light green leaves on cucumber plant
Pick off the badly affected brown foliage. It is not uncommon for the older foliage to yellow and brown. Cucumbers are susceptible to some leaf spots. Monitor the plants. They also looks like they need some fertilizer. Cucumbers are a long season crop. In containers they may need to be lightly fertilized every two weeks or so to produce a continuous harvest. Here is our publication on container gardening, fertilizers, and our vegetable profile on cucumbers http://extension.umd.edu/sites/extension.umd.edu/files/_images/programs/hgic/Publications/HG600%20Co...