Anthracnose crown rot and fruit rot caused by species of the fungal genus Colletotrichum produce significant losses in strawberry (Fragaria ×ananassa) production with crown rot, resulting in plant death and fruit rot, which in turn results in some percentage of unmarketable fruit (Maas, 1998). Anthracnose crown rot has been a major disease problem in the strawberry producing regions of the southeastern United States since the 1970s. Although initially considered a southeastern United States problem, anthracnose disease has increasingly become a significant problem in other strawberry production regions in the United States and the world (Freeman et al., 1998; Howard et al., 1992; Maas, 1998).
Anthracnose is incited by the fungal species Colletotrichum fragariae, Colletotrichum gloeosporioides (teleomorph Glomerella cingulata) (Smith and Black, 1990), and Colletotrichum acutatum (teleomorph Glomerella acutata). Colletotrichum fragariae causes crown and fruit rot but is rarely found outside the southeastern United States, and its host range is limited (Smith, 1998a, 1998b). Colletotrichum gloeosporioides most commonly causes crown rot but can also cause fruit rot. Colletotrichum acutatum causes a destructive fruit rot (Howard et al., 1992; Smith, 1998a) in addition to runner and petiole lesions, and it can also cause crown rot (Smith, 1998b). Both C. gloeosporioides and C. acutatum have broad host ranges, including apple (Malus ×domestica Borkh.), key lime [Citrus aurantiifolia (Christm.) Swingle], mango (Mangifera indica L.), peach [Prunus persica (L.) Batsch], sweet orange [Citrus sinensis (L.) Osbeck], blueberry (Vaccinium corymbosum L.), and almond [Prunus dulcis (Mill.) D. A. Webb] (Farr et al., 1989; Freeman et al., 1998; Peres et al., 2005; Smith, 1998a, 1998b).
For control of various plant diseases, integrated pest management and sustainable agriculture strategies often incorporate the use of resistant or tolerant cultivars. However, very few cultivars resistant to anthracnose crown rot currently are available in North America. Only ‘Pelican’ is resistant to both anthracnose fruit and crown rot (Shuman, 2001; Smith et al., 1998). Though four parent clones (Galletta et al., 1993) are resistant to anthracnose crown rot, ‘Gem Star’ (Chang, 2002a) and ‘Treasure’ (Chang, 2002b) are the only cultivars other than ‘Pelican’ reported to be resistant to anthracnose crown rot.
Selection for additional cultivated strawberry genotypes resistant to anthracnose crown rot is a credible option for managing this disease (Ballington et al., 2002; Denoyes-Rothan and Guérin, 1996; Gupton and Smith, 1991; Hancock et al., 1991, 1996; Simpson et al., 1994; Smith et al., 1996). However, apparent “field resistance” (Olcott-Reid and Moore, 1995) may be due to genotype by environment interactions unfavorable to infection or spread, rather than to true genetic disease resistance (Smith and Black, 1987). Expression of resistance is strongly affected by interactions among environment, genotype, and isolate (Ballington and Milholland, 1993; Olcott-Reid and Moore, 1995; Smith and Black, 1987, 1990). In addition, individual isolates within and among the three Colletotrichum species vary in pathogenicity to Fragaria genotypes (Smith and Black, 1990).
A reliable assay has been used for 20 years to identify resistant strawberry genotypes (Smith and Black, 1987; Smith et al., 1990). Multiple isolates of Colletotrichum spp. may be used in this assay to screen for resistance to pathogenic strains collected from various hosts and geographic locations. Anthracnose-resistant genotypes (including US 70, US 159, US 292, US 438, and ‘Pelican’) selected using this assay have been grown in replicated field trials at the Small Fruit Research Station, Poplarville, MS, for 10 to 20 years under heavy anthracnose pressure without dying from anthracnose crown rot. Plants of commercial cultivars, including Chandler, grown in the same trials often died from anthracnose crown rot by late spring. While C. fragariae was the most common causal agent of anthracnose crown rot in these field trials, C. acutatum and C. gloeosporioides also were isolated from crowns of plants dying in the field. In addition, all three species have been isolated from symptomatic fruit on plants grown in these trials (B.J. Smith, unpublished data).
Only a small portion of F. ×ananassa germplasm has been screened for resistance to anthracnose crown rot (Smith and Black, 1990). The germplasm base of the cultivated strawberry is very narrow (Dale and Sjulin, 1990; Hancock and Luby, 1995; Sjulin and Dale, 1987); thus, there is not a large reservoir of germplasm among cultivars from which to draw resistance genes for developing new resistant cultivars. Hancock et al. (1993) suggested that the genetic diversity of F. ×ananassa should be expanded by introgressing genes from elite clones of wild native octoploid Fragaria species. Very little attention has been given to evaluation of wild octoploid Fragaria germplasm as potential sources of resistance to anthracnose. Moreover, the germplasm that has been evaluated has been screened with only a small number of Colletotrichum isolates that ultimately originated either from the southeastern United States or California. Only recently have some Fragaria virginiana clones from the southeastern U.S. been reported to be potential sources of genes for resistance to both C. acutatum and C. fragariae (J.R. Ballington, personal communication).
A core subset of the Fragaria collection maintained at the U.S. Department of Agriculture National Clonal Repository, Corvallis, OR, has been constructed to contain an elite group of native F. virginiana and Fragaria chiloensis. This core subset is being characterized for many horticultural traits useful to breeders (Hancock et al., 2001a, 2001b), including reactions to several common foliar diseases, resistance to black root rot (causal organisms unknown) (Hancock et al., 2001b, 2002), and resistance to northern root-knot nematode (Meloidogyne hapla) and root-lesion nematode (Pratylenchus penetrans) (Pinkerton and Finn, 2005). Our objective in the present work was to evaluate many of the clones represented in the core collection for resistance to a selection of isolates of the three Colletotrichum species that can cause anthracnose crown rot.
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