Studies on the chemical control of Fusarium ear blight of winter wheat (Triticum aestivum L.).
The fungicides prochloraz and tebuconazole (at concentrations of2 J.lg ml-I
) were shown to
reduce the mycelial growth of Fusarium culmorum, F. avenaceum, F. poae, F. gramineanlm
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 J.lg ml-I
, 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
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.