Answer: In my experience, a larger sized chip mulch should be fine in general for roses in terms of keeping the soil moist and cool and slowing weeds. However, a smaller size of mulch particle will break down faster and help feed the soil sooner.
The large chips are very attractive to slugs, as you noticed, although any mulch can harbor them. Slugs seem to be worse in a damp year, but they can be controlled in a number of ways. They can be handpicked at night and dropped into soapy water, trapped with commercial traps or saucers of stale beer, or killed with commercial baits. Be very careful to read and follow the label instructions if you use baits, especially if you have children or pets. In my experience it also helps to disturb the top inch or so of soil as well as the mulch layer in early spring so that the emerging slugs are exposed to the elements.
Black spot is a fungal infection and spores do overwinter on the soil surface and splash back up onto the plants the next year. Some gardeners will replace the mulch each winter or will simply add a new surface layer on top of the old to cover the spores. Careful attention to fall clean up in the rose bed as well as careful disposal of all prunings and removal of any overwintering foliage can also help. Black spot is difficult to control and unfortunately, some varieties are simply more prone to it than others. You may want to look into some of the resistant varieties. Also be sure your roses have full sun and good air circulation, are properly pruned and spaced, and avoid wetting the foliage especially at night.
Rose midge is truly a pest to reckon with. Rose midges are small, nearly invisible insects that attack new growth, primarily flower buds but also the hips. If the first generation makes it, you'll have problems all summer long. Keep a close watch all season and cut off and destroy any infested buds or hips immediately. Insecticidal soap may also be effective or can be used as a safeguard measure. You might also apply beneficial nematodes (such as Grub Guard from Gardener's Supply Co. http://www.gardeners.com; ph# 800/863-1700) to the soil around the base of your roses. They'll feast on the pupae of any surviving midges. If there are wild roses nearby that harbor a reservoir population of midges, cut them down if you can or apply nematodes to the soil around them, as well. Good luck with your roses!
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