I.  Introduction
Well, first, roses get blackspot and humans get colds (as opposed to the play on words in the title), but the question why is the thing. The two chief factors which determine whether a rose is susceptible to blackspot are growing conditions and lineage. We'll discuss these factors, completing the discussion with suggestions on how to deal with this aggravation. What is blackspot? Diplocarpon rosae, common name, blackspot, is a fungus which is the most widely distributed and most destructive disease in rose culture. Symptoms start with black or gray spots, followed by chlorosis, or yellowing of the remainder of the leaf. Defoliation follows, and in severe cases, the rose may actually perish. Blackspot thrives in damp, humid, warm weather, and by the way, by the time you see the black spots on the leaves, the fungus is well established on the plant. Sounds dire, doesn't it? Well, the good news is that blackspot is usually a visual nuisance (who wants bare, leafless, rose canes in their garden?), and if proper care is taken, blackspot need be nothing more than a minor headache from time to time. In order to not add too many chemicals to the environment, I limit the amount of spraying I do (my wife might suggest laziness is a more probable motive for my lack of effort with regard to sprays); I can attest that through selection of resistant varieties and the occasional spray placed on my more delicate roses that blackspot is not a real problem in my own rose gardens. We should strive to move towards a philosophy in the garden that to grow roses successfully does not mean a resignation to constant spraying and fuss.

II. Stress for Humans Can Be Good; Stress for Roses Is Bad. (Growing Conditions)
My father, a good and wise man, often told (tells) me that stress can be a good thing, the thought being that stress can stimulate us to greater heights. Or, to quote some wise sage: 'The hotter the fire, the harder the steel.' Clearly, heavy stress, instead of causing us to grow strong can have a negative effect upon the quality of our lives and our ability to solve life's inevitable challenges. Another side effect of stress, such as an environmental factor like cool, wet weather, is that it can cause our immune systems to weaken, leading to the dreaded winter Cold. The frosty weather of winter affects our immune systems, leading to invasion by Cold or Flu. The snowball keeps going, as sneezing, sniffling hosts spread the germ to other humans. Anyone who keeps fish as a hobby can attest that overfeeding or a broken heater or improper filtering will soon lead to 'ick' or some other affliction which if not treated leads to the death of the finny pets. The overfeeding or improper filtering don't directly cause the demise of the fish, but contribute to stress upon their immune systems.

Right now you might be wondering if you have stumbled onto a immunology or fishkeeping webpage cleverly disguised as a rose page, but you might also see where this is all headed. The lesson to be learned from the common cold as it relates to rose culture is that we must either eliminate as much stress as possible from our roses or we must select roses which are less susceptible to blackspot. It must be stressed that 'blackspot immune' and 'blackspot resistant' are two different things; virtually every rose variety may get blackspot, but a healthy rose or a rose with good lineage will often shrug off blackspot without trouble. Improper growing conditions do not usually directly cause the death of a rose, but improper growing conditions weaken the plant's ability to withstand onslaught from blackspot (and by the way, rust, mildew, insects, and the occasional golden retriever).

III. Balance In All Things Is Important.
Let's plant the majority of our roses with disease resistance in mind; let's encourage breeders to select plants for disease resistance, and to divert a little, let's dedicate ourselves to using no pesticide which eliminates bees and butterflies while aiming for Japanese Beetles. In other words, let's use a marksman's rifle, not a shotgun, as our weapon.

IV. What Can Be Done? (How to Prevent and Treat Blackspot)

Implement a Fungicidal Spray Program (If you must)
There are two types of sprays, surface-protectants (broad spectrum or 'contact') and systemic. Surface-protectant sprays coat the leaf on the outside. Systemic fungicides penetrate to the interior of the leaf, interrupting the life-cycle of the fungus and preventing the spread of blackspot.

note:  The mob does not support using insecticidal sprays in the ornamental garden.  The ecological risk is real.

Pro's and Con's of Implementing a Spray Program
There are two schools of thought on this subject. One, buy blackspot resistant plants and only spray when an outbreak occurs...Or Two, implement a regular spray program. The advantage to only spraying when an outbreak occurs is that there is not a regular influx of chemicals being shot all over the garden; the downside is that when an outbreak of blackspot does occur it is very difficult, if not impossible, to heal damaged leaves, leaving the only alternative to strip the infected leaves from the plant and begin spraying. Implementing a regular spray program has the definite advantage of prevention, so important in combating blackspot.

The Garden Mob Recommendation
We prefer not to spray on a regular basis, finding that taking the risk of a blackspot outbreak or two is worth the benefits of not having to spray chemicals all over the place. However, there is a risk, so if you are the type that can't sleep at night worrying about an outbreak, or you have a large investment in plants, or your garden is an exhibition garden, then a spray program may well be worth the peace of mind.

Surface-protectants compared to Systemic Sprays

• Neither surface-protectants nor systemics sprays will heal infected leaves.
• Neither surface protectants nor systemics will protect new growth until it has been sprayed.
• Systemic sprays will not be washed off by rain; surface protectants will.
• Blackspot spores may build up a resistance to systemic sprays; generally, blackspot will not build up resistance to surface protectants.

Rotating Sprays
Blackspot fungus is able to build up a resistance to spray agents. What happens is that a spray may kill a significant percentage of fungal spores, but the surviving spores will soon fill the space left by the killed spores to become the dominant, and often, resistant strain.

This problem can be alleviated to a certain extent by alternating fungicides.

Recommended Sprays and Schedules
Surface-protectants: Daconil (Recommended), Fungi-Gard, Maneb
Systemic: Triforine, Funginex (Recommended), Strike, 3336, Rubigan, Immunex

First, follow the directions exactly as indicated on spray labels.  This will ensure your safety and the most effective regimen.  If you choose a systemic program, complement with a surface-protectant spray.  Spray on a seven day schedule and rotate systemic sprays.