I'd never heard of that particular potato disease, so can't really tell you how to deal with it. Sounds pretty serious, though, and is spreading throughout the world. The use of fungicides seems to have caused it to mutate in some areas so that those fungicides no longer work. That's generally what happens when people reach for chemicals first in any kind of food production~be it animals and plant~they only make the baddies stronger.
http://www.potatoes.co.za/SiteResources/documents/Alternaria alternata population study in relation to management of brown spot (malroes).pdf
Also not sure how to distinguish between cherry and other tomatoes at a young age...usually takes me awhile and then I can tell by the way the blossoms are growing on the stem....if aligned neatly on either side of the little stem, cherry. That's all I have for information. Also, all my cherry varieties have a much more slender plant stem altogether.
I have a weird tomato thing going on....pulled one plant earlier on due to it stopped growing, the leaves turned very dark green and curled. Pulled it and threw it out...figured some kind of weird disease. Now, a few plants down from that one, another very thick stemmed and seemingly healthy plant(had large bloom on and the coloration of everything was good) stopped growing and I noticed some ooze coming from the top of the plant....come to find that whole main stem was just turned to mush inside. I split it all the way to the roots, which also looked great, and the mush went all the way down. I can only figure some kind of fungal disease. Hope it doesn't take the whole row of plants.
Sourced these from a source I hadn't used in many years and most of these plants came from that nursery....hope they didn't bring a fungal disease along with.