ABSTRACT In 2025 a survey was conducted to determine the distribution and impact of Calonectria fungi associated with leaf spot, stem canker and lower crown defoliation of containerised Nandina domestica nursery stock in Virginia, United States. In total, 47 symptomatic plants representing seven cultivars were sampled from 19 nurseries in 16 counties, resulting in 27 Calonectria isolates. Three of these isolates were identified as Calonectria kyotensis and they were recovered from the roots of three nandina plants. The remaining 24 isolates belonged to a previously unknown species in the C. kyotensis species complex. This novel species resides on a distinct branch in an eight‐locus phylogeny. It also produces 3‐septate macroconidia, a unique morphological feature distinguishing it from other species in the species complex. This new species is hereby designated as Calonectria nandinae sp. nov. This new species was isolated mostly from diseased leaves and stems, and twice from roots. To confirm its pathogenicity, one foliar (spray) and two root (dip and drench) inoculation assays were performed on N. domestica ‘Gulf Stream’ and ‘Lemon Lime’ sapling plants. C. nandinae was re‐isolated at high rates (mean 96%) from stem cankers and leaf spots of plants in the foliar inoculation assay, demonstrating C. nandinae as the causal agent of foliage blight on N. domestica . Its significance as a root pathogen is unclear due to less defined root rot symptoms, low re‐isolation rate (mean 3%) and ambiguous root‐weight‐loss results between the two root inoculation assays.
ABSTRACT Temperature is a key environmental factor influencing the development and severity of plant diseases. In the rice blast pathogen Pyricularia oryzae , successful infection depends on appressorium formation, a process that is strongly influenced by temperature. In this study, we investigated strain‐specific responses to elevated temperature through phenotypic characterization, pathogenicity assays and transcriptomic analysis. Appressorium formation was strongly temperature‐dependent, with optimal development at 25°C and near‐complete inhibition at 35°C. At 30°C, strains showed contrasting responses, allowing classification into thermotolerant and thermosensitive groups based on their capacity to maintain appressorium formation. Thermotolerant strains retained pathogenicity at elevated temperature, whereas thermosensitive strains exhibited variable effects on pathogenicity. Time‐course analyses further demonstrated that appressorium development in one thermosensitive strain was delayed rather than permanently inhibited under elevated temperature. Transcriptomic analysis revealed that thermotolerant strains exhibited relatively limited changes in gene expression, while thermosensitive strains showed extensive transcriptional responses, particularly in metabolic and transport‐related pathways. Chemical complementation experiments further indicated that thermosensitive strains differed in their responses to surface‐associated cues under heat stress, which may contribute to reduced appressorium formation. These findings indicate that thermotolerance in P. oryzae is associated with the maintenance of infection‐related development under elevated temperature and highlight the potential impact of climate warming on the behaviour of rice blast populations.
ABSTRACT Downy mildew (DM) caused by the oomycete Hyaloperonospora diplotaxidis (= Peronospora diplotaxidis ) is a devastating foliar disease that affects wild rocket ( Diplotaxis tenuifolia ) worldwide. Understanding the diversity of pathogen populations and the existence of resistance host‐specific responses is crucial, enabling the selection of well‐adapted cultivars. This study aims to clarify the taxonomy, host range and genetic relationships of different H. diplotaxidis isolates, reinforcing its status as a distinct species from other Hyaloperonospora pathogens of the Brassicaceae family. The isolates obtained in three plant species ( D. tenuifolia , Raphanus sativus and Brassica oleracea ) and distinct locations were evaluated for virulence on 17 Brassicaceae genotypes at the seedling stage. Hyaloperonospora sp. isolates were separated into three groups according to the species of origin. H. diplotaxidis isolates were pathogenic only in the species of origin D. tenuifolia , and the same behaviour was observed in H. brassicae isolates collected in B. oleracea . In relation to H. brassicae isolates collected in R. sativus , some sporulation was observed in a broccoli ( B. oleracea var. italica ) accession. With the set of host genotypes evaluated, no differential responses were detected among H. diplotaxidis isolates. Four wild rocket genotypes were identified as potential sources of resistance. Molecular identification using ITS and cox2 barcoding regions confirmed the separation of Hyaloperonospora isolates into three distinct groups identified in pathogenicity trials. These findings contributed to the understanding of host specificity and genetic structure within Hyaloperonospora sp., supporting future breeding in wild rocket.
