Podosphaera xanthii causing powdery mildew on Impatiens balsamina in China

Abstract

Impatiens balsamina (garden balsam) is an annual herb and widely grown for ornamental and medicinal use in China. In 2020 and 2021, powdery mildew symptoms were found on I. balsamina in Xinxiang City, Henan Province, China. The pathogenic fungus was identified based on its morphological characteristics and molecular analysis. The internal transcribed spacer (ITS) region was amplified and sequenced. The sequence obtained for the fungus showed 100% identity with the previously reported Podosphaera xanthii. The virulence of the pathogen on I. balsamina was confirmed by utilizing Koch’s postulates. Therefore, we identified the causal organism of this disease as P. xanthii. This is the first report of powdery mildew caused by P. xanthii on I. balsamina in China. The identification of P. xanthii on I. balsamina will support efforts towards its future control and management.

Résumé: Impatiens balsamina (balsamine des jardins) est une herbe annuelle cultivée extensivement en Chine pour ses propriétés tant ornementales que médicinales. En 2020 et 2021, des symptômes du blanc ont été décelés chez I. balsamina à Xinxiang, dans la province du Henan. Le champignon pathogène a été identifié en fonction de ses caractéristiques morphologiques et par analyse moléculaire. La région de l’espaceur transcrit interne (ITS) a été amplifiée et séquencée. La séquence obtenue pour le champignon a affiché 100% d’identité avec le champignon Podosphaera xanthii précédemment rapporté. La virulence de l’agent pathogène à l’égard de I. balsamina a été confirmée en vérifiant les postulats de Koch. En conséquence, nous avons identifié l’organisme causal de cette maladie comme étant P. xanthii. Il s’agit de la première mention du blanc causé par P. xanthii chez I. balsamina en Chine. L’identification de P. xanthii chez I. balsamina contribuera à appuyer les futurs efforts de lutte et de gestion.

Keywords:

• fungal pathogen
• Impatiens balsamina
• internal transcribed spacer (ITS)
• pathogenicity
• Podosphaera xanthii
• powdery mildew

Mots clés:

• Agent pathogène fongique
• Impatiens balsamina
• espaceur transcrit interne (ITS)
• pathogénicité
• Podosphaera xanthii
• blanc

Introduction

Impatiens balsamina L., garden balsam, is an annual plant that is cultivated widely in home gardens due to its value as an ornamental. In China, the flowers of this plant are traditionally utilized for preparing a red dye to paint fingernails. Recently, Impatiens balsamina was found to be a suitable plant for the bioremediation of contaminated soils due to its phytostabilization of polychlorinated biphenyls, lubricant oil and lead (Gamage et al. 2021; Liu et al. 2021). In addition, it has been extensively used in traditional Chinese medicine to treat various ailments (Ding et al. 2008). Previous studies demonstrated that various components isolated from multiple I. balsamina organs showed antimicrobial, antitumor and anti-inflammatory activities (Sakunphueak and Panichayupakaranant 2012; Szewczyk 2018; Lee et al. 2020).

Common fungal diseases of Impatiens include mainly brown spot (Cercosporafu mshian), powdery mildew (Sphaerotheca balsaminae = Podosphaera balsaminae), wilt (Rhizoctonia solani), ring spot (Ascochyta phaseolorum), downy mildew (Plasmopara obduce), and anthracnose (Phytophthora nicotianae) (Zhou et al. 2015). Powdery mildew was repeatedly reported as a disease on I. balsamina and severely affected plant health (Farr and Rossman 2020). Previously, various powdery mildew fungi were demonstrated to infect I. balsamina. For example, Golovinomyces biocellatus and P. balsaminae were reported to cause powdery mildew on I. balsamina in Argentina, China, Germany, India, Japan and Korea (Hanlin and Amano 1986; Akram 1995; Wolcan 2004; Choi et al. 2021), while Podosphaera xanthii was reported as the cause of powdery mildew on I. balsamina in India (Mulpuri et al. 2016) and Thailand (Meeboon 2016). Generally, powdery mildew infection of I. balsamina resulted in senescence, colour changing, the loss of leaves and even plant death.

