This seems to spread from one leaf to another. It started on my Datura and and is now on my pepper plants too. Is there an insect causing this or something else. I don't see anything.

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.

Can you post a photo of the plant and tell us more about your water, fertilizing, and pest management regimen? Based on the information so far the problem could be any number of things. There several different disease possibilities each with their own cause and treatment but the odds favor other causes before jumping to the "must be a disease" conclusion. Dave

I'm thinking underwatering for the pots is the culprit, although overwatering can also cause the yellow leaf symptoms, though usually not so quickly here. Agree with Mad, going with heat stress & find it nearly impossible to overwater a potted rose in active growth as long as the pot size is adequate, the potting mix good & drainage holes functioning. Like Valrose, potted roses here are watered at least once a day & often twice when the day temps get to 80 & above. I water each one & then go back & water each again to make sure they're thoroughly watered each time. If we get a soaking rain, I skip the watering. Hot days after cooler ones increase water needs here. And it's not only the warmer temps, but the greater intensity of sunlight every year that affects plants whether potted or planted. In the last few years, plants that were formerly happy in part sun part shade conditions have needed more shade as well, which I also chalk up to the intensity cranking up. With such frequent watering of pots, I subscribe to the "weakly weekly" fertilizing routine, using Neptune's Harvest liquid at half rates for larger plants & quarter rates for younger ones, on a second watering of the day. A good bit of rain usually falls here year round, with cloudbursts & storms frequent over the growing season. Though the land is clay based loam, the roses were never more lush & happy than a recent Summer when storms came by daily. Agree with Mad here again, in those circumstances, even with the soil saturated to puddles, no roses (or other plants) seemed to mind at all. What we see as red pigments in rose leaves, such as apparent in new leaves, actually protect the leaf by dissipating heat. Mildew can be brought on by underwatering in heat here, but it's opportunistic, does't bleach leaves & treated by washing the leaves with water wand & making sure the roots have adequate water. Occasionally see older leaves bleach out quickly right ahead of or at the same time as a growth spurt occurs. Just some random thoughts over a late lunch break & likely not well organized, but hopefully food for thought. Are your pots protected from heating up in the sun? I'd water the pots a good bit more, the planted out roses too, and start them on the Neptune's once satisfied their water needs were met daily. The Neptune's also great as a foliar spray applied in early evening when the temp's below 75 & leaf stoma open to absorb it completely. Sue