Development and Evaluation of Laboratory Bioassays to Study Powdery Mildew Pathogens of Phlox In Vitro

Bedding Plant Production and the Challenge of Fungal Diseases

Bedding plants are a major group of ornamentals produced in greenhouses or nurseries worldwide and planted outdoors. Their economic importance has increased continuously in the last four decades in both the United States and the European Union. These plants are subject to a broad number of diseases that can negatively impact their production and cultivation. The initial steps of production strongly influence the health status of these plants and, consequently, their aesthetic appeal, which is a strong requisite for consumers. Seeds, cuttings, and other forms of propagative material, along with production systems and growing media, can influence the phytosanitary status of the final product. In this article, case studies of soilborne and foliar diseases are presented together with preventive measures to achieve innovative disease management strategies. Quarantine restrictions and eradication measures are also discussed, in consideration of the high likelihood for ornamental plants to be long-distance vectors of new pathogens and pests.

First Report of Golovinomyces orontii Causing Hydrangea macrophylla Powdery Mildew in Qinghai-Tibet Plateau, China

Hydrangea macrophylla (Thunb.) Ser. (Hydrangeaceae) is the most popular hydrangea species grown in home gardens and landscapes in China. Plants of H. macrophylla with symptoms of powdery mildew were found in a commercial wholesale nursery in Huzhu, Haidong (36°49'11.87" N, 101°57'03.36″E, alt. 2490 m), in May 2020, with disease incidence reaching 80%. Symptoms included yellowing and necrosis of leaves. Upon microscopic observation, masses of conidia and mycelium were observed covering the symptomatic tissues. Fungal isolates displayed nipple-shaped hyphal appressoria, often poorly developed, conidiophores erect, arising laterally or from the upper surface of hyphal mother cells, and positioned almost centrally or towards one end of the cells, up to about 160 μm long (n = 30), with foot cells straight or flexuous, 32 to 86 × 8 to 13 μm (n = 50), followed by one to three shorter cells about 11 to 24 × 10 to 15 μm (n = 50), forming catenescent conidia in usually predominantly chains, conidia doliiform to limoniform, hyaline, 24 to 35 × 13 to 25μm (n = 50). Conidial germination was of the Euoidium type. Chasmothecia were not observed. To confirm fungal classification, single spores were isolated and cultured on detached leaf bioassay following the protocol described in Farinas et al. (2019). Total DNA was extracted directly from single-spore cultures using a Chelex extraction method (Walsh et al. 1991). The rDNA internal transcribed spacer (ITS) regions were amplified by polymerase chain reaction (PCR) utilizing the universal primer pairs ITS1/ITS4 (White et al. 1990). The sequences (726-727 bp) were deposited in GenBank (accessions no. MT568633, MT757924 and MT757925). The ITS sequences showed 99.9-100% identity with a sequence of Golovinomyces orontii reported on Papaver rhoeas (AB769466) in Switzerland. Based on the ITS rDNA phylogenetic tree, the sequences retrieved from the specimen clustered within a strongly supported clade with G. orontii (AB769466), confirming the identity of the pathogen (Takamatsu 2013). Cladistic trees were constructed using the neighbor-joining method with the Kimura 2-parameter substitution model in MEGA 6.0. Branch robustness was assessed via bootstrap analysis with 1,000 replicates. To confirm pathogenicity, eight H. macrophylla plants were sprayed until run-off with a suspension containing 1 × 105 conidia/ml. Four plants were used for fulfilling Koch's postulates and four plants were used as mock-inoculated controls sprayed only with sterile distilled water. Inoculated and non-inoculated plants were covered with plastic bags separately and maintained overnight in a greenhouse at 25 ± 2°C and 50 to 60% relative humidity. Typical powdery mildew colonies developed on inoculated plants 10 to 15 days after inoculation, which were morphologically identical to those originally observed on the diseased plats, whereas the control plants remained symptomless. To our knowledge, this is the first report of powdery mildew caused by G. orontii on H. macrophylla in Qinghai-Tibet Plateau, China (Braun and Cook 2012).

Phlox Species Show Quantitative and Qualitative Resistance to a Population of Powdery Mildew Isolates from the Eastern United States

Ornamental plants in the genus Phlox are extensively planted in landscapes and home gardens around the world. A major limitation to a more widespread use of these plants is their susceptibility to powdery mildew (PM). In this study, we used multilocus sequence typing (MLST) analysis to gain insights into the population diversity of 32 Phlox PM pathogen (Golovinomyces magnicellulatus and Podosphaera sp.) isolates collected from the eastern United States and relate it to the ability to overcome host resistance. Low genetic diversity and a lack of structure were found within our population. Whole genome comparison of two isolates was used to support low genetic diversity evidence found with the MLST analysis. Recombination was suggested by the incongruences observed in the six phylogenetic trees generated from the housekeeping genes TEF-1α, CSI, ITS, IGS, H3, and TUB. Contrasting with low genetic diversity, we found high phenotypic diversity when using 10 of the 32 isolates to evaluate host resistance in four different Phlox species (P. paniculata 'Dunbar Creek', P. amoena OPGC 3598, P. glaberrima OPGC 3594, and P. subulata OPGC 4185) using in vitro bioassays. We observed quantitative and qualitative resistance in all Phlox species and a consistent low disease severity in our control, P. paniculata 'Dunbar Creek'. Taken together, the results generated in this study constitute a robust screening of popular Phlox germplasm that can be incorporated into breeding programs for PM resistance and provides significant information on the evolution of PM pathogens.

Whole-Genome Sequence of the Phlox Powdery Mildew Pathogen Golovinomyces magnicellulatus Strain FPH2017-1

Powdery mildew (PM) fungi are obligate biotrophs capable of infecting diverse plant hosts, ranging from monocotyledonous agricultural crops to dicotyledonous ornamental crops. The PM lifestyle poses significant challenges for studying these pathogens in isolation from their host. We present a draft genome of Golovinomyces magnicellulatus, a host-specific PM on Phlox species.

Powdery mildew (PM) fungi are obligate biotrophs capable of infecting diverse plant hosts, ranging from monocotyledonous agricultural crops to dicotyledonous ornamental crops. The PM lifestyle poses significant challenges for studying these pathogens in isolation from their host. We present a draft genome of Golovinomyces magnicellulatus, a host-specific PM on Phlox species.