Downy Mildew on coleus (Plectranthus scutellarioides) caused by Peronospora belbahrii sensu lato in Tennessee

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 on the cultivar Wasabi in a botanical garden (∼100 plants affected) and on an unknown cultivar in a private garden, both in Nashville, TN. Disease signs and symptoms were characterized by abundant gray sporulation on the abaxial surface of leaves and irregular chlorotic lesions that became necrotic with age. Microscopic examination of diseased tissue (BPI 893222 and BPI 893223) showed straight hyaline sporangiophores, monopodially branched, ending with curved branchlets bearing single sporangia. Sporangia were ellipsoid to ovoid with pale brown coloration. Sporangiophores measured 292 to 689 μm (n = 17) in length, while sporangial measurements ranged from 20.5 to 29.0 × 17.6 to 22.8 μm (mean 24.9 × 19.7 μm, n = 31). Morphological characteristics were consistent with the genus Peronospora, with sporangial sizes smaller than P. belbahrii but larger than P. lamii (Saccardo 1888). The mtDNA cox2 and rDNA ITS were PCR amplified and sequenced bidirectionally 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 GenBank BLASTn searches, the cox2 sequence (Accession No. KT828759) shared 100% identity with a specimen of P. belbahrii on coleus (FJ394339) and 98% with P. belbahrii on basil (Ocimum basilicum; KJ654229). The ITS sequence (KT828757) also showed 100% nucleotide identity with P. belbahrii on coleus (KP164987 and FJ394336). Morphological and molecular characteristics of our sample 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 10 detached leaves of the cultivar Pineapple. Inoculated and negative control leaves (n = 20) were placed on moist paper towels and incubated in a closed container under a 12-h photoperiod at 23°C. Disease symptoms and signs on inoculated leaves were observed 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 United States, Peronospora spp. on coleus have been reported since 2006 from Florida, Louisiana, and New York, and P. belbahrii sensu lato has been reported from Michigan (Farr and Rossman 2015). Previous records indicated the presence of a Peronospora sp. on coleus from various landscapes of Tennessee in 2006 (http://www.sna.org/Resources/Documents/07resprocsec09.pdf). The occurrence and rapid spread of Peronospora species affecting coleus presents a significant threat to the ornamental plant industry.