when and how short do you cut rose bushes in the fall?
Also, can I spray for black spot?
Since your roses will be dropping leaves shortly, it doesn't make sense to treat them for black spot at this time. Make sure to clean up all leaves after leaf fall and destroy them. In black spot has been a problem for several years in a row, consider replacing mulch and top one inch of soil early next spring. Also, put your plants on a mildew preventative next spring. It is easier to prevent than cure rose black spot. Prune annually in spring just as leaf buds are starting to swell on the canes. This often coincides with the forsythia blooming. Proper pruning leaves a basic framework but removes many smaller canes. This allows better air circulation and sunlight penetration which, in turn, help prevent black spot. Typical rose bushes are not pruned/cut back in fall. Tall grandiflora roses that receive lots of winter wind should be cut back about 50% to keep canes from whipping around in the wind and rocking the plant base out of the ground. (If your area has high winter winds.) Hybrid tea roses probably need winter protection by mounding shredded bark 4-6 inches over the top of the crown. I've provided links several articles on pruning, winterizing and black spot disease control.