When you get into roses with more petals, more chance of rust due to the depletion of calcium and potassium after blooming. Lots of calcium and potassium is used up to produce that many petals. With my 100+ varieties of high-petals-count roses, I use sulfate of potash and gypsum to fix my alkaline tap water (pH 9): fill a large bottle with 3/4 vinegar, then keep adding POWDER potassium 1st, then gypsum in a 2 to 1 ratio, until those solids can no longer dissolve. Then use 1 tablespoon of that per 2 gallons of tap-water.

I spent hours researching the cause of rust after Sonia Rykiel (over 100 petals) rust on me in a dinky pot in hot summer. If you check with agricultural abstracts, here are the causes of rust: 1) excessive rain which leaches out calcium and potassium 2) low potassium and high manganese 3) high-salt tap water 4) hot & dry weather which induce potassium deficiency. Potassium deficiency is also linked to mildew.

The logic of using sulfate of potash plus gypsum to fix alkaline tap water is: gypsum (calcium sulfate) is known to de-salt saline soil, so gypsum flushes out the salt. Sulfate of potash is to provide sulfur to lower the high pH of tap-water, plus to provide potassium to prevent mildew and rust. Austin roses with zillion petals have a higher need for calcium and potassium.

RMV was, at one time, a major issue with American raised roses. It reached its zenith in the late eighties to mid nineties, and it has been theorized that something like 80% of the American crop was infected with one (or more) types of the plant viruses which comprise Rose Mosaic Virus (RMV), including PNRSP (Prunus Necrotic Ring Spot Virus). Some strains are more virulent than others and some roses are much more susceptible than others. As there are different types/strains, as a rose passed through different companies, some were actually infected with multiple strains/types, increasing their viral load. With very popular, long-produced types such as Queen Elizabeth, Double Delight, Iceberg and many others, at one time, it was impossible to find stock of them which was NOT infected. Fortunately, there are and have been programs in various universities across the country which have "indexed", tested and heat treated many varieites to clean them of the RMV group. Because of these programs, it is possible to find plants of them which are not infected. Unfortunately, the last American source which actually identified their cleaned stock was Vintage, which is no longer in active business. Pickering, in Canada, which is also now out of the nursery business, indexed, "cleaned", all of their roses they sold. Heirloom Roses has long had a policy of indexing their roses. Presumably, a variety purchased from Heirloom SHOULD be free from RMV.

For older types which were never passed through any indexing program, it is virtually impossible to find examples not infected. Occasionally, a very old plant is discovered somewhere, propagated and introduced back into circulation. Also, occasionally, it finds its way into a virus testing program and is found to not be infected. Schmidt's Smooth Yellow, a thornless, yellow Polyantha discovered in an old garden in San Rafael, CA and Belmont Yellow, another yellow poly type discovered back east, were both tested and determined to carry the same strain of PNRSV, indicating they had both probably been produced by the same source at one time. George Washington Richardson, a found rose in an old cemetery in California, was tested and found to be free from RMV. It has also been observed to be virtually identical to the rose now determined to be the original Mlle de Sombreuil, not the large flowered climber in commerce as Sombreuil, but the Tea rose. Atmore Lamarque, the plant brought to Ventura County from Placerville, CA by covered wagon by the Atmore brothers in 1869 and planted at the Queen Anne style mansion one of the brothers built, the family still occupies and at which the plant still grows, has been tested and found to be free of RMV.

For anyone living in a warmer climate, where "killing freezes" are not an issue, growing a virused rose is much less of an issue than for someone living where winter weather can more easily take out a plant. Where a variety is able to be found without viral infection, of course it is preferable. But if you want a particular rose and it is only available with RMV infection, and you live where "winter" won't be expected to potentially kill it, it's your choice whether or not to grow it. Yes, I do have a very few roses I know are virused. If and when I am asked for cuttings of them, I do indicate to the person asking for them I know they are infected. That permits them to make the choice whether or not they still desire them. I know my Lilac Charm has RMV. It occasionally expresses the characteristic leaf patterns, but I love the rose and I choose to grow it. The RMV may perhaps inhibit its performance. I don't know for sure as I have never seen a plant of it anywhere which did not express the virus symptoms at some period of the year. For several others, I grew them with infection in the past and determined that if clean stock were to become available, I would grow only the indexed material to grow them again. Sterling Silver is an excellent example of that. I grew it for years and it expressed symptoms. When indexed material presented itself, I obtained it and am now nursing the own root plant along until I can bud it to indexed root stock. The point is, living in Zone 10, most roses with RMV will likely grow acceptably for you, as very many had for many years at The Huntington Library. Would an uninfected plant of the same variety perform better? Possibly, possibly not. Would an uninfected plant be desirable over one known to be infected? Personally? Absolutely! But, what if you can't obtain that specific rose known to be free of RMV? It's your choice whether to deprive yourself of growing a rose you desire because it is only available with RMV, or not. As I have stated, I choose and have chosen for several decades, to grow the roses I wanted, whether infected or not and have replaced those with infection with clean stock, if and when it has become available to me. Had I lived where a weakened plant would more likely to have been killed by severe cold, I may have made a different choice...or not.