I have recently purchased five rubber plants potted together in one pot. They are approx 5ft high. One of the plants is slightly smaller than the others and the leaves are dropping and yellowing. The first 12 inches or so of the stem is brown and dry, with a soft feel. I think it may be infected with red spider mites as I have noticed a few webs. Could this be causing the above symptoms and what is the best treatment to get this plant back in good condition?
I would be grateful for your advice.

thank you

Caroline