Pseudoidium Neolycopersici Research Articles

Occurrence and epidemiological consequences of Erysiphe neolycopersici on tomato plants in Mauritius

Leveillula taurica has until recently been believed to be the causal pathogen of powdery mildew of tomatoes in Mauritius. However, in the year 2022, another powdery mildew species was detected on tomatoes under culture in various tomato growing localities of the island. Based on morphological comparison and molecular analysis, its identity was confirmed as Erysiphe neolycopersici (anam. Pseudoidium neolycopersici). This species has a worldwide distribution causing severe epidemics mainly on greenhouse tomatoes. There is evidence of the rapid spread of this pathogen during 2022–2023 causing serious infection of all tomato cultivars in greenhouse culture in Mauritius. The origin of these infections and its epidemiological consequences are discussed.

ZED1-related kinase 13 is required for resistance against Pseudoidium neolycopersici in Arabidopsis accession Bla-6.

To explore specific components of resistance against the tomato-adapted powdery mildew pathogen Pseudoidium neolycopersici (On) in the model plant Arabidopsis, we performed a disease assay in 123 accessions. When testing the resistance in the F1 from crossings between resistant accessions with susceptible Col-0 or Sha, only the progeny of the cross between accession Bla-6 and Col-0 displayed a completely resistant phenotype. The resistance in Bla-6 is known to be specific for Pseudoidium neolycopersici. QTL analysis and fine-mapping through several rounds of recombinant screenings allowed us to locate a major resistance QTL in an interval on chromosome 1, containing two candidate genes and an intergenic insertion. Via CRISPR/Cas9 targeted mutagenesis, we could show that knocking out the ZED-1 RELATED KINASE 13 (ZRK13) gene compromised the On resistance in Bla-6. Several polymorphisms are observed in the ZRK13 allelic variant of Bla-6 when compared to the Col-0 protein.

Simultaneous Detection of Plant- and Fungus-Derived Genes Constitutively Expressed in Single Pseudoidium neolycopersici-Inoculated Type I Trichome Cells of Tomato Leaves via Multiplex RT-PCR and Nested PCR

Type I trichomes of tomato leaves (Solanum lycopersicum Mill. cv. Moneymaker), as outgrowths of the plant epidermis, are suitable for monitoring infection processes of powdery mildew species using a high-fidelity digital microscope (DM) without fungal staining. On the trichomes, tomato powdery mildew (Pseudoidium neolycopersici L. Kiss) isolate KTP-03 produced a maximum of four vigorously elongated hyphae per conidium, which stopped growth approximately 12 days after inoculation. Single trichome cells, invaded by fungal hyphae at various fungal infection stages during the 12-day period after the inoculation of single conidia, were cut at the bases and directly collected with small precision scissors (i.e., microscissors) held by the manipulator under a DM. Subsequently, suc-polymerase chain reaction (PCR) (reverse transcription (RT)-PCR followed by nested (N)-PCR) was conducted to explore gene expression in the infected trichome. We selected intron-containing genes from tomatoes and powdery mildew fungi for the detection of constitutive gene transcripts, namely plasma membrane H+-ATPase (LHA2) and β-tubulin 2 (TUB2) genes. In suc-PCR, a single band from spliced mRNAs of both LHA2 and TUB2 genes were detected, suggesting that both genes were successfully transcribed in single KTP-03-infected trichomes. With combined primers for both LHA2 and TUB2 (multiplex RT-PCR/N-PCR), two bands were detected through the amplification of intron-spliced mRNAs of both genes. Therefore, our single-trichome cell PCR amplification method is effective for detecting the expression patterns of genes from both tomato and powdery mildew fungus. Combinations of digital microscopy, microscissors, and multiplex RT-PCR/N-PCR amplification techniques will be useful for simultaneously analysing the molecular interactions between plants and powdery mildew fungi at the level of single tomato leaf trichome cells. Also, this employed technique will be of benefit in other plant species and crops, possessing leaf trichome cells, to elucidate the molecular interactions between plants and pathogens.

