First Report of Powdery Mildew on Physocarpus opulifolius Caused by Podosphaera physocarpi in Tennessee

Abstract

Eastern ninebark (Physocarpus opulifolius (L.) Maxim.) is a popular native perennial plant used in landscapes because of its colorful foliage and spring flower display. Powdery mildew symptoms were observed on container-grown eastern ninebark ‘Mindia’ Coppertina® plants in a commercial nursery in DeKalb County, TN in May 2016. The disease severity was nearly 40% and the disease incidence was nearly 60% of 1,000 plants. Affected plants displayed witches’-brooms with cream to white colored, thickened shoots with stunted, curly leaves as well as patches of white powdery fungal growth on the surface of young and old leaves, inflorescences, infructescences and stems (Figures 1 and 2). Microscopic observation revealed masses of conidia and mycelium covering symptomatic tissues. Conidiophore foot cells measured 19.2 to 66.7 μm (mean = 38.3 μm) × 5.4 to 15.1 μm (mean = 9.7 μm) (n = 30). Conidia were ovoid and measured 11.4 to 28.5 μm (mean = 20.9 μm) (n = 30) in length and 8.2 to 14.8 μm (mean = 11.7 μm) (n = 30) in width. Conidiophores produced two to six conidia in chains. Fibrosin bodies were observed after treating conidia with a 3% KOH solution. Chasmothecia were numerous, 60.0 to 85.0 μm (mean = 74.2 μm) (n = 30) in size and contained one ascus [60.0 to 82.0 × 52.0 to 69.0 μm; mean = 73.4 × 59.4 μm (n = 30)] with 8 ascospores [25.2 to 28.0 × 14.8 to 16.0 μm; mean = 26.5 × 15.5 μm (n = 30)]. To confirm pathogen identity, total DNA was extracted directly from plant tissue with the UltraClean Microbial DNA Isolation Kit (MO BIO Laboratories, Inc., Carlsbad, CA) following the manufacturer's instructions. The ITS region of the ribosomal DNA was amplified by PCR using primer pair ITS1 and ITS4 (White et al. 1990). The sequence (GenBank acc. no. MT605142) of the amplicon had 100% coverage and 100% identity to that of Podosphaera physocarpi (U. Braun) U. Braun (= Podosphaera aphanis var. physocarpi (U. Braun) U. Braun & S. Takam.) (GenBank acc. no. MT106654). Pathogenicity was confirmed three times by inoculating leaf surfaces of five eastern ninebark ‘Mindia’ Coppertina® plants by tapping fungal spores from infected eastern ninebark leaves onto the surfaces of healthy leaves. Inoculated plants were maintained in a greenhouse (21 to 23°C) using drip irrigation system until symptoms developed. Five non-inoculated control plants were maintained in the same greenhouse. After two weeks, typical symptoms of powdery mildew developed on the inoculated plants and microscopic examination revealed the same pathogen morphology as the original isolate. All non-inoculated control plants remained disease-free. To our knowledge, this is the first report of powdery mildew caused by P. physocarpi on P. opulifolius in Tennessee. Powdery mildew is known to be a disease problem on eastern ninebark grown in its native range in landscape plantings. Lubell et al. (2011) reported varying levels of powdery mildew resistance among eastern ninebark cultivars. Timely application of fungicides with no phytotoxic effect will be necessary to manage this disease on susceptible eastern ninebark cultivars in affected nurseries.

Management of Powdery Mildew on Ninebark Using Sanitizers, Biorational Products, and Fungicides

“…The presence of powdery mildew in container-grown 'Mindia Coppertina V R ' ninebark was confirmed in a commercial nursery in Dekalb county, TN in 2016(Baysal-Gurel et al, 2020. The disease severity was 40%, and the disease incidence was found to be 60% after observing 1000 plants.…”

“…Ninebark is considered a hardy and persistent ornamental shrub; however, it can be severely affected by powdery mildew caused by Podosphaera physocarpi (Baysal-Gurel et al, 2020;Zlesak, 2012). The fungus is highly specialized and forms a close association with ninebark, and a condition of high relative humidity but dry leaves favors the incidence of powdery mildew (Pscheidt and Ocamb, 2000).…”

“…The fungus is highly specialized and forms a close association with ninebark, and a condition of high relative humidity but dry leaves favors the incidence of powdery mildew (Pscheidt and Ocamb, 2000). The symptomatic plants exhibit witches'-broom with cream-to whitecolored, thickened shoots, curly leaves, and patches of a white, powdery fungal colony growing on the surface of leaves, stems, flowers, and fruits (Baysal-Gurel et al, 2020;Lubell et al, 2011;Pscheidt and Ocamb, 2000), reducing the ornamental characteristics of plants in landscapes. The cultivars showing resistance to powdery mildew were 'Nanus', 'Seward Summer Wine V R ', and 'Luteus' for green, purple, and yellow foliage, respectively; whereas the yellow foliage cultivars such as 'Dart's Gold', 'Morning Star', and 'Nugget' were highly susceptible to powdery mildew (Lubell et al, 2011).…”