Baking Soda
Recently there has been much news about the use of baking soda. Intuitively, it seems to me that if it is a compound we use in cooking, its level of toxicity must not be too bad. Baking soda was tested by the National Rose Society in England. There it was determined that a single spraying at the recommended dosage delayed the onset of blackspot by about two weeks, whereas a single spraying of a conventional petrochemical spray was good for much more than twice that long.
I don't know the recommended dosage for that trial. I used 1 tablespoon per gallon and sometimes get some leaf deformity at this level. I also had protection that lasted well over six weeks. This spray is not recommended for rugosa hybrids; they seem to dislike most sprays.
Neem Oil
Another material touted for preventing black spot is extract of neem oil. Neem is a plant native to India. Where it grows in India its twigs are chewed and used sort of like toothbrushes to clean teeth and kill oral bacteria that cause bad breath.
Neem oil is also touted as a good organic pesticide. I sprayed extract of neem oil on my roses, mixed with baking soda. One week later the roses had the most impressive colony of aphids I saw the whole season. "If this concoction is as good at preventing blackspot as it is at preventing aphids," I grumbled to myself, "I'm in for a long summer."
As it turned out, two sprayings were enough to totally stop black spot on my most blackspot-prone rose, Don Juan. It is impossible to know to what extent the fungal protection was due to neem oil and to what extent it is due to the baking soda. In any case, I can certainly live with a program requiring that I spray these very non-toxic items two or three times per season.
Elemental Sulfur
At the height of the industrial revolution in England coal-fired furnaces sprewed tons of sulfurous chemicals into the air. Then, early in the twentieth century England started cleaning up its air. As this happened, roses began having ever more trouble with fungal diseases. Sulfur, as it turned out, had been preventing the blackspot.
Today you can buy elemental sulfur in spray form. This last season I used this material. It was by far the dampest season I've seen in New Jersey, so any comparison is not fair, but the compound seemed to just barely work against black spot. And I had by far the worst infestaton of downy mildew ever.
Recommendaton
I recommend that you concentrate on roses that have no need of spraying. (Fragrant red Hybrid Teas, as it turns out, are almost all prone to fungus - Etoile de Hollande might be an exception. And who can grow roses without including at least one fragrant red Hybrid Tea? So maybe just a little spraying is fine. )
If you must grow roses that get fungal diseases, start first by using bi-weekly sprays of sodium bicarbonate diluted at the rate of 1 tablespoon per gallon of water. And if you want to use some neem oil, that's okay, too.
If baking soda proves insufficient, I'd advocate using Bordeaux mixture every other season or so, following the lable instructions.
If you make your living on the exhibition of roses, then I suppose you should consider using petrochemical fungicides on roses most prone to disease; I can't help but imagine that the roses themselves and the people spraying them would be better off using less of the stuff. Besides being toxic, I find the smell of a chemical-sprayed rose garden repulsive.
To reiterate, I have to recommend against spraying for pests, except for spot spraying in the direst of emergencies. Preventative spraying for pests done at a low level throughout a large area is perhaps the most effective way of increasing the population of pests. The spray accumulates in pest preditors often killing them before it kills the pests. With fewer predators, the target pests multiply many times faster. In most cases pests can be washed or picked off a rose well enough to to bring it through an infestation. Before long predatory insects will move in and help things along.
There are pests that are exceptions. Japanese beetles seem to have no above-ground predators. Traps with lures can help at least divert them to another location. Aphids are sometimes protected by ants, so some special treatment is required in this case. Deer, of course need special treatment, but sprays are not designed to deter them.
When faced with fungal disease, go ahead and spray baking soda or neem oil for fungus prevention, but whatever you spray, always carefully follow the directions and use a dust mask to prevent inhaling the material. Even chemically inert materials if inhaled over long periods of time can cause lung problems. Asbestos and silica, two of natures's most inert materials have been linked to terminal lung diseases.
Until I moved to New Jersey I had not sprayed a rose. And until half of my roses nearly died of downy mildew my second season here I imagined I never would. At that point I realized that fungal diseases were a real threat to rose health and if I did not come to terms with this fact, I'd lose some good roses.
