I always thought it was alkaline soil that mf wasn't very fond of, not sandy loam soils. But then again if all your roses were own-root, there's no telling what they all prefer. If you have any mf hybrids or polyanthas, that might give a clue how mf will do in your area. There is really only what, 3 rootstocks that are commonly used, mf, fortuniana, and dr. Huey. Of course some roses don't advertise what for sure you are getting, and fortuniana seems to be confined to the deep south, mainly Florida. Rootstocks are used primarily for their vigor, so how well is well, would depend on what each was compared to.
I wouldn't be too concerned about it.

MF IS sensitive to alkaline conditions. I don't know about sandy loam.
In our row of Golden Celebrations, the huge topmost one is (or was) on MF.
The lower plants are all on Huey, and presumably virused.
The top one is far more vigorous, but to this day it is sometimes troubled by chlorosis.
The plants on Huey are not.

I'd have THOUGHT the one on MF would be on its own by now, but maybe not.

Jeri

If you are anywhere close to Tyler Texas and the piney woods, it might be useful to know that in the early 1900s the huge rose industry there was centered on roses buded onto multiflora rootstock that were grown out two years in the fields before shipping out.
Some of the old Plant Propagation textbooks have lots of photos of the work being done back then.

My ignorance re: grafts is showing.
Lens, The soil is pretty alkaline, but polyanthas do well here.
Jeri, I guess I will have to tx them with sulfur, hope for the best and get get some cuttings started asap.
Ann, I'm on the TX/OK border but thanx for the info.

Yes. Soil sulfur helps a great deal.
Of all the remedies I've tried, it is the most effective and longest-lasting.

The polyantha thing is funny. The only one that's ever been much affected by it is 'Excellenz von Schubert,' whose leaves occasionally turn ivory white.

Jeri

There are a fair number of articles in the 1930's ARAs listing the qualities of rootstocks and the effect that they have on the roses in question. Not only can rootstocks influence the size and vigor of the rose, but also other material conditions such as the color of the rose and the prospective life span. JimD

Startling (to me) that rootstock can influence the color of a rose!

Greybird hope u don't mind if i slip in a question i've been wondering about for awhile. Can anyone comment on how Fort can be expected to perform on acid clay?

Mauirose, I think the only issue with fortuniana is hardiness, but used as a rootstock you could probably go a zone or two lower. Unless you are extreme
On the acid or clay, I would think it would do great as most rose roots would.
If you are in FL they use it because it seems least affected by nematodes that are in some FL soils. If you are in Maui, HAwaii, I've not heard but you never mentioned lava in the soil,LOL!

I do grow a bush of R. x fortuniana on acidic clay. And it's at the base of a volunteer walnut tree (which doesn't seem to bother it.) It's big, blooms well and makes it through winter.
It's stronger than my plant of R. laevigata (which grows at the bottom of our hill where it averages five degrees colder.)

I've also had a number of roses grafted on fortuniana and they don't mind the acidic clay; here they share fort's habit of not slowing down for winter and that leads to massive loss of most new growth come spring. Fort winters better. I don't understand why, but after a decade it seems proven that way in my back yard.