The role of saprophytic microflora in the development of Fusarium ear blight of winter wheat caused by Fusarium culmorum

Saprophytic microflora may interact with ear blight pathogens of wheat and contribute to the poor performance of fungicides against this disease in vivo. A glasshouse experiment and in vitro experiments were conducted to determine the relationship between saprophytic microflora and Fusarium culmorum and to determine fungicide effects on Alternaria alternata, Botrytis cinerea, Cladosporium herbarum and Fusarium culmorum. Inoculation of winter wheat ears (cultivar Avalon) in the glass-house with A. alternata, B. cinerea or C. herbarum at GS 69 before inoculation with F. culmorum at GS 69 led to a decrease in the severity of Fusarium ear blight (FEB). The mean percentage of spikelets infected was reduced from 19% for ears which had been inoculated with the pathogen alone to 4%, 6% and 5% for ears which had been inoculated with A. alternata, B. cinerea and C. herbarum, respectively. In vitro, the mycelial growth of F. culmorum was reduced when inoculated opposite A. alternata, B. cinerea and C. herbarum. The antagonism was shown to be due to production of non-volatile and volatile antibiotics by the saprophytes. In vitro experiments showed that F. culmorum and the three saprophytes showed differential sensitivity to a range of fungicides including benomyl, chlorothalonil, fluquinconazole, flusilazole, flutriafol, prochloraz, pyrimethanil and tebuconazole. These experiments showed that A. alternata, B. cinerea and C. herbarum suppressed the mycelial growth of F. culmorum in vitro and reduced the severity of FEB when inoculated onto plants before the pathogen. The four species showed differential sensitivity to fungicides in vitro. It is possible then that fungicides disrupt the normal suppression of FEB by saprophytic microflora on the ears of wheat. (C) 1997 Elsevier Science Ltd.