That's pretty simple, Yvette. Soil in pots heats up much faster and much hotter than the ground. Soil in pots experiences water stress much more quickly, more severely than the ground. Rose roots weren't "made" for hot, dry conditions. Even on a triple digit day, the soil just a few inches under ground level is likely not much above seventy degrees, if that, and it is often damper than the average pot. Heat up the root zone of a potted plant of any kind and you accelerate the plant growth until it reaches a stressed state. Add reduced water availability and you have a plant whose immune system is not functioning as fully as it should. You can force almost any rose to mildew and rust, probably black spot, by stressing it for water. It can occur with only a few such stress situations. If you were to plant the offending Malmaisons (and any other rose in its position) in the ground and treat it like the others you have planted and they should respond the same way the others in the ground are responding.

I never believed the statements made by Jack Harkness and Edward LeGrice, two well respected, prolific British rose breeders, that water stress can force roses to mildew even in high, dry heat, until I DID it. I can force several roses in my garden to rust by keeping them too dry. I did it repeatedly to a potted R. Arkansana just to prove it to myself. If otherwise resistant varieties can be forced to mildew and rust, even when conditions don't support the infections, why not black spot? They're functions of conditions affecting the immune system of the plant.

I'm not surprised one of your plants is expressing the desire to climb. It could have been an accident in propagation, grabbing a climbing cutting instead of the bush, or it could have mutated while in your possession. It isn't uncommon for either to happen, particularly when someone who may not be that familiar with the variety is sent out to collect material for propagation. Kim

It probably doesn't help that the pot is huge and painted blue as that color would increase the temperature of the pot.

First I will water them more and if that doesn't work I'll move them into a bed.

Yvette

That depends upon what shade of blue. If it's light, it should reflect more of the heat than a darker shade would and could actually reduce some of the stress a bit. It's what the pot is made of that counts more. A "huge" pot would suffer less heat extremes to the center of the pot. Large soil masses insulate the roots better than smaller ones. You can more easily kill a plant in a six inch pot than you can the same plant in a twenty-four inch one. The larger the soil ball, the longer it will hold water and the greater the insulation value to the roots as a whole.

But, the material the pot is made from makes an extreme difference. Ideally, if you live where there is extreme sun, extreme heat, extreme aridity, they should be made from materials which do no absorb high levels of heat; do not retain the absorbed heat for long periods of time and don't efficiently radiate that heat to the pot interior. Plant roots grow outward and eventually wrap themselves around the interior of the pot. When the sides of the pot get really hot, remain really hot for long periods of time and efficiently transmit that heat to the soil/roots inside, the plant gets quite stressed, quite quickly and can easily literally be cooked to death.

Terra cotta (clay) and glazed ceramic are cooking utensils. You can buy terra cotta bake ware and Corning Ware is "glazed ceramic". Both are tremendously efficient at cooking food well. Those materials are also tremendously efficient at cooking plant roots. There is little difference between a Corning Ware casserole dish sitting on an electric or gas burner (or inside an oven) and a glazed ceramic pot with the sun shining directly on its surface or sitting on a hot concrete patio.

If you live where it is generally cooler and you want to push plants and make fruit sweeter, use those kinds of planters/pots and put them on masonry surfaces so you can increase the soil temps. If you live where those conditions are extreme, you need to think of the container more as an ice chest...something which will protect the roots against the extreme heat sources. Foam, fiberglass, concrete, wood, even plastic nursery cans can actually be safer to use than the more desirable types so often selected due to their "beauty". Black plastic does absorb heat, but it doesn't hold it nearly as long as clay or ceramic do, so the soil and pot begin to cool as soon as the direct sun moves from its surface. If the pots are actually shaded against any direct sun, they don't heat much at all compared to clay and ceramic. If you use lighter colored plastic pots, they don't heat nearly as much as the black plastic nursery cans, even when the sun shines directly on their surfaces.

If you want to continue using those blue painted pots for the roses, you can literally insulate their interiors using bubble wrap or even thin sheets of Styrofoam. Simply line them with those materials as you would insulate the ceilings of your house with fiberglass to create an "ice chest" effect. It will reduce the actual soil/root area, but it will also significantly reduce the heat transferred to the soil/root ball. Your roses should perform better than they have in uninsulated pots. But, I would still expect them to do even better in the ground than potted. Kim

I had no idea that clay and ceramic pots can cook the roots of plants. I learn something new every time I visit this forum. Thank you for educating me :)

The pot is either clay or ceramic so at this point I'm not sure what I'm going to do.

You could put it in a plastic pot and then put that plastic pot inside the nice ceramic one. Possibly with the insulation that roseseek mentioned.

If you use the method of a plastic pot inside the ceramic, definitely put insulation between them if you live in a high heat, high sun climate. Kim