Master Gardener: Diagnosing black spots on roses

Q: I live on the peninsula in OC.  My rose bushes’ leaves have developed black spots. My soil is very sandy.  Could a lack of nutrients in the soil cause this problem?  
A: Black spot on roses is caused by a fungus (Diplocarpon rosae). If left uncontrolled, it can weaken the plant and make it susceptible to other diseases.
Black spot can be prevented by early morning watering and taking care not to wet the leaves. Rake up fallen leaves and prune away diseased foliage. D