Variation in UV-Mediated Damage Recovery Among Pseudoidium neolycopersici Isolates: Possible Mechanisms

The cryptochrome/photolyase family (CPF) consists of a diversified class of flavoproteins that are evolutionarily related. Although their domain architecture is highly conserved, they perform entirely different physiological functions. Previous studies have confirmed the presence of a functional photolyase in Pseudoidium neolycopersici, which repairs UV-C-induced DNA damage by using near UV-A/blue light as an energy source. Similar doses of UV-C treatment followed by incubation with dark or blue light was tested on conidia germination of five isolates of P. neolycopersici collected from different regions of Norway and the Netherlands. The results showed variations in the effect of UV on germination and germination recovery under blue light incubation after UV treatment. Evolutionary studies confirmed that P. neolycopersici photolyase is highly conserved among different isolates of P. neolycopersici and among different species. All CPF members have a core domain consisting of an identical cofactor, flavin adenine dinucleotide (FAD), and an additional photoantenna chromophore. An amino acid sequence analysis demonstrated that CPF members have highly conserved C terminals compared with their N terminals, because FAD binds in the C-terminal region. When compared with other CPF members, varying lengths of N and C terminals were noticed in P. neolycopersici photolyase and putative cryptochrome, respectively. Further research using comparative genomics targeting pyrimidine base composition, the role of regulatory elements, including promotor architecture, and the characterization of optical properties of native photolyase among isolates may help to explain the differences in the biological responses of conidia germination of P. neolycopersici treated with UV. [Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license .

Differential modulation of S-nitrosoglutathione reductase and reactive nitrogen species in wild and cultivated tomato genotypes during development and powdery mildew infection

Nitric oxide plays an important role in the pathogenesis of Pseudoidium neolycopersici, the causative agent of tomato powdery mildew. S-nitrosoglutathione reductase, the key enzyme of S-nitrosothiol homeostasis, was investigated during plant development and following infection in three genotypes of Solanum spp. differing in their resistance to P. neolycopersici. Levels and localization of reactive nitrogen species (RNS) including NO, S-nitrosoglutathione (GSNO) and peroxynitrite were studied together with protein nitration and the activity of nitrate reductase (NR). GSNOR expression profiles and enzyme activities were modulated during plant development and important differences among Solanum spp. genotypes were observed, accompanied by modulation of NO, GSNO, peroxynitrite and nitrated proteins levels. GSNOR was down-regulated in infected plants, with exception of resistant S. habrochaites early after inoculation. Modulations of GSNOR activities in response to pathogen infection were found also on the systemic level in leaves above and below the inoculation site. Infection strongly increased NR activity and gene expression in resistant S. habrochaites in contrast to susceptible S. lycopersicum. Obtained data confirm the key role of GSNOR and modulations of RNS during plant development under normal conditions and point to their involvement in molecular mechanisms of tomato responses to biotrophic pathogens on local and systemic levels.

Functional Characterization of Pseudoidium neolycopersici Photolyase Reveals Mechanisms Behind the Efficacy of Nighttime UV on Powdery Mildew Suppression.

