Development of a reliable and rapid detached leaf assay to detect resistance to the fungal disease phomopsis leaf blight, caused by Diaporthe toxica, in Lupinus albus

Abstract

A detached leaf assay was developed to assess disease response of Lupinus albus to phomopsis leaf blight caused by Diaporthe toxica. A spore concentration of 1 × 107 spores mL−1 using spray inoculation gave the greatest discrimination between genotypes. The effect of leaf position was studied using the first five fully expanded leaves above the cotyledons. Leaf position did not alter the disease response for genotypes with a resistant reaction, but older leaves were more susceptible in less-resistant genotypes. There was good agreement between disease response using detached leaves and leaves inoculated in planta, indicating the utility of the detached leaf assay. In this study, 110 Lupinus albus cultivars were evaluated for resistance to phomopsis leaf blight caused by Diaporthe toxica in two detached leaf experiments. In one experiment, 95 genotypes were assessed for phomopsis leaf blight resistance and phomopsis stem blight resistance after the leaves used in this study were removed. There was no correlation between the stem scores and the leaf scores, indicating that a detached leaf assay cannot be used to predict the reaction of L. albus to phomopsis stem blight. Genetic resistance to phomopsis stem and leaf blight may be under independent genetic control. Sixteen genotypes were identified as having useful resistance to phomopsis leaf blight. Relying on a detached leaf assay to detect overall disease resistance in L. albus may lead to some false positives (apparent resistance) but the assay was generally capable of detecting susceptible genotypes.