Fusarium Wilt is a devastating soil-borne fungal disease that enters through root wounds.  It slowly chokes out the plant's vascular water-transport system. A classic diagnostic indicator is the "one-sided flag" symptom—where leaves or an entire branch turn golden-yellow and wilt on just one side of a single branch or plant first. The fungus thrives in hot summer weather and can survive indefinitely in the soil as hardy spores, even without live host plants present.

How to Fix It 

Rip up the plants and burn them.  Get all the roots.  Then, cover infected garden beds with clear UV-stabilized plastic sheeting during the peak heat of summer for 4 to 6 weeks to trap intense heat and bake out underlying spores. Rotate crops and do not plant eggplant, potatoes or peppers in those areas for several years to starve the spores.  Sterilize equipment with alcohol.  

Apply organic biocontrol treatments containing Trichoderma harzianum or beneficial Bacillus strains to build up competitive colonies around root zones and suppress pathogen activity.  Drench the soil with a garlic drench.  Use a neem oil  drench around neighboring plants to boost underlying defenses against root nematodes, as microscopic root injuries from pests often create open gateways for Fusarium spores.