Powdery mildews can be controlled by brief exposure to ultraviolet (UV) radiation with devastating effect on their developmental stages including conidia germination. The treatment effect can be impaired by subsequent exposure to UV-A/blue light. UV-A/blue light-activated photolyase may be responsible for this and therefore we tested the function of three cryptochrome/photolyase family (CPF)-like genes (OINE01015670_T110144, OINE01000912_T103440, and OINE01005061_T102555) identified in the obligate biotrophic fungus Pseudoidium neolycopersici, the cause of tomato powdery mildew. A photolyase-deficient mutant of Escherichia coli transformed with coding sequence of OINE01000912_T103440 and exposed to brief (UV)-C treatment (peak emission at 254 nm) showed photoreactivation and cell survival when exposed to subsequent blue light, indicating complementation of photolyase activity. In contrast, the same photolyase-deficient E. coli transformed with the coding sequences of other two CPF-like genes did not survive this treatment, even though their expression were confirmed at protein level. This confirmed that OINE01000912_T103440 is a gene encoding photolyase, here named PnPHR1, with functionality similar to the native photolyase in E. coli, and classified as a class I cyclobutane pyrimidine dimer (CPD) photolyase. Modeling of the 634-amino acid sequence of PnPHR1 suggested that it is capable of binding flavin adenine dinucleotide (FAD) and methenyltetrahydrofolate (MTHF). However, spectroscopic data of the protein produced in an E. coli expression system could only reveal the presence of a reduced form of FAD, i.e., FADH– as an intrinsic chromophore. Within the tested wavelength range of 365–525 nm, the survival of photolyase-deficient mutant E. coli transformed with PnPHR1 showed a broad action spectrum from 365 to 454 nm. This was very similar to the previously characterized action spectrum for survival of P. neolycopersici conidia that had been treated with UV-C. Quantitative RT-PCR revealed that the expression of PnPHR1 in P. neolycopersici conidia was induced by UV-C, and peak expression occurred 4 h after brief UV-C treatment. The expression of PnPHR1 was repressed when incubated in red light after the UV-C treatment, but not when incubated in UV-A/blue light. The results may explain why the disease-reducing effect of short wavelength UV is impaired by exposure to UV-A and blue light.

Correlation between total hypha length and haustoria number of Pseudoidium neolycopersici in type I trichome cells of tomato leaves

Powdery mildew haustoria are easier to be observed by light microscopy in trichome cells compared to epidermal cells of infected leaves. The objective of this study was to explore the relationship between the hyphal length and the number of haustoria in type I trichome cells of tomato (Solanum lycopersicum Mill.) leaves. The trichome cells of tomato cv. Moneymaker were inoculated with conidia of tomato powdery mildew (Pseudoidium neolycopersici L. Kiss), isolate KTP-04. On these cells, the P. neolycopersici isolate produced a maximum of four vigorously elongated hyphae per conidium. At 12 days after inoculation, KTP-04 formed two to five haustoria per conidium. Field-emission scanning electron microscopy showed that the haustorium consisted of a haustorial body and several lobes embedded in an extrahaustorial matrix. The number of haustoria per hypha and hyphal length on trichomes were positively correlated. Also, the tips of one to four hypha per conidium (excluding germ tubes and primary appressoria) were injured using a minute glass needle installed on micromanipulator under a high-fidelity digital microscope, and their total hyphal lengths were compared. Wounded hyphae possessed the same number of haustoria in trichome cells as non-wounded hyphae, and total hyphal lengths were similar between treatment groups. In this study, a new model was developed to study the infection mechanisms of powdery mildews that will be useful in future gene expression studies.

First Report of Powdery Mildew Caused by an Erysiphe sp. on Aristolochia debilis in China

