Caracterización de las propiedades de la calidad de agua

Investigations into possible fungicide control measures for F. graminearum and

F. culmorum have provided some promising yet mixed results. The success of fungicide application is complicated by its effects being apparently dependent upon other fungal species present and the effect of the fungicide upon these species (Aldred and Magan, 2004). Another problem arises from reports that certain fungicides may actually stimulate mycotoxin production when applied under certain conditions (Aldred and Magan, 2004). While several fungicide studies have reported a correlation between disease extent and mycotoxin level (Homdork et al., 2000; Mennitti et al., 2003; Mesterhazy et al., 2003; Haidukowski et al., 2005), one point of concern is that fungicide application

reduces visible disease symptoms while having little effect on mycotoxin level, leading to apparently healthy but contaminated grain (Aldred and Magan, 2004).

An example of such a situation was presented by Nicholson et al. (2003). When azoxystrobin (which inhibits respiration by binding to the Qo site of cytochrome

bc1) was applied to fields infected with both Fusarium species and the non-toxin

producing FEB species Microdochium nivale, disease levels were reduced but DON level in grain increased. This was thought to be due to selective inhibition of M. nivale by azoxystrobin, reducing competition on the toxigenic Fusarium

species. In contrast, other fungicides including tebuconazole (a sterol biosynthesis inhibitor) were selective against the Fusarium species only and effects were generally dose- dependent but none of the applications used could reduce DON to a level below 0.75 ppm. Other fungicides that have been found to increase DON production include tubiconazole, difenoconazole, epoxiconazole and propiconazole (Simpson et al., 2001; Magan et al., 2002; Aldred and Magan, 2004).

Tebuconazole has been suggested to be a useful fungicide against FEB in several other studies (Homdork et al., 2000; Cromey et al., 2001; Mennitti et al.,

2003; Mesterhazy et al., 2003), but applications may be insufficient when disease pressure is high (Mesterhazy et al., 2003). The extent of disease and DON reduction by tebuconazole appears to vary with conditions, sometimes being highly effective – two applications at 189 g ai/ha around the flowering stage by Cromey et al. (2001) were reported to reduce FEB incidence by up to 90%. However, Milus and Parsons (1994) found that tebuconazole application at 140 g ai had little effect on disease or DON levels in heavily inoculated plants. In addition, Covarelli et al. (2004) found tebuconazole to be poorly effective against F. culmorum DON production in vivo.

Tebuconazole has very poor solubility (Mauler-Machnik and Suty, 1997). Its effects are short-lived and a slow-release method is required to allow increased solubility and longer periods of availability (Balmas et al., 2006). Balmas et al.

(2006) synthesised a complexation of tebuconazole with β-cyclodextrin for controlling foot and crown rot of Durum wheat in soil inoculated with F. culmorum. Applied as a seed dressing, the complexation reduced disease and

increased grain yield comparable to a commercial tebuconazole formulation. It allowed release of the fungicide without hampering its effectiveness.

Other fungicides reported to show efficacy against FEB include trifloxystrobin, which, in vitro, may act to inhibit initiation of trichothecene biosynthesis (Covarelli et al., 2004), the sterol biosynthesis inhibitors prochloraz and bromoconazole (Mennitti et al., 2003), the mitosis β-tubulin assembly inhibitors

carbendazim, (Jones, 2000; Cromey et al., 2001), the osmotic signalling MAP kinase inhibitor fludioxonil (Jones, 2000) and a range of fungicide mixes. The development of resistance against such fungicides is, however, an ongoing concern. A recent study of carbendazim resistance in F. graminearum in China noted a correlation between resistance and increased trichothecene production (Zhang et al., 2009).

Currently, no clear fungicidal preventative or cure has so far been found. Disease and mycotoxin reduction is often only partial or requiring several applications at high dosage, leading to high cost and potential safety concerns. Grain may still contain mycotoxins at levels above those set by regulations and so be hard to market (Jones, 2000). The effects of fungicide application are also very dependent on application rate and timing and prevailing conditions in the field (discussed in Jones, 2000), with the complexity of Fusarium biology meaning application at the correct time is difficult to achieve (Mauler-Machnik and Suty, 1997). Systemic fungicides may prove more successful as they are absorbed into tissues and less easily removed by moisture (Jones, 2000). New fungicides, combinations and fungicide targets need to be investigated to obtain a much better degree of FEB control at a suitable cost and safety level.

In addition, the application of fungicides to a vertical cereal ear is difficult with substances easily running or washing off the ears. Flowering also tends to occur over a period of around two weeks and to control this period of maximum susceptibility with a single spray is problematic. Finally, with Fusarium exhibiting symptomless spread through ears (Brown et al., 2010), by the time symptoms are noted, then fungicide application will be too late with a large percentage of the ear already infected.

In the future, new EU regulations revising Directive 91/414/EEC which controls the use of plant protection products will further limit the number of chemistries permissible for use on crops. Substances previously used to treat wheat crops that could be subject to exclusion include the triazoles, mancozeb and prochloraz among others (Clarke et al., 2008). As a result, alternative methods of control need to be found and combating wheat diseases may prove much more difficult. According to a report by the environmental consultancy ADAS, the percent loss of UK wheat crop will greatly increase under such proposals (from around 6-16% to 20-30%), with at least a 20% decrease in production, an increase in the land required to maintain current UK production of over 500,000 hectares and a significant economic impact (Clarke et al., 2008).