First Report of Phytophthora Aerial Blight Caused by Phytophthora nicotianae on Vinca, Lobelia, and Calibrachoa in Ohio

Abstract

Vinca (Catharanthus roseus), lobelia (Lobelia erinus), and calibrachoa (Calibrachoa × hybrida) are important edging and bedding plants to Ohio’s floriculture industry. In July 2015, Ohio State University’s C. Wayne Ellett Plant and Pest Diagnostic Clinic received plant samples of vinca cultivars Dark Red and Titan Burgundy, calibrachoa cultivar Konga Rose Kiss, and an unknown lobelia cultivar from a botanical garden in Hamilton County, Ohio, exhibiting symptoms of foliar blight, stem dieback, and collapse of the aboveground tissues. The samples were collected from spatially separated beds within the same garden, and disease incidence in each bed varied between 30 and 60%. The samples tested positive for Phytophthora using an ImmunoStrip test (Agdia). Isolations from the edge of leaf lesions onto pimaricin, ampicillin, rifampicin, and pentachloronitrobenzene-V8 medium (Ferguson and Jeffers 1999) consistently yielded typical Phytophthora spp. isolates. All subcultured isolates produced a nondescript growth pattern on lima bean agar (LBA) but a stoloniferous one on potato dextrose agar. Morphological characteristics of one representative isolate from each plant species were determined using 18-day-old colonies on LBA incubated at 22 to 23°C under continuous florescent light. Cultures were heterothallic and produced abundant chlamydospores, which were intercalary or terminal and measured 25.66 to 54 × 27.2 to 67.76 μm (n = 128). Sporangiophores were unbranched. Sporangia varied in shape from globose to ovoid, ellipsoid, or pyriform measuring 29.03 to 55.96 × 33.6 to 69.08 μm (n = 137), were papillate, occasionally bipapillate, and noncaducous. Papillae measured 5.06 to 7.88 × 7.17 to 11.67 μm (n = 90). Based on morphological characteristics, the pathogen was identified as Phytophthora nicotianae Breda de Haan (Erwin and Ribeiro 1996). The identity of isolates was confirmed through BLAST searches of their internal transcribed spacer region (ITS1-5.8S-ITS2) sequences (GenBank accession nos. MF471219, MF784254, and MF784255) in the Phytophthora-ID database, all resulting in 99% similarity with 100% coverage to P. nicotianae (accession no. AF266776). Pathogenicity of the three isolates was determined by spraying a 1 × 10⁴ to 9 × 10⁴ zoospores/ml solution until run-off, directly on the foliage of three of each of the respective hosts (10-week-old vinca cultivar Titan Burgundy, calibrachoa cultivar Crave Sunset, and 12-week-old lobelia cultivar Riviera Blue Eyes). Three control plants per species were sprayed with sterile distilled water. Following inoculation, plants were individually placed in plastic bags for 48 h and were kept in a greenhouse at a temperature of 23 to 24°C during the day and 19 to 22°C during the night. Leaf spots were observed on all inoculated plants in as little as 2 days. Leaf and shoot blight were observed after 4 to 5 days, whereas wilting developed after 10 days. Control plants remained asymptomatic. The pathogen was successfully reisolated and identified morphologically as described above, therefore fulfilling Koch’s postulates. Phytophthora nicotianae is one of the most common Phytophthora species attacking ornamental plants (Olson et al. 2013). The pathogen has been reported on vinca throughout the eastern United States and on calibrachoa in North Carolina (Lamour et. al. 2003; Olson and Benson 2011), but it has never been reported on lobelia in the United States. This is the first reported case of P. nicotianae infecting all three hosts in Ohio. Due to environmental characteristics, incidence of Phytophthora diseases is high in Ohio. This report contributes to increased awareness of the potential of new hosts for the pathogen to thrive on.