The fungus Diplocarpon rosae (anamorph = Marssonina rosae Lind.) is the causal organism of rose black spot, which is the most damaging disease of rose worldwide (Dobbs, 1984). Symptoms include lesions on leaves and stems as well as frequent leaf yellowing and defoliation that significantly compromise plant growth and appearance. Consequently, numerous topical and systemic fungicides are used by homeowners and landscapers to combat this disease. Diplocarpon rosae is classified as an ascomycete in the family Dermateaceae and is a hemibiotrophic fungus that is restricted to the genus Rosa L. (Blechert and Debener, 2005). It is spread primarily through waterborne, two-celled asexual spores (conidia) that require free water to germinate. Visual symptoms may develop in susceptible cultivars in as little as 4 d after spore germination and penetration. The sexual stage of this fungus has been reported only twice in North America and twice in England (Horst, 1983), although frequency of sexual recombination in D. rosae populations has not been determined.
Phenotypic variability among single-conidial isolates suggests that D. rosae is a genetically diverse pathogen (Wenefrida and Spencer, 1993). Pathogenicity tests have revealed the presence of numerous races. Five races were discovered in Germany (Debener et al., 1998) among 15 isolates, four have been found in England (Yokoya et al., 2000), four were described in Ontario, Canada (Svejda and Bolton, 1980), and seven races were described in Mississippi (Spencer and Wood, 1992a, 1992b). Although it is not known whether any of these races from different studies are the same, the occurrence of multiple races in each location suggests a large total number of races in the world. The Mississippi isolates were collected from a seven-county area demonstrating diversity in pathogenicity within a relatively narrow geographic range. Considerable variability in conidial size and colony color among single-conidial isolates has also been documented (Wenefrida and Spencer, 1993).
Genetic diversity has been documented in D. rosae using molecular marker analyses. Lee et al. (2000) examined 10 isolates using restriction fragment length polymorphism (RFLP) analysis of the internal transcribed spacer region of rDNA. Distinct RFLP patterns were observed and isolates clustered into three groups. Initial studies by British researchers with simple sequence repeat and amplified fragment length polymorphism (AFLP) indicated that AFLP was the more reproducible and sensitive marker system for their isolates, although final results of their work have not yet been published (Drewes-Alvarez, 2003). Carlson-Nilsson (2002) conducted a randomly amplified polymorphic DNA analysis of 15 D. rosae isolates collected from locations in Sweden, Canada, France, and the United States, which were roughly separated by geographic origin in a cluster analysis. The survey did not allow in-depth analysis of geography, host of origin, pathogenic race, fungicide selection, or other factors on genetic similarity among isolates, although diversity among isolates was demonstrated. The current lack of knowledge about such factors and their influence on diversity complicates host resistance breeding efforts.
The objective of this study was a preliminary examination of the genetic diversity present in 50 single-spore isolates of D. rosae collected from eastern North American using AFLP analysis (Vos et al., 1995) and the manner in which diversity was distributed in relation to geographic origin, host of origin, or pathogenic race structure of the isolates under study. Such information is useful for the screening and evaluation of host resistance in rose germplasm targeted for North American markets.
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