First Report of Powdery Mildew Caused by Podosphaera leucotricha on Photinia serrulata in China

First Report of Powdery Mildew on Photinia × fraseri Caused by Podosphaera leucotricha in Guizhou Province, China

The evergreen shrub Photinia × fraseri is a Photinia glabra × Photinia serrulata hybrid belonging to the family Rosaceae that is widely used in ornamental landscaping. In March 2022, severe powdery mildew symptoms were observed on shrubs of Photinia × fraseri in Huaxi University Town, Guiyang, Guizhou Province, China. All observed Photinia × fraseri plants in the green belts of both roads and parks in University Town showed powdery mildew symptoms. Almost all young branches of each Photinia × fraseri individual was infected. Powdery mildew colonies covered twig tips entirely, including the stems, petioles, and the adaxial and abaxial surfaces of leaves. Infected leaves were rolled up and had irregular, dark red spots. Fungal hyphae were straight to flexuous, branched, septate, 3 to 6 μm in width, and had nipple-shaped appressoria. Conidiophores were erect, straight or somewhat flexuous, and measured 90 to 300 µm × 7 to 10.5 µm (n = 30). Foot-cells were cylindrical or subcylindrical, straight or somewhat flexuous, and measured 25 to 50 × 7 to 9.5 µm (n = 30). Foot-cells were followed by one to two shorter cells, these being 10 to 16 × 7 to 9.5 µm in size (n = 50). Shorter cells were followed by one to six conidia (most often five conidia). Conidia formed in chains, ellipsoid to ovoid in shape, having dimensions of 22.5 to 30 × 12.5 to 16 µm (n = 50), and containing fibrosin bodies. No chasmothecia (fruiting bodies) were observed. Based on these morphological characteristics, the pathogen was identified as Podosphaera leucotricha (Ellis & Everh.) E.S. Salmon (Braun & Cook 2012). To confirm this species-level identification, the ribosomal DNA internal transcribed spacer (ITS) was amplified using the primers ITS1/ITS4 (White et al. 1990). The resulting sequence was deposited in GenBank (Accession No. ON325389). When the query coverage is 100%, the obtained ITS sequence showed 99.8% identity with P. leucotricha (AB027231, MT180425, MZ298746, KX842350, and KY661036) and 100% with P. leucotricha (HM242221, KY661017, KY661028, KY661050, KY661076, KR048110, MW364489, MW364490, MZ343479, OM022112, ON073894, and ON325389), respectively. Based on the ITS sequences of Podosphaera spp., phylogenetic tree was constructed with MEGA7.0 using the Maximum Likelihood method. The ML analysis supported our isolate's putative identification as P. leucotricha. To fulfill Koch's postulates, pathogenicity testing was conducted by gently pressing naturally diseased leaves onto young leaves of three healthy, potted 1-year-old Photinia × fraseri plants; three non-inoculated healthy plants served as control. Powdery mildew symptoms were observed on 100% inoculated Photinia × fraseri plants after 12 days (in a growth chamber at 21°C under a 12 h/12 h light/dark cycle), whereas the control plants remained symptomless. The powdery mildew colonies on inoculated leaves were morphologically identical to those observed on the original diseased leaves. It is known that P. leucotricha causes powdery mildew on Photinia × fraseri in Italy (Garibaldi et al. 2005). Moreover, this fungus reportedly infected Photinia serrulata in New Zealand, Ukraine, Italy, the United States, Japan, and in East China's Shandong Province (Liang et al. 2012). To the best of our knowledge, this is the first report of powdery mildew caused by P. leucotricha on Photinia × fraseri in Southwest China's Guizhou Province. This finding is significant as P. leucotricha is the causal agent of powdery mildew on apple and pear (Strickland et al. 2021). The occurrence of said disease on Photinia × fraseri could pose a potential disease threat to these fruit crops if nearby ornamental shrubs were able to act as reservoirs for the fungus, and a means to escape agricultural management efforts.

Identification and Characterization of Nothophoma quercina Causing Bud Blight on Photinia × fraseri in China

Photinia (Photinia × fraseri Dress) is a well-known green plant that has high ornamental value and is widely distributed around the world. An outbreak of typical bud blight disease was observed between May and August in photinia in 2017 in Qingdao, China. The causal agent for this blight was subsequently isolated from symptomatic samples and identified as Nothophoma quercina based on morphological characterization and molecular analyses (ITS, LSU, RPB2, and TUB2). Results of pathogenicity tests on isolated fungi also supported the conclusion that N. quercina is the pathogen responsible for this condition. To our knowledge, this is the first report of bud blight on P. fraseri caused by N. quercina in China.

A Genome Resource for the Apple Powdery Mildew Pathogen Podosphaera leucotricha

Powdery mildew, caused by Podosphaera leucotricha, is an economically important disease of apple and pear trees. A single monoconidial strain (PuE-3) of this biotrophic fungus was used to extract DNA for Illumina sequencing. Data were assembled to form a draft genome of 43.8 Mb consisting of 8,921 contigs, 9,372 predicted genes, and 96.1% of complete benchmarking universal single copy orthologs (BUSCOs). This is the first reported genome sequence of P. leucotricha that will enable studies of the population biology, epidemiology, and fungicide resistance of this pathogen. Furthermore, this resource will be fundamental to uncover the genetic and molecular mechanisms of the apple-powdery mildew interaction, and support future pome fruit breeding efforts.