Answer: All vascular wilts, regardless of which pathogen causes them, are devastating to many annual and perennial vegetables and ornamentals. Fusarium is a soil inhabitant, while Verticillium is more of a soil invader. Both infect plants through the roots. Fusarium overwinters in the soil or plant debris. Verticillium overwinters in the perennial parts of plants that are left in the soil (such as dahlia tubers or roots of perennial plants). Once established, both of these wilt pathogens remain indefinitely, although their populations may vary. There are some tomato cultivars that are resistant to to Fusarium and Verticillium, such as Burpee's Early Pick, Heatwave II, and Red October Hybrid. You can try growing them instead of the varieties you're now growing. The dahlia tubers are probably infected with verticillium so tossing them would at least remove the hosts from the garden bed. There has been some success in defeating the wilt pathogens through summertime soil solarization. The process is simple - spread clear plastic over the area, and anchor it down. The heat from the sun will penetrate the first few inches of soil and kill the pathogens. The process may have to be repeated after turning the soil to bring the next few inches of soil to the surface for solarization. You may want to consider growing your most wilt-susceptible plants in containers filled with commercial potting soil rather than soil from your garden.
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