Answer: Milky spore disease infects the grubs while they are in the soil so it will not stop adult beetles which fly in. It needs to be applied over a large area (contiguous acres) and be allowed a number of years to take full effect. Once applied, it remains effective for years. For a more immediate result on your roses (a Japanese beetle favorite) you might consider using Neem applied according to the label instructions.
Yellowing leaves can be caused by lots of things. Heat stress, water stress (either too much or too little), lack of fertilizer, diseases and insect feeding. There are several sprays on the market especially for roses but you'll need to figure out whether the problem is cultural, a disease, or caused by insect feeding.
Best wishes with your roses!
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