Comparison of Anthracnose Resistance With the Presence of Two SCAR Markers Associated With the Rca2 Gene in Strawberry
Strawberry anthracnose diseases are caused primarily by three Colletotrichum species: C. acutatum J.H. Simmonds, C. fragariae A.N. Brooks, and C. gloeosporioides (Penz.) Penz. & Sacc. Molecular markers are being used in breeding programs to identify alleles linked to disease resistance and other positive agronomic traits. In our study, strawberry cultivars and breeding germplasm with known anthracnose susceptibility or resistance to the three anthracnose-causing Colletotrichum species were screened for two sequence characterized amplified region (SCAR) markers linked to the Rca2 gene. The Rca2 resistant allele SCAR markers were associated with varying degrees of significance for a strawberry plant’s anthracnose resistance to C. fragariae but not to C. acutatum or C. gloeosporioides. Although the presence or absence of the markers associated with the Rca2 resistance gene is an imperfect indicator of anthracnose resistance, it may serve as a useful starting point in selecting germplasm for breeding programs.
In the southeastern United States, anthracnose diseases of commercial strawberry (Fragaria ×ananassa Duch.) are caused primarily by three Colletotrichum species: C. acutatum, C. fragariae, and C. gloeosporioides (Maas, 1998). Colletotrichum acutatum incites two major anthracnose diseases on strawberry: anthracnose fruit rot and root necrosis (Howard et al., 1992; Mertely and Peres, 2012; Mertely et al., 2005), and has been found to remain latent on symptomless strawberry plants (Leandro et al., 2001). The devastating Colletotrichum crown rot (anthracnose crown rot) may be caused by C. gloeosporioides or C. fragariae (Howard et al., 1992; Peres et al., 2007; Smith, 1998; Smith and Black, 1987; Ureña-Padilla et al., 2002). Both C. gloeosporioides and C. fragariae may also cause anthracnose symptoms on all aerial parts of the strawberry plant. Fungicides are used routinely to control anthracnose diseases; however, frequent use of the same fungicides has resulted in their failure to control anthracnose as a result of pathogen resistance (Forcelini and Peres, 2018; LaMondia, 1995; Smith and Black, 1992, 1993). The need for fungicides can be reduced by growing strawberry cultivars resistant to anthracnose diseases.
Strawberry breeders and plant pathologists are working to develop strawberry germplasm resistant to anthracnose crown rot and fruit rot diseases using traditional and classic techniques (Denoyes and Guerin, 1996; Salinas et al., 2018; Whitaker et al., 2017), and are exploring native germplasm for sources of anthracnose resistance in strawberry (Lenne and Wood, 1991; Lewers et al., 2007). Various protocols have been developed to screen strawberry seedlings and mature plants for resistance to the different Colletotrichum species. Denoyes and Guerin (1996) screened for resistance to C. acutatum by immersing whole plants into a conidial suspension of the pathogen. Horowitz et al. (2004) developed a foliar-dip method for large-scale screening of 12- and 15-week-old strawberry seedlings for resistance to C. gloeosporioides and C. acutatum. A protocol for identifying strawberry germplasm with resistance to C. fragariae based on petiole and crown symptoms was developed at the U.S. Department of Agriculture–Agricultural Research Service, Poplarville, MS (Smith and Black, 1987; Smith and Spiers, 1982) and was used to select anthracnose-resistant germplasm with desirable horticultural characteristics. Selections from this program were given the prefix MSUS, and one anthracnose resistant cultivar, ‘Pelican’ (Smith et al., 1998), and four anthracnose resistant breeding lines—US70, US159, US292, and US438 (Galletta et al., 1993)—developed in this program were released. Recently the anthracnose resistance/susceptibility of 31 strawberry cultivars was compared with that of 50 anthracnose-resistant MSUS selections developed at Poplarville, MS (Miller-Butler et al., 2018).
Most strawberry cultivars are octoploids, which presents difficulties in determining their genetics in classic breeding programs (Anciro et al., 2018; Hancock et al., 2008). Disomic inheritance for at least part of the strawberry genome has been demonstrated by some researchers (Folta and Davis, 2006; Lerceteau-Köhler et al., 2003), which allows breeders to identify dominant and recessive alleles for these loci. The advent of molecular genetics provided plant breeders with molecular markers to use for identification of alleles linked to disease resistance and other positive agronomic characteristics (Longhi et al., 2014). Genetic marker-assisted detection of resistance genes has been used in various Fragaria–pathogen systems, including the following: locating the Rpc1 gene that provides resistance to crown rot disease caused by Phytophthora cactorum in Fragaria vesca (Davik et al., 2015); screening strawberry seedlings in field trials for resistance to V. dahlia, the causative agent of verticillium wilt (Antanaviciute et al., 2015); identifying the Rpf1 gene for resistance to red stele root rot caused by Phytophthora fragariae (Van de Weg et al., 1997); locating a quantitative trait locus (QTL) conferring resistance to C. gloeosporioides (Anciro et al., 2018); and detecting the subgenome-specific locus FaRCa1 conferring resistance to C. acutatum (Salinas et al., 2018). SCAR markers were used to demonstrate a resistant:susceptible segregation ratio of 1:1 in strawberry progeny from crosses between red stele-susceptible × -resistant germplasm (Gel Vonauskienė et al., 2007).
Several studies have tested strawberries for resistance to C. acutatum, which has been classified as either pathogenicity groups 1 or 2, based on its pathogenicity on five strawberry cultivars (Denoyes and Baudry, 1995). Classification of additional C. acutatum isolates was performed by Denoyes-Rothan et al. (2003), who conducted pathogenicity tests on two strawberry cultivars—Belrubi (resistant to group 2 and susceptible to group 1) and Elsanta (susceptible to both groups)—using a subset of 34 European C. acutatum isolates and the American C. acutatum isolate Goff. Denoyes-Rothan et al. (2005) examined the inheritance of high- and intermediate-level plant resistance to C. acutatum isolates of pathogenicity group 2 and found that the inheritance of a dominant gene (Rca2) controlled strawberry resistance to that group. Lerceteau-Köhler et al. (2005) identified two SCAR markers and used them to screen European and American strawberry genotypes, reported in the literature as resistant or susceptible, for the presence of the Rca2 gene. They found 13 of the 28 resistant genotypes had both SCAR markers and none of the 14 susceptible genotypes had either of the SCAR markers. They considered their results to be indicative of monogenic control for resistance to C. acutatum pathogenicity group 2.
In our study we investigated the possibility that strawberry germplasm containing the Rca2 gene with resistance to group 2 C. acutatum isolates might also confer a level of resistance to C. fragariae and C. gloeosporioides, as well as to C. acutatum isolates for which a pathogenicity group has not been determined. The objective of our research was to screen 31 strawberry cultivars and 50 MSUS selections for the presence or absence of the two SCAR markers linked to the Rca2 gene and to determine whether their presence or absence in each germplasm line correlates with the resistance/susceptibility to each of the three Colletotrichum species as reported by Miller-Butler et al. (2018). Knowledge of the presence or absence of the Rca2 gene and a germplasm’s resistance to anthracnose should improve breeders’ decisions concerning which strawberry germplasm is most desirable to incorporate into breeding programs.
Miller-Butler, M. A.,
Smith, B. J.,
Smith, B. J.,
Kreiser, B. R.,
Blythe, E. K.
(2019). Comparison of Anthracnose Resistance With the Presence of Two SCAR Markers Associated With the Rca2 Gene in Strawberry. HortScience, 54(5), 793-798.
Available at: https://aquila.usm.edu/fac_pubs/16425