**5. QTLs related to pathogens resistance in strawberry**

One of the most important shortcomings in strawberry breeding is the lack of commercially cultivated strawberry cultivars with high resistance to many destructive pathogens. Developing disease-resistant cultivars or improving existing cultivars in breeding programs is a priority objective for many strawberry breeding programs around the world [86]. Diseases based on pathogens are not only a very big issue

limiting strawberry production, but also restricts the breeding programs. Thus, resistance to diseases has become a significant focus point for strawberry breeding due to its large effects. In this regard, many major and minor QTLs linked to resistance to pathogens have been identified in cultivated octoploid strawberries so far.

The first QTL related to disease resistance in a strawberry was performed and several RAPD markers associated with the Rfp1 locus, a gene that confers resistance to *Phytophthora fragariae* var. *Fragariae* have been reported by Haymes et al. [87]. Then, two sequence-characterized amplified region (SCAR) markers closely associated with the Rpf1 gene were found to be linked with resistance to *P. fragariae* [88]. For resistance to *Colletotrichum acutatum* pathogenicity, SCAR markers associated with the Rca2 gene were developed [68]. The fruit quality traits were one of the first QTL analyses among the agronomically important traits in strawberries. The population derived from Capitola and CF1116 had a total of 213 genotypes that had many opposite traits and were used for the creation of the mapping population. QTLs related to 34 fruit quality traits were identified in this population [89]. The phenotypic evaluation was carried out over two consecutive years and a total of 22 QTLs were detected, among them 8 QTLs on the female and 14 on the male map and phenotypic variance (PV) values were calculated ranging from 6.5% to 16% in these traits. Denoyes-Rothan et al. [67] determined QTL regions related to resistances of *Cerastium acutatum* and *Phytophthora cactorum* using the same map developed by Lerceteau-Kohler. Although all progenies were screened for *P. cactorum* reaction levels, selected 185 plants were screened in terms of their resistance and susceptibility to *C. acutatum*, while the entire progeny was screened for reaction to *P. cactorum*. Totally, five QTLs with a PV ranging from 5.8% to 10.2% were identified for the resistance to *C. acutatum*.

A total of 39 full-sib families derived from octoploid strawberries were utilized in order to test for resistance to anthracnose fruit rot (AFR). Additionally, a validation population consisting of 77 advanced selections and 10 cultivars was tested in the second season. The phenotyping of accessions was performed every week, and genotyping data was obtained using the IStraw35 SNP array. A major QTL linked by resistance to *C. acutatum* that is named FaRCa1 QTL was located at 55–56 cM of LG VI and the phenotypic variation of this locus was calculated as 50%. It was identified that this locus has a dominant allelic nature in all trials. In this study, a locus linked by AFR was detected in LG VI, while markers associated with resistance to *C. acutatum* were found in LG VII in the previous finding gene, Rca2 [76].

QTLs linked with resistance to *Verticillium dahlia* were identified in the cultivated octoploid strawberry [71]. Researchers investigated resistance to the crown and root rot diseases caused by *Colletotrichum gloeosporioides*, *P. cactorum,* and leaf spot caused by *Xanthomonas fragariae* in strawberry. The clonal replicates were derived from 139 full-sib families. More than 1100 strawberry plants were monitored for crown and root rot symptoms in two successive seasons [61].

Roach et al. [73] reported that defined QTL associated with resistance to bacterial leaf spot led by *X. fragariae* (FaRXf1) and that this QTL was controlled by a single dominant allele and demonstrated a 1:1 segregation. According to pedigree-based QTL analysis, major and minor QTLs such as Cg1 and Cg2 were determined as linked with resistance to *C. gloeosporioides* and verified in two successive years [74, 75]. Although total phenotypic variation belongs to Cg1, it explained 26% of the total PV, while Cg2 was identified as minor and explained 5% of the total PV. The associated QTLs that resistance to pathogens such as *P. cactorum* or phytophthora, FaRPc2, were identified and tested [74, 79]. It was noticed that this locus represented 35% of total

#### *Quantitative Trait Loci Associated with Agronomical Traits in Strawberry DOI: http://dx.doi.org/10.5772/intechopen.108311*

the phenotypic variation in *P. cactorum*. Three major QTLs for resistance to *P. cactorum*, a water-borne disease, were detected using the segregation population. These QTLs account for 37% phenotypic variation in the population. On the other hand, the researchers reported that several loci associated with crown rot disease caused by *P. cactorum* using a genome-wide association study (GWAS) of 114 individuals were identified in the present study. The loci linked by crown rot were mapped within the LG VI and VII, while the manhattan plot obtained from GWAS demonstrated that associated loci were located on chromosomes 5 and 7 [77].

Gray mold is a pathogenic disease that causes postharvest decay in many different species. This disease, caused by *Botrytis cinerea*, infects different plant tissues and is more destructive, especially in mature fruits and this increases post-harvest economic losses. Petrasch et al. [90] detected the QTLs related to gray mold using the 50 K Axiom SNP array [91] and five full-sib families. A total of nine QTLs were found most significant according to LOD score and these QTLs were located on chromosomes 3, 4, 5, and 7 different sub-genomes. Since the identified QTLs were minor QTLs and distributed among the different sub-genomes of chromosomes, the genes responsible for gray mold can be controlled by polygenes.