Aristolochia debilis is a perennial twining ornamental plant in China that is used as a food source for butterflies and occasionally sold as an ornamental plant. A. debilis was originally used in traditional Chinese medicine; however, a chemical produced by the plant, aristolochic acid, is a potent human carcinogen, and consequently treatment with A. debilis is no longer recommended (Chen et al. 2012). In September and October 2019, typical symptoms of powdery mildew were found on leaves of A. debilis plants at Henan Normal University (35.18°N; 113.52°E), Henan, China. White colonies were observed on adaxial and abaxial surfaces of leaves. Infected leaves were collected and then monitored to determine the morphological characteristics of the pathogen. Foot cells (n = 25) were 35 to 70 μm long, and conidiophores (n = 50) were 70 to 150 × 6 to 10 μm (length × width) with three to four cells. Singly formed conidia were oblong-elliptical and were 27 to 49 × 10 to 16 μm (average [n = 50], 33 × 13 μm) with a length/width ratio of 2.5. No chasmothecia were observed. These morphological features indicate that this fungus belongs to Pseudoidium (asexual morph of the genus Erysiphe) (Braun and Cook 2012). Genomic DNA was extracted from hyphae attached to leaves using the method described previously (Zhu et al. 2019). The rDNA internal transcribed spacer (ITS) regions were amplified by polymerase chain reaction (PCR) utilizing fungi universal primer pairs ITS1/ITS4 and PMITS1/PMITS2 (Cunnington et al. 2003; White et al. 1990). PCR products were sequenced by Invitrogen (Shanghai, China), and the resulting sequences of 641 bp from ITS1/ITS4 and 774 bp from PMITS1/PMITS2 were deposited in GenBank (accession nos. MN700999 and MN701000, respectively). BLASTn analysis revealed that MN700999 was 99% identical to Erysiphe takamatsui (MK478870, 639 of 641 nucleotides), E. macleayae (JQ681217, 638 of 641 nucleotides), E. sedi (MN147817, 637 of 640 nucleotides), and E. circaeae (KY660882, 633 of 637 nucleotides) and that MN701000 was 99% identical to E. catalpa (KY653194, 676 of 679 nucleotides), Pseudoidium neolycopersici (AB032484, 672 of 676 nucleotides), and P. hortensiae (MG654731, 744 of 748 nucleotides). These results suggested that this fungus belongs to the E. aquilegiae clade (Takamatsu et al. 2015). Therefore, the causal organism of the powdery mildew was confirmed by morphological characteristics and ITS sequence data as Erysiphe sp. Pathogenicity was tested by gently pressing adaxial surfaces of infested leaves onto young leaves of three healthy A. debilis plants. Three noninoculated plants were used as a control. Inoculated and control plants were separated and maintained in growth chambers (light/dark, 16 h/8 h; temperature, 18°C). White powdery mildew colonies appeared 9 to 12 days after inoculation, whereas control plants remained asymptomatic. The fungus on the inoculated leaves was morphologically identical to that observed on the original diseased leaves. Erysiphe sp. has been reported from a number of hosts in China (Zheng and Chen 1981; Zhou et al. 2015). In Australia and Korea, Erysiphe sp. was reported on Catharanthus roseus, the first reported host outside the Ranunculaceae (Cho et al. 2017; Liberato and Cunnington 2006). To our best knowledge, there has been, so far, no report of Erysiphe sp. infecting A. debilis, and this is the first report of powdery mildew on A. debilis in China, as well as worldwide. Because this powdery mildew could affect the ornamental value of A. debilis plants and reduce the populations of a critical food source for butterflies, control management and strategies should be developed and followed.

First Report of Powdery Mildew of Excoecaria cochinchinensis Caused by Pseudoidium neolycopersici in China

Excoecaria cochinchinensis Lour., a small evergreen shrub in the family Euphorbiaceae, is usually grown as a native ornamental plant in South China. In April 2019, severe powdery mildew infection was observed on E. cochinchinensis in a garden (23.9°N, 113.23°E) in Guangzhou, China. Nearly 60% of the Excoecaria plants were infected, and the foliage of infected plants turned yellow and necrotic, followed by rapid defoliation. Voucher specimens were deposited in the Herbarium of Mycology of Guangdong Institute of Traditional Chinese Medicine under accession number HMGDTCM18. White to gray rounded irregular patches, consisting of abundant mycelia and conidia of the fungus, were found on both leaf surfaces and green stems. Hyphae were septate, branched, and with moderately lobed appressoria. Conidia were solitary, borne on top of conidiophores, and were ellipsoidal, hyaline, 38 to 48 × 13 to 17 μm (mean 42 ± 3 × 15 ± 1 μm, n = 50), containing no distinct fibrosin bodies. Conidiophores were erect, unbranched, consisted of three cells, and 84 to 117 μm long (mean 106 ± 7 μm, n = 50). Chasmothecia were not observed on sample plants. The pathogenicity of this fungus was confirmed by gently pressing infected leaves onto healthy leaves and stems of three healthy seedlings in separate pots, and two noninoculated seedlings were used as a control. After maintaining at 25°C in a greenhouse for 10 days, similar disease symptoms were observed in the inoculated plants, whereas no symptoms developed on control plants. Molecular identification was conducted by extracting genomic DNA from conidia washed from infected leaves and using a Fungal DNA Kit (Omega Bio-tek, Guangzhou, China). The rDNA internal transcribed spacer region was amplified using primers ITS1/ITS4 (Glass and Donaldson 1995). The obtained sequence (GenBank accession no. MK811439) analysis by BLASTN search showed>99% similarity with Pseudoidium neolycopersici (GenBank accession nos. LC009912.1 and AB032484.1). On the basis of the morphological and molecular analyses, the fungus was identified as P. neolycopersici (Kiss et al. 2001). This pathogen has only been reported on Codiaeum variegatum, a euphorbiaceous host (Liu et al. 2015). To our knowledge, this is the first report of this fungus infecting Excoecaria. In addition, P. neolycopersici has been reported on tomato, papaya, and Physalis pubescens in China. Therefore, powdery mildew infection on E. cochinchinensis not only poses a potential threat to health and beauty of this ornamental plant but may also serve as a reservoir of the pathogen that could allow spread to other economically important hosts.

