European Mycotoxin Awareness Network - EMAN

Wheat Production in Europe 1 - Pre-harvest

5. Wheat production in Europe 1 - pre-harvest

Introduction

In this article we look at the pre-harvest situation, i.e. the growing crop together with the relevant environmental conditions, the disease pressure and the methods for managing this.

1. Mycotoxin hazards

There are two main mycotoxin hazards associated with wheat in Europe. The first is the production of mycotoxins as a result of the development of fungi belonging mainly to the genus Fusarium in the growing crop. Mycotoxins produced by these fungi include zearalenone (ZEA) as well as the trichothecenes, such as nivalenol (NIV), deoxynivalenol (DON) and T-2 toxin. Fusarium species are also responsible for a serious disease called Fusarium ear blight (FEB) which can result in significant losses in crop yield and quality. Measures that are taken against Fusarium development in the field are currently mainly because of FEB rather than the mycotoxin risk. However, with EU legislation imminent the consideration of mycotoxins is becoming increasingly important.

The second mycotoxin hazard is the production of ochratoxin A (OTA) by the fungus Penicillium verrucosum growing on wheat grain in storage conditions (P. verrucosum is a "storage" fungus which is capable of developing in conditions of reduced moisture). Another important consideration is that, although Fusarium infection is generally considered to be a pre-harvest problem, poor drying practices can lead to susceptibility in storage and mycotoxin contamination.

2.The pre-harvest situation

We have already outlined the general factors that are important in mycotoxin control pre-harvest, and described how many of these factors are incorporated into pre-requisite programs such as Good Agricultural Practice (GAP). A number of these factors are of special relevance for wheat production and are discussed below.

2.1 The nature of FEB

It is necessary first to consider the nature of Fusariuminfection of wheat in the field. FEB is in fact a disease complex,which means that it may be caused by individual species, or a combination of related species. In northern Europe this disease complex is normally dominated by Fusarium culmorum, and in southern Europe by F. graminearum, although this species is currently steadily spreading northwards. Other species implicated are F. avenaceum, and F. poae (both toxin producers) and related Microdochium nivale varieties (not known to produce toxins). The problem of FEB seems to have been exacerbated by the increasing cultivation of bread wheats in Europe (Magan et al., 2002). At present no durable, fully FEB 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). However the development of resistant cultivars is an extremely important area since it has been shown that wheat cultivars with resistance to the most aggressive, high DON producing strains of F. graminearum and F. culmorum inhibited both disease progression and toxin production (Mesterhazy, 2002). The relationship between FEB development and mycotoxin production can be complex, and this will be discussed in 2.3 below.

2.2 Field management

Appropriate field management and preparation is particularly relevant to FEB and mycotoxin control. Strategies include deep ploughing as a method of removing residual fungal material present in previous crop debris from the surface. Crop rotation is also important and is intended to break the chain of production of infectious material, for example by using wheat / legume rotations. The use of maize in a rotation is to be avoided as maize is also susceptible to Fusarium infection and can lead to carry-over onto wheat via stubble/crop residues. In Mediterranean climates it is good practice to leave ploughed land exposed to autumn sunshine as a means of destroying fungal material that could otherwise infect the following crop.

There is evidence that infection of wheat flowers (the usual initiation point of FEB infection) can be brought about by water splash of soil-borne material onto the plants during anthesis, so irrigation methods that prevent excessive splashing may be an important control point in some instances. This may be managed by the type of irrigation used, or more practically by the timing of irrigation to avoid the anthesis stage.

Lodging also seems to be an important risk factor in mycotoxin production. A recent study carried out for the Home Grown Cereal Authority (HGCA) showed that when lodging occurred, DON production was very high, irrespective of any fungicide treatment used in the study (Nicholson et al., 2003).

2.3 Environmental conditions

Environmental conditions such as relative humidity and temperature are known to have an important effect on the onset of FEB. For example, it has been shown that moisture conditions at anthesis are critical in Fusarium infection of the ears, and Lacey et al. (1999) has shown that Fusarium infection in the UK is exacerbated by wet periods in the summer. Equally, drought damaged plants are more susceptible to infection. Little was known until recently about the threshold limits for mycotoxin production in the FEB system.

Hope and Magan (2003) have carried out in-vitro studies on DON and NIV production by a strain of F. culmorum over a range of water activity (a_W) and temperature conditions, using a wheat grain-based substrate. They found that the conditions under which both toxins were produced were far more restrictive than the conditions allowing growth of the fungus. For example, production occurred in the relatively narrow a_W range 0.995-0.95 while growth persisted to 0.90 a_W. However, the optimal conditions for DON and NIV production; 25⁰C at 0.995 and 0.981 a_W respectively, were within the range optimal for growth. These a_W levels correspond to water contents of approximately 30% and 26% respectively, and are within normal ranges for harvested grain in wet years. Toxin production was significantly higher at 25⁰C compared to 15⁰C. Both toxins continued to accumulate for the duration of the studies (40 days).

2.4 Fungicide use.

Part of the integrated control of FEB in wheat production involves the use of fungicides, but this introduces a complication as far as mycotoxins are concerned, as there is evidence that under certain conditions fungicide use may actually stimulate toxin production. This is a particularly undesirable situation since circumstances may arise where the obvious manifestations of FEB are reduced or even eliminated, and yet high levels of mycotoxins may be present. Clearly grain affected in this way cannot be identified by visual inspection for signs of FEB (e.g pink grains) and in fact cannot be identified until a specific mycotoxin analysis is carried out. As we have pointed out, mycotoxin analysis does not figure routinely in the grain supply chain at present.

