Powdery mildew (Erysiphales) on ornamental plants in the Czech Republic

In this study, two new species of powdery mildews together with disease severities measured at four different growing stages were identified on three ornamental species in the Ažanliurfa district of Turkey. Observations and measurements were made on natural and experimental stands. On natural stands, 15, 85 and 100% disease rates were determined on Fuchsia regia, Calendula officinalis and Zinnia elegans, respectively. Powdery mildews of C. officinalis and Z. elegans were identified as Podosphaera xanthii(Oidium subgenus Fibroidium). Disease symptoms were observed from early seedling to late maturing stages on Z. elegans, and from pre flowering to maturing stages on C. officinalis. The mildew pathogen of Fuchsia regia was identified as Leveillula taurica. This ornamental species appear to be resistant to this respected powdery mildew agent during summer, but somewhat susceptible in the early autumn. In addition, upper leaves were observed the only section displaying the disease. Key words: Podosphaera xanthii, Leveillula taurica, ornamentals, disease severity.

Powdery mildew is a serious disease and causes considerable decline in production and fruit quality of melon (Cucumis melo L.) grown in Java, Indonesia. The fungal species that cause powdery mildew in Indonesia have never been reported. Therefore, the lack of knowledge about the biology of these fungi will inhibit in the handling of the eradication of powdery mildew further. A survey was conducted in 58 locations of melon cultivation in Central Java, East Java and West Java to study the occurrence of powdery mildew in Java. Powdery mildew species involved in the disease development were identified morphologically based on the anamorph characters such as the mycelium/hyphae, germ tube, appressoria, conidia and fibrosin bodies. In order to confirm the identification, the complete ITS (internal transcribed spacer) region of rDNA of powdery mildew species was amplified using ITS 1 and ITS 4 primers, sequenced directly and compared with sequences from the GenBank. Twelve isolates of powdery mildew species were collected from 58 melon cultivations in Java. Based on the morphological characters, the powdery mildew species had similarity characters to Podosphaera xanthii (Castagné) U. Braun & N. Shishkoff (syn. Podosphaera fusca (Fr.) U Braun & N Shishkoff). Molecular characterization of ITS rDNA produced 562 bp to 704 bp sequences. Homology search from the Genbank showed that the powdery mildew species in Java had 98 % to 99 % similarity to a sequence from the USA and it confirmed that the identity of powdery mildew species in Java as P. xanthii.

Powdery mildews were first recognized as white powdery appearance on the leaves of Humulus, Acer, Lamia, Galeopsis, and Lithospermum by plant pathogens infection as early as 1753 which was named as Mucor erysiphe. Initially powdery mildews were a fascinating subject of research for mycologists who reported several species of powdery mildews on different hosts on the basis of minor differences in their morphological characteristics. The economic importance of powdery mildews was realized in 1850 with the most wide spread, and disastrous losses attributable to powdery mildew of grapes in France. Later, during nineteenth century, powdery mildew diseases of barley, gooseberry, peaches, cucurbits, and several other crops have been found to cause economic losses in many countries of the world. The crucifer’s powdery mildew has gained its importance as an economically damaging disease during last two decades of twentieth century. In total, upto now, more than 873 species of powdery mildews have been reported to cause infection to nearly 10,000 plant species all over the world. The powdery mildew of crucifers (Arabidopsis) is considered as one of the “Top ten fungal pathogens for molecular, and genetical studies all over the world”. The classification, taxonomy, nomenclature, and identification of powdery mildew fungal genera, and species has now been resolved with the use of morphological data of both anamorph, and teleomorph stages, host range, using light, and scanning electron microscopy, and phylogenetical analysis. In future, under the new rules, powdery mildew fungi will be treated for nomenclaturaly like plants, and all other groups of organisms ruled by the “International code of nomenclature for algae, fungi and plants” i.e., with one correct name for each species. Recently, powdery mildews have shown their potential as biological weed control agent. Some hyperparasites of powdery mildews have been identified with practical application under field conditions for powdery mildew disease control. With global warming, powdery mildews may become threatening, and more devastating disease all over the world. The information revealed on genetical, and molecular mechanisms of host-parasite interaction will be helpful in planning strategies of powdery mildew management under field conditions.

