Trichothecene production preharvest

The key preharvest considerations are the environmental conditions and the range of inputs into intensive monoculture cereal crop systems. The key hazard associated with natural contamination with trichothecenes, especially deoxynivalenol, in wheat preharvest in Europe is the presence of strains from one or more Fusarium spp. infecting the growing crop. Fusarium spp. are plant pathogens, some of which cause Fusarium head blight, also known as ear blight or head scab, which results in significant losses in grain quantity and quality (Jennings et al., 2004). Measures taken against Fusarium development in the field are determined by the need to control Fusarium head blight rather than by the need to reduce the mycotoxin risk. However, with European Union legislation imminent and regulations already existing in many countries world-wide (FAO, 2004) the consideration of mycotoxin contamination has become more important. Although Fusarium infection generally is considered a preharvest problem, inadequate grain drying may result in colonization by Fusa-rium species in damp layers of drying grain and an increase in mycotoxin contamination.

The nature ofFusarium head blight

Fusarium head blight may be a disease complex, which means that it can be caused by individual species or by a combination of related species. In northern Europe the disease usually is caused by F. culmorum, and in southern Europe by F. graminearum, although recent studies suggest that the latter species is becoming more common across Europe, including the United Kingdom (Edwards and Ray, 2005; Waalwijk et al., Chapter 17). Studies suggest that F. graminearum is very competitive and may be able to outcompete F. culmorum (Hope et al., 2005a). Other species implicated are F. avenaceum, and F. poae (both mycotoxin producers) and some closely related Microdochium nivale varieties that are not known to produce toxins. The problem with Fusarium head blight seems to have been exacerbated by the increasing cultivation of bread wheats in Europe (Magan et al., 2002). At present no durable, fully Fusarium head blight resistant wheat cultivars exist, so control relies on the use of cultivars with partial resistance together with field management and fungicide use (Diamond and Cooke, 2003). The development of resistant cultivars is an extremely important area, since wheat cultivars that are resistant to the most aggressive, high deoxynivalenol-producing strains of F. graminearum and F. culmorum have less disease progression and lower levels of toxin contamination (Mesterhazy, 2002).

Resistant cultivars

Recently, this research area has attracted a great deal of attention. Mesterhazy (2002) showed that the wheat varieties most resistant to Fusarium head blight also were contaminated by little, if any, deoxynivalenol. In fact, resistance appeared to depend on inhibition of toxin production directly, since the most aggressive disease-causing fungal strains were those that produced the highest levels of deoxynivalenol. We think that increased availability of such resistant varieties, coupled with the use of appropriate fungicides are a critical part of any integrated approach to controlling Fusarium diseases and mycotoxin contamination. Such a strategy, with emphasis on mycotoxin control, could represent an important critical control point at the preharvest stage.

Field management

Appropriate field management and preparation is particularly important for Fusarium head blight and trichothecene control. An important part of the strategy is deep plowing to re move from the surface residual fungal material, which is present in debris from the previous crop. Crop rotation also is important and can break the chain of production of infectious material by reducing sporulation on crop residue. For example, wheat/legume rotations decrease the amount of inoculum present. The use of maize in such rotation should be avoided as this crop also is susceptible to infection by the same Fusarium strains that can infect wheat, and maize debris can be a very good source of carry-over onto wheat via stubble/crop residues. In Mediterranean climates it is a good practice to leave plowed land exposed to autumn sunshine to help destroy fungal material that could otherwise infect the following crop.

There is a narrow window (anthesis) during which ripening heads are susceptible to infection by splash dispersed soil-borne material onto the plants during periods of rain. If irrigation is used, then excessive splashing should be avoided and this practice may be an important control point in some instances. Effective management of the irrigation regime and the timing of irrigation applications to avoid infection at anthesis are very important. Lodging also is an important risk factor in trichothecene production. If lodging occurs then, deoxynivalenol production is very high irrespective of any fungicide treatment used as part of a control strategy (Nicholson et al., 2003; Jennings et al., 2004).

Environmental conditions

Environmental conditions such as relative humidity and temperature have an important effect on the onset of Fusarium head blight, with the moisture conditions during anthesis being particularly critical for Fusarium infection of the heads. Lacey et al. (1999) showed that F. culmorum infection in the UK increases if it is wet during the early flowering period in the summer, which is the optimum time for infection. Drought stress also can damage plants and make them more susceptible to infection. However, until recently, very little was known about the threshold limits for trichothecene production in the Fusarium head blight system. Agro-meteorological information for the period preceding and during ripening can be used to predict the risk of deox-ynivalenol contamination of wheat by F. graminearum and F. culmorum [Hooker and Schaafs-ma (2003) in Canada; Detrixhe et al. (2003) in Belgium; Rossi et al. (2003) in Italy].

