Symptoms and Signs
Mature plants infected with Fusarium gradually decline in productivity and growth. During the summer, infected plants are characterized by one to several stunted, bright yellow ferns. A reddish-brown vascular discoloration, which may extend into the crown, is present at the base of stalks infected by Fusarium oxysporum f. sp. asparagi. Crowns and belowground portions of stems exhibit reddish flecks or sunken brown lesions, which can be seen by cutting them open. Reddish-brown, elliptical lesions occur on storage roots of infected plants. Feeder roots, most of which may be rotted off completely, show reddish-brown discoloration.
Comments on the Disease
Fusarium crown and root rot is the major disease of asparagus worldwide. The fungus survives in the soil indefinitely, and may spread as a seedborne contaminant. Spread is by movement of infested soil within the field, on transplants grown in infested soil, and on plants grown from contaminated seed. Infection occurs at any point below ground. All three fungi can colonize the crown and roots; F. oxysporum f. sp. asparagi can also invade the xylem tissue. Adverse environmental factors and the interactions of Fusarium crown and root rot with other diseases or insects add stress to plants and further reduce plant vigor. Excessive cutting periods (greater than 75 days) may also weaken asparagus and increase susceptibility to wilt.
Use clean seed in noninfested soil to produce disease-free seedlings, transplants, or crowns. Long rotations out of asparagus are beneficial. Minimize plant stress as much as possible. Hybrid varieties such as UC 157, Apollo, and Jersey Giant have increased plant vigor, which provides a degree of tolerance, but not resistance, to this disease. Avoid extended harvest periods, and end harvest when production declines to 70% of the season's highest yield to maintain crown vigor. Maintain crown vigor with proper irrigation and fertilization.
Organically Acceptable Methods
Good field sanitation, resistant varieties, and good cultural practices are all acceptable to use in an organically certified crop.