ABSTRACT Fusarium wilt, caused by Fusarium oxysporum f. sp. cubense (Foc) Tropical Race 4 (TR4), represents a significant threat to global banana production. This study investigates the genetic diversity and phylogenetic relationships of Foc TR4 populations in South America, crucial for developing effective region‐specific containment and management strategies. We report a genomic analysis that combines sequencing of new Venezuelan isolates with previously sequenced Colombian and Peruvian isolates. A high‐resolution genome‐wide SNP‐based phylogenetic analysis was conducted using 32 Foc genomes. The results show that the Venezuelan isolate VNZ_F3 and Colombian isolates were grouped in the same monophyletic clade with 100% bootstrap support, suggesting they have a shared introduction source or a direct transmission event. This study also confirms at least two distinct incursions of Foc TR4 into South America, with Peruvian isolates forming a separate, genetically distinct clade. Interestingly, Venezuelan isolates VNZ_F29 and VNZ_F32 showed a separated lineage indicating rapid evolutionary divergence within Venezuela. Comparative genomic analysis, visualized in a Circos plot, revealed changes consistent with the SNP‐based phylogeny. Gapped regions, representing missing DNA sequences, were observed, supporting a closer genetic relationship between Colombian and most Venezuelan isolates. This study highlights the usefulness of genomic surveillance to identify genetic changes as Foc TR4 evolves and spreads, helping in the development of targeted quarantine measures.
ABSTRACT Xanthomonas campestris , the causative agent of black rot and leaf spot disease in Brassicaceae , is a bacterial pathogen of international importance. It is seed‐borne and can cause a disease outbreak at low infection percentages of seed batches. Effective measures for controlling this pathogen rely mainly on early detection. In this study, we developed a loop‐mediated isothermal amplification (LAMP) identification assay for Xanthomonas campestris pv. campestris and pv. raphani in plant leaf material. We show that the assay is highly specific for these two pathovars and can detect approximately 10 4− 10 5 cells/mL in Brassica plant material extract. In addition, an amplification control for the plant cytochrome c oxidase gene was added in a multiplex assay, effectively preventing false negative results. Finally, the multiplex assay was evaluated at a seed‐producing company using freeze‐dried primers, enzyme mixes and a portable detection device. In all cases, the pathogen could be identified in symptomatic plant material. Thus, the developed assay was confirmed as a quick and on‐site identification tool for X. campestris for use in seed production systems.
ABSTRACT Although standard area diagrams (SADs) are widely used to reduce subjectivity in visual disease assessment, their design has often prioritised accuracy metrics. Here we developed a two‐colour, 10‐diagram SAD to aid severity assessment of coffee leaf rust caused by Hemileia vastatrix , and evaluated its performance in terms of accuracy and implications for assessment time and field sampling efficiency. Seventeen raters provided visual estimates that were either unaided, aided by an existing true‐colour 21‐diagram SAD and aided by the new SAD. Estimates using the new SAD achieved the numerically highest mean accuracy (Lin's CCC > 0.93) and bias correction coefficient (Cb = 0.97). CP (0.70) was higher than for unaided assessments (CP = 0.29) and the existing SAD (CP = 0.65). Inter‐rater reliability was consistently high across methods (> 0.90). Field evaluations confirmed that both SADs were similarly effective (Lin's CCC > 0.86). The minimum number of plants required to achieve a 95% confidence interval half‐width of ±2 percentage points increased with mean field severity, and depended on sampling design and SAD method, ranging from 3 to 8 plants in the low disease‐severity field to > 40 plants under the least intensive design in the high disease‐severity field. Predicted total field scoring time ranged from < 10 to 50 min across scenarios and the new SAD reduced assessment time by approximately 11%–13% while maintaining the same precision target. Overall, a simplified SAD can preserve statistical performance while improving operational efficiency, enabling the same level of precision with reduced in‐field disease scoring effort.
ABSTRACT Huanglongbing (HLB), a disease of citrus associated with several species of ‘ Candidatus Liberibacter’, remains the most severe biotic threat to global citrus production. Since the emergence of the disease in the Americas in the early 2000s, particularly in Brazil and the United States, research investment and coordinated programmes involving research, technology development and extension have intensified dramatically. Hundreds of millions of dollars have been allocated to HLB‐related research and management efforts, yet no effective, sustainable and scalable solution has emerged. This letter argues that a substantial part of this failure is not due to insufficient resources, but rather to a persistent conceptual error in research prioritisation. HLB has been predominantly approached as a classical plant–pathogen interaction, with strong emphasis on bacterium‐centred strategies and long‐term genetic improvement. However, HLB is fundamentally a vector‐driven pathosystem, sustained by the interaction among ‘ Candidatus Liberibacter’ spp., the psyllid vectors Diaphorina citri and Trioza erytreae , and the citrus hosts. I critically examine the current state of knowledge of these interactions and highlight the major experimental and conceptual limitations that have hindered progress. Furthermore, I discuss why reliance on conventional breeding is unlikely to deliver timely solutions. Finally, I propose a strategic reorientation of research investment toward transmission‐focused approaches that target vector competence. By prioritising interventions capable of disrupting acquisition and transmission rather than attempting to cure infected trees, future research efforts may achieve meaningful epidemiological impact.