Botanical gardens host diverse plant assemblages that provide valuable opportunities to study fungal biodiversity and plant–fungal interactions. Powdery mildews ( Erysiphaceae ) are common pathogens in these settings, yet little is known about how they co‐occur with culturable fungi present on infected leaves. To document fungi recovered from infected tissue, we surveyed powdery mildew infections at the JC Raulston Arboretum, which is part of North Carolina State University. We identified the powdery mildew species present and the fungi that could be cultured from the infected phyllosphere. Eighteen powdery mildew species representing three genera were documented and confirmed through morphological and molecular methods. Using a dilution‐to‐extinction culturing approach, we recovered 147 isolates corresponding to 57 fungal species across 36 genera. Several genera including Aspergillus and Cladosporium were repeatedly recovered from powdery mildew‐infected leaves, and six isolates represent potentially novel lineages. Several of these genera have previously been shown to possess biological control potential against powdery mildew. These findings demonstrate that botanical gardens harbor rich, underexplored fungal diversity and that powdery mildew–infected tissues provide access to a broad community of co‐occurring fungi. Our results highlight the potential of culture‐based surveys in botanical gardens to uncover novel fungal taxa and identify candidate biological control agents for powdery mildew management.

The Fabaceae family has been reported to host more than fifty species of powdery mildew worldwide. Despite being commonly found on fabaceous hosts throughout Australia, the accurate identification of many powdery mildew species remains uncertain. The objective of this study was to identify powdery mildew species that naturally occur on fabaceous hosts in Australia and provide insight into those native and weedy species that may host crop pathogens and contribute to disease in cropping systems. The ribosomal DNA internal transcribed spacer (ITS) sequences and morphology of 34 fresh and 40 herbarium powdery mildew specimens infecting diverse Fabaceae species in Australia were characterised in this study. Altogether, a total of eleven powdery mildew species were identified from 51 Fabaceae species. Podosphaera xanthii was the most common powdery mildew in this study and was detected on 18 host species across ten genera. Ten species of Erysiphe were confirmed on 37 host species covering 17 host genera, with E. diffusa and E. cf. trifoliorum the most prevalent. This work provides the most comprehensive catalogue of powdery mildew species infecting legume hosts throughout Australia.

