Niels, my guess would be something left over by insects. I've never heard of rose sweat. Sorry that I can't help you. Hope you solve this mystery, hey there's a working title The Mystery Of The Sweating Rose.

Carla

Are there trees in the area? Could be honeydew or sap from them. Or did someone splash a soda?

I did feel kind of silly even asking this (Newbie ..hey we are all still learning)question, but I have never noticed this in warm areas of the world before. So perhaps some in parts of the world with scorching sun have noticed this too? We are having record many hours of sunlight here in May. It is probably going to be the sunniest month ever here in Denmark with more than 330 hours of sunlight...YEAHHH!!! Never seen that before! We were lucky to get a little rain today, before the sun comes back and it gets even warmer (80F).

There are no trees around this part of the garden and I have not been drinking sodas on the patio. It suddenly dawns on me what could be the explanation for the the first picture ... I have a birdbath. Perhaps all the birds bathing leave some of the oily stuff on their feathers and when they take of they splash some of the greasy water (Even though I change the water 2 times every day)on the rose leaves! And then when the sun is burning greasy drops burns the leaves a bit causing small purplish spots.

But it still does not explain why It is only on the edges of the leaves in the 2. picture? Well we don´t find answers to all the things we observe in or gardens.

Things that make us go "Hmmmmmm......"

invisible aphids...

Niels, roses have a organs on the leaf margins that emit accumulated salts. Sometimes you will see droplets of moisture evenly dotting the serrations of the leaf margin. This is an indication that the leaves are trying to exude excess salts through these organs. If they cannot emit sufficient salts, the organs in the margins can discolor or even die back. This commonly occurs when fertilizer salts build up in soils. It occurs less commonly when water or soils are high in salts roses cannot effectively emit, such as Boron. The message is clear: water more.

Ahhhh Thank you Cass for explaining excactly what I see in the second picture. I only use organic fertilizers so I don´t think I have overdone it? The "organs" at the leaf margins are emitting this substance when the shade comes to that part of the garden. I have never noticed it before, but it has also been the sunniest month in 88 years. And I do water a lot since we are getting no rain and have already reached the drought stage ... got less than half an inch of rain during the last month, which evaporate very quickly and is no use for the plant roots since it is not getting to them. So I will keep deepwatering about 50 Litres of water per square Metre/ 12 gallons of water per 9 square feet about once a week. The roses really thrive getting this much water and sun. They are strutting with new growth and are so healthy, but I am on the lookout for signs of powdery mildew, that is worse here than blackspot, since it harder to control. Perhaps douching the plants early in the morning will help as some are suggesting to rinse away the fungals spores.
Your explanation also explains why many mineral deficiencies shows at the leaf margins. I still have a lot to learn about plant physiology! - Always learning new things.

What Cass said. The process is called "guttation", a word I learned on the forum a while back.

Here is a link that might be useful: Wikipedia and Guttation

Thank you, this was very helpful. Gean

Now, correct me if I am nuts. I admit I made a C in chemistry, one of the reasons I never made it to Med School, but is transpiration (also known as dew in my area) not similar to what is going on and sweat (transpiration/dew) is what causes our bodies to "sweat electrolytes?"