Some, but not all, of my roses are experiencing a disease or insect problem. I had thought it was a foliage disease but I have been told by two people that it was more likely insects (although I got different opinions on what insect). I can't figure out how to post photos here, but I can email them to anyone who would like to help me.

I had thought it was foliage disease because:

1) Some are affected badly. Some aren't affected at all. And in at least one case, there is a terribly affected rose right next to an unaffected one.

2) Rose 'The Fairy' has a moderate case - and its all around the lower perimeter of the plant, encircling it. So, I thought it was a disease perhaps being spread upwards from the foliage I didn't clean up well enough last year?

3) I can't actually see any insects.

Here's my question , in any case. I was told that modified Cornell formula didn't work on insects. But then why is there horticultural oil in it? Should I go ahead and try it? Does it make a difference what brand hort. oil I use (I have "All Seasons")? And can I use this on hybrid rugosas as I've heard rugosas don't like to be sprayed - but I wasn't sure if that just referred to the chemical sprays.