Certain mineral and nutrient deficiencies can seriously affect the roses in your garden. Soil with high alkaline content, as are most desert soils, can prevent the plants from absorbing iron, and will result in iron chlorosis.
Other deficiencies include lack of nitrogen and phosphorous. Application of proper fertilizer can correct those problems.
There are several bacterial, viral and fungal infections that exhibit symptoms similar to mineral deficiencies.
There are subtle differences, however between nutrient deficiency and plant infection. The article, Fertilize Desert Garden Roses, describes how you can spot mineral and nutrient deficiencies and how to fix the problem.