My first knockout rose has been in the ground since last Wed. and has blackspot already. What is the best treatment? I thought they were foolproof!

You might want to try feeding and protecting your roses weekly with a diluted foliar tea application mixed with aerated compost tea and/or fish emulsion mixed with garlic/hot pepper teas.

Powdered sulfur is also good as a fungicide.
I like to use corn meal products mixed in my compost around the plants as a soil food and soil fungicide.

Thanks for the advice. I have all the products mentioned on hand. I am a rookie at these roses.

How are they being watered? Try not to let a sprinkler system spray water all over the leaves. Roses are best if watered at the roots.

Thanks. The rose is in a new perennial garden. I intend to use a soaker hose for occasional watering, as I tried to put in plants that are drought tolerant.

this is interesting,
i logged on tonight to ask about knockout roses myself: my older knockouts (three years old) are beginning to have yellow leaves on the older growth, and reduced new growth (they used to produce new growth all over, now it is just one or two shoots from the base)
I have also noticed a general 'yellowing' of other plants, not limited to older growth: daylillies, rosemary, carolina jasmine...seemingly unrelated.
Any ideas to turn things around?
thanks, mary

Just because it's drought tolerant doesn't mean they can never have a fungal disease. Make sure you're watering it at the roots and incorporate a couple of hand fulls of corn meal around the base of each plant. Spray weekly with Garrett Juice or something similar.

Thanks for the treatment CaptainCompostAL. Thanks for giving your valuable replay on Knockout Roses....

Regards,
Bhuvana