I can start off by saying that there are no fungicides that are affective in controling cytospora canker. With that said this disease is often associated with growing conditions that affect the health of the tree and result in putting the tree under stress. So that means that things like poor drainage, over watering, under watering, hot, dry, compacted soils, long periods of droughty soils can reduce vigor of the tree and make it easier for the disease to attacka the tree and cause damage. The only thing that is often suggested is to try and releve stresses to the tree. That means timely watering to make sure the soils stay cool and moist, fertilizaton to stimulate new growth, pruning affected portions when the weather is dry and the plant material is dry. And even when all the right things are done many trees still are affected. My only suggestion is to try and remove affected portions, make sure there is an ample mulch ring all the way out to the edge of the limbs, water well, and fertilize to stimulate new growth that might fill in some of the missing portions of the tree .