First report of Plasmopara halstedii on Coreopsis grandiflora in the United States

Authors: SALGADO-SALAZAR, CATALINA - Orise Fellow; CRESWELL, TOM - Purdue University; RUHL, GAIL - Purdue University; Crouch, Jo Anne

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/14/2018
Publication Date: 11/15/2018
Citation: Salgado-Salazar, C., Creswell, T., Ruhl, G.E., Crouch, J. 2018. First report of Plasmopara halstedii on Coreopsis grandiflora in the United States. Plant Disease. https://doi.org/10.1094/PDIS-09-18-1501-PDN.
DOI: https://doi.org/10.1094/PDIS-09-18-1501-PDN

Interpretive Summary: Downy mildew diseases cause significant damage to crop plants in the United States and worldwide. This research describes a new downy mildew disease outbreak affecting coreopsis, a popular perennial plant valued by gardeners and the horticultural industry for its long period of bloom, showy yellow flowers, and ease of care. This downy mildew was found on two different cultivars of coreopsis grown in a commercial nursery in Indiana, causing severe damage to affected plants. The mold was identified as Plasmopara halstedii, a common pathogen of sunflower that has not been found to infect coreopsis in the United States. This work provides new information about the host range of the mold that causes downy mildew disease of sunflower and coreopsis. This knowledge will be useful to growers, the horticultural industry, plant pathologists, and quarantine officials in the detection and management of this disease.

Technical Abstract: Coreopsis grandiflora, also known as large-flowered tickseed, is a popular ornamental plant in the Asteraceae, highly regarded due to their long-blooming nature, yellow blossoms and glossy foliage. Large-flowered tickseeds are widely distributed in central and eastern North America as both native plants and as cultivated types. They are easy to grow, require low maintenance, and are popular among gardeners, nursery growers and landscape professionals. In June 2018, disease symptoms were observed on 27 C. grandiflora ‘Golden Sphere’ and on 39 C. grandiflora ‘Presto’ potted plants at a commercial nursery in Monroe Co., Indiana. Symptoms included necrotic gray to black leaf lesions followed by complete necrosis of some leaves. Symptoms were more severe on ‘Golden Sphere’ than on ‘Presto’. Infected leaves turned dark green, with abundant white downy growth in the abaxial leaf surface. Microscopic examination of the diseased specimens revealed hyaline sporangiophores, 344 – 579 × 7 – 13 µm (average 459.6 × 10.6 µm, n = 20), straight, monopodially branched at right angles. Sporangia cylindrical to ellipsoid, hyaline to light yellow, 15 – 22 × 13 – 19 µm (average 18.5 × 15.9 µm, n = 40) were observed. The morphological characteristics agree with those of Plasmopara halstedii (Farl.) Berl. & De Toni (Saccardo 1888). To confirm the pathogen’s identification on cv. ‘Golden Sphere’, the rDNA LSU, mtDNA cox1 and mtDNA cox2 regions were PCR amplified and bidirectionally sequenced using primer sets LR0R/LR6-O, OomCoxI-Levup/OomCoxI-Levlo and Cox2-F/Cox2-RC4, respectively (Choi et al. 2015, White et al. 1991). BLASTn comparison of cox2 and LSU sequences showed 99% similarity to P. halstedii described from Helianthus annuus and Rudbeckia fulgida cv. ‘Goldsturm’ hosts (KU232285 and EF553469 - KF927154, respectively). Cox1 sequences of P. halstedii were absent from GenBank, but BLASTn queries using the cox1 sequence revealed 93% similarity to an unidentified Plasmopara species on Plumeria pudica (KC774621). Based on morphology and sequence data, we identified this organism as P. halstedii. To our knowledge, this is the first description of P. halstedii on C. grandiflora ‘Golden Sphere’ in the United States. Although P. halstedii has been reported on several genera of flowering plants in the Asteraceae and is widely accepted as a species complex, it has only been listed once from Coreopsis spp. in USA (Florida, 1984; Farr and Rossman 2018) and once from C. lanceolata in Korea (Choi et al. 2009). Primary preventive management should be reinforced at the production level given this disease has the potential to cause significant economic losses in nurseries and landscape business.