An updated host differential due to two novel 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; HOKANSON, STAN - University Of Minnesota; SMITH, KRISTEN - Star Roses & Plants; Zurn, Jason; Bassil, Nahla; BRADEEN, JAMES - University Of Minnesota

Submitted to: HortScience
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/31/2020
Publication Date: 9/23/2020
Citation: Zlesak, D.C., Ballantyne, D., Holen, M., Clark, A., Hokanson, S.C., Smith, K., Zurn, J.D., Bassil, N.V., Bradeen, J.M. 2020. An updated host differential due to two novel races of Diplocarpon rosae wolf, the causal agent of rose black spot disease. HortScience. 55:1756–1758. https://doi.org/10.21273/HORTSCI14902-20.
DOI: https://doi.org/10.21273/HORTSCI14902-20

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: Rose black spot, caused by the pathogenic fungus Diplocarpon rosae Wolf, is considered the most widespread and troublesome disease of outdoor grown roses. Infections can lead to rapid defoliation, and repeated cycles of defoliation can severely weaken susceptible cultivars. A host differential was challenged with D. rosae isolates from North America and Europe, and eleven races were identified to form the international race collection of D. rosae that is housed at the University of Minnesota (Whitaker et al. 2010). Three of these races (3, 8, and 9) were used to challenge cultivated roses (Zlesak et al. 2010). Brite EyesTM (‘RADbrite’) and Oso Easy® Paprika (‘CHEWmaytime’) demonstrated race-specific resistance to these three races. Infected leaves with typical black spot symptoms, including sporulating lesions, were obtained from plants growing in landscape settings of both Brite EyesTM (from West Grove, PA in September 2009; 39.49572° N, 75.53001° W) and Oso Easy® Paprika (from Minneapolis, MN in July 2016; 44.92691° N, 93.29671° W). A single spore isolate was generated from infected leaves of each rose cultivar and designated BEP and PAP, respectively. These isolates were maintained and multiplied on detached leaves of the cultivars from which they were collected and ‘Morden Blush’ (a universal susceptible). The isolates were characterized via a detached leaf assay using a previously developed host differential set (Whitaker et al. 2010). Inoculations were conducted as previously described (Whitaker et al. 2010), with the modification of pipetting inoculum onto leaves versus misting (Zurn et al. 2018). Leaves for detached leaf assays were collected from greenhouse grown plants free of rose black spot symptoms. Reactions were scored as susceptible if acervuli were present. At least two inoculation containers containing two leaves each were assessed for each rose cultivar by isolate combination. Brite EyesTM, Oso Easy® Paprika, and an additional rose, Lemon FizzTM (‘KORfizzlem’), were challenged with an isolate of each of the eleven races in the international collection, BEP, and PAP. BEP and race 7 shared the same infection pattern on the host differential. However, Lemon FizzTM, distinguished between them, being susceptible to race 7 but resistant to BEP. Therefore, BEP represents a newly identified race we designate as race 12. The rose cultivar Lemon FizzTM should be added to the standard host differential to differentiate races 7 and 12. PAP had a unique infection pattern on the original host differential and also represents a new race we designate as race 13. Oso Easy® Paprika shared the same race infection pattern as Baby LoveTM (‘SCRivluv’) in the host differential, with both roses being susceptible to races 11 and 13 and resistant to all other races. Interestingly, Brite EyesTM and Lemon FizzTM were susceptible to only a single race (race 12 and race 7, respectively). Inclusion of these two newly identified races of D. rosae into the international race collection and Lemon FizzTM as a new differential host expands this valuable resource for use in applications such as characterizing geographical race distribution and host resistance gene discovery (Zurn et al. 2018).