Identification of two new races of Diplocarpon rosae Wolf, the causal agent of rose black spot disease

Authors: ZLESAK, DAVID - University Of Wisconsin; BALLANTYNE, DARCY - University Of Minnesota; HOLEN, MATTHEW - University Of Minnesota; CLARK, ANDREA - University Of Minnesota; SMITH, KRISTEN - Star Roses & Plants; Zurn, Jason; Bassil, Nahla; BRADEEN, JAMES - University Of Minnesota; HOKANSON, STAN - University Of Minnesota

Submitted to: American Society of Horticulture Science Meeting
Publication Type: Abstract Only
Publication Acceptance Date: 3/1/2019
Publication Date: 7/21/2019
Citation: Zlesak, D.C., Ballantyne, D., Holen, M., Clark, A., Smith, K., Zurn, J.D., Bassil, N.V., Bradeen, J.M., Hokanson, S.C. 2019. Identification of two new races of Diplocarpon rosae Wolf, the causal agent of rose black spot disease. American Society of Horticulture Science Meeting. American Society of Horticulture Science Meeting.

Interpretive Summary: Rose black spot is one of the most devastating diseases of cultivated roses for both the home and commercial market. The use of genetic resistance is the most economic and environmentally friendly management strategy for controlling the disease. In order to make the best use of genetic resistance it is important to understand how pathogen populations are changing and if any new strains emerge that would render a resistance gene useless. In this study, two new strains of the black spot pathogen were identified and characterized on a set of rose varieties that contain different resistance gene combinations. The first isolate was collected from the cultivar Brite EyesTM in West Grove, PA. The second isolate was collected from the cultivar PaprikaTM in Minneapolis MN. Each of these isolates have a uniqe response when compared to the 11 strains previously known and represent the 12th and 13th strain. More work is needed to discover the distribution of these strains across North America.

Technical Abstract: The fungal pathogen, Diplocarpon rosae Wolf, infects only roses (Rosa spp.) and leads to rose black spot disease. Rose black spot is the most problematic disease of outdoor grown roses worldwide due to the potential for rapid leaf yellowing and defoliation. Plants repeatedly defoliated from black spot become weakened and may eventually die from lack of energy reserves. Eleven races of the pathogen were previously characterized from isolates collected in North America and Europe. Isolates of D. rosae obtained from infected leaves of Brite EyesTM (‘RADbrite’; isolate BEP; collected in West Grove, PA) and PaprikaTM (‘CHEwmaytime’; isolate PAP; collected in Minneapolis, MN) proved to have unique infection patterns using the established host differential and Lemon FizzTM (‘KORlem’). The new races are designated race 12 (BEP) and 13 (PAP), respectively. A differential infection pattern on Lemon FizzTM is what distinguished race 7 from 12. Expanding the collection of D. rosae races provides an ever more valuable resource for efforts towards assisting ongoing work to identify and characterize unique race-specific resistance genes in Rosa and virulence genes in the pathogen as well as for supporting rose breeding programs.