ABSTRACT Black spot disease, caused by the fungus Diplocarpon rosae , is a major threat to garden roses, leading to significant defoliation and reduced plant vigour. Several isolates originating from the United States and Europe have been characterised as races with different pathogenicity using a differential set of 10 plant hosts. However, no isolates from France have been tested on this set, and the races present in this country remain unknown. This study investigated the pathogenicity of seven monoconidial D. rosae strains isolated in France on 17 rose genotypes, including the previously published differential host set. Our results revealed that four of these strains have the virulence pattern of race 8, the prevalent race in North America. However, the commercial rose cultivars: Surrey (cultivar: KORlanum), High Voltage (cultivar: BAIage) and Ramblin’ Red (cultivar: RADramblin) showed intermediate reactions to three strains, suggesting additional infection mechanisms. The four resistance genes available in the host set ( Rdr3 , Rdr4 , Rdr5 , Rdr6 ) remained effective against all tested strains. This study provides the first information on the races of D. rosae present in France and will be useful in breeding programmes for black spot‐resistant roses.
Lambelin et al. (Sun,) studied this question.
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