Can you take a large diameter length of pipe and do some "core samples"? Can you take strong, smooth stakes and drive them into the soil several places in each plant's root zone? Pull them out and touch them, smell them, see if they appear dry, damp or smell "soured" like a pot that's stopped draining. Until you can see what's happening under ground, you can't know the answer to that question. What you're seeing COULD be the beginning of black spot (yes, Knock Out CAN black spot). Or, it could be a water issue. How does the new growth look? Either under watering or over watering would affect the new, softer, sappier growth first. If it appears shriveled, it could be either. Too little water wouldn't support the new, soft growth tips and they would stop, either not forming or shriveling. Too much water, depending upon the drainage and water retention of your soil, could lead to root rot where the new growth would also stop, but appear like limp, wilted celery or lettuce that's been left too long in the crisper.

If it's black spot or some other fungal attack, the new growth COULD still be looking decent, normal, what you expect it to look like. Extreme heat can cause foliage drop, too. As the plant experiences water stress, usually new growth halts and as it worsens, the plant reabsorbs the nutrients from the older foliage first, often causing them to take on "autumn foliage" type colors depending upon the genetics of the rose. To reduce the transpiration of water (water loss through sweating), the foliage begins to be shed. The logical conclusion is that all foliage is shed, leaving a bare plant which appears to just be sitting there.

Once you can SEE and smell core samples (or the soil dug from areas in the root zone of the plants, but that disturbs more roots making water stress worse), or strong stakes shoved several places in the root zone of the plants, you won't be able to know if it seems too dry, too wet, smells "soured" from too much standing water too long. Determine what the soil conditions are below ground where the roots are and you have a better chance of figuring out what's going on with the plants. Kim

One thing you can do is measure the amount of water you are applying by counting seconds to fill a container of known size. 6 gal/week/sq. yd. would be enough for the most extreme conditions in NC in the absence of rain. Then, say 5 gal. for normal summer weather and 4 gal. for spring and fall. If you have sandy soil, divide the amount into twice-weekly or thrice-weekly doses, and apply a little extra. If you have clay, build a dike to allow water to sink into the root zone rather than running off.

Kim's advice is always good. Do explore the soil condition, but I know from experience that it will be OK if I apply the appropriate quantity

I don't see any disease symptoms on the leaf, so it could just be old, worn-out leaves or a reaction to the heat. Knock Outs in NC can get a small amount of black spot or cercospora spot, but I've never seen it serious enough to need treatment. Sometimes leaves just die.

Thank you, Kim & michaelg.

Neither, but it depends on how you want to define the term, "heat stress." If you define heat stress as ultimately, a condition caused by a lack of water, then yes - one might say it's under-watering.

However, I have trees in my yard that are dropping leaves that look exactly like that. I also have about 3 roses that are very affected - one almost half defoliated, when it's normally one of the healthiest, most vigorous that I have.

Almost every rose I have is exhibiting the same appearance of your leaves at the juncture between a pruning joint and the new stem growth. These leaves are the ones that suffer the most stress this time of year. Existing right at the junction point of the prune, they seem to have little substance with which to receive nourishment from the overall bush. At the same time, any of those canes I thought perhaps questionable at Spring pruning - the leaves on those are turning into this appearance as well (and then in many cases, the cane dies for good).

You decide whether if your watering regimen could prevent it and allow it to be called over-watering/under-watering. I think NOT for either. Unless you're willing to stand out there every day and spray water whenever the temperatures get too high, I'd just say, "heat-stress." I water only once a week - but it's about 10 gallons per rose. One might say that's over-watering, but it doesn't explain all my other roses and the dropping leaves on my trees.

So, bottom line - heat stress.

I've heard that the time it takes to fill a tuna fish can is one inch of water. I know I've been watering more than that though. The hard thing is that the results of both over and under watering will look pretty much the same in leaf symptoms. And heat stress could be a major factor this year. Check your soil as Kim and Michale suggested and go from there. You can also purchase a very inexpensive soil moiture meter. I have one and it's helped me cut down on my water bills! I was watering way more than I needed to.

In our yard, overwatering or roots standing in water is evidenced by yellowing at the edges of the leaves. If it is long and persistent enough, the yellowing will progress to the center of the leaves. To me, the color looks very much like RMV streaks.

We have largely fixed the problem by growing our roses in raised beds, or by adding compost to raise the beds. Interestingly enough, this fix takes a while. Because we need to bury the roots deeply for winter protection, the early feeder roots are too deep, and have drainage issues. Thus, we have overwatering issues early in the first year, until the plants grow feeder roots closer to the surface.