I live in Lake Alfred and have the same problem. Some of the yellow can be from to much rain and this is probably black spot because the roses are not drying off by nightime and if you know roses you should never water later then in the morning. Another reason is because they are growing new leaves and others naturally die off. Hope this helps!

There are many causes for yellow leaves. Heat stress, water stress, salt stress, phytotoxicity, nutrient deficiencies, nutrient toxicity, disease and insects all cause yellow leaves.

You mentioned afternoon rains. Is there good drainage? Water displaces air in the soil. With the high summer heat, the roots need to support rapid transpiration right now. If they are too wet, that won't happen and leaves will yellow. Other symptoms...are your blooms 'balling'? Do the center of your canes appear soft and brown?

Another common cause is phytotoxicity, or 'soffocation' caused by spraying. When chemicals clog leaf pores, plant tissues cannot transpire moisture and gases quick enough. They cease producing chlorophyll. The cell walls deteriorate and the leaves turn yellow and fall off.