First Report of Powdery Mildew Caused by Pseudoidium neolycopersici on Moringa oleifera in China

First Report of Powdery Mildew Caused by <i>Pseudoidium neolycopersici</i> on <i>Moringa oleifera</i> in China

Occurrence of Powdery Mildew Caused by Pseudoidium neolycopersici on Papaya (Carica papaya) in China

Papaya (Carica papaya L.) is an economically important fruit tree in tropical and subtropical countries. In late March 2017, symptoms of powdery mildew were observed on 80% of papaya plants that were grown at the campus of Fujian Agriculture and Forestry University, Fuzhou, China. The initial symptoms of powdery mildew on papaya appeared as circular to irregular white colonies on both sides of leaves of papaya. As the disease progressed, the leaves became covered with white mass of fungal colonies; eventually leaves showed chlorosis and necrosis followed by desiccation and rapid senescence. Fungal mycelium was amphigenous, white, and effuse. Hyphae were hyaline, septate, branched, and 5 to 8 μm wide with lobed appressoria. Conidiophores were approximately 90 to 150 × 8 to 10 μm with cylindrical foot cells of 30 to 60 × 8 to 10 μm followed by one to three short cells of 14 to 25 μm. Conidia were hyaline, ellipsoidal-ovoid, or doliform, 30 to 60 × 14 to 20 μm in size, and were produced singly on the tip of conidiophores. Germ tubes were terminal or subterminal, with lobed ends. The sexual morph was not found. Based on these characteristics, the pathogen was identified as a Pseudoidium sp. (Kiss et al. 2001). A voucher specimen (HMAS 247192) was deposited at the Institute of Microbiology, Chinese Academy of Sciences Herbarium Mycologium, Beijing, China. DNA was extracted and the nrDNA internal transcribed spacer (ITS) region was sequenced. The ITS sequence (deposited in GenBank as KY887788) was identical to that of Pseudoidium neolycopersici isolates AB032484, KY776199, and JQ972700. Pathogenicity was confirmed through gently pressing infected leaves onto the leaves of three asymptomatic papaya plants (approximately 90 cm high). The inoculated papaya plants were covered with plastic bags placed outdoors for 7 days at 25°C under natural daylight conditions in April. Typical symptoms of powdery mildew were observed after 7 days, whereas noninoculated plants remained asymptomatic. To confirm pathogen identity, an eggplant cultivar (unknown) and two tomato cultivars (Zaofen and Fenguo) were also cross inoculated (five plants of each cultivar) with inoculums from infected papaya plant. Only tomato cultivars showed powdery mildew symptoms. The ITS sequence of sample MG846017 from the tomato host plant was identical to that of the Pseudoidium sp. on papaya plants (KY887788). The inoculated eggplant cultivar remained asymptomatic. To our knowledge, this is the first report of papaya powdery mildew caused by P. neolycopersici in mainland China. The only report of this disease caused by P. neolycopersici along with Erysiphe diffusa and Podosphaera xanthii was in Taiwan (Tsay et al. 2011). Other powdery mildew species have been reported on papaya in many tropical and subtropical areas (Liberato et al. 2004). Papaya is a popular and widely grown crop in China, and the presence of a new pathogen could have an impact on its production. P. neolycopersici has been reported on tomato in China (Li et al. 2008). Powdery mildews with similar ITS sequences have recently been found on other plant species as well (Liu et al. 2015; Southwell et al. 2018).