In October 2020 and July 2021, powdery mildew-like signs and symptoms were observed on leaves of I. balsamina grown in home gardens in Henan Province, China. White to greyish powder-like masses were abundant on both sides of the leaves and covered up to 90% of the leaf area (Fig. 1). Some of infected leaves were chlorotic or senesced. More than 90% of the examined plants exhibited these signs and symptoms. However, the causal organism of the pathogen was unclear. Therefore, examination of morphological characteristics and molecular analysis were used to identify the pathogenic fungus.

Fig. 1 Signs and symptoms of powdery mildew disease on Impatiens balsamina.

Materials and Methods

Collection and morphological characterization

Mildew-infected plant leaves were collected and transported to the lab. In order to morphologically characterize the fungi, the leaves were bleached, and fungal structures on the leaves were stained with trypan blue and observed under a light microscope (Sunny Optical, EX30, Zhejiang, China) (Zhu et al. 2017).

DNA extraction and amplification

To collect fungal conidia, mildew-infected leaves were painted with 5% (w/v) cellulose acetate dissolved in acetone. After evaporation of the acetone, cellulose-acetate strips were harvested. Total fungal genomic DNA was extracted according to a previous method (Zhu et al. 2019). The fungal ribosomal ITS segment universal primers, ITS1 (5ʹ-TCCGTAGGTGAACCTGCGG-3ʹ) and ITS4 (5ʹ-TCCTCCGCTTATTGATATGC-3ʹ) (White et al. 1990), were used for polymerase chain reaction (PCR) amplification, using the following program: 94°C for 5 min, followed by 35 cycles of 94°C for 30 s, 54°C for 30 s, 72°C for 1 min, and final elongation at 72°C for 5 min. PCR products were detected by electrophoresis in 1.5% agarose gels. The amplicon was sequenced and the resulting sequence was deposited in GenBank (NCBI) as accession no. MW301281.

Phylogenetic analysis

To build of a phylogenetic tree, ITS sequences of Podosphaera sp. were retrieved and analyzed with the ITS sequence of the isolated fungus (Table 1) in MEGA software. The evolutionary tree was constructed using the neighbor-joining method with the following options: 1000 bootstraps replicates, p-distance, 50% site coverage cut off (Zhu et al. 2021a). The ITS sequences of Erysiphe sp. and Arthrocladiella mougeotii were included as outgroups (Zhu et al. 2020a; Zhu et al. 2020b).

