problem in my rose plants...

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i hav a pot containing three rose plants...i hav been observing since a month or so tht some leaves(mostly lower ones)in each plant dried up turning yellowish brown with dark green spots on thm before thy r shed...the foliage at the top is normal and healthy..flowering also seems normal...i want to knw if there is any problem with any of the rose plants...has it been infected by a bacteria or fungus??...if there is any problem..pls suggest me any remedy to cure this...