Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/31/2018
Publication Date: 2/1/2018
Citation: Salgado-Salazar, C., Ismaiel, A., Crouch, J. 2018. Downy mildew of Double Knock Out® rose caused by Peronospora sparsa in Maryland. Plant Disease. 102(7):1464. https://doi.org/10.1094/PDIS-11-17-1802-PDN.
Interpretive Summary: Knock Out® roses are an extremely popular landscape plant, prized for their prolific, season-long blooms and resistance to most diseases that plague other roses. The research describes the first outbreak of a downy mildew disease on Knock Out® roses in Maryland. The mildew disease caused unsightly black and purple leaf spots over most of the infected plants, and caused the flower buds to collapse and never open. This disease is very destructive to other cultivars of roses, but it is still unknown how it might affect Knock Out® rose health and production in commercial nurseries and landscape plantings. This finding of downy mildew on Knock Out® roses alerts gardeners, growers and plant pathologists to this existence of the disease, and provides photographs and descriptions that can be used to diagnose future outbreaks if they occur.
Technical Abstract: Roses are one of the most popular and economically important ornamental plants worldwide. In the last 17 years, Knock Out® roses (Rosa x 'Radtko') have been widely used in public and private gardens across the U.S. due to their disease resistance, self-cleaning, drought tolerance and multiple-blooming nature. In October 2017, downy mildew-like symptoms were observed on leaves of four Double Knock Out® rose bushes in a residential garden in Anne Arundel County, MD. More than 90% of the leaves of the surveyed plants were symptomatic. Disease symptoms included yellowing and purplish red to dark brown or black angular leaf spots. Several collapsing unopened flower buds were also observed. Microscopic examination of diseased tissue (BPI 910538) revealed sparse hyaline sporangiophores on the underside of leaves, 236 to 653 × 5 to 10 µm (mean 368 × 7.7 µm, n = 30), branching 3 to 4 times, with branch ends 8 to 27 µm (mean 15.2 µm, n = 50), each pair with one branch curved inward and one reflexed. Sporangia were slightly ellipsoidal, pale yellow-brown with short stalk sometimes present, measuring 17 to 24 × 16 to 20 µm (mean 20.1 × 17.9 µm, n = 30). Sporangiophores and sporangia were also observed on floral bud sepals. Oospores were not observed. These morphological characteristics were consistent with Peronospora sparsa Berk. Identification was confirmed by PCR amplification and bidirectional sequencing of rDNA ITS, rDNA LSU, mtDNA cox1 and mtDNA cox2, using primers ITS-O/LR-O, LR0R/LR6-O, OomCoxI-Levup/OomCoxI-Levlo and Cox2-F/Cox2-RC4, respectively (Choi et al. 2015, Riethmüller et al. 2002, Robideau et al. 2011). Using GenBank BLASTn, ITS and LSU sequences (MG552675-76) showed 99% similarity to P. sparsa described from Rosaceae hosts (DQ874342 and AY035488, respectively). Cox1 and cox2 sequences of P. sparsa from rose were absent from GenBank, but BLASTn searches using these data (MG552673-74) showed 99% similarity to P. rubi (a synonym of P. sparsa), P. potentillae-repantis and P. sanguisorbae, all of them downy mildew pathogens of Rosaceae hosts other than Rosa spp. Based on morphology, host association and sequence data, we identified this organism as P. sparsa. To our knowledge, this is the first description of P. sparsa infecting any species of rose in Maryland, including Double Knock Out® roses (Farr and Rossman 2017). Although CABI (cabi.org) lists P. sparsa from Maryland, the underlying publications cited do not support the entry. The popularity of Knock Out® roses is partially due to their resistance to diseases affecting other roses, such as black spot (Diplocarpon rosae) and powdery mildew (Podosphaera pannosa). Previous reports of downy mildew from other rose varieties highlight the highly sporadic nature of this disease (Aegerter et al. 2003). However, the impact and distribution of downy mildew on Knock Out® roses is currently unknown. Since Knock Out® roses tend to be established in mass plantings in landscape settings, this host may be conducive for increased disease incidence and spread, and should be included in primary prevention activities.