Effect of heat-shock pre-treatment on tomato plants infected by powdery mildew fungus

The effect of plant heat-shock (HS) pre-treatment (40.5°C, 2 h) on Pseudoidium neolycopersici development in the susceptible and moderately resistant Solanum spp. genotypes was studied together with biochemical responses (endogenous concentrations of salicylic (SA), jasmonic (JA), abscisic acid (ABA), and peroxidase (POX) activity). In HS pre-treated S. lycopersicum, an acceleration of pathogen, chlorosis and necrosis development, strong SA, JA accumulation, and increased POX activity were detected. In S. chmielewskii, HS pre-treatment caused a slight suppression of pathogen development, increase in JA, ABA concentrations, and POX activity. HS accelerated and strengthened the development of symptoms and biochemical responses to the infection in the susceptible genotype in contrast to moderately resistant genotype with a robust defence response to an infection per se.

The elicitin β-cryptogein’s activity in tomato is mediated by jasmonic acid and ethylene signalling pathways independently of elicitin–sterol interactions

The level of resistance induced in different tomato genotypes after β-CRY treatment correlated with the upregulation of defence genes, but not sterol binding and involved ethylene and jasmonic acid signalling. Elicitins, a family of small proteins secreted by Phytophthora and Pythium spp., are the most well-known microbe-associated molecular patterns of oomycetes, a lineage of fungus-like organisms that include many economically significant crop pathogens. The responses of tomato plants to elicitin INF1 produced by Phytophthora infestans have been studied extensively. Here, we present studies on the responses of three tomato genotypes to β-cryptogein (β-CRY), a potent elicitin secreted by Phytophthora cryptogea that induces hypersensitive response (HR) cell death in tobacco plants and confers greater resistance to oomycete infection than acidic elicitins like INF1. We also studied β-CRY mutants impaired in sterol binding (Val84Phe) and interaction with the binding site on tobacco plasma membrane (Leu41Phe), because sterol binding was suggested to be important in INF1-induced resistance. Treatment with β-CRY or the Val84Phe mutant induced resistance to powdery mildew caused by the pathogen Pseudoidium neolycopersici, but not the HR cell death observed in tobacco and potato plants. The level of resistance induced in different tomato genotypes correlated with the upregulation of defence genes including defensins, β-1,3-glucanases, heveins, chitinases, osmotins, and PR1 proteins. Treatment with the Leu41Phe mutant did not induce this upregulation, suggesting similar elicitin recognition in tomato and tobacco. However, here β-CRY activated ethylene and jasmonic acid signalling, but not salicylic acid signalling, demonstrating that elicitins activate different downstream signalling processes in different plant species. This could potentially be exploited to enhance the resistance of Phytophthora-susceptible crops.

Involvement of S-nitrosothiols modulation by S-nitrosoglutathione reductase in defence responses of lettuce and wild Lactuca spp. to biotrophic mildews.

Resistant Lactuca spp. genotypes can efficiently modulate levels of S-nitrosothiols as reactive nitrogen species derived from nitric oxide in their defence mechanism against invading biotrophic pathogens including lettuce downy mildew. S-Nitrosylation belongs to principal signalling pathways of nitric oxide in plant development and stress responses. Protein S-nitrosylation is regulated by S-nitrosoglutathione reductase (GSNOR) as a key catabolic enzyme of S-nitrosoglutathione (GSNO), the major intracellular S-nitrosothiol. GSNOR expression, level and activity were studied in leaves of selected genotypes of lettuce (Lactuca sativa) and wild Lactuca spp. during interactions with biotrophic mildews, Bremia lactucae (lettuce downy mildew), Golovinomyces cichoracearum (lettuce powdery mildew) and non-pathogen Pseudoidium neolycopersici (tomato powdery mildew) during 168h post inoculation (hpi). GSNOR expression was increased in all genotypes both in the early phase at 6hpi and later phase at 72hpi, with a high increase observed in L. sativa UCDM2 responses to all three pathogens. GSNOR protein also showed two-phase increase, with highest changes in L. virosa-B. lactucae and L. sativa cv. UCDM2-G. cichoracearum pathosystems, whereas P. neolycopersici induced GSNOR protein at 72hpi in all genotypes. Similarly, a general pattern of modulated GSNOR activities in response to biotrophic mildews involves a two-phase increase at 6 and 72hpi. Lettuce downy mildew infection caused GSNOR activity slightly increased only in resistant L. saligna and L. virosa genotypes; however, all genotypes showed increased GSNOR activity both at 6 and 72hpi by lettuce powdery mildew. We observed GSNOR-mediated decrease of S-nitrosothiols as a general feature of Lactuca spp. response to mildew infection, which was also confirmed by immunohistochemical detection of GSNOR and GSNO in infected plant tissues. Our results demonstrate that GSNOR is differentially modulated in interactions of susceptible and resistant Lactuca spp. genotypes with fungal mildews and uncover the role of S-nitrosylation in molecular mechanisms of plant responses to biotrophic pathogens.

