Effect of fungicides on Fusarium ear blight and mycotoxin accumulation in winter wheat (Triticum aestivum L.)

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In order to determine the effect of a range of fungicide products, dose rate and time of application on the development of Fusarium head blight (FHB) and mycotoxin contamination of winter wheat, a series of field experiments (1998-2001) were carried out. A mixture of the fungicides metconazole+azoxystrobin provided the most significant reduction of (FHB) severity (63%) in comparison with control treatments followed by mectonazole (39%) and tebuconazole (45%) applied at both full and half the manufacturers recommended dose rate. Quantification of Tri5 DNA and deoxynivalenol (DON) concentration in harvested grain indicated that metconazole and tebuconazole were the most efficacious fungicides at reducing colonisation of grain by trichothecene-producing Fusarium species and DON content. Azoxystrobin had no effect on Tri5 DNA and DON concentration in grain, but significantly reduced the incidence of Microdochium nivale infected wheat grain. Fungicides applied early in the growing season (GS 31 and 39) were less effective at controlling disease than those applied later in the season (GS 39 and 59). Indeed, it was evident that the timing of fungicides was critical for the effective control of FHB and DON with the best control achieved when fungicides were applied either two days before or two days after the artificial inoculation of wheat ears with FHB causing pathogens. Regression analysis on field trial data revealed that fungicides were able to influence DON content in harvested wheat grain by affecting the extent of grain colonisation by mycotoxin producing species. Since published literature has suggested that fungicides can directly influence DON contamination by imparting a stress influence on FHB pathogens, glasshouse studies were undertaken to determine the effect of a range of dose rates of the fungicides azoxystrobin and metconazole against FHB and DON in wheat plants inoculated with either Fusarium culmorum or F. graminearum. Results revealed that metconazole was effective at reducing FHB, Tri5 DNA and DON and that a rate response was observed as dose rate was increased from one quarter to double the manufacturer's recommended dose rate. Although azoxystrobin reduced FHB, Tri5 DNA and DON content compared to untreated controls, no dose rate response was observed. Regression analysis of glasshouse data supported those obtained from field studies, suggesting that fungicides are able to influence DON content indirectly by affecting the extent of grain colonisation by F. culmorum and F. graminearum. Further glasshouse studies were conducted to determine the interactions between Alternaria tenuissima, Cladosporium herbarum, M. nivale and F. culmorum and the effect of fungicides on these interactions. Introducing C. herbarum or M. nivale to wheat ears at GS 57, before inoculation with F. culmorum at GS 65, resulted in a significant increase in FHB severity, Tri5 DNA and DON concentration in grain. Applying azoxystrobin to wheat ears after the introduction of M. nivale at GS 57, increased DON concentration in grain by 56%. The significance of interactions between saprophytic fungi, M. nivale and Fusarium species on fungicides efficacy against FHB and DON is discussed.