Question
I have French perfume, Golden zest (hybrid of Golden Celebration) and a generic rose (camerlot hybrid tea).  French perfume and especially Golden Zest have new buds much smaller than before.  New leaves are less than half the size of the older ones with powerdry mildews.  Camerlot grows very slow.  As I cut the stem, there's almost no new growth for a month.  I also notices there's so many flies living between the mulch and the soil.  All these plants are in the pot it came in, between 2 and 5 gallons.  I've given organic fertilizer about a month ago. I wonder whether it's insufficient sunlight, fertilizer has leached out, or something else.  

Answer
who decided to plant them pot and all? planting them w/pots on is probably stunting their growth and causing the roots to rot. take them out of the pots & replant them w/out pots. the roots need to stretch out into the fertile soil.  roses do best in full sun.  don't get water on the foliage when you water them. this normally is what causes powdery mildew.
rosecare depends on where you live in this large world and what climate zone you are in. i'm in southern NJ, USA.
before you do anything else visit a local garden center/nursery and buy a basic rose book(ortho, sunset, etc.). quiz them on rose care in your area. the book will tell you how to care for them(plant, prune, fertilize, water, treat for pest, etc.) so you can be a successful rose grower.
good luck
rick
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