Table 1. Podosphaera sp. and their corresponding hosts

	Powdery mildew fungus	Host	Location and year	Accession No.	Reference
1	Podosphaera xanthii	Impatiens balsamina	Henan, China; 2020 and 2021	MW301281	This study
2	Podosphaera xanthii	Abelmoschus esculentus	Iksan, Korea; 2014	MG754404	(Choi et al. 2018)
3	Podosphaera hibiscicola	Hibiscus mutabilis	Chiba, Japan; 2000	AB040308	(Hirata et al. 2000)
4	Podosphaera macrospora	Tellima grandiflora	United Kingdom; 2015	KX032536	(Ellingham et al. 2016)
5	Podosphaera tridactyla	Sorbaria sorbifolia		MK425648
6	Podosphaera clandestina	PM Phlox paniculata		MW168800
7	Podosphaera ampla	Prunus salicina x persica	Australia; 2021	MW364541	(Smith et al. 2021)
8	Podosphaera erigerontis	Crepis sancta-A		MW703889
9	Podosphaera aphanis	Rubus pringlei Rydb	Chiapas and Michoacán, Mexico; 2020	MW988591	(Solano-Báez et al. 2021)
10	Podosphaera leucotricha	Photinia x fraseri	Torino, Italy; 2004	MW364490	(Garibaldi et al. 2005)
11	Podosphaera pannosa	Citrus ×reticulata-sinensis	Coquimbo; 2017	MK260048	(Cornejo et al. 2019)
12	Podosphaera xanthii	Impatiens balsamina	Aomori, Japan; 2001	AB462803	(Ito and Takamatsu 2010)
13	Podosphaera xanthii	Impatiens balsamina	Kagoshima, Japan; 2000	AB462799	(Ito and Takamatsu 2010)
14	Podosphaera xanthii	Impatiens balsamina	Mie, Japan; 2000	AB462800	(Ito and Takamatsu 2010)
15	Podosphaera balsaminae	Impatiens noli-tangere	Hokkaido, Japan; 2003	AB462805	(Ito and Takamatsu 2010)
16	Podosphaera balsaminae	Impatiens noli-tangere	Hokkaido, Japan; 2003	AB462807	(Ito and Takamatsu 2010)
17	Podosphaera balsaminae	Impatiens noli-tangere	Iwate, Japan; 2003	AB462806	(Ito and Takamatsu 2010)
18	Podosphaera balsaminae	Impatiens noli-tangere	Niigata, Japan; 1996	AB040318	(Ito and Takamatsu 2010)
19	Podosphaera xanthii	Impatiens textori	Nagano, Japan; 2000	AB462801	(Ito and Takamatsu 2010)
20	Podosphaera xanthii	Impatiens textori	Akita, Japan; 2001	AB462802	(Ito and Takamatsu 2010)
21	Podosphaera xanthii	Impatiens textori	Niigata, Japan; 1996	AB040344	(Ito and Takamatsu 2010)

Pathogenicity assay

Briefly, three healthy plants were placed in a settling tower and then inoculated by blowing pathogen conidia from diseased leaves with pressurized air into the settling tower (Zhu et al. 2021b). Three non-inoculated plants served as controls. The controls and inoculated plants were maintained separately in two growth chambers (temperature 18°C; humidity 60%; light/dark 16 h/8 h) and disease signs and symptoms were checked daily. After signs and symptoms appeared, the morphological characteristics of the powdery mildew on infected leaves were determined to confirm whether these were consistent with the disease on originally infected plants.

Results

Morphological characteristics of the powdery mildew on Impatiens balsamina

White to greyish powder-like masses were abundant on both sides of I. balsamina leaves and covered up to 90% of the leaf area. Some infected leaves were chlorotic or senesced. More than 90% of plants exhibited these signs and symptoms. Cylindrical conidiophores (n = 50) were 90–220 × 10–13 μm and composed of foot cells, followed by shorter cells and conidia. Foot cells (n = 50) were straight and 40 to 80-μm long. The ellipsoid-ovoid-shaped conidia (n = 50) with conspicuous fibrosin bodies were 26–39 × 15–25 μm with a length/width ratio of 1.6 to 2.0 (Fig. 2). No chasmothecia were observed. Based on these morphological characteristics of fungal structures by microscopy analysis, the pathogen was initially identified as P. xanthii.

Fig. 2 Fungal structures of P. xanthii on host leaf surface. The structures were stained with trypan blue.

Confirmation of the powdery mildew fungus by molecular analysis

BLASTn analysis using the ITS fragment amplified in this study (MW301281) indicated that the sequence was 100% identical (563/563) to P. xanthii on Abelmoschus esculentus. The same results were obtained with the powdery mildew collected in 2021.

The phylogenetic tree clearly showed that the ITS sequence of the powdery mildew fungus on I. balsamina clustered with P. xanthii on A. esculentus (MG754404) and on I. balsamina (AB462803, AB462799, AB462800) (Fig. 4).

Fig. 3 PCR detection of the powdery mildew on I. balsamina samples collected in 2021 (A) and 2020 (B). M: marker; 1–4: diseased samples.

