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Title: Downy mildew on coleus (Plectranthus scutellarioides) caused by Peronospora belbahrii sensu lato in Tennessee

item RIVERA, YAZMIN - Rutgers University
item SALGADO-SALAZAR, CATALINA - Rutgers University
item WINDHAM, ALAN - University Of Tennessee
item Crouch, Jo Anne

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/24/2015
Publication Date: 3/3/2016
Citation: Rivera, Y., Salgado-Salazar, C., Windham, A.S., Crouch, J.A. 2016. Downy mildew on coleus (Plectranthus scutellarioides) caused by Peronospora belbahrii sensu lato in Tennessee. Plant Disease. 100(3):655.

Interpretive Summary: Downy mildews are destructive plant diseases caused by fungal-like water molds. These molds are the cause of extensive damage to crop plants around the world. Coleus are showy annual plants, primarily grown for their vibrant, long-lasting and colorful mounds of foliage. This research reports two downy mildew disease outbreaks on coleus plants in Tennessee during 2015. This disease on coleus was caused by the same type of water mold that causes downy mildew disease on basil. This is the first time that downy mildew disease of coleus has been officially reported from Tennessee. Knowledge of this disease will be useful to plant regulatory officials working to control the spread of downy mildew diseases, and provides information about this new disease to growers and gardeners.

Technical Abstract: Coleus (Plectranthus scutellarioides [syn. = Solenostemon scutellarioides]) is a popular ornamental plant in the mint family (Lamiaceae), prized for its colorful and showy foliage. In August 2015, disease symptoms typical of downy mildew were observed at two sites in Nashville, Tennessee: (i) at the Cheekwood Botanical Garden, where ~100 coleus plants of the cultivar ‘Wasabi’ were affected; and (ii) in a private homeowner garden on an unknown cultivar of coleus. Disease signs and symptoms were characterized by abundant gray sporulation on the abaxial surface of leaves and irregular chlorotic lesions that ultimately turned necrotic. Diseased tissue (BPI 893222, BPI 893223) showed the presence of straight hyaline sporangiophores, monopodially branched, ending with curved branchlets bearing single sporangia. Sporangia were ellipsoid to ovoid with pale brown coloration. Sporangiophores were 292-689 µm (n = 17) in length, and sporangia measured on average 24.9 × 19.7 µm (20.5-29.0 × 17.6-22.8 µm; n = 31). Morphological characteristics were as described for the genus Peronospora, with sporangial sizes smaller than P. belbahrii but larger than P. lamii. The mtDNA cox2 and rDNA ITS were PCR amplified and sequenced bi-directionally using primers Cox2-F/Cox2-RC4 and ITS-O/LR-O, respectively (Choi et al. 2015) from DNA extracted from infected tissue using the DNeasy Plant Mini kit (QIAGEN, Gaithersburg, MD). Based on BLASTn searches of NCBI GenBank, the cox2 sequence (XX00000) shared 100% identity with a specimen of P. belbahrii on coleus (FJ394339) and 98% with P. belbahrii on basil (Ocimum basilicum; KJ654229) and P. salvia-plebeia on sage (Salvia plebeia; KJ654299). The ITS sequence (XX00000) also showed 100% nucleotide identity with P. belbahrii on coleus (KP164987 and FJ394336). Morphological and molecular characteristics of our samples were consistent with the description of P. belbahrii sensu lato (Thines et al. 2009). Peronospora belbahrii is described as a complex of species, likely defined by plant host. Pathogenicity tests were conducted by rubbing fresh sporangia onto six detached leaves of the cultivar ‘Pineapple’. Inoculated and negative control leaves (n = 3) were maintained under a 12-h photoperiod at 23°C. Downy mildew signs and symptoms were observed on inoculated leaves within 5 days and confirmed through morphological examination and sequence analysis. No symptoms were observed on the controls. To our knowledge, this is the first report of P. belbahrii on coleus in Tennessee (Farr and Rossman, 2015). In the U.S., Peronospora sp. on coleus has been reported since 2006 from Florida, Louisiana and New York, and P. belbahrii s.l. has been reported from Michigan (Choi et al. 2009; Daughtrey et al. 2006; Palmateer et al. 2008). The occurrence and rapid spread of Peronospora species affecting coleus presents a significant threat to the ornamental plant industry.