Banksiae seedlings and hybrids are notoriously mildewy. At least the straight seedlings out grow it and achieve the high degree of resistance it demonstrates. Not all hybrids do. Minutifolia seedlings are highly mildew susceptible but maturity appears to eliminate that pressure. So far, all of my modern X Minutifolia hybrids are resistant. Stellata mirifica X Minutifolia seedlings appear EXTREMELY mildew susceptible. Hopefully, they will mature out of it as neither species is affected.
Do they grow out of it without assistance? Or do you intervene?
I get a lot of mildewy seedlings from some things (even things that surprise me like rugosa or blue for you and because itās mentioned in the very first post, Iāve never had a seedling from Carefree Beauty that isnāt mildew ridden to the point they death before reaching 5cm/2inch if no intervention) while others very few (polysā¦even though the parent/s get powdery mildew). Never quite sure if I should be intervening, I tend to not intervene and generally those that were runty their first year continue to be runty their second year if they survive (one heatwave with 40c+/104f+ temps and most things with noticeable mildew fry to a crisp and donāt recover).
On the flip side, after flipping through the new foliage (spring starting in a few days, southern hemisphere), a lot of my earlier rugosa seedlings (2018 sown) that are still around are starting to bud up for the first time (which may be an indicator for a lot of once bloomer types from around that timeā¦like the thorny tangled hell pit of Lord Penzance seedlings down the backā¦yay but also boo I may need to get in there, Iām going to get stuck on something).
I forgot to add that cutting back immature plants will not hasten their maturity.
IF you can maintain them until they develop enough root system and foliage mass, they outgrow the mildew issues. That can be a HUGE āifā. Iāve recently obtained potassium bicarbonate which supposedly works well. I havenāt used it yet because I used very old Serenade on the Stellata X Minutifolia seedling and burned the daylights out of it. Itās putting out more growth and once it appears not to teeter on the edge, I will treat it with the potassium chloride. Itās interesting stuff, available from wine and beer making suppliers to reduce acid in brews. A friend steered me to an on line company which had a decent price for a pound but wanted a ton for shipping, so I found one on Amazon who sold it for $8 a pound with free shipping.
I gave up on Carefree Beauty years ago because I refused to spray and it refused to NOT mildew. Not its seedlings as I never raised any. If the parent tightly holds fungal issues to its bossom as a sign of pride, I donāt care what its seedlings might do. Itās gone. I know CB is ācarefreeā elsewhere but it never was here. I figure, for the most part, if the seedlings are so diseased they require chemical intervention, perhaps they shouldnāt live. But, Stellata mirifica X Minutifolia? No one has ever done that before, at least that I can discover, so Iām treating it when it feels safe. The Banksiae self seedlings have mostly outgrown the mildew, except for the few which didnāt and literally died from it. The first batch of Minutifolia selfs all died from it, the one from this batch of seed is (so far and fingers ARE crossed) remaining clean.
Itās a shame when roses that are selected and distributed for their disease resistance eventually become susceptible. āCarefree Beautyā is one example, āTropicanaā is another.
There was a report in the ARA sometime in the 1980s that collected reports from around the U.S. on the disease resistance of various popular roses. Among the combined reports for HTs and Floribundas, āTropicanaā ranked highest. And it was also in the '80s that I learned that āTropicanaā was susceptible to a specific strain of mildew. Pity!
But it seems likely that āTropicanaā would still be useful for breeding if another variety could be found with resistance to that special mildew.
Breeders face the same problem with blackspot: āSemeniuk estimates there may be as many as 50 to 100 strains of the blackspot fungus in the United States alone, but he chose seven strains that he had closely studied to test for resistance during his breeding program. The strains, which he maintained in the laboratory, had been cultured from infected leaves from Beltsville, Md.; University Park, Pa.; Ithaca, N.Y.; Tifton, Ga.; Tyler, Texas; Ames, Iowa; and Delaware, Ohio.ā
Back in the 1930s there was some discussion of āvigor-relatedā resistance and ātrueā(i.e. genetic) resistance. āGolden Bantamā sweet corn, for example, had the vigor to defeat most infections. The genetic resistance was desired by some breeders because it was inherited as a āunit characterā. Mendelism was all the rage back then. But this type of resistance was often defeated when those pesky microbes mutated themselves.
The answer is not either/or. Vigor is always important, and will reinforce the resistances conferred by specific genes.