Genetic architecture of tipburn resistance in lettuce.

KEY MESSAGE: Two major QTLs for tipburn were identified in LGs 1 and 5 contributing to resistance in cv. Salinas. The findings suggest pleiotropic effects between leaf crinkliness/savoy and tipburn. Tipburn is a physiological disorder in lettuce that is thought to be caused by a localized deficiency of calcium in leaf tissues. To elucidate the genetic architecture of resistance to tipburn in lettuce, seven recombinant inbred line populations were analyzed in multiple environments and years to identify quantitative trait loci (QTLs) for tipburn. Core height, head firmness, head closure, leaf crinkliness, plant fresh weight, and leaf savoy were also analyzed to investigate whether QTLs for these morphological traits collocated with QTLs for tipburn, which would be indicative of pleiotropic effects. Twenty-three major, intermediate, and minor unique QTLs for tipburn were identified in one or more populations scattered throughout the genome. Two major QTLs for tipburn incidence were identified in linkage groups (LGs) 1 and 5, which determined up to 45 and 66% of the phenotypic variance. The major QTL in LG 1 collocated with the head firmness QTL. The major QTL in LG 5 collocated with the QTL for core height, leaf crinkliness, and head firmness. Further research is needed to determine whether these associations are due to pleiotropic effects of the same gene or if the genes determining these traits are tightly linked. The beneficial alleles at the QTLs in LGs 1 and 5 are present in Lactuca sativa cv. Salinas, the genotype sequenced for the reference genome assembly. Therefore, these QTLs are good targets to identify genes causing tipburn as well as regions for marker-assisted selection to improve resistance to tipburn in lettuce.

The expansion of Phytophthora clade 8b: three new species associated with winter grown vegetable crops.

Despite its association with important agricultural crops, Phytophthora clade 8b is a poorly studied group of species. The clade currently consists of three officially described species (Phytophthora porri, P. brassicae and P. primulae) that are host-specific pathogens of leek, cabbages and Primula spp., respectively. However, over the past few decades, several other clade 8b-like Phytophthoras have been found on a variety of different host plants that were all grown at low temperatures in winter seasons. In this study, a collection of 30 of these isolates was subjected to a phylogenetic study using two loci (the rDNA ITS region and the mitochondrial cox1 gene). This analysis revealed a clear clustering of isolates according to their host plants. To verify whether these isolates belong to separate species, a detailed morphological study was conducted. On the basis of genetic and morphological differences and host specificity, we now present the official description of three new species in clade 8b: Phytophthora cichorii sp. nov., P. dauci sp. nov. and P. lactucae sp. nov. Two other groups of isolates (Phytophthora taxon castitis and Phytophthora taxon parsley) might also represent new species but the data available at this time are insufficient for an official description. This brings Phytophthora clade 8b to a group of six species that are all host-specific, slow-growing and specifically infect herbaceous crops at low temperatures.