ABSTRACT The potyvirus pepper yellow mosaic virus (PepYMV), transmitted by aphids in a non‐persistent manner, causes pepper yellow mosaic disease in sweet pepper ( Capsicum annuum ). The disease is largely avoided in pepper crops by planting hybrids carrying the Pvr4 gene. However, a resistance‐breaking isolate, pathotype 2 1 .2 3 , was recently detected in the Sumaré region, São Paulo, Brazil, affecting resistant hybrids (Dahra R and Heloísa). Accordingly, the aim of this study was to quantify the damage caused by the PepYMV pathotype on plant growth and yield, using both field and greenhouse experiments. In the field, plants were naturally infected at three developmental stages, whereas greenhouse trials involved mechanical inoculation at 30 and 50 days after transplanting (DAT). Leaf gas exchange variables were also evaluated in greenhouse plants. Field results showed that early infection significantly reduced yield, with Dahra R and Heloísa plants showing 51.1% and 61.5% yield losses, respectively, when infected at an early stage (score 3) compared to plants with later infections (score 1). Greenhouse results confirmed these findings. In both field and greenhouse experiments, fruit quality was not significantly affected. Photosynthetic variables, including net photosynthesis, stomatal conductance and transpiration, were notably reduced in plants infected at 30 DAT, correlating with more severe disease symptoms. These results highlight the importance of monitoring sweet pepper fields for the emergence of new PepYMV pathotypes. Early detection is crucial for minimizing losses and preventing the establishment of resistance‐breaking pathotypes in production areas.
ABSTRACT Witches' broom disease, caused by the fungus Taphrina wiesneri , severely affects cherry trees ( Prunus spp.); however, the pathogen's micromorphology remains poorly understood. Here, we combined field observations and field‐emission scanning electron microscopy (FESEM) to characterise infected Yoshino cherry leaves and document the developmental stages of the naked asci of T. wiesneri . Diseased trees produced dense shoot clusters in place of normal blossoms, and infected leaves displayed distortion, browning, necrotic lesions and hypertrophy of spongy mesophyll cells. Asci were observed exclusively on the abaxial surface of diseased leaves, where FESEM revealed dense populations of naked asci and yeast cells, a continuous yeast layer up to ~25 μm thick, and markedly elongated trichomes. These observations identified four putative morphological stages of naked ascus development: asci beneath the cuticle, asci protruding through the cuticle, asci fully protruding beyond the host surface and asci with ruptured walls exposing internal ascospores. Mature naked asci released ascospores via terminal rupture, whereas ascospores displayed both polar and lateral budding. Despite extensive tissue collapse, guard cells remained structurally intact. Collectively, these observations provide the first detailed structural framework of T. wiesneri infection and reveal previously unrecognised features of naked ascus development and ascospore release.
ABSTRACT Resveratrol, a polyphenolic stilbene phytoalexin found in high concentrations in red grape skins ( Vitis vinifera ), was evaluated for antimicrobial efficacy against key plant‐pathogenic bacteria and fungi through both in vitro and in planta assays, while exploring its role in modulating plant basal immunity. In vitro growth kinetics demonstrated a clear dose‐dependent inhibition of four major bacterial pathogens: Pseudomonas syringae pv. tomato , Xanthomonas campestris pv. campestris , Acidovorax citrulli and Clavibacter michiganensis . Complete growth arrest for all tested species was achieved at a concentration of 100 μg/mL. Time‐killing assays revealed that resveratrol's activity is primarily bacteriostatic rather than bactericidal, an attribute that may limit the selective pressure for the development of pathogen resistance. Regarding antifungal activity, resveratrol significantly inhibited the mycelial growth of Fusarium oxysporum f. sp. radicis‐cucumerinum and Alternaria alternata . However, Botrytis cinerea exhibited notable tolerance, probably due to its specialized ability to enzymatically degrade or metabolize stilbene compounds. In planta experiments with Arabidopsis thaliana plants showed that foliar pretreatment with resveratrol significantly reduced the proliferation of P. syringae pv. tomato . Molecular analysis via reverse transcription‐quantitative PCR provided evidence of immune ‘priming’ as resveratrol induced the expression of pattern‐triggered immunity marker genes. This research highlights the dual functionality of resveratrol as both a direct antimicrobial agent and a potent inducer of plant defence responses. These findings position resveratrol as a viable, eco‐friendly tool for integrated disease management, although further studies on field stability and bioavailability are required for practical agricultural implementation.
