Question
Last year my 2 year old double knockout roses succombed to the perfect storm of an extremely wet/humid spring, close proximty to an evergreen (Sitka Spruce), and Black spot.  The bushes  developed black spot, and dropped some leaves at the bottom of the shrubs.  I attempted to use a Bayer(tm) systemic gel to treat them at the roots, and had limited success.  I have just pruned them to about 18 inches and noticed some powdery mildew and black spots on the canes.  I trimmed the worst canes to 6 inches (below any black or white spots)and removed the dead leaves and old mulch on Feb. 28.  My question is : What is the best way to prevent these diseases, cut the canes severely, or treat with a fungicide like Immunox(tm)?

Answer
Because Black spot is caused by fungal spores, it is impossible to prevent it as the spores float around in the air. When conditions are right for this disease, the spores are knocked to the ground by either wind or rain. If they land on a rose leaf they then start their life cycle. If there was only one type of fungus,  a gardener might be able to control it but in some areas there can be  different varieties of spores. So what keeps one type under control one year may not the next. Also certain rose can be prone to different strains of infection. When the spores are knocked to the ground, not only do they sit on the rose leaves and canes, they also sit on the soil. Then they can be transferred to the rose canes by watering, touching the rose after weeding etc the list goes on. That is why a mulch around the base of the rose bush, such as wood chips, will help make the area unattractive for the spores to start.
As you can see controlling black spot is not easy, especially if you live in an area that has the ideal conditions for it to start.
Therefore cutting the rose canes back will not help as you only see black spot when it has become a black mould. By that time it has released many more spores on the rose.  Your Knock Out rose has what they call good resistance to this disease. What that means is the rose can drop 60% of it's leaves and still grow well. It doesn't mean it won't get the disease. You have done all what is necessary to help control black spot but if you still have problems with the rose then Immunox is one of the better fungicides on the market and should give you good protection.
Also if you are getting mildew then the rose is not getting one or two things. One, not enough sunlight or two, not enough air circulation around the plant when mildew conditions are perfect such as your spring weather.  If a rose gets dry roots for a long time this will also cause mildew. Perhaps the rose is too close to the tree. If you are in an area that is prone to fungal problems, then giving a rose more sunlight is a good idea. Although roses only really need 5 hours of full sun, more in an area prone to black spot will help.Try spraying using 1 tablespoon baking soda to 2 quarts of water daily until the mildew has gone. If that doesn't help then you will have to resort to a chemical spray for rose mildew and all work well.