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First Report of Plasmopara halstedii on Coreopsis grandiflora in the United States
- Salgado-Salazar, C., Creswell, T. C., Ruhl, G., Crouch, J. A.
- Plant disease 2019 v.103 no.4 pp. 775
- Coreopsis grandiflora, Helianthus annuus, Plasmopara halstedii, Plumeria, Rudbeckia fulgida, business enterprises, container-grown plants, cultivars, financial economics, hosts, indigenous species, landscapes, leaves, microscopy, mitochondrial DNA, necrosis, oligodeoxyribonucleotides, ornamental plants, pathogens, polymerase chain reaction, ribosomal DNA, sporangia, Florida, Indiana, Korean Peninsula
- Coreopsis grandiflora, also known as large-flowered tickseed, is a popular ornamental plant in the Asteraceae, highly regarded owing to its 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. These plants 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 County, Indiana. Symptoms included necrotic gray to black leaf lesions and brown to gray 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 on the abaxial leaf surface. Microscopic examination of the diseased Presto specimens (BPI 910721 and BPI 910722) revealed hyaline sporangiophores, 344 to 579 × 7 to 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 to 22 × 13 to 19 µm (average 18.5 × 15.9 µm, n = 40). The morphological characteristics agree with those of Plasmopara halstedii (Farl.) Berl. & De Toni (Saccardo 1888). To confirm the pathogen’s identification on cultivar Presto, the rDNA LSU, mtDNA cox1, and mtDNA cox2 regions were polymerase chain reaction 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. 1990). BLASTn comparison of cox2 and LSU sequences (MH807829 and MH807830) showed 99% similarity to P. halstedii described from Helianthus annuus (KU232285) and Rudbeckia fulgida ‘Goldsturm’ (EF553469 and KF927154) hosts. Cox1 sequences of P. halstedii were absent from GenBank, but BLASTn queries using the cox1 sequence (MH807828) 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 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 the United States (Reported in Florida in 1984; Farr and Rossman 2018), and it has furthermore also been reported once from C. lanceolata in Korea (Choi et al. 2015). Primary preventive management should be reinforced at the production level given that this disease has the potential to cause significant economic losses in nurseries and landscape businesses.