While a not so healthy plant may show some disease, it is more likely that the conditions in your area have changed to make your roses more susceptible to blackspot and mildew. Both are dependent on weather conditions. You said you used to spray, what did you use? Not spraying is not going to help, but you have to use the right product. Also, if your plants don't go through a real dormancy, they can carry the spores for disease over winter. That's why most rose growers start spraying as soon as the roses leaf out in the spring. The fact that the roses are fine after the spring tells me they are healthy and that the conditions are just right for disease when the first flush is going on.

Number 1: These are mostly evergreen roses so they're bound to change their leaves eventually. They do that mostly in the spring. The rate at which they manage to grow new leaves depends on number 2 and the weather.
Number 2: Yes, their performance may be affected by under-fertilizationa and/or underwatering.
Number 3: Old leaves that are about to be shed are the first to be aflicted by disease. That's nature at work

Think of 1 & 2 & 3 in tandem and maybe you'll notice some consistency in their behaviour.

'Old Blush' here in Southern Appalachia had blackspot all season. Usually it was not fully defoliated because it grew so fast, but the lower 2/3 was bare much of the time. In Tuscany it may be too hot and dry at midsummer for blackspot to spread much.

In addition to the sensible suggestions above, there's another reason why a rose that resisted blackspot could become susceptible. That is the arrival in the garden of a new race of the disease that can affect a different set of roses. I don't know whether mildew has that kind of variability.

In our SoCal garden, Cl. Old Blush never blackspotted. But like the bush form, it mildewed constantly. A rose that big that is covered with mildew is NOT an asset, and so after a number of years of non-improvement, it left us.
It is to-die-for gorgeous in Texas. Here, not so much.

Tiny bits of BS here in the worst of years affecting mostly the older leaves. Mildew used to be a problem when my 4 OBs were young, now they get some at the height of the mildew season (early spring and late fall) but they shrug it off in a couple of weeks unless they are thirsty. I've never sprayed them. They bloom throughout the year pausing just enough between flushes to make new growth. Really a monthly rose.

Thank you all for your replies; I feel encouraged. Looking around in my garden, I see that several other roses are behaving in a way similar to Clbg. OB: Fellemberg,Camellia Rose, Mt. Vernon Purple Noisette, and Deschamps. None are as mature as Clbg OB,however,but it's interesting to note that HMF classifies two of these as Chinas, like Clbg. OB. My garden is such a "particular case",since I literally started from scratch about 16 years ago,on completely uncultivated soil, and with no gardening experience either. Of these 16 years, about 10 or 11 were very dry,and from about 2010 to 2013,we had a real drought. Since then, we have been blessed with good rain. I am seeing different reactions amongst my oldest roses. My oldest Mme. Isaac Pereire kept on declining,cane after cane died; I dug it up,and found that the root-stock had completely died off, and what was left was the beginning of an own-root MIP. Clair Matin 's root-stock has probably also died, but it already had enough"own-roots" to permit the plant to keep on going; I just lost it's tallest climbing canes, and now have a nice, healthy shrub which will hopefully soon climb again. This year, both Gloriana and Crepuscule looked like they were in suspended animation,up until recently,when suddenly they began putting out lots of new growth (VERY late in the season for this,in my climate, please note!) In other words, I think the great improvement in the weather conditions, along with the work I've done on my soil,has affected even the established roses dramatically, in different ways,so perhaps this is part of what's going on with Clbg OB. Hopefully, it's working on it's roots, and once they improve, so will it's health.It's funny , because another rose that I truly wanted to grow-Reine des Violettes-had sort of the opposite problem: very healthy, vigorous plant,but the flowers were stunted and ugly. I moved it into well-prepared soil, and it is blooming very nicely now! though,ironically, now it does have a touch of PM... but there again, I moved it only about 6-7 months ago, so it's by no means established in it's new home. I do hope that I can improve Clbg OB's soil sufficiently without moving it though.
I actually am by no means sure that spraying is a good thing,since when you spray, you are also killing off all the good fungi as well as the bad. It sure worked that way with the insects: things improved dramatically after I stopped spraying with insecticide. However, it did take a year or two to re-establish the balance. regards, bart

When we quit spraying, it took a little more than a year to restore a natural balance. Things are a bit less perfect, now, but much healthier.

Yeah,maybe I wasn't doing it frequently enough, or was using overly weak products (just copper and sulphur), but frankly I didn't see any benefits at all really. Seriously, this is the second year without spraying, and in general my roses are pretty much as they were in former years. I really don't think that Clbg OB's health issues have anything to do with my not spraying, since it began having these issues while I was still spraying: now that I've been thinking about it, I'd say the health issues have been going on say about 4 years. They definitely did NOT start last year ,once I stopped spraying!!! I do remember clearly that, before I started spraying, my garden had PM ,but no bspot,so I'm wondering if perhaps beneficial fungi were present before, keeping the bspot away? In any case, it's a big relief not feeling obliged to do that enormous and difficult job.