tomatoe stems brown

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  

Possibly fungal infection as a result of slug damage?