Powdery mildew (PM) is one of the most common plant diseases in the world, occurring in many economically important food and ornamental crops. The characteristic symptoms of this disease are easily recognizable as a white powder that extends across the aerial parts of the plants. The causal pathogens of PM are obligate biotrophic fungi from the Erysiphales order that obtain nutrients from the living tissue of their hosts. To this day, chemical control of PM remains a common practice in many crops. However, the use of fungicides has a negative impact on the environment. For this reason, breeding for resistant varieties stands out as an essential tool to achieve a sustainable management of this disease.In tomato, PM is caused by Oidium neolycopersici (On), a polyphagous fungal species that threatens the production of this crop worldwide. The current understanding of this disease and the mechanisms through which plants are able to withstand it comes from the research done in two pathosystems. On the one hand, in the late 1980’s, an outbreak of On in cultivated tomato in Europe urged researchers to find natural sources of resistance to this disease. This drove the research on the identification of Resistance (R) loci in wild relatives of tomato. On the other hand, On can also infect the model plant Arabidopsis thaliana (Arabidopsis). This model organism offers many advantages for research due to the availability of a large number of scientific resources and information. Apart from On, Arabidopsis is a host for three more PM species. Hence, the Arabidopsis-PM pathosystems have been widely used to identify genetic components of resistance and susceptibility. These latter components are particularly interesting, as they seem to be conserved for several species of PMs and can potentially be extrapolated to other plant species. The plant genes whose absence results in the inability of the pathogen to complete their life cycles, are named Susceptibility (S) genes. The use of S genes is proposed as a suitable alternative to obtain more durable, broad-spectrum resistance and is particularly important in the absence of R genes in certain crops. The work of this thesis focuses on the exploration and identification of new genetic components of resistance and susceptibility to On in both the model plant Arabidopsis and the crop plant tomato.In the first half of this thesis, we make use of Arabidopsis to identify genetic components of the natural resistance against On. In Chapter 2, I report the finding of a dominantly-inherited resistance in accession Bla-6. Through a series of recombinant analyses, we fine mapped the locus responsible for the resistance and used CRISPR/Cas9 to knockout the candidate genes. We show that targeted mutagenesis of the ZED1-RELATED KINASE 13 (ZRK13) in Bla-6 results in a susceptible phenotype, thus identifying this gene as a novel component of resistance against On. Furthermore, in Chapter 3, we found two major QTLs explaining resistance to On in accession Litva. Through an independently-executed fine-mapping effort we identified ZRK13 to be also required for resistance in this accession. However, an additional locus in chromosome 3 is needed for full resistance in Litva. In these two chapters I discuss the identity of this gene and suggest further steps to investigate the molecular mechanisms of this previously uncharacterized resistance component.The second half of this thesis focuses on the tomato-On interaction to study susceptibility and resistance components. In Chapter 4, I report the generation, through CRISPR/Cas9-targeted mutagenesis, of tomato knockouts of the susceptibility gene PMR4. This gene was first found in a forward genetic screening in Arabidopsis and resistance through its impairment was later found to be conserved in the tomato-On pathosystem. I describe the characterization of five different mutation events in the target gene and report a reduced but not complete loss of susceptibility upon inoculation with On. With this study, we set the basis for the CRISPR/Cas9-based protocol for gene identification that was later used in the other chapters of this thesis.In Chapter 5 I describe the fine-mapping and characterization of the candidate genes for Ol-1, a dominant resistance found in S. habrochaites accession G1.1560. We show that targeted mutagenesis of Solyc06g060800, a gene encoding a 2-oxoglutarate and Fe(II)-dependent oxygenase superfamily protein, results in increased susceptibility to PM in near-isogenic Ol-1 background. In this chapter we further discuss the implications of this finding.In Chapter 6 I give an overview of the results presented in this thesis and discuss their implications in the plant immunity model. Altogether, the work presented in this thesis contributes to the understanding of the diversity of genetic components of resistance against On in both Arabidopsis and tomato, with the aim of aiding in the development of breeding strategies that result in a sustainable management of this disease.