Trichomes: interaction sites of tomato leaves with biotrophic powdery mildew pathogens

The present study aimed to explore the possibility of using the type I trichomes of tomato (Solanum lycopersicum) to monitor the infection processes of powdery mildews by microscopy. Individual trichome cells of two tomato genotypes were inoculated with pathogenic and non-pathogenic powdery mildew species, Pseudoidium neolycopersici, Erysiphe trifoliorum and Podosphaera xanthii. On the trichome cells of the tomato cultivar Moneymaker, hyphae of the pathogenic Pseudoidium neolycopersici (isolates KTP-03 and KTP-04) grew vigorously; whereas hyphal growth of the non-pathogenic Erysiphe trifoliorum and Podosphaera xanthii ceased after appressorium formation, which was associated with papilla formation and hypersensitive cell death, respectively. Similar infection processes of the tested powdery mildews were seen in Moneymaker epidermal cells. Therefore, tomato trichomes are suitable for analysing, at individual cell level, the infection processes of different pathotypes of powdery mildews and for observing the cytological responses of plants by non-pathogenic powdery mildews. On the other hand, it was observed that both isolates KTP-03 and KTP-04 failed to produce conidiophores on the hyphae elongating on Moneymaker trichomes. Similarly, no conidiophores were produced on the hyphae elongating on trichomes of Solanum peruvianum LA2172, which is resistant to KTP-03 and susceptible to KTP-04. Interestingly, delayed cell death occurred in LA2172 epidermal cells, which were attacked by KTP-03 hyphae elongating from trichomes and conidiophores were formed on new hyphae growing from the leaf epidermal cells. Thus, leaf trichomes and epidermal cells of the wild tomato species LA2172 reacted differently to the avirulent isolate KTP-03.

Review of tomato powdery mildew – a challenging problem for researchers, breeders and growers

Tomato powdery mildew (Pseudoidium neolycopersici (syn. Oidium neolycopersici)) is one of the most devastating diseases of cultivated tomatoes worldwide. Although the first epidemics were recorded nearly 30 years ago, many aspects of the host-pathogen interaction are still not well understood. Detailed morphological and molecular studies of the anamorphs confirmed that P. neolycopersici is phylogenetically close to Erysiphe aquilegiae var. ranunculi. Host range is rather broad; apart from the family Solanaceae, hosts were found in very distant plant families. Screening of wild Solanum sect. Lycopersicon spp. (previous Lycopersicon spp.) germplasm revealed valuable sources of resistance (Solanum habrochaites, Solanum pennellii, Solanum cheesmaniae, Solanum chilense, Solanum peruvianum). The main resistance mechanism was found to be a hypersensitive response (HR), in some cases followed by limited development of the pathogen. However, there is a broad variation in resistance response at the histological and cytological levels. Biochemical studies focusing on production of reactive oxygen species (ROS), reactive nitrogen species (RNS) and corresponding enzymes during infection of P. neolycopersici showed that all these play a crucial role in activation of defense responses in genotypes of wild Solanum sect. Lycopersicon.

First Report of Powdery Mildew (Pseudoidium neolycopersici) on Croton (Codiaeum variegatum var. pictum) in China.

