to the Rose Forum.
Frank, I hope you don't mind if I step in with a couple of corrections to the post above ...
I have a "thing" about rose history ...
'Peace' was and is certainly a break through rose which was introduced by three different nurseries after World War II, but it was not the first hybrid tea. 'La France', which was introduced in 1867 is often designated as the first hybrid tea, although some experts cite 'Victor Verdier' which is generally classified as a hybrid perpetual as the first HT. 'Victor Verdier' was introduced in 1859.
Rose (Rosa 'La France')
When conditions are right, 'Peace' can be very susceptible to black spot, but it can also be disease free in other climates.
That kinda brings me to another question, does having all the plants around the standards base cause problems e.g. Lack of airflow to the base/roots? Battle for nutrients from the soil/feed?
You are on the right track asking this question. (btw your rose and under plantings are beautiful.)
When you take the anatomy of the root system into consideration, under plantings do impact the health of a rose. The feeder roots of a rose are very near the surface of the soil. As the rose grows, they are constantly growing outward from the crown of the rose seeking more nutrients and moisture.
Margie was correct when she wrote "Cultivating and digging the annuals/perennials around the rose bush each year could be damaging the the feeder roots of the rose bush." except that she used "could be" ...
There is no doubt that any digging / cultivating in the root zone of a rose will damage the feeder roots. If the rose has to put energy into growing new roots, it will sacrifice putting energy into the top growth and your plant will not be as healthy as a plant where the roots are not regularly disturbed.
The competition for moisture and nutrients does have a real impact on the rose. Not only because the other plants are grabbing the moisture and nutrients, but also because the rose's root system has been damaged.
Some roses can deal with this more effectively than others. It depends on the rose. Also, in your case, they use a different root stock in Europe which may not be as strong as the one that is common in the United States.
As for black spot, yes, those plants can harbor bs spores. Actually, even if they do, bs spores are in the air and if a rose is susceptible to bs, it doesn't matter if you have under plantings. The spores are there and if the rose is prone to getting bs, it will get bs.
Healthy roses are more disease resistant than stressed roses. My preference is to always protect the root system of the plant as much as possible because that is the foundation of the rose.
There are a lot of right ways to grow roses and a lot of people will disagree with me on that one ...
In your garden, where you know you have bs the common practice of changing out the old mulch for new mulch will go a long way to help manage bs. It will not prevent bs, but it will make a difference.
As for the fungicides you might use to control black spot, I know we have different products available to us here in the United States than you have across the pond. I think it is best to contact a reliable rose nursery for that kind of information.
As for the lichen, I can't begin to address whether or not it will harm the rose in the long term. I don't know enough about your growing conditions. I don't know what type of lichen is on your rose or how to treat it.
I do hope this helps. Please let us know how the rose is doing this season.