Genetic mapping reveals a major locus for red leaf blotch tolerance in almond

Abstract

Red leaf blotch (RLB) of almond, caused by the fungus Polystigma amygdalinum, is a common foliar disease in Mediterranean and Middle East countries. RLB symptoms are characterized by orange spots on leaves that eventually lead to early defoliation, reducing tree photosynthetic activity and fruit production. This study aimed to investigate the genetic basis of RLB tolerance in almond. For this purpose, an F1 population (N=79) derived from a cross between two cultivars with different levels of disease tolerance (‘Tarraco’, highly susceptible; and ‘Vairo’, tolerant) was phenotyped for disease severity and incidence over four years and genotyped with the almond Axiom™ 60K SNP array. Linkage maps were constructed and used for QTL analyses. A major QTL (qP-RLB-3.1m), detected across all years and accounting for up to 21 and 31% of phenotypic variation for disease incidence and severity respectively, was found at the lower end of linkage group (LG) 3. Minor QTLs, only detected in particular years and explaining low percentages of the phenotypic variations, were also detected on LGs 1, 7, and 8. qP-RLB-3.1m was targeted for haplotype analysis in the individuals of the population, and in 19 almond cultivars with different levels of disease susceptibility, revealing an interesting allele associated with increased disease tolerance. In the region spanning qP-RLB-3.1m, there were 505 annotated genes in the almond reference genome. Fourty-nine of them were related to plant defense mechanisms in other species. The possible involvement of these genes and the implementation of marker-assisted selection strategies in almond breeding programs are discussed.