Croton (Codiaeum variegatum (Linn.) var. pictum (Lodd.)) is an ornamental plant commonly grown in southern China. In March 2014, severe powdery mildew infections were observed on crotons in gardens of Hainan University (20.1°N and 110.3°E), Haikou, Hainan province. Disease incidence was estimated in a random batch of 100 plants in three replicates, with the average value approaching 80%. Symptoms first appeared as white circular patches on the adaxial surface and expanded to the abaxial surface, petioles, and stems. The top leaves were the most affected. Upper surfaces of the infected leaves were covered by white, dense mycelia. As the disease progressed, infected leaves turned purple on the lower surfaces and curly before becoming necrotic and abscising from the plant. Powdery mildew was more severe in shaded environments, especially during rainy or foggy weather in early spring. Two hundred conidiophores and conidia were observed microscopically. The conidiophores were straight or occasionally flexuous, 62.3 to 127.6 × 6.2 to 10.2 μm, consisting of two to three straight cells. Conidia were born in solitary on the top of conidiophores. Conidia were hyaline, ellipsoidal, 26.4 to 42.2 × 11.7 to 23.4 μm (average 32.5 × 16.5 μm), contained no distinct fibrosin bodies, and produced a subterminal germ tube. The wrinkling pattern of the outer walls of older conidia was angular or reticulated. Appressoria were single and multilobed. Cleistothecia were not observed. Based on morphological characteristics, the fungus was identified as Oidium neolycopersici (2), which was recently renamed Pseudoidium neolycopersici (L. Kiss) (3). The identity was confirmed by sequence analysis. Genomic DNA was extracted from the foliar powdery mildew colonies using Chelex-100 (Bio-Rad, Shanghai, China). The rDNA internal transcribed spacer (ITS) region was amplified with primers ITS1 and ITS4 (5). The ITS sequence of the representative isolates C01 (GenBank Accession No. KJ890378.1) and four other powdery mildew samples collected from crotons in Hainan University was 100% identical to that of P. neolycopersici isolates from tomato plants such as JQ972700 and AB163927. Inoculations were made by gently pressing diseased leaves onto leaves of five healthy plants of croton and tomato ('Money maker'). Five non-inoculated croton and tomato plants served as controls. Inoculated and non-inoculated plants were maintained in an incubator at 25°C with a 12-h photoperiod. After eight days, typical powdery mildew symptoms developed on 93% of the inoculated plants, while no symptom developed on the non-inoculated plants. The pathogenicity tests were repeated three times. The same fungus was always re-isolated from the diseased tissue according to Koch's postulates. The pathogenicity tests further confirmed that the pathogen from crotons is P. neolycopersici (Basionym. Oidium neolycopersici (KJ890378.1)), which is commonly known as the tomato powdery mildew. P. neolycopersici is also a pathogen of Normania triphylla (1) and papaya (4). To our knowledge, this is the first report of P. neolycopersici infecting croton. The avenue of this pathogen entering gardens of Hainan University remains unknown. The gardens are located far away from tomato farms. Also no symptom of powdery mildew on croton was observed during surveys in other locations in Haikou. The origin of the pathogen warrants additional research.

Erysiphe takamatsui, a powdery mildew of lotus: Rediscovery of teleomorph after 40 years, morphology and phylogeny

The teleomorph of Erysiphe takamatsui, a powdery mildew of lotus, was found in 2012 at a lotus pond of Niigata-shi, Japan, where this species was first found in 1974. This is the second record of teleomorph of E. takamatsui. Morphological and molecular analyses revealed that the asexual stages of the lotus powdery mildews found in Aichi, Osaka and Tokyo represent the anamorph of E. takamatsui. The nucleotide sequences of the internal transcribed spacer (ITS) and 28S rRNA gene were identical to those of E. aquilegiae, E. catalpae, and E. macleayae. These species formed a homogeneous clade together with E. sedi, Pseudoidium neolycopersici, and Pseudoidium spp. occurring on a wide range of plant families with only five base substitutions of ITS sequences in maximum. These results suggest that E. takamatsui is a fungus that appeared as a result of recent host expansion.