Fusarium solani f. sp. phaseoli
A few weeks after sowing, the leaves of infected seedlings can turn yellow and start to wilt. The plants may be stunted and can die shortly after emergence if the environmental conditions favor the disease. Underground symptoms appear as reddish lesions or streaks on the main root only one week after emergence. These lesions may turn dark brown, coalesce and as they dry, they develop into cracks along the root axis. Lateral roots and root tips shrivel and die but persist on plants. New fibrous roots can develop above these lesions, closer to the soil line. The tissues do not become soft and moldy, thereby the other common name of the disease "dry root rot". If they survive the adverse conditions, plants only grow a few pods with few seeds.
Fusarium root rot is caused by the fungus Fusarium solani, which can survive in debris in the soil for several years. The fungus penetrates the growing seedling shortly after germination and settles in the water and nutrient transport tissues. The presence of the fungus there usually causes little damage to unstressed, healthy plants. However, if the environmental conditions are adverse (drought, flooded soils, poor nutrition, deep planting, compacted soils, herbicide injury), the blocked water and nutrient transport leads to added stress and to the appearance of symptoms. Important yield losses can be expected in this case.
Seed treatments with biocontrol agents such as Bacillus subtilis with Rhizobium tropici might work. Other treatments with microorganisms include solutions based on Trichoderma harzianum.
Always consider an integrated approach with preventive measures along with biological treatments. Fungicides are generally not effective in controlling fusarium root rot.