Enivornmental issues and disease often cause similar symptoms. For example, yellowing of leaves could be from too much water, and this fungal disease called Fusarium wilt causes similar symptoms. Usually the first symptom is a slight yellowing of lower leaves on just one side of the plant or one stem, or a slight wilting and drooping of the lower leaves. A distinct brown discoloration of the water and food channels in the stem can be seen by cutting a stem close to the base of the plant. Since soil retains this fungus indefinitely and is more prevelant in over-watered soils/plants its recommended that you don’t re-plant tomatoes (or related plants) and other plants that are susceptible to this disease in the same spot, especially year after year.