Linkage of Vfa4 in Malus × domestica and Malus floribunda with Vf resistance to the apple scab pathogen Venturia inaequalis

The Vf locus from Malus floribunda clone 821 is an important source of resistance to apple scab disease caused by Venturia inaequalis , and has been introduced into numerous cultivars of domesticated apple, Malus  ×  domestica . Cloning of the putative Vf locus has revealed that it contains several receptor-like Vf candidate genes. In order to determine which of these genes is most closely linked to Vf resistance, primers were designed based on conserved regions in the Vf candidate genes adjacent to a variable portion of the leucine-rich repeat (LRR) domain, to yield PCR product length polymorphisms. PCR products were obtained from 31 cultivars of M.  ×  domestica , of which 19 are known to contain Vf resistance, and from 10 selections of M. floribunda . PCR products corresponding in size to Vfa1 and Vfa2 were found in all the plants tested. However, a PCR product with 100% predicted amino acid identity to Vfa4 was found only in M.  × domestica cultivars known to have Vf scab resistance. This PCR product was also found in most, but not all, selections of M. floribunda tested, including the original source of Vf resistance, M. floribunda 821. The PCR product matching Vfa4 appears to be the most closely linked to Vf resistance and should be a valuable tool for monitoring Vf inheritance in apple.     

Identification of the mutation responsible for resistance to QoI fungicides and its detection in Ascochyta rabiei (teleomorph Didymella rabiei)

This study characterized a fragment of the cytochrome b gene from Ascochyta rabiei isolates collected in North Dakota, USA, that varied in sensitivity to quinone‐outside inhibitor (QoI) fungicides. The sequenced genomic DNA fragment contained a group I intron immediately after codon 131. The size of the cytochrome b gene was estimated to be over 4·6 kb. Multiple alignment analysis of cDNA and protein sequences revealed a mutation that changed the codon for amino acid 143 from GGT to GCT, introducing an amino acid substitution from glycine to alanine (G143A), which is frequently associated with QoI resistance. Based on this mutation, a diagnostic PCR assay was developed using an approach called mismatch amplification mutation assay. This method was successfully validated by testing a total of 70 A. rabiei isolates, of which 38 isolates were found to be QoI‐resistant. This fast and accurate PCR assay provides a very useful and simple screening method for QoI resistance in A. rabiei isolates.