If the white is like a saw bug attached on the stem you can swab alcohol if the problem is First off, when it is cooler you will get fungus or if you water at night. The best time to water the plants  would early morning but if you can not water at that time you can water late afternoon. Mother Nature has to do her part and dry the water or else you have fungus and fungus also  attracts bugs. Now the white stuff can be quite a lot of things but, let’s see what it could be. If it is like a white powder that is  nothing major it just looks unsightly. That is usually caused my night watering . If you have the mildew on the leaves there is really nothing to worry about unless you have alot. However, if you have fungus the fungus may spread to another plant that you have so, you should take care of it before that happens. If the white is like a saw bug attached on the stem you can swab alcohol if the problem is not severe. If the problem is severe i rec commend malathion. Aphids are not white but they attack roses too and you can also use malathion on aphids. Another thing that helps is if your rose is regularly fed it is less likely to be infested with disease and bugs