Zinnia elegans L., known as common zinnia, is an annual flowering plant belonging to the Asteraceae family and native to North America. The plant has colorful flowers and is one of the popular ornamental bedding plants for gardening. In March 2020, powdery mildew symptoms were observed in a zinnia floral field with an incidence of >70% in Dacun Township, Changhua County, Taiwan. The symptoms were spotted on the stems, flower petals and leaves which appeared as irregular colonies and white patches on the surfaces. When disease progressed, most of the plant surfaces were covered by the white fungal colonies and became yellowish. Under microscopic examination, hyphal appressoria of the fungus were indistinct or slightly nipple-shaped. The conidiophores were unbranched, erect, straight, smooth to slightly rough, 75.0 to 200.0 × 10.0 to 15.0 µm (n=10), composed of a cylindrical, flexuous foot cell, 40.0 to 100.0 × 8.8 to 15.0 µm (n=10), and following 1 to 5 shorter cells. The conidia were ellipsoid to ovoid, 25.0 to 37.5 × 15.0 to 23.8 µm (n=60), with an average length-to-width ratio of 1.8 and contained fibrosin bodies. No chasmothecia were found. Three voucher specimens (TNM Nos. F0033680, F0033681, and F0033682) were deposited in the National Museum of Natural Science, Taichung City, Taiwan. To confirm the identification, the internal transcribed spacer (ITS) regions of the three specimens were amplified using primer pairs ITS1/PM6 and PM5/ITS4 (Shen et al. 2015) and sequenced from both ends. The resulting sequences were deposited in GenBank under Accession Nos. MT568609, MT568610, and MT568611. The sequences were identical to each other and shared a 100% identity with that of Podosphaera xanthii MUMH 338 on Z. elegans from Japan (Accession No. AB040355) (Ito and Takamatsu 2010) over a 475 bp alignment. Accordingly, the fungus was identified as P. xanthii (Castagne) U. Braun & Shishkoff (Braun and Cook 2012) based on its morphological and molecular characters. Pathogenicity was demonstrated through inoculation by gently pressing naturally infected leaves onto leaves of three healthy potted common zinnia that had been sprayed with 0.02% Tween 20. Additional three non-inoculated plants treated in the same way without inoculating the powdery mildew served as the controls. Powdery mildew colonies were observed on inoculated leaves after 10 days at room temperature, later the diseased leaves became yellowish and deteriorated. The morphological traits of the fungus on the inoculated leaves were similar to those of the first observed. In addition, the ITS sequence from a colony on the inoculated leaves was 100% identical to MT568609-MT568611, fulfilling the Koch's postulates. All the controls remained symptomless. Z. elegans is known to be a host for different species of powdery mildew in the genus Erysiphe, Golovinomyces, and Podosphaera (Farr and Rossman 2020). In Taiwan, powdery mildew has been briefly reported on zinnia without detailed descriptions (Hsieh 1983). This study confirmed P. xanthii as a causal agent of powdery mildew in Taiwan and the awareness of the disease may benefit the floral industry. To our knowledge, this is the first confirmed report of P. xanthii on Z. elegans in Taiwan.

First report of <i>Erysiphe palczewskii</i> on <i>Caragana arborescens</i> in the Czech Republic

Since 2001, several isolates of Blumeria graminis, the causal agent of cereal powdery mildew, maintained on detached leaves at the John Innes Centre, Norwich, UK, have spontaneously become infected with an unknown filamentous fungus whose mycelia have quickly overgrown the powdery mildew colonies and destroyed them completely. A total of five isolates of the contaminant were obtained and identified as Paecilomyces farinosus based on morphological characteristics and rDNA ITS sequence data. To determine whether these P. farinosus isolates can be considered as biocontrol agents (BCAs) of powdery mildews, we studied the interactions between P. farinosus and the following four powdery mildew species: B. graminis f.sp. hordei infecting barley, Oidium neolycopersici infecting tomato, Golovinomyces orontii infecting tobacco and Podosphaera fusca infecting cucumber. The powdery mildew colonies of all these four powdery mildew species were quickly destroyed by P. farinosus in leaf cultures but neither conidial suspensions nor cell-free culture filtrates of P. farinosus isolates could suppress the spread of powdery mildew infections on diseased barley, tomato, tobacco or cucumber plants in the greenhouse. It is concluded that P. farinosus cannot be considered as a promising BCA of powdery mildew infections although it can destroy powdery mildew colonies in detached leaf cultures and can be a menace during the maintenance of such cultures of cereal, apple, cucurbit and tomato powdery mildew isolates.

A total of 103 tree and shrub samples infected with powdery mildew were collected during 2002–2019 from locations within parks, botanical gardens and urban green areas within the Czech Republic and the powdery mildews were morphologically analyzed and identified. The most frequently represented genera were: Erysiphe (27, including former genera Microsphaera and Uncinula), Podosphaera (11, including former genus Sphaerotheca), Phyllactinia (3), Sawadaea (2) and Arthrocladiella (1). New records for the Czech Republic were: E. (U.) arcuata, E. (M.) deutziae, E. (M.) euonymicola, E. (U.) flexuosa, E. (M.) platani, E. (M.) symphoricarpi, E. (M.) vanbruntiana var. sambuci-racemosae, E. (U.) ulmi, Po. amelanchieris, Po. (Sph.) pruinosa and Po. (Sph.) spiraeae. The results were compared with the spectrum of powdery mildew species in the surrounding countries (Slovakia; Hungary; Poland and Germany (Bavaria)).

