Culm discolouration as an indicator of Fusarium pseudograminearum biomass

Abstract

Disease severity of crown rot (Fusarium pseudograminearum) in winter cereal culms is typically assessed at harvest maturity by recording incidence of infection, whitehead production and discolouration of basal internodes. This study has investigated the relationship between the proportion of discolouration of individual internodes and the extent of host tissue colonisation by the pathogen, based on species-specific quantitative PCR (qPCR). Field-grown plots were used to compare visual ratings and F. pseudograminearum DNA content of eight cereal genotypes (six hard white spring wheats, one durum wheat and one barley) at 16 and 22 weeks after planting (WAP) in 2009 and 2013. At 16 WAP (post anthesis – early milk development) strong correlations were present between visual discolouration and fungal DNA content per unit of tissue weight for internodes one, two and three in 2009 and 2013. At 22 WAP (harvest maturity) these relationships were only significant for internode two in 2009 and internode three in the 2013 trial. Differences in both visual symptoms and fungal DNA content between the genotypes occurred at both sample times, but the most significant distinction between genotypes was at 16 WAP. We conclude that visible discolouration of cereal culms is a useful indicator of fungal biomass in culm tissue, as estimated by qPCR. Furthermore, assessment of diseased culms at 16 WAP provides a stronger correlation with fungal colonisation and discriminates more clearly between disease reactions in different host genotypes, in comparison to assessment at harvest maturity.