Tomato Stalks turning dark

[Mr. Greenjeans]

Hello Tomatovillians!

A friends garden I saw today had the bottom six inches or so of main stalk of her tomato plants turning a dark color, almost black (or a VERY dark green). Sorry, no pictures. She's been away for several days, & can't really say when it started or if it's progressing. The rest of each plant look very normal & healthy. She planted early for southern Connecticut & her plants are about 4' high now. NOTHING on the leaves at all.

A stroll through the interwebs suggests this could be caused by Alternaria stem canker, bacterial canker and both early and late blight.

But I don't see lesions or anything like what is described ....
http://homeguides.sfgate.com/stems-t...nts-25580.html

Is this likely to spread up the plant?

Thanks!