In late summer 2022 and 2023, powdery mildew symptoms were found on female plants of hemp (Cannabis sativa) growing in a private garden in Hradec nad Moravicí (Silesia, Czech Republic). On the upper sides of the leaves, white powdery colonies were observed which coalesced and finally covered the whole leaf surface. The stems and flowers were not affected. Microscopic examination revealed the formation of asexual (anamorphic) states characterized by conidiophores forming catenescent conidia (Euoidium type), but with two types of conidia, viz., ellipsoid to ovoid or doliiform conidia with fibrosin bodies (resembling conidia of Podosphaera spp.), and ellipsoid to doliiform conidia without fibrosin bodies (resembling conidia of Golovinomyces spp.). Molecular genetic analyses confirmed co‐infections in the studied hemp samples by two powdery mildew species, viz., Podosphaera macularis and Golovinomyces ambrosiae, which represent first records from the Czech Republic, as well as Europe and in the world. The authors discuss the latest information and ideas regarding this pathosystem.

The occurrence of Erysiphe cichoracearum (Ec) and Sphaerotheca fuliginea (Sf), causal agents of cucurbit powdery mildew&lt;br /&gt;in the Czech Republic (CR) was studied in the period of 1995–2001. Nearly 800 leaf samples with disease symptoms&lt;br /&gt;were microscopically examined. Ec is the predominating species, detected on 98% of locations. It was accompanied by Sf&lt;br /&gt;on 24% of locations. The occurrence of Sf as the only powdery mildew species was proved on 2% of locations. Recent&lt;br /&gt;occurrence of Sf in the CR corresponds with data on Sf fast spreading and prevailing on cucurbits in West and South&lt;br /&gt;Europe. However, Ec is the strongly predominating powdery mildew species largely distributed throughout the country.&lt;br /&gt;The hyperparasitic fungus Ampelomyces quisqualis was detected on 30% of samples.

A total of 26 Ampelomyces strains were isolated from mycelia of six different powdery mildew species that naturally infected their host plants in Japan. These were characterized based on morphological characteristics and sequences of ribosomal DNA internal transcribed spacer (rDNA-ITS) regions and actin gene (ACT) fragments. Collected strains represented six different genotypes and were accommodated in three different clades of the genus Ampelomyces. Morphology of the strains agreed with that of other Ampelomyces strains, but none of the examined characters were associated with any groups identified in the genetic analysis. Five powdery mildew species were inoculated with eight selected Ampelomyces strains to study their mycoparasitic activity. In the inoculation experiments, all Ampelomyces strains successfully infected all tested powdery mildew species, and showed no significant differences in their mycoparasitic activity as determined by the number of Ampelomyces pycnidia developed in powdery mildew colonies. The mycoparasitic interaction between the eight selected Ampelomyces strains and the tomato powdery mildew fungus (Pseudoidium neolycopersici strain KTP-03) was studied experimentally in the laboratory using digital microscopic technologies. It was documented that the spores of the mycoparasites germinated on tomato leaves and their hyphae penetrated the hyphae of Ps. neolycopersici. Ampelomyces hyphae continued their growth internally, which initiated the atrophy of the powdery mildew conidiophores 5 days post inoculation (dpi); caused atrophy 6 dpi; and complete collapse of the parasitized conidiphores 7 dpi. Ampelomyces strains produced new intracellular pycnidia in Ps. neolycopersici conidiophores ca. 8–10 dpi, when Ps. neolycopersici hyphae were successfully destroyed by the mycoparasitic strain. Mature pycnidia released spores ca. 10–14 dpi, which became the sources of subsequent infections of the intact powdery mildew hyphae. Mature pycnidia contained each ca. 200 to 1,500 spores depending on the mycohost species and Ampelomyces strain. This is the first detailed analysis of Ampelomyces strains isolated in Japan, and the first timing and quantification of mycoparasitism of Ps. neolycopersici on tomato by phylogenetically diverse Ampelomyces strains using digital microscopic technologies. The developed model system is useful for future biocontrol and ecological studies on Ampelomyces mycoparasites.