Research carried out on fungicide use in terms of FEB and mycotoxin development has produced very interesting results. In particular, fungicides in common use have been shown to have differential effects against toxin forming Fusarium species and related non-toxin forming pathogens such as Microdochium nivale on ears (Simpson et al., 2001).

The outcome of the use of fungicides seems to depend on the fungal species present, and the effect that the particular fungicide has on these species. For example, in recent work commissioned by the UK Home Grown Cereal Authority (HGCA), the use of azoxystrobin showed a significant reduction in disease levels while increasing the levels of DON present in grain in an experimental situation where F. culmorum and M. nivale were both present, (Nicholson et al., 2003). This was believed to be the result of selective inhibition of the non-toxigenic M. nivale, as azoxystrobin was very effective at inhibiting this fungus. M. nivale is a natural competitor of toxin forming Fusarium species, particularly F. culmorum. Removal of M. nivale by the fungicide probably allowed development of the toxigenic species in its place with concomitant increase in toxin formation. This is an important finding since it indicates that the effects of the fungicides are not directly on toxin production. In the same study the fungicides tebucanazole, metconazole and HGCA 2 appeared to work in an opposite way so that they selectively inhibited F. culmorum, while having a far less marked effect on M. nivale. The efficacy of the fungicides in these situations was found to be directly correlated with dose level. It follows from these findings that where FEB is caused by Fusarium species in the absence of Microdochium that disease development is associated with higher levels of toxin.

Simpson et al. (2002) also carried out work with azoxystrobin and reported similar findings. Importantly, mixtures of azoxystrobin with both prochloraz and fluquinconazole were found to be less effective against F. culmorum than M. nivale.

Other fungicides that have been shown to stimulate toxin production under certain conditions include tridemorph, which stimulated T-2 toxin production by F. sporotrichoides; tubiconazole, which stimulated production of monoacetyl deoxynivalenol (3-AcDON) and difenoconazole, which stimulated production of the same toxin by F. culmorum (Magan et al., 2002). Recent in-vitro studies by the same author, looking at a range of F. culmorum strains from across Europe, showed a stimulation in DON production in the presence of epoxiconazole and propiconazole.

Further work in the same series of studies carried out for HGCA looked at lower dose levels of fungicides, since it has been suggested that sub-optimal additions may be a cause of high levels of production of mycotoxins in grain. In fact results obtained, at least for some fungicides, did not bear this out. Fungicides such as tebuconazole, metconazole and HGCA 2 were able to inhibit toxin production at levels that were insufficient to completely inhibit FEB symptoms. Sub-optimal levels of fungicides were, as may be expected, more effective in situations where disease levels were low. However, in situations where both Fusarium and Microdochium species were present, significant disease and toxin reduction was not always achieved by these fungicides, even for full dose application

In general, the studies found that full dose application of fungicides with main activity against Fusarium species, and timed at mid-anthesis (mid-flowering) were most effective at inhibiting both FEB and DON and NIV production. Fungicides used in the studies appeared to be equally effective against DON and NIV producing strains of F. culmorum.

This series of studies also looked at fungicide application methods and found that the use of double fan nozzles and a reduced tractor speed resulted in significantly improved coverage of wheat ears, and improved control of FEB.

This work on the effects of fungicides on FEB and toxin development has interesting implications for dealing with field development of Fusarium toxins. It is clear that the situation is complex, this stems from the nature of FEB itself, depending as it does on interactions between a group of fungal species. The work also suggests the presence of a number of important risk factors concerned with the use of fungicides in the field.

2.5. Biological control agents

The EC Project CONTROL MYCOTOX FOOD (see Newsletter 2) is currently looking at the potential for the use of biological control agents (BCA�s) for the management of FEB and mycotoxin production in the field. The BCA approach involves the application of antagonistic fungi which inhibit the disease and toxin producing species. This approach is in principle very attractive since it could lead the way to the reduction of the use of chemical fungicides in crop production. The work has involved both glasshouse and field trials and has yielded a number of candidate antagonists that show the ability to decrease both FEB and DON production significantly. Overall BCA�s may be more effective at decreasing mycotoxin production than eliminating FEB. Much more work is required in this area before a useable commercial product can be developed.

References

Diamond, H. and Cooke, B. M. (2002) Preliminary studies on biological control of the

Fusarium ear blight complex of wheat. Crop Protection 22, 99-107.

Hope, R. and Magan, N. (2003). Two-dimensional environmental profiles of growth, deoxynivalenol and nivalenol production by Fusarium culmorum on a wheat -based substrate. Letters in Applied Microbiology 37, 1-5.

Lacey, J. Bateman, G. L. and Mirocha, C. J. (1999) Effects of infection time and moisture on the development of ear blight and deoxynivalenol production by Fusarium spp in wheat. Annals of Applied Biology 134, 277-283.

Nicholson, P., Turner, J. A., Jenkinson, P., Jennings, P., Stonehouse, J., Nuttall, M., Dring, D., Weston, G. and Thomsett, M. (2003) Maximising control with fungicides of Fusarium Ear Blight (FEB) in order to reduce toxin contamination of wheat. Project Report No. 297, HGCA, London.

Magan, N., Hope, R., Colleate, A. and Baxter, E. S. (2002) Relationship between growth and mycotoxin production by Fusarium species, biocides and environment. European Journal of Plant Pathology 108, 685-690.

Mesterhazy, A. (2002) Role of deoxynivalenol in aggressiveness of Fusarium graminearum and F. culmorum and in resistance to Fusarium head blight. European Journal of Plant Pathology 108, 675-684.

Simpson, D. R., Weston, G. E., Turner, J. A., Jennings, P and Nicholson, P. (2001) Differential control of Head Blight pathogens of wheat by fungicides and consequences for mycotoxin contamination in grain. European Journal of Plant Pathology 107, 421-431.

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