Hope and Magan (2003) compared two dimensional environmental profiles for growth and deoxynivalenol production by F. culmorum and F. graminearum to water availability (water activity, aw) and temperature. They found that the range of conditions over which deoxynivalenol was produced was far more restricted than were the conditions under which growth could occur on wheat grain. Toxin production occurred in the relatively narrow aw range 0.995-0.95 while growth persisted to 0.93-0.90 aw. The optimal conditions for deoxyni-valenol production (25°C, 0.995-0.98 aw) were within the range that also was optimal for growth. These aw levels correspond to water contents of approximately 30% and 26% respectively. In wet years, grain with > 20% moisture content can be present. Toxin production was significantly higher at 25°C than at 15°C. The significant difference between the species was temperature, with the aw ranges being similar. Recent studies of F. graminearum strains from other regions of the world are consistent with these conclusions (Ramirez et al., 2006).

Fungicide use

Part of the integrated control of Fusarium head blight in wheat involves the use of fungicides, but such a use also complicates predictions of trichothecene production, as under some conditions the fungicide may stimulate toxin production (Ramirez et al., 2004). This situation is particularly dangerous since circumstances may occur in which the disease levels of Fu-sarium head blight are reduced or even eliminated, and yet high levels of mycotoxin contamination may still occur. Clearly grain affected in this way cannot be identified by visual inspection for signs of Fusarium head blight, e.g., pink grain or tombstone kernels, and, in fact, cannot be identified as a problem at all until specific mycotoxin analyses are conducted.

Research on fungicide use in terms of Fusarium head blight and mycotoxin development has produced very interesting results. In particular, fungicides in common use have differential effects against toxin-forming Fusarium species and related non-toxin forming pathogens, e.g., Microdochium nivale on wheat heads (Simpson et al., 2001). The effectiveness of the use of a fungicide depends on the fungal species present, and the effect that a particular fungicide has on these species. For example, in work commissioned by the Home Grown Cereal Authority (HGCA), azoxystrobin significantly reduced disease levels but increased the levels of deoxynivalenol present in grain under experimental conditions when F. culmorum and M. nivale were both present (Nicholson et al., 2003). These data were thought to result from the selective inhibition of M. nivale by azoxystrobin. Microdo-chium nivale is a natural competitor of toxin-forming Fusarium spp., particularly F. culmorum. Removal of M. nivale by the fungicide probably allowed the toxigenic Fusarium species to fill the niche with a concomitant increase in toxin formation. This result is an important finding because it indicates that the impact of the fungicide is only indirectly related to mycotoxin production. Other fungicides such as tebuconazole and metaconazole appeared to work in the opposite way, i.e., they selectively inhibit F. culmorum, while having far less effect on M. nivale. The efficacy of the fungicides in these situations was directly correlated with the applied dose. Thus, whenever Fusarium head blight is caused by Fusarium species in the absence of Microdochium, disease severity can be associated with higher levels of toxin.

Simpson et al. (2001) also worked with azoxystrobin and reported similar findings, i.e., mixtures of azoxystrobin with either prochloraz or fluquinconazole were less effective against F. culmorum than they were against M. nivale. Other fungicides that stimulate toxin production under some conditions include tridemorph, which stimulates T-2 toxin production by F. sporotrichioides (Moss and Frank, 1985), and tubiconazole and difenoconazole, which both stimulated production of monoacetyl deoxynivalenol (3-AcDON) by F. culmorum. In-vitro studies of several strains of F. culmorum from across Europe found that increased deoxyniva-lenol production occurred in the presence of low concentrations of epoxiconazole and propi-conazole (Magan et al., 2002; Ramirez et al., 2004). Thus, it is important to use the recommended rate of fungicide to ensure full effects against Fusarium spp. Fungicides applied at early to mid-anthesis (mid-flowering) were the most effective at inhibiting both Fusarium head blight and deoxynivalenol/nivalenol contamination (Edwards and Ray, 2005).

Biological control agents

In a comprehensive screening of biocompetitive microorganisms to control Fusarium pathogens of cereal ears and maize, potential microorganisms were identified that decrease sporulation by Fusarium species on cereal stubble, and thus decrease the pool of inoculum for infection (Dawson et al., 2002a,b; Luongo et al., 2005). Studies to control Fusarium head blight and deoxyni-valenol production have identified a number of microorganisms that can effectively decrease both toxin and disease levels. There is potential for further progress in this area, but there is a very narrow window of only 5-10 days during which protection is needed. Targeted spraying of biocontrol agents on flowering heads could provide the necessary protection. The potential for commercialization of some of these microbes is being evaluated (Dawson et al., 2004).

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