When the lower leaves on rose bushes turn yellow and drop off, that's not necessarily cause for alarm. This is perfectly normal for most varieties of rose bushes, and happens when older leaves outlive their usefulness. However, this natural process should only occur with those leaves that are closest to the ground. If leaves at the top part of the bush's lower section show similar symptoms before the leaves below them have fallen off, that could indicate a magnesium deficiency.
An iron deficiency is the most likely cause of yellowing upper leaves on a rose bush. It isn't normal for rose bushes to lose their upper leaves because those leaves are newer and still of some use to the plant. If you notice the upper leaves of the bush turning a light green while the veins are remaining dark green, you can take action to prevent them from turning yellow. Treat the plant with iron chelate, which you can purchase at nurseries and garden centers.
Uneven yellowing of leaves that includes dark spotting generally means that the rose bush is infected with a fungus called blackspot. Blackspot is activated by very wet conditions; as the fungus spreads on the plant, it creates large, dark circles ringed with yellow areas on the leaves. The leaves eventually turn yellowish brown and drop off, killing the plant.
If a rose bush is infected with blackspot fungus, it's best to prune away all infected areas as soon as possible. If the leaves are turning yellow and have red spots, this generally points to a nitrogen deficiency. Re-balance the soil by adding nitrogen-rich fertilizer or a gardening compound designed to correct the issue.
Sometimes, the veins of rose bush leaves will turn yellow. Often, the leaves will then become brown and dry and drop off the plant. Oxygen deficiency causes this in rose bushes, and is often present in plants that have poor drainage and are constantly sitting in soggy soil. To save a rose bush that has oxygen deficiency, you'll most likely have to raise the bed, and loosen and aerate any compacted soil.