Your arborist is correct in saying that there is no cure for cytospora canker. This fungal disease (Cytospora kunzei) is most likely to attack Norway or Colorado blue spruce under stress-growing on an unfavorable site, stressed from drought, overcrowded, or otherwise in decline. Symptoms of cytospora canker include browning needles on one or more branches. A canker develops on the stem. The canker oozes amber-color pitch that dries to white. There is no fungicide labeled for the control or prevention of cytospora. Your best bet is to keep the tree in good health. Fertilize the tree if growth is poor. Water during dry periods. Control insect pests that may infest the tree. And avoid injury to the trunk and branches. If the disease gains a foothold, prune out dead and dying branches several inches beyond the cankered area. Disinfect pruning shears between cuts with 70 percent rubbing alcohol or a 10 percent chlorine bleach solution.