The relationship between root and leaf infection in 11 cultivars of barley (Hordeum vulgare) by different isolates of Bipolaris sorokiniana was investigated in young plants. Roots of 10‐day‐old seedlings, grown in filterpaper rolls, and the third leaf of 17‐day‐old seedlings were inoculated with the different isolates and a Disease Development Index (DDI) was calculated. The rate of lesion development in leaves was higher than in roots, indicated by generally higher DDI after leaf inoculation than after root inoculation. Significant differences in resistance were found among the barley cultivars. Inoculation with different isolates of B. sorokiniana caused significant differences in DDI for both roots and leaves. In the leaves, but not in the roots, a significant cultivar–isolate interaction was found. No significant correlations, neither in isolate aggressiveness nor in cultivar reaction between root and leaf, were observed.

Ergosterol content in the plant pathogenic fungus Bipolaris sorokiniana was determined in different matrices including mycelium, spores, culture filtrate and infected barley leaves. Ergosterol was extracted with methanol, hydrolysed with KOH and quantified by reverse phase high performance liquid chromatography (HPLC). Our procedure was used to study how the ergosterol concentration of B. sorokiniana varied due to fungal age and nutrient availability when growing in liquid medium. It was found that the ergosterol content decreased with fungal age. The decrease was not due to leakage. It was also found that a change to a less nutrient-rich medium caused an increase in ergosterol content whereas a change to a rich medium led to a decrease. The procedure was also used for quantification of fungal infections in complex matrices (e.g. leaves). The development of fungal infection in barley leaves was followed during 10 days. Visual grading of leaf spots was also compared to ergosterol content in three varieties of barley. The ergosterol content in the leaves increased exponentially until day 7, and the grading of the leaf spots was correlated to the ergosterol content. Our results show that, despite a great variation, ergosterol may be used as a biomarker to detect and quantify fungal infections in a given matrix.