Roses, which are distributed throughout the temperate regions of the Northern Hemisphere (Krüssmann, 1981), have been important ornamental plants for more than 5000 years. There are thousands of cultivars for the garden, floriculture, medicinal, fragrance, and culinary industries (Gudin, 2000; Marriott, 2003; Shepherd, 1954). The rose industry contributes a value of about $400 million from garden roses, which is the major crop in the $2.81 billion wholesale U.S. shrub market (AmericanHort, 2014). Compared with the rose market in the United States 35 years ago, the production of garden roses has decreased (Byrne et al., 2010) from 40 million roses down to 12 million field-grown and 15–18 million pot-grown rose bushes in 2012 (Hutton, 2012). This is thought to be because many rose cultivars have low tolerance to disease and abiotic stress (Waliczek et al., 2015). In response to a consumer demand for low-maintenance roses, many rose breeding programs are working toward developing cultivars resistant to the common rose diseases (Byrne, 2015; Debener and Byrne, 2014).
Black spot, the most important disease affecting garden roses globally, is caused by the fungus D. rosae Wolf (Marssonina rosae anamorph) (Nauta and Spooner, 2000). The typical symptoms of this disease include dark rounded spots with a feathery edge on the adaxial side of the leaves while the abaxial epidermis remains unaffected. The disease can lead to the development of chlorosis around the lesion and eventually defoliation (Blechert and Debener, 2005; Gachomo et al., 2006; Horst and Cloyd, 2007). Eleven unique races of D. rosae have been identified among the isolates obtained from North America and Europe (Whitaker et al., 2010b).
Two types of disease resistance to black spot have been characterized in roses. Vertical or complete resistance, which blocks sporulation and severely restricts the mycelial growth of the pathogen, is usually controlled by major dominant genes (Rdr or Rosa disease resistance genes) (von Malek and Debener, 1998; Whitaker et al., 2010a; Yokoya et al., 2000). In rose, the dominant resistance genes are pathogen race-specific, indicating a gene-for-gene interaction pattern (von Malek and Debener, 1998).
Partial or horizontal resistance that appears to be non-race-specific has also been identified in roses (Shupert, 2005; Whitaker and Hokanson, 2009a, 2009b; Xue and Davidson, 1998). This resistance does not prevent infection of the pathogen, but rather delays disease development and results in reduced lesion size, reduced sporulation, and/or delayed infection after inoculation (Parlevliet, 1981; Whitaker and Hokanson, 2009a, 2009b; Xue and Davidson, 1998). Compared with complete resistance, partial resistance is generally more durable over the range of pathogenic races (Noack, 2003). The ideal disease-resistant genotype should have both highly effective and long-lasting resistance to a broad spectrum of pathogenic races (Blechert and Debener, 2005), which can be achieved by pyramiding dominant complete resistance genes, obtaining strong partial resistance or by combining both types of resistances.
Black spot resistance is commonly evaluated in field trials in different geographic regions to expose the rose to a greater number of pathogenic races. These trials typically last 2–3 years to ensure sufficient disease pressure to properly assess the resistance of the plants (Carlson-Nilsson, 2002; Debener and Byrne, 2014; Noack, 2003; Shupert, 2005). The laboratory-based DLA is a tool for observing disease development efficiently under uniform and well-controlled environmental conditions and inoculum levels (Hattendorf et al., 2004; von Malek and Debener, 1998; Whitaker and Hokanson, 2009a, 2009b). Because single-conidial isolates are used in laboratory screening, the combination of compatible and incompatible interactions that are caused by various races in nature can be avoided (Blechert and Debener, 2005). However, as DLA only allows one cycle of disease development, the differences among genotypes might not be as accentuated as compared with a field trial in which multiple cycles of pathogen development are common (Xue and Davidson, 1998). Other factors such as the physical status of the host plant, degradation of the leaves, missing observations on leaf abscission/defoliation in DLA, and low or nonuniform inoculation levels in field assessment could all cause low correlation between these two methods of phenotyping (Johansson et al., 1992; Palmer et al., 1966; Zlesak et al., 2010).
The objectives of this research were to 1) evaluate two methods (DLA and WPI) of artificial inoculation for black spot disease evaluation and characterize rose genotypes for black spot resistance and 2) characterize partial black spot disease resistance derived from RW in laboratory tests in diploid populations to estimate the components of genetic variances, and the heritability of this partial resistance.
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