Monitoring surveys, conducted in four Italian regions (Apulia, Marche, Sardinia and Veneto), revealed the widespread occurrence of young and mature fig trees (Ficus carica) showing sudden death, crown thinning, shoot blight, branch dieback, cankers and root rot symptoms. Given the widespread distribution and severity of these symptoms, a study was conducted to identify the main causal agents. To achieve this goal, 86 samples including fine roots with rhizosphere (11) and branches with bleeding and sunken cankers (75) were collected from 11 symptomatic fig trees. Isolations performed on universal and selective growth medium yielded colonies belonging to three families: Botryosphaeriaceae, Diaporthaceae (Ascomycetes) and Peronosporaceae (Oomycetes). Based on morphobiometric data and DNA nucleotide sequences, seven species, namely Botryosphaeria dothidea (14 isolates), Diaporthe cinerascens (10), Neofusicoccum australe (9), Neofusicoccum mediterraneaum (10), Neofusicoccum parvum (28), Phytophthora citricola (3) and Phytophthora plurivora (6) were identified. In addition, two isolates of a new putative Phytophthora species obtained from the rhizosphere including symptomatic fine roots of fig trees in Apulia region are described here as Phytophthora messapica sp. nov. For Phytophthora species, N. australe and N. mediterraneaum (reported here for the first time on fig trees worldwide), Koch's postulates were satisfied by inoculating 5-year-old fig trees under controlled conditions. Sixty days after inoculation, all inoculated plants showed the same symptoms as those observed in the field. Overall, the data obtained highlights the involvement of multiple Botryosphaeriaceae and Phytophthora species in the aetiology of the emerging diseases affecting fig trees in Italy.
Viruses in legume crops constitute one of the main biotic stresses affecting their production worldwide. Currently, more than 160 viruses are known to infect legumes, with most being transmitted by aphids. Moreover, these viruses often occur in mixed infections, the epidemiological significance of which is poorly understood. In this study, monitoring of legume crops in Poland was carried out in 2020, 2023 and 2024 growing seasons. Over 100 plants, with varying disease symptoms, were collected from various locations. From the collected plants, 12 pooled samples were selected for RNAseq, comprising 10 pea (Pisum sativum) samples, one faba bean (Vicia faba var. minor) sample and one lupin (Lupinus albus) sample. We detected the presence of numerous viruses in mixed infections in these Polish legumes. Moreover, in samples from P. sativum and V. faba var. minor collected in 2024, we detected the presence of pea necrotic yellow dwarf virus (PNYDV) and clover yellow vein virus (ClYVV), posing a threat to legume cultivation in Europe. The results of the phylogenetic analysis revealed that the two Polish isolates of PNYDV were diverse and grouped with the isolates from the Czech Republic, Austria and Denmark. Moreover, we also observed the widespread presence of pea enation mosaic virus 1 (PEMV1), pea enation mosaic virus 2 (PEMV2) and pea seed-borne mosaic virus (PSbMV) in the majority of tested samples. These results suggest that the spread of viruses, primarily transmitted by aphids, could have far-reaching consequences for food production and ecosystem functioning.
ABSTRACT Powdery scab of potatoes ( Solanum tuberosum ) is caused by the soil‐borne protist Spongospora subterranea f. sp. subterranea (Sss). Although Sss is considered a ‘non‐standard’ plant pathogen, it poses an increasingly significant threat to potato production worldwide. Effective management strategies to control this disease are unavailable. The development of control methods is particularly challenging due to the ability of Sss to produce robust resting spores, which persist in soil for extended periods, and the limited understanding of its biology and interaction with the host. This review summarises the current knowledge on the life cycle and epidemiology of Sss and outlines the key barriers for effective control. Particular emphasis is placed on recent advances in host resistance, drawing on molecular genetics and genomic studies that reveal potential resistance in potato. Comparative insights from Plasmodiophora brassicae , a closely related protist and the causal agent of clubroot in brassicas, are also discussed to enhance understanding of host resistance mechanisms. Collectively, these insights provide a foundation for developing durable resistance and integrated management approaches to mitigate the impact of powdery scab in potato production systems.
