Blackspot of roses is a fungus disease that can cause leaves to yellow and eventually defoliate. The first signs of blackspot, usually on lower leaves working its way up the plant, are leaves with dark, roughly circular spots with uneven edges. The most severe cases of blackspot occur when the relative humidity is above 85% with temperatures above 75 degrees and the foliage is wet for an extended period of time.
If you have roses that are naturally susceptible to blackspot, plan to undertake a regular spray program (check with your local nurseryman or garden center for the most recent control options.)
Or, plant roses that are resistant to blackspot. For a list of these go to: http://www.ppdl.purdue.edu/ppdl/weeklypics/3-22-04.html. There are also several cultural practices that will help alleviate the occurrence and spread of blackspot. These include the following: 1) Plant roses in sunny locations that have good air movement which limits the time the foliage is wet. 2) Avoid overhead irrigation that wets the foliage as this fungus is spread primarily by splashing water. If you do need to use a sprinkler do it early in the day so that the foliage will dry as quickly as possible with the morning sun. 3) Remove diseased leaves that have fallen to the ground to limit the source and subsequent spread of ongoing disease.
Photos courtesy of Clemson University and Yahoo.