Fig. 4 Phylogenetical analysis of P. xanthii and Podosphaera sp. The neighbour joining tree was constructed in MEGA software with 1000 bootstrap replicates and p-distance method. The bar indicates a distance of 0.020. Red dot highlights P. xanthii. Arthrocladiella mougeotii and Erysiphe sp. were included as outgroup.

Pathogenicity of identified powdery mildew

Pathogenicity assays were performed using an inoculation test. Signs of powdery mildew were visible on the inoculated plants 10 days after inoculation (Fig. 5), while the controls remained healthy. The same results were obtained from the two repeated pathogenicity experiments. Therefore, the pathogen was identified and confirmed as P. xanthii based on morphological and molecular analysis.

Fig. 5 P. xanthii infected I. balsamina and signs of powdery mildew 10-days post inoculation.

Discussion

Impatiens balsamina is an important plant with high ornamental, bioremediation and medical value (Ding et al. 2008; Sakunphueak and Panichayupakaranant 2012; Szewczyk 2018; Lee et al. 2020; Gamage et al. 2021; Liu et al. 2021). However, its quality and survival are significantly affected by fungal pathogens, including powdery mildews. Previously, Golovinomyces biocellatus and P. balsaminae were identified as the causal agents of powdery mildew on I. balsamina in many countries, including China (Tai 1979; Hanlin and Amano 1986; Akram 1995; Wolcan 2004). However, P. xanthii was recently reported to cause powdery mildew on I. balsamina in India (Mulpuri et al. 2016) and Thailand (Meeboon 2016). Podosphaera xanthii is known to cause powdery mildew on many ornamental plants (Farr and Rossman 2020) and is one of the most important limiting pathogens in the production of cucurbits (Pérez-García et al. 2010). Whether P. xanthii could cause powdery mildew on I. balsamina in China was not known.

In 2020 and 2021, powdery mildew was observed on I. balsamina in Xinxiang City, Henan province, China (Fig. 1). Initially, we microscopically examined the morphology of the fungus (Fig. 2) and found that its characteristics were consistent with those of P. xanthii (Braun and Cook 2012). To confirm this identification, we used molecular and phylogenetic analysis (Figs. 3 and 4). The sequence of our powdery mildew was 100% identical to that of P. xanthii on A. esculentus (Choi et al. 2018), and our putative P. xanthii clustered with P. xanthii previously reported on I. balsamina (Ito and Takamatsu 2010). Earlier, it was demonstrated that isolates of different powdery mildew species on Impatiens formed distinct clades (Ito and Takamatsu 2010). While the ITS sequence of the powdery mildew we identified from I. balsamina also showed 100% identity with the previously reported P. balsaminae on I. balsamina, in agreement with Ito and Takamatsu (2010), our phylogenetic analysis confirmed that the powdery mildew fungus identified in this study was P. xanthii. The pathogenic ability of the powdery mildew was tested with inoculation assays, which showed that P. xanthii successfully infected I. balsamina and that signs of powdery mildew developed 10-days after inoculation (Fig. 5). Therefore, the causal organism of powdery mildew disease on I. balsamina was identified and confirmed as P. xanthii.

To the best of our knowledge, this is the first record of P. xanthii causing powdery mildew on I. balsamina in China. The sudden outbreak of the disease caused by P. xanthii may adversely affect plant health and thus reduce the ornamental and medical value of I. balsamina. Therefore, the identification and confirmation of P. xanthii infecting I. balsamina provides additional insights into the hosts of this pathogen in China and will support disease control efforts in the future.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was financially supported by the Foundation for the Key Research Program of Higher Education of Henan (20A210026), Doctor Initiative Foundation of Henan Normal University to Mo Zhu (5101049170196) and the Open Project of Key Laboratory of Integrated Pest Management on Crops in Central China, Ministry of Agriculture and Rural Affairs, P. R. China and Hubei Key Laboratory of Crop Disease, Insect Pests and Weeds Control (2020ZTSJJ3).