- Agronomic: sorghum (milo), wheat
- Crop Production: conservation tillage
- Pest Management: biological control
- Soil Management: green manures, soil analysis
This study investigated the use of green manures for reducing Fusarium graminearum survival in association with wheat residues in greenhouse and field experiments. Green manures promoted the development of higher densities and Fusarium-antagonistic abilities of F. graminearum-antagonists in soils. The use of green manures did not significantly impact the survival of F. graminearum in wheat residue. However, streptomycete densities and F.graminearum-antagonist densities were significantly positive correlated with reduced survival of Fusarium. These results suggest that the use of green manures can enhance populations of indigenous soil microorganisms antagonistic to the survival of F. graminearum in wheat residue.
Fusarium head blight (FHB or scab) is one of the most economically devastating diseases of wheat (Triticum aestivum L.) (Windels, 2000). Studies have documented a significant increase in the frequency of FHB epidemics worldwide over the last 15 years (McMullen, 2005; Windels, 2000). Some of the proposed causes for the upsurge of FHB epidemics include an increase in the frequency of wet weather, especially between anthesis and harvest; the planting of highly susceptible wheat cultivars; and the widespread adoption of conservation tillage practices (Dill-Macky, 1996; McMullen et al., 1997).
Fusarium head blight is caused by several species from the genus Fusarium. F.graminearum Schwabe (Gibberella zeae (Schwein.) Petch [telemorph]) is most frequently associated with this disease in the United States (Wilcoxson et al., 1988; Wong et al., 1992). Crop residues are reported as the major source of ascospores, which are considered the primary inoculum in FHB epidemics (Reis, 1988; Sutton, 1982). The residues of corn (Zea mays L.) and small grain cereals (Champeil et al., 2004; Dill-Macky and Jones, 2000; Sutton, 1982; Wiese, 1987), are particularly important contributors of the inoculum initiating FHB epidemics. Although F. graminearum readily produces conidia on the infected spikes of cereal plants, FHB is generally regarded as a monocyclic disease (Bai and Shaner, 1994; Khonga and Sutton, 1988). Because residues are the principal inoculum reservoir, inoculum densities are associated with the amount of crop residue and the degree of infestation (Shaner, 2003; Dill-Macky and Jones, 2000; Pirgozliev et al., 2003). Pereyra and Dill-Macky (2004) concluded that the node tissues of wheat, being slower than other tissues to decompose, provide an ideal site for inoculum production over extended periods. The survival of F. graminearum in residues will however be affected by other microbes, either directly by competition, parasitism and predation, and/or indirectly by influencing the rate of residue decomposition.
Several studies have focused on reducing the inoculum produced by crop residues (Dill-Macky and Jones, 2000; Khonga and Sutton, 1988; Pereyra and Dill-Macky, 2004; Sutton and Vyn, 1990). A relatively new approach to reduce pathogen populations in crop residues is managing indigenous soil microbial communities (Peters et al., 2003; Sturz et al., 1997; Weller et al., 2002; Zaitlin et al., 2004). Organic amendments may be used to increase the activity of indigenous microbes that act as biocontrol agents (Lockwood, 1988). Bossio et al. (1998), Mazzola (2004) and Govaerts et al. (2006) reported that cultural practices, soil type, plant species, and plant genotype are significant factors determining the composition of soil microbial communities. The soil community requires carbon energy sources, and different sources may promote some microorganisms over others (Bailey and Lazarovits, 2003). Cover crops and green manures have been reported as especially effective in increasing the proportion of pathogen inhibitory microorganisms within indigenous microbial populations (Davis et al., 1994; Lupwayi et al., 1998; Mazzola et al., 2001; Wiggins and Kinkel, 2005a,b).
The incorporation of green manures has been shown to increase the density and diversity of microbes in soil, particularly the density and the pathogen-inhibitory activity of bacteria such as fluorescent Pseudomonas spp. (Mazzola et al., 2001; Bulluck and Ristaino, 2002), non-pathogenic Fusarium spp. (Davis et al., 1994; Alabouvette et al., 1996), streptomycetes and actinomycetes (Mazzola et al., 2001; Wiggins and Kinkel, 2005a, b). Several authors have reported effective disease control using green manures to promote the increase of Streptomyces populations acting as biocontrol against diseases caused by other Streptomyces (Wiggins, 2003; Lazarovits et al., 1999), Phytophthora (Jones and Samac, 1996; Xiao et al., 2002), Pythium (Chamberlain and Crawford, 1999; Jones and Samac, 1996) and Rhizoctonia (Chamberlain and Crawford, 1999; Wiggins, 2003). Furthermore, Chamberlain and Crawford (1999) demonstrated that inoculation with Streptomyces hygroscopicus effectively controlled Fusarium oxysporum Schlechtendahl amend. Snyder & Hansen in turfgrass production. Wiggins (2003), found that survival of Fusarium oxysporum was significantly lower in soils following the incorporation of a buckwheat green manure compared to control treatments and that greater pathogen inhibitory activity of the streptomycete population was consistently associated with this result. There is however no literature on the use of green manures to promote indigenous streptomycete populations to control FHB.
Fusarium head blight is one of the most difficult diseases to control due to the lack of effective genetic resistance, the wide host range of the pathogen, and the long distance dispersal of inoculum. The available control measures, including the use of moderately resistant varieties, fungicides and cultural practices are only partially effective (Luz et al., 2003). Due to the lack of a single effective practice to control FHB, the integration of several different management practices appears necessary. By improving our knowledge of how FHB-antagonists might be manipulated using green manures, it may be possible to expand the available management options.
- Determine the effects of green manures on the frequency of soilborne antagonists inhibitory against pathogenic F. graminearum, and on the intensity of their F. graminearum inhibition.
Quantify the impacts of green manures on the survival of F. graminearum on wheat residues.
Evaluate the effects of green manures on the rate of decomposition of F. graminearum-infected residue.