Garden roses (Rosa sp.) are among the most popular flowering shrubs in the world. Diversity for traits such as form, color, and fragrance of flowers, plant habit, size, environmental adaptability, and extended season of flowering all contribute to their widespread cultivation and versatility (Zlesak, 2006). Lower maintenance cultivars that can tolerate regional environmental conditions without routine dependence on pesticides and excessive care are especially increasing in popularity (Harp et al., 2009; Lonnee, 2005). Factors fueling this trend include negative consumer attitude toward pesticides, emerging legislation putting greater limits on pesticide availability and use, busy lifestyles, and greater availability of lower maintenance rose cultivars (Harp et al., 2009).
Earth-Kind® Rose Trials are a component of the overall Earth-Kind program, started at Texas A&M (the term Earth-Kind® and associated logo are trademarks of the Texas AgriLife Extension Service, Texas A&M System) and help to serve the horticulture community by identifying the most adaptable landscape roses through regional cultivar trials (Harp et al., 2009). Pesticides are not used during the trials and cultural management practices use techniques that support environmental stewardship. This includes pre-plant incorporation of compost, maintenance of a 7.6- to 10.2-cm layer of organic mulch, and irrigation methods that conserve water. Before a rose can earn regional Earth-Kind designation, it must exhibit consistent, superior performance across multiyear and multilocation trials representing different soil types and other environmental conditions typical in the region. A large number of cultivars are advertised as low maintenance by nurseries wanting to capitalize on the popularity of lower maintenance roses. It is difficult for consumers to know which roses truly possess the highest levels of pest and environmental tolerances. Earth-Kind designation gives consumers confidence that they are choosing roses that will have a high likelihood of success when basic plant care is provided.
Disease susceptibility poses a major challenge to roses and limits their success as low-maintenance landscape shrubs. Fungi that attack roses include Diplocarpon rosae (Wolf) (causal agent of black spot), Podosphaera pannosa (Wallr.: Fr.) de Bary (causal agent of powdery mildew), and Cercospora puderi B.H. Davis (one of the causal agents of rose leaf spot) (Horst and Cloyd, 2007). Of these, black spot is the most serious in the outdoor landscape across most regions as a result of the potential for rapid disease development that typically leads to leaf yellowing and defoliation (Dobbs, 1984). Black spot has been the most prevalent and widespread disease in the Earth-Kind rose trials (Mackay et al., 2008). Plants repeatedly defoliated from black spot become weakened and quickly fall out of contention for Earth-Kind designation.
Diplocarpon rosae is capable of infecting only the genus Rosa. Asexual spores (condia) overwinter on stems and fallen leaves and are transported to new growth in the spring through water droplets. If free water remains present, the conidia form germ tubes that penetrate the leaf epidermis. Lesions may appear in as little as 4 d as sub-cuticular mycelia radiate from the point of infection. Condia-bearing acervuli burst through the leaf cuticle followed by leaf abscission in susceptible cultivars (Horst and Cloyd, 2007).
Multiple studies have been conducted to characterize the pathogenic race structure of D. rosae and then to use the characterized races to identify genes conferring host resistance (Debener et al., 1998; Whitaker et al., 2007a, 2007b, 2010a; Yokoya et al., 2000). Isolates are distinguished from one another based on their differential ability to infect a common set of rose genotypes. Those isolates with the same host infection pattern are designated as a race. Collections that have been preserved for continued research include six races discovered in Germany (Debener et al., 1998), four from Great Britain (Yokoya et al., 2000), and three races from a genetically diverse set of 50 isolates originating in eastern North America (Whitaker et al., 2007a, 2007b). Whitaker et al. (2010b) evaluated this international collection, identified 11 unique races among them, and standardized the race nomenclature. These studies and others have uncovered genetic resistance within rose species and cultivars that is race-specific. Such race-specific resistance can only be characterized with controlled inoculations of roses with individual races as opposed to a field setting where the presence and prevalence of specific races are not known. Using characterized races for inoculations under controlled conditions is also advantageous when surveying for partial resistance (Whitaker and Hokanson, 2009a; Xue and Davidson, 1998).
Detached leaf assays have been an efficient tool to characterize the resistance of rose seedling populations, and they have been found to be strongly correlated with whole plant inoculations (Hattendorf et al., 2004; Von Malek and Debener, 1998; Whitaker and Hokanson, 2009a). Detached leaf assays are preferable as a result of greater ease in controlling humidity and inoculum levels (Whitaker and Hokanson, 2009b). To the best of our knowledge, single-spore isolates of D. rosae have only been used for race characterization, comparison of pathogenicity of isolates, and to study the segregation of resistance to particular races in genetic studies and the characterization of partial resistance components. Single-spore isolates representing different races to the best of our knowledge have not been used to individually challenge commercial cultivars for widespread race-specific and horizontal resistance characterization. Schulz et al. (2009) challenged rose accessions uninfected with black spot in two field locations with a mixture of single-spore D. rosae isolates using detached leaf assays. However, the number of races represented by these isolates is not known. Recently, Whitaker and Hokanson (2009a) reported using detached leaf assays to characterize the partial resistance to black spot of segregating populations using the measurement of LL, defined as the diameter of the lesion at its widest point. LL was chosen by the authors because of the ease of measurement and its significant correlations with three other measures of partial resistance (Whitaker et al., 2007b; Xue and Davidson, 1998).
Using a collection of races to characterize roses marketed for low-maintenance landscape use would be helpful beyond highlighting the resistance of roses for growers and consumers. Knowledge of the genetic resistance of these roses would guide the selection of cultivars that could serve as controls to help differentiate the presence of known races in field trials where race composition is not known. Additionally, when cultivars with resistance to all known races become infected, pathologists may characterize the infective isolate(s) in search of new pathogenic races.
A widespread cultivar screen with known races would also allow breeders to identify desirable genetic resistances among a core set of highly resistant cultivars. If knowledge of resistance could be coupled with knowledge of rose ploidy level, this would help breeders develop disease-resistant cultivars more efficiently because of a greater likelihood of reproductive success. Preferential crossability and fertility as a consequence of ploidy level has been well documented in roses (El Mokadem et al., 2001; Leus, 2005; Rowley, 1960; Shahare and Shastry, 1963).
The objectives of this study were to characterize roses within or being considered for the Earth-Kind rose program according to 1) resistance to three North American races of D. rosae using detached leaf assays; and 2) ploidy through direct chromosome counts.
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