Dave - the rose fertilizer shouldn't have caused a problem if all of the nutrients were in moderation. I would wonder if the weak stems are a result of growth being too rapid if it there's an excessive amount of foliage on the tops of the stems. It's possible that the weed and feed contributed but hard to say. I would keep any fertilizers away from the plants for the rest of the growing season and make sure they have adequate moisture. I do have a few roses in one of my beds and I don't normally fertilize them except that I did do a remineralization project this spring for the entire bed. Your soil type might indicate how often your roses need nutrition. I have silty loam on top of clay so generally there's enough nutrition in the soil. Sandy soil might need more nutrients.
