The white roses are planted alternately with pink ones, and two members of my family think it is cross pollination from the bees. Me, well I think it is something else. Cross pollination can create all kinds of weird and wonderful things, but that is usually done through the seeds. My roses keep coming back each year from the same plant.
Some other responses have been 'lady bugs' ( which I know eat the aphids) , and 'fairy dust being sprinkled' which is rather a creative answer.
We have had some rain and I think it may be some kind of fungus, but I am not sure. Another thought is some kind of bug.
Whatever it is though, it has resulted in a very pretty flower, which I would like to have all the time.
