Isolation and identification of eight races of powdery mildew of roses (Podosphaera pannosa) (Wallr.: Fr.) de Bary and the genetic analysis of the resistance gene Rpp1

Abstract

Powdery mildew, caused by Podosphaera pannosa, is one of the most-severe diseases of roses grown under glass. The differentiation into physiological races and the genetic analysis of resistance in a segregating host population was investigated using single conidial isolates of the pathogen. Using ten rose genotypes, all eight isolates of the pathogen could be ascribed to different races. Five races were isolated from one location, which indicates that populations of P. pannosa exhibit a high racial diversity. Infection experiments in a backcross-population of 114 rose plants resulted in a 1:1 segregation, suggesting control by a single dominant gene. Rpp1 is the first resistance gene against rose powdery mildew to be described.

Assessment of Partial Resistance to Powdery Mildew (Podosphaera pannosa) in a Tetraploid Rose Population Using a Spore-suspension Inoculation Method

“…Two well-defined monospore isolates, isolate 2 from Ahrensburg, Germany (Linde and Debener, 2003) and isolate F1 from Lesdain, Belgium (Leus et al, 2002), were kindly provided by the authors. The monospore isolates were maintained in vitro as described by Linde and Debener (2003).…”

“…The development of cultivars with improved resistance has been a major breeding challenge for a long time, but successes are scarce. This is due to the lack of highly resistant genetic sources in cultivated roses, a limited knowledge of resistance mechanisms and a large diversity in the pathogen species (Linde and Debener, 2003). There is also a need for more reliable assessment methods for screening.…”

“…Morphological changes such as the vacuolization of epidermal cells (Temmen et al, 1980), physical barriers such as leaf cuticle thickness (Ferrero et al, 2001) and the formation of papillae in plant cells (Mence and Hildebrandt, 1966) were found to influence fungal penetration. Hypersensitive response (HR) (Conti et al, 1985), simple monogenic control (De Vries and Dubois, 2001), a single dominant gene (Linde and Debener, 2003) and horizontal resistance (Schlosser, 1990;De Vries and Dubois, 2001) confer powdery mildew resistance. It seems that vertical resistance (race-specific, HR or monogenic resistance) is very common in roses.…”

“…Leus et al (2002Leus et al ( , 2003 studied eight isolates collected in Belgium and showed a differential host response, indicating differences in virulence among isolates. Linde and Debener (2003) recently classified eight different races in northern Germany and concluded that the pathogen harbours a high diversity of virulence genes.…”

“…An improvement of the blowing method using a vacuum-setting tower (Francisco et al, 1988;Linde and Debener, 2003) has made it possible to address this problem but the inoculation method is not adequate for screening large numbers of plants, especially in field and greenhouse tests. The use of spore suspensions in water has been extensively employed for the inoculation of powdery mildew in cucumber (Zijlstra et al, 1995), tomato (Bai et al, 2003) and pepper (Lefebvre et al, 2003), but has never been used in rose since research has shown that water adversely affects the viability and infectivity of the conidia (Yarwood, 1939;Wheeler, 1973).…”