NPIPM:Fusarium root rot on soybean
Authors: Buyung Hadi and Dean Malvick
Fusarium root rot is an important and widespread disease in the US soybean producing regions. However, this disease may be difficult to diagnose because the causal agent(s) may either act as the primary pathogen or merely colonizing the root along with other pathogenic fungi (e.g. Phytophthora, Pythium, Rhizoctonia).
Causal Organism
Various Fusarium species infect and colonize soybean roots and stems. Fusarium root rot is caused by several different species of Fusarium fungi, two of the most common species are Fusarium oxysporum and Fusarium solani. Other Fusarium species shown to cause root rot of soybean include F. acuminatum, F. chlamydosporium, F. compactum, F. culmorum, F. equiseti, F. graminearum, F. proliferatum, F. redolens,and F. semitectum, F. sporotrichioides, F. subglutinans, and F. verticillioides.
Symptoms and Signs
The lower portion of infected tap root and lateral roots are often rooted and brown or black in color. The infected tap root may eventually be destroyed, leaving a shallow and fibrous root system. Consequently, severely infected plants may wilt when subjected to low moisture and high temperatures.
Soybean seedlings infected with Fusarium root rot may not emerge. If the infected seedlings emerge, they may be stunted and weak with chlorotic cotyledons.
Disease Impacts
A combination of root rot infection and limited soil moisture later in the season can reduce the size and quality of produced seeds considerably. In severe cases, yield reduction can be as significant.
Life Cycle and Epidemiology
Fusarium species can survive in the soil and plant debris as chlamydospores or mycelium for a long period of time. Some dicotyledonous weeds have been shown to serve as symptomless hosts for F. oxysporum.
F. oxysporum enters the soybean host either by directly penetrating the host's epidermis or through natural openings or wounds. Some Fusarium infection is favored by cool temperatures (57-73˚F or 14-23˚C), damp conditions and saturated soils, whereas other types of Fusarium infection are favored by warmd and dry soils. Later in the season, the severity of Fusarium root rot is exacerbated by dry soil conditions. There are other factors that aggravate the severity of Fusarium root rot. Herbicide injuries can predispose the plants to greater severity. The presence of nematodes may also increase the severity of Fusarium root rot.
Management Approaches
Cultural Methods
When the infection is favored by cool soil temperatures and saturated soil, planting in well drained soils during a warmer soil period should reduce the risk of Fusarium root rot. Reducing soil compaction by tillage may result in greater aeration, reducing the favorable conditions for infection.
Host Plant Resistance
No cultivars resistant to Fusarium root rot are currently available. Yet, there appear to be varying degrees of susceptibility to various Fusarium species causing root rot among soybean cultivars.
Chemical Control
Fungicidal seed treatment is recommended in fields with history of Fusarium root rot, although current seed treatments may have limited efficacy for this disease complex.
