Question
I live slightly in the hills in Sherman Oaks, CA,  I have 30 rose bushes and 4
climbers.  They have been pruned back already and are starting to get new
leaves.  When is the best time to spray (preferably with a dormant oil spray?)
and is it o.k. to do that even though they've started to get new leaves?

Answer
Dormant oil spray is best for keeping black spot and mildew under control. However it should be done when the roses are dormant and show no growth. Because the dormant oil has sulphur in it, this will burn the new leaves maybe even the mature ones. If you want to spray now, on the package there are two strengths, one for winter and one for summer. Use the summer one as this will be half the strength. I should warn you to do it on a cool day as heat will act on he sulphur and could possibly burn the foliage.
Some rose myths have been destroyed and one of them is about black spot. Those that study this disease have found that black spot like a certain PH level to grow and if you can change that PH level the black spot won't grow. To do this you can use anything that coats the leaf and mamke it less acid or hard to grow on. I like to use a foliar spray that is a fertilizer as it solves two problems. So any liquid fertilizer should coat the leaves  and keep black spot under control. Because you have waited a bit long to do the dormant oil spray, it might be worth to try the foliar spray. As the saying goes, what have you to lose.