Black spot of roses, caused by the fungus Marssonina rosae (teleomorph = Diplocarpon rosae), is the most important and damaging disease of roses in landscapes worldwide. The pathogen causes round to irregular coal black spots with fringed margins, primarily on the upper leaf surfaces. Infected leaves can turn yellow and often fall off. M. rosae survives unfavorable periods on fallen leaves and diseased canes. Repeated defoliation weakens plants, leading to fewer blooms, reduced growth, and heightened sensitivity to environmental stresses (e.g., drought conditions).
Although cultural practices such as planting in sunny locations, avoiding overhead irrigation, and removing fallen, diseased leaves can suppress black spot to a limited extent, the most effective control measures are applying fungicide sprays and planting resistant cultivars. Intensive fungicide spray programs are unacceptable to many rose gardeners, however, and have drawn increased public concern due to perceived health and environmental hazards of chemical pesticides.
Most modern hybrid tea roses are highly susceptible to black spot. Attempts to transfer resistance from species to hybrid roses by crossing have often failed, however, due to chromosomal mismatch of the diploid species with the tetraploid hybrids (Walker et al., 1996). Despite decades of concerted effort, developing rose cultivars that combine meaningful levels of black spot resistance with desirable horticultural traits remains a major challenge (Carlson-Nilsson and Davidson, 2006).
In the north-central United States, the challenge of breeding for black spot resistance is compounded by the need for tolerance to the extremely low winter temperatures typical of the region. Although many cultivar evaluations for black spot resistance have been conducted in the southern and eastern United States (e.g., Hagan et al., 2005), northern Europe (e.g., Carlson-Nilsson and Davidson, 2006), and Japan (Chatani et al., 1996), only a few such trials have focused on cultivars adaptable to the north-central United States (Palmer and Salac, 1977; Watkins and Steinegger, 1986). Rose producers and gardeners in this region need clearer guidelines to select cultivars that not only resist black spot but also survive the climate.
While he was a professor at Iowa State University, Griffith Buck developed more than 80 new rose cultivars, primarily shrub types, which were released between 1962 and 1997. A primary aim of his breeding program was to develop cultivars that were sufficiently robust not only to survive Iowa winters (with minimum temperatures as low as −35 °C), but also to retain their foliage all season without fungicide sprays. Buck's breeding program was among the first in the world to focus on roses that were relatively easy to care for, even in challenging environments. Recently, rising public interest in low-maintenance roses has led to renewed interest in Griffith Buck roses, and many Griffith Buck roses are becoming more widely sold and planted.
Although Griffith Buck roses were not selected explicitly for black spot resistance, many of his releases possess some resistance or tolerance to the disease. However, no published studies have compared black spot resistance in relation to other resistant or susceptible cultivars, and Buck never compared levels of black spot resistance among his releases. The objective of our study was to assess resistance to black spot in 24 commercially available Griffith Buck roses that were rated as disease resistant when released, and to compare their resistance and landscape performance to highly black spot-resistant and -susceptible control cultivars.
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