[lanky lurcher]

hi everyone,
 I'm growing 5 beef tomatoe plants outdoors in grow bags, the fruit seems to be growing well,however the main stem of 3 of the plants are turning from green to brown. the browness began at the bottom of each plant and is getting higher up the plants ,even to the branches where the growing fruit is !
what is the problem? can it be stopped? will it effect the fruit ripening? will it kill the plants?
it is very worrying  

[Annie]

Does the brown have a kind of powdery look to it or release powder if you flick the sterms gently,also what do the leaves look like?If it`s moving up the plant though it doesn`t sound good but these are outdoor toms so....

[Fat Hen]

Possibly fungal infection as a result of slug damage?