This is either a minor leaf spot infection or environmental stress; it's hard to tell from the symptoms, since there can be lots of overlap when leaves develop a purplish or reddish splotching. Usually environmental stress this time of year revolves around heat and/or soil moisture (too damp or too dry). The withering stem visible on the right side may be dying back, though, and should be pruned off. We can't tell why it is withering, but make sure the mulch around the base of the plant is not smothering the crown (where stems emerge from the roots)...pull it back a few inches around the crown if it's currently touching the stems. This improvement in airflow around the crown will reduce the risk of a fungal outbreak like Southern Blight.

If the leaf spotting is due to a fungal or bacterial infection, there is no cure, as fungicides cannot reverse existing infections and don't often treat bacterial pathogens. In weather this dry, though, generally the only leaf spotting we've seen is on plants that are being irrigated often, so unless you are watering with a sprinkler regularly that gets the leaves wet (as opposed to only watering the soil in the root zone), a severe leaf infection is not likely.

Miri