HPIPM:Fusarium Root Rot

Author: Howard F. Schwartz, David H. Gent, Gary D. Franc and Robert M. Harveson

Taxonomy

Domain	Eukarya
Kingdom	Fungi
Phylum	Ascomycota
Subphylum	Pezizomycotina
Class	Sordariomycetes
Subclass	Hypocreomycetidae
Order	Hypocreales
Family	Nectriaceae
Genus	Fusarium
Species	Fusarium oxysporum

Scientific Name

Fusarium oxysporum f.sp. phaseoli

Common Name

Fusarium wilt

Identification and Life Cycle

Fusarium root rot is caused by the fungus Fusarium solani f. sp. phaseoli. The Fusarium root rot fungus occurs in most cultivated soils of the region where dry beans have been grown previously; and most plants grown in these soils will become infected to some degree before they mature. Root rot severity depends on factors such as cropping history, plant spacing, moisture and temperature stresses, and soil compaction. In general, any factor which contributes to a reduced rate of root growth increases the plant's susceptibility to Fusarium root rot. Early planting in cool, moist soil favors the disease, as does soil compaction in the form of tillage or hardpan layers. In dryland production areas the disease can be more serious, especially in years of drought. High plant populations also increase plant stress and favor infection. Improper cultivation, other soil-borne pathogens, and various herbicides are also known to induce injury of young roots and aggravate Fusarium root rot damage. The effect of Fusarium root rot is most apparent during blossoming and early pod set when the plant and its productivity are more sensitive to stress. The pathogen is disseminated within and among fields by the movement of contaminated soil by wind, irrigation water, and on equipment. Infested soil can contaminate seed lots. The pathogen can survive between dry bean crops as dormant chlamydospores or in infested crop debris.

Plant Response and Damage

Fusarium root rot initially appears as red to reddish-brown streaks (lesions) on the hypocotyl and primary root two to three weeks after planting. These affected areas enlarge with age, may merge and gradually become brown. The symptoms may extend up the main root to the soil surface, and longitudinal cracks can develop in older lesions. Severely infected primary and secondary roots are commonly killed but can persist on the plant as dried remnants. Lateral roots can develop from the hypocotyl above the initial infection site if sufficient soil moisture is available. Infected plants are stunted with yellow leaves, but are seldom killed by the fungus. Yield losses associated with Fusarium root rot are difficult to assess, but may exceed 50 to 75% in severe cases.

Biological Control

Deny (Burkholderia cepacia) is registered for protection of roots from Fusarium, Rhizoctonia, and Pythium spp. T-22 Planter Box (Trichoderma harzianum strain KRL-AG2) is registered, but cannot by used in alkaline or cold soils. The efficacy of these biological control agents in the High Plains is not known. Incorporation of organic matter may reduce Fusarium root rot by improving soil tilth and water holding capacity.

Cultural Control

Plant high quality seed in warm (greater than 60°F), well-prepared seedbeds under conditions favorable to rapid seedling emergence. Some varieties are less susceptible or tolerant to Fusarium root rot. Avoid planting in fields with a history of root rot problems. A three-year or longer rotation to non-hosts such as small grains or field corn reduces root rot severity. Chiseling (sub-soiling) 10 to 20 inches deep between dry bean rows promotes root penetration and improves plant tolerance to root rot; but avoid root injury during field operations. Manage irrigation to eliminate moisture stress to the developing plant, but avoid excessive water that may deprive roots of oxygen. Avoid dense plantings. Space plants at least two to three inches apart within the row to reduce plant competition for water. Till soil up around the base of infected plants to enhance lateral root development above infected portions of the hypocotyls.

Chemical Control

Seed and in-furrow treatments may delay root infection, but they must be used in combination with sound cultural practices to reduce Fusarium root rot losses.

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