Lisa,

That yellow flagging is very characteristic of Fusarium wilt. It may be that Opalka is more susceptible than your other varieties, but I suspect that if it is Fusarium wilt, the other plants (assuming they're non-hybrids) will start showing symptoms as well.

It's not always fatal, though it can be, and it can impact yield anywhere from a little to completely. It's hard to manage, though many people have good luck with growing in containers, and most hybrids are resistant to most or all of the Fusarium races out there (there are three). If you ask questions specifically geared to growing tomatoes despite Fusarium, I'm sure people here will be able to give you their experiences.

Sorry to be the bearer of bad tidings.

--Alison

Thanks Alison

I was kind of hoping that was not the answer!

I'll start researching Fusarium

Lisa