Studies on the chemical control of Fusarium ear blight of winter wheat (Triticum aestivum L.)

Liggitt, Joanne (1997). Studies on the chemical control of Fusarium ear blight of winter wheat (Triticum aestivum L.). PhD thesis The Open University.



The fungicides prochloraz and tebuconazole (at concentrations of 2 µg ml-1 ) were shown to reduce the mycelial growth of Fusarium culmorum, F. avenaceum, F. poae, F. graminearum and Microdochium nivale in vitro by over 90 % compared to the untreated control. In addition, chlorothalonil inhibited spore germination of all species and pyrimethanil reduced the mycelial growth of M. nivale by over 60 % at 2 µg ml-1, although it was ineffective against the other species. In the glasshouse, prochloraz and tebuconazole were moderately effective in reducing the severity of fusarium ear blight (FEB) caused by F. culmorum and M. nivale. The fungicides gave less effective control of FEB in the field. There was a significant relationship between the incidence and severity of FEB in 1995 but there was no significant relationship between ear blight and yield in either 1995 or 1996.

It was proposed that the interactions between saprophytic microflora and ear blight pathogens may account for the poor performance of fungicides against FEB in vivo. Glasshouse and laboratory studies showed that Alternaria alternata, Botrytis cinerea and >Cladosporium herbarum reduced the severity of FEB caused by F. culmorum and this antagonism was attributable to both volatile and non-volatile antibiotic production. The saprophytic species showed inherent variability in their sensitivity to the fungicides tested in vitro and in the glasshouse. It was shown that certain fungicides (e.g. pyrimethanil) which reduced mycelial growth of the saprophytic species in vitro allowed the pathogen to grow by reducing the antagonism of the microflora species against the pathogen. This may not be true for all fungicides in practice.

It was also proposed that the inefficacy of fungicides to control FEB was due to a failure of the fungicide to reach the site of infection. It was shown, using a fluorescent tracer that retention 11 of spray was influenced by cultivar, time of application and fungicide. The amount of tracer retained was significantly correlated with the number of extruded anthers of wheat. When radio-labelled prochloraz was applied to the ears of wheat, the prochloraz was retained predominantly on the outer glumes, with very small amounts being retained by the florets and rachis. There was no movement of prochloraz between tissues and the half-life of the active ingredient was 48 hours.

This work illustrates the efficacy of fungicides against Fusarium spp. and Microdochium nivale in vitro, under glasshouse conditions and in the field, and provides some evidence to explain their poor performance. It is proposed that future work should investigate environmental and biological factors which contribute to ear blight epidemics, in order that a forecasting system for fungicide application can be devised. Also, studies of fungicide activity against antagonistic ear microflora species and studies of fungicide retention and penetration may help to optimise fungicide application to control this disease.

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