did you both buy the plants at the same place? or were they grown from seed?

a few of us locals had some problems with green zebra heirlooms last year. random branches were wilting and falling off. we all bought at the same place. i believe it was verticillium wilt, personally. I pulled the plants to prevent infecting the other tomatoes and subsequent tomatoes in that bed. I dug out the bed totally in that area, and replaced all of the soil :\

Walter Reeves was talking Saturday on his call in show about a couple of different kinds of wilt that effect tomatos. It is something to think about especially with the damp weather we've been having. You could look for his program notes or search his website for tomato wilt and see if any of the symptoms he describes sound familiar. Walterreeves.com

Good luck

We did not buy the plants at the same place but I am pretty sure they came from the same grower. I don't know much about growing tomatoes... if this plant has a disease will it spread to the other plants? Should I pull it out?

Just leave it for now. I it can be a number of things, and Mortgage Lifter is one of the heirlooms that is significantly less prone to wilts than some others. It could simply be soggy soil. Did you amend your soil so that it has good drainage?

soggy soil, too much nitrogen, planting under black walnut tree?!!

Even planting close to a pecan tree!

Actually there is a pecan tree in the yard next door. What kinds of problems does that cause? It actually looks better today. I think maybe it just had too much water from all of the rain.

Seedling pecan trees are noted to be allopathic at times - they have properties which can make it difficult for some other plants to grow and or reproduce except themselves.

I think it is fairly close to the tree though this can be a problem and not with well bred pecans.

I have noted this problem here... however today I also noted that I have a yellow pear tomato that is just about dead as a doornail! I planted it not long ago so I'm going to scoop it and the dirt around it out. Hopefully that will be enough so I can plant another in it's place!