With Fusarium crown rot, the leaves often turn brown or black and eventually wilt. With Fusarium wilt, the yellow leaves turn downward and droop. Fusarium oxysporum , the cause of both diseases, is a common tomato fungus that lives in the plant's vascular system, which carries water from the roots to the leaves. Both over- and underwatering can mimic disease symptoms. Check the roots.
Discolored roots indicate root rot.
Cut the lower or main stem and look inside at the vascular tissue. Fusarium wilt causes a dark brown discoloration within the vascular tissue. Fusarium crown rot causes a rot or canker at the base of the stem and possibly a root rot. However, if vascular discoloration is noted during the diagnostic process, pull and destroy affected plants. Do not plant tomatoes, potatoes or eggplant in the affected area for two or three years.