Studies on the epidemiology and chemical control of fusarium seedling blight of wheat using molecular techniques

Fusarium seedling blight is an important disease of wheat caused primarily by seed-borne Microdochium nivale of which there are two sub-species; var. nivale and var. majus. Though important pathogens of UK wheat crops, little is known about the relationship between seed-borne inoculum of each sub-species and subsequent disease, annual and regional distributions of inoculum, their pathogenicity as seed-borne pathogens or the relative effectiveness of the main method used to control seedling blight (seed treatments). This investigation aims to answer questions relating to these important factors of M. nivale as a seed-bome pathogen. Nucleotide sequences for the elongation factor 1-a gene were used to produce primers specific to Microdochium nivale var. nivale and var. majus. Internal standards were constructed and quantitative PGR assays developed using standard assay curves. Microdochium nivale var. nivale and var. majus inoculum was quantified on seed harvested in 1997, 1998 and 1999. Total M. nivale DNA was significantly related to the determination of M. nivale contamination using standard agar plate counts in each year, both estimates of M. nivale contamination were significantly related to seedling blight disease symptoms. A positive correlation was observed between fungal biomass of M. nivale var. nivale and var. majus in each year. Microdochium nivale var. majus was the dominant sub-species present in Scottish samples from each year though only in English samples from 1999. Microdochium nivale var. majus was the more pathogenic seedling blight pathogen as either inoculated surface-borne or natural seed-bome inoculum. Beret Gold was most active of six fungicide seed treatments towards isolates of each sub-species in vitro. Field trials showed that the amount of M. nivale DNA quantified in seed was reflected in the amount present in the resultant seedlings. This was reduced significantly through the use of the seed treatments Beret Gold or Sibutol. Significant differences between the treatments were evident when disease pressure was high.Microdochium nivale var. majus was the most effective coloniser at the seedling stage whereas var. nivale predominated prior to flowering. These results show that (i) PCR assays are a useful determinant of M. nivale contamination of wheat seed and seedling blight symptoms, (ii) seasonal and regional fluctuations occur in sub-species predominance on seed samples (iii) M. nivalevar. majus is a more pathogenic seedling blight pathogen than var. nivale (iv) Sibutol and Beret Gold are effective against both sub-species at the seedling stages of crop development