A total of 26 Ampelomyces strains were isolated from mycelia of six different powdery mildew species that naturally infected their host plants in Japan. These were characterized based on morphological characteristics and sequences of ribosomal DNA internal transcribed spacer (rDNA-ITS) regions and actin gene (ACT) fragments. Collected strains represented six different genotypes and were accommodated in three different clades of the genus Ampelomyces. Morphology of the strains agreed with that of other Ampelomyces strains, but none of the examined characters were associated with any groups identified in the genetic analysis. Five powdery mildew species were inoculated with eight selected Ampelomyces strains to study their mycoparasitic activity. In the inoculation experiments, all Ampelomyces strains successfully infected all tested powdery mildew species, and showed no significant differences in their mycoparasitic activity as determined by the number of Ampelomyces pycnidia developed in powdery mildew colonies. The mycoparasitic interaction between the eight selected Ampelomyces strains and the tomato powdery mildew fungus (Pseudoidium neolycopersici strain KTP-03) was studied experimentally in the laboratory using digital microscopic technologies. It was documented that the spores of the mycoparasites germinated on tomato leaves and their hyphae penetrated the hyphae of Ps. neolycopersici. Ampelomyces hyphae continued their growth internally, which initiated the atrophy of the powdery mildew conidiophores 5 days post inoculation (dpi); caused atrophy 6 dpi; and complete collapse of the parasitized conidiphores 7 dpi. Ampelomyces strains produced new intracellular pycnidia in Ps. neolycopersici conidiophores ca. 8-10 dpi, when Ps. neolycopersici hyphae were successfully destroyed by the mycoparasitic strain. Mature pycnidia released spores ca. 10-14 dpi, which became the sources of subsequent infections of the intact powdery mildew hyphae. Mature pycnidia contained each ca. 200 to 1,500 spores depending on the mycohost species and Ampelomyces strain. This is the first detailed analysis of Ampelomyces strains isolated in Japan, and the first timing and quantification of mycoparasitism of Ps. neolycopersici on tomato by phylogenetically diverse Ampelomyces strains using digital microscopic technologies. The developed model system is useful for future biocontrol and ecological studies on Ampelomyces mycoparasites.

Understanding the mechanisms responsible for divergence and specialization of pathogens on different hosts is of fundamental importance, especially in the context of the emergence of new diseases via host shifts. Temporal isolation has been reported in a few plants and parasites, but is probably one of the least studied speciation processes. We studied whether temporal isolation could be responsible for the maintenance of genetic differentiation among sympatric populations of Ampelomyces, widespread intracellular mycoparasites of powdery mildew fungi, themselves plant pathogens. The timing of transmission of Ampelomyces depends on the life cycles of the powdery mildew species they parasitize. Internal transcribed spacer sequences and microsatellite markers showed that Ampelomyces populations found in apple powdery mildew (Podosphaera leucotricha) were genetically highly differentiated from other Ampelomyces populations sampled from several other powdery mildew species across Europe, infecting plant hosts other than apple. While P.leucotricha starts its life cycle early in spring, and the main apple powdery mildew epidemics occur before summer, the fungal hosts of the other Ampelomyces cause epidemics mainly in summer and autumn. When two powdery mildew species were experimentally exposed to Ampelomyces strains naturally occurring in P.leucotricha in spring, and to strains naturally present in other mycohost species in autumn, cross-infections always occurred. Thus, the host-related genetic differentiation in Ampelomyces cannot be explained by narrow physiological specialization, because Ampelomyces were able to infect powdery mildew species they were unlikely to have encountered in nature, but instead appears to result from temporal isolation.