In some parts of the country where the air is dry, the soil is moist, the sun is bright,and the soil pH is slightly alkaline, roses grow without fungal diseases. I think maybe there is a strip of soil in Texas that meets this requirement, and probably there is another in California that does. This is where America's rose breeders and growers are located. It is my guess that much of the rest of the country needs to use sprays to combat fungal diseases, especially on disease-prone cultivars.
As for spraying of pesticides, this is a practice we recommend against. Mathematical population models of preditors and prey (see Differential Equations and their Appications by Braun) indicate that pesticides will likely increase most pest problems because sprays kill predators more readily than prey. There are exceptions to the rule. For instance, if one can lure most pests to a single area in the garden and kill them locally without affecting preditors, then the negative effects of pesticides can be minimized. Lagerfeld is a powerful magnet for thrips. One might get good control planting one at each corner of the garden and dusting each new bud with rotenone or pyrethrum daily.
Sprays for fungal diseases make a little more sense. The good news is that there are a lot of choices. The bad news is that none is perfect.
Other Things to Consider First
Before we talk about sprays it is important to realize that there are a number of things that come first. First, choose a cultivar that is resistant to all fungal diseases, or at least one that is resistant to the ones in your area. East of the rocky mountains rust is not much of a problem. Where the relative humidity rarely gets above 50% blackspot is often not a problem. This means that you may not need to consider rust resistance if you are buying a rose for Connecticut, and you may not need to consider black spot resistance if you are buying a rose for SantaFe or SanDiego.
In any case, remember that choosing a fungal-resistant plant is the first line of defense. It is doubly so if you grow a lot of roses. One rose that is particularly susceptible to a fungal disease can act as a breeding ground for the fungus and accellerate its attack on other plants. I once grew Gloire des Mousseusses but pulled it up when I concluded that it infected Constance Spry and Fantin Latour much earlier than they would suffer without it.
Next consider the soil. Is it the correct pH? Does it provide enough trace minerals? Is it fertile enough? Is there plenty of organic material? Do you see a lot of earthworms crawling around in it when you dig it up? Be sure that the soil provides enough calcium and phoshorous and that the pH of the soil is high enough that these minerals are not bound up by soil acids.
Are there external stresses that predispose the rose to disease? Is it getting enough light? Enough water? Is it getting enough shade to prevent it from being cooked in the midday sun?
If you've done all these things well and your roses are getting fungal diseases, it's time to consider sprays .
Before you do, recall that even chemically inert substances like dust and asbestos can cause serious lung damage. So always use a respirator when spraying
Spray Types
There are two general types of sprays; systemmic fungicides and barrier sprays. Systemmic fungicides work by being absorbed into the rose tissue. Then, when a fungal spore sprouts and invades the structure of the plant the fungicide kills the intruder. Barrier sprays work by preventing intimate contact between the fungal spore and the leaf. The spore either does not sprout because it does not have an appropriate environment, or if it sprouts, the structures it has to break into the rose's structure fail.
If a systemmic material is sprayed onto the rose, then the highest concentration will be at the surface where the fungus attacks - at least until the first over-head watering or the first rain shower. After this only fungicide that has been absorbed through the plant wall will be active. Most of the treatments listed below are systemmic.
Bordeaux
The earliest fungicidal spray is known as Bordeaux mixture. It's a mixture of copper sulfate and calcium carbonate (lime). It is generally held that the copper delivers an effective knockout punch to the fungus, and the lime does little other than perhaps act as a mild surfactant. The good news about this mixture is that it is not terribly toxic, it is water soluable, it is inexpensive. The bad news is that copper can build up in the soil and, at least in theory, eventually become toxic. It is possible that the lime and the sulphate ions have some beneficial actions also.
Petrochemicals
After World War II a number of chemical compounds derived from petroleum were introduced. These had the benefit of being highly toxic to fungi. Unfortunately they had to be delivered in some kind of oil-derivative solvent, for they were not soluable in water. Also these compounds are fairly toxic to beneficial flora and fauna, and if used without a mask can lead to serious lung, liver or kidney problems. Most rose exhibitionists depend to these compounds. And most rosarians will swear by them.
I have deliberately avoided learning about them because I personally would like to grow a garden without them. If a rose cultivar cannot thrive when helped along by spraying a benign compound, it is best in the long term if that cultivar goes extinct. I'm afraid that my personal conviction is so strong that I have difficulty recommending them to others.
Companies like Rosemania make their living off selling material for rose culture and they offer good advice on using the chemicals that they sell.