If there are no spots it's probably not black spot. My guess is that the rose is just shedding older leaves due to heat and water stress. Leaves perspire and the rose loses water so it sheds leaves to maintain a better water supply to the parts that are growing. If they are other wise healthy and blooming I wouldn't worry...just water, a lot!

Lower leaves are the oldest leaves and will drop for any number of reasons but in heat and water stress, roses will sacrifice old growth for new. An annual dropping of leaves isn't uncommon as we enter a period of natural dormancy or significant weather changes that impact uptake or availability of nutrients to the plant.

Increasing the water during periods of heat and drought will limit the leaf-drop but also erecting some type of shade structure will limit it substantially as the intensity of the sunlight and heat is reduced.

Limiting or reducing fertilizer applications during extremely hot weather will curtail some new growth and not stress the bush as much.

Keep in mind that when leaves turn yellow and fall in a forest, we call it Autumn and expect the trees to go dormant as winter moves in. Some roses react the same way as they sense changes in humidity and temperature. In this area, we do a light pruning to remove summer growth and encourage new growth and fall blooms then do a hard-pruning in January. At that time, we remove all leaves anyway.

I believe blackspot is a tough disease for arid climates to have, especially when the average daily temperatures are over 90F and humidity less than 15%. Phoenix even saw a couple days where humidity was less than 0% this summer.

I agree with the others that it is probably aging leaves turning yellow and dropping off, but just to be sure, check and see how wet the soil is around those roses--like stick you finger in 4 inches and see if it is wet that deep down. If it is, you may be losing leaves from too much water. Perhaps you are overcompensating, given the drought and water problems, and now are over-watering. But if the soil is not wet, don't worry about it. Probably just an aging problem.

Kate

The stem even gets yellow at the point of connection to the cane.

That would be a concern, if the stems are turning yellow then I would wonder if the plant is either going dormant for the winter or you are having problems below the surface. It could be spider mites since you said it has been very dry. Are you seeing any webbing near the yellow leaves?

I definitely now think it's a combination of our intense heat this past summer and the lack of rain. Our city water tends to contain a lot of calcium and relying solely on it is difficult. With even just a 1/2 inch of rain you can notice a distinct difference in the landscape. I'm going to try using soaker hoses weekly to do a deep watering and see if that helps.