspot, rust, and lack of summer bloom. The latter is probably caused by heat and drouth.

My mistake. She said the bud union, graft area.

Yes, the theory is that old barky stuff on the bud union discourages new shoots.

I see the dosage of SulPoMag is quite moderate, so it wouldn't be a problem unless magnesium levels in the soil are quite high. Overall it's a reasonable program and she's not telling you to overfeed.

But the business of wirebrushing the bud union -- that's usually done at winter/spring pruning. I wouldn't do it in the summer.

Nor would I necessarily use Sul-Po-Mag.
In much of Southern California, soil conditions are alkaline, and epsom salts are the reverse of helpful.
From personal experience I would using it unless you've had at least a pH test done.
If you're in one of those blessed pockets of more acidic conditions, well and good.

In the height of summer heat, the best thing you could do for your roses is a dose of alfalfa tea, some nice, deep mulch, and regular deep watering.
They're going to bloom less for the next month. And that's -- OK.

Jeri