Roses planted in the ground need to be fed three times a year for absolute maximum results. (September,December then February) 150gms of a good organic based feed. The best fertilizers are the 4 in 1 mixes which consist of chicken manure, seaweed extract, fish meal and blood&bone. There are lots of companies making these all in 1 mixes now. Which is great so no more adding liquid fertilsers and other so called wonder feeds. Feeding consistently with these quality organic based fertilizers will always ensure top healthy gardens. It creates stronger growth to all your roses - organically.
Therefore they will get less fungal problems such as black-spot during the humid months of Spring. Even if they did get black-spot - if you have good growth from all the lovely organic feed, the roses will grow through it and repair themselves much quicker.
Cow manure will be okay as a feed but using it as a mulch with the 4 in 1 mix as the main feed works together beautifully. With this cocktail - you watch those worms go crazy even in the poorest of soil conditions. (We do not recommend fresh horse manure as it throws to much weed.) We highly recommend mulching but only with established roses as newly planted roses need at least 6 months of settlement before they get the works thrown on top. It can be to much for freshly planted bare rooted roses so go easy in the first year. Don't dig fertilizer in either. Just spread over the top and let nature work it in.