One of the interesting things that you can observe in the garden is a colorful disease that affects roses called rose mosaic. It is caused by viruses that infect roses.
There a number of "mosaic" diseases caused by viruses that affect plants. In general, they cause parts ot the leaf to turn color—usually yellow. To someone with an artful eye, the pattern can sometimes resemble a tile mosaic, but this is highly variable. (See some pics on Google here.)
In roses the symptom is usually angular, splotchy yellowing on the leaves, but sometimes it can take on a pale yellow band on a leaf that zig-zags around. It's not so much the shape of the symptom that alerts you to its presence, but more that in spring the whole plant will have a similar yellow pattern on most of the leaves of the same age and sun exposure.
In roses there are 2 primary viruses that cause mosaic: Rose Necrotic Ringspot Virus and Apple Mosaic Virus. In addition there are many unknown causes and at least another virus that can cause these symptoms. Often there are multiple causes.
Most grafted roses in the Eastern Sierra that are about 20 years or older show mosaic symptoms. I usually see symptoms in spring, but once the weather warms up, the mosaic pattern vanishes. April and May seem to be most pronounced in effect.
Why do we have this so commonly here? This is a disease that is usually spread through propagation. I suspect most of our roses ultimately came from 1 or 2 wholesale nurseries that had the virus in their stock plants used to bud or graft onto the ubiquitous Dr. Huey rootstock. Today this virus is less common in the trade. Nurseries have means to remove virus from their stock plants through tissue culture, if they so choose.
The virus doesn't do much harm in the landscape other than making early season leaves look weird. It may also reduce vigor and stem length. I don't lose much sleep about it, and there really aren't any control measures if I did! If mosaic bothers you, the best course of action is to remove the plant and start anew.
For more information: The UC IPM program has a page on rose diseases. This old article (1977) from California Agriculture also has information and some pictures about rose viruses. The Marin county Master Gardeners also have some information on mosaic diseases in general in the garden.