Almond ( Prunus dulcis ) canker diseases and decline syndromes have been reported during the past decade in the main almond‐growing regions worldwide. The notable expansion of almond cultivation in intensive farming systems to increase yields, particularly in countries across the Mediterranean Basin such as Italy and Spain, as well as in Australia, United States (California) and South Africa has led to their emergence. This review focuses on the different canker diseases and decline syndromes affecting almond crops and their symptoms, causal agents and life cycles. The influence of biotic and abiotic factors on the emergence and development of some of these diseases and syndromes under altered environmental conditions because of climate change is discussed. Finally, the challenges that the control of these complex diseases and syndromes poses for global food security are presented, including an overview of the integrated management strategies currently available. This comprehensive review aimed to elucidate the aetiology of almond cankers and decline syndromes as well as raise awareness within the scientific community of the need to deepen our knowledge of their aetiology and epidemiology. This should help in developing effective management strategies to reduce their high economic impact on almond (and other woody crops) worldwide, in the current era of climate change.
Marinomyxa is a genus of obligate intracellular parasitic protists known to infect tropical seagrasses of the genus Halophila. The genus has attracted increasing attention since the rediscovery of Marinomyxa galls in the invasive seagrass Halophila stipulacea in the Eastern Mediterranean Basin in 2015, with numerous subsequent reports documenting its spread throughout the Mediterranean and Caribbean Seas alongside its host. To date, two species have been formally described: Marinomyxa halophilae, parasitising Halophila ovalis and Halophila major; and Marinomyxa marina, infecting H. stipulacea. Here, we present molecular and morphological data on Marinomyxa infections in two additional Halophila species. We report galls in the cosmopolitan Halophila decipiens from Puerto Rico caused by M. halophilae, and in the vulnerable intertidal seagrass Halophila beccarii from southeast Asia caused by a previously undescribed parasite. Phylogenetic analyses based on 18S and 28S rRNA gene sequences reveal that the parasite found in tissues of H. beccarii represents a distinct lineage within the genus, which we designate as Marinomyxa denhartogi sp. nov. Through analysis of a previously published environmental sequencing dataset from New Caledonia, we provide additional evidence that the presence of Marinomyxa can be traced even in remote areas of its hosts' geographical range. Our findings indicate that Halophila and Marinomyxa are persistently co-occurring symbionts with a tightly linked co-evolutionary history, suggesting that additional host-parasite associations remain to be discovered through broader sampling of seagrass populations.
Variety choice is one of the key operational decisions a wheat farmer makes. A recent farm survey showed that farmers consider the disease resistance rating to be the primary factor when deciding which variety to grow. Here we analyse data from a survey of winter wheat crops between 1985 and 2019 in England to understand what factors affect the variety choice when faced with variety with different resistance ratings against Septoria tritici blotch, caused by Zymoseptoria tritici. We find that the mean Septoria tritici blotch resistance rating of the varieties grown showed considerable variations, with periods where the mean resistance rating decreased followed by periods with increased mean resistance rating. We explore the causes of these fluctuations and the factors that affect variety choice by the farmer. In this analysis we consider (i) the availability of variety with a preferred resistance rating, (ii) the long-term Septoria tritici blotch disease pressure at the location of the farmers' farms, (iii) the availability of effective fungicides, (iv) whether farmers respond to a decrease in the resistance rating of a variety and (v) whether the Septoria tritici blotch severity in previous years affects the choice of the resistance rating of the variety in the subsequent year. The ultimate aim of this work is to understand the choices farmers make when selecting a wheat variety and how the choice of resistance rating is influenced by a range of aspects relating to the Septoria tritici blotch pathogen.
Single-spore isolation of Plasmodiophora brassicae, the obligate parasite responsible for clubroot disease in crucifers, is essential for generating genetically homogeneous pathogen collections. However, the small size of its resting spores (3-4 mu m diameter) and the inability to culture the pathogen make isolation challenging and time-consuming. This study evaluated a novel laser microdissection technique for P. brassicae single-spore isolation. In Experiment 1, the efficiency of this method was compared with two previously published protocols. The older methods achieved success rates of 18% and 14%, respectively, requiring approximately 20 min per attempt. In contrast, the initial trial using laser microdissection yielded a lower success rate (4%) but significantly reduced the required time (similar to 7 min per attempt). Experiment 2 examined the effects of centrifugation duration (30-120 s) and incubation time (24-72 h) on isolation success across two years (2023-2024). Success rates varied across treatments (0%-21.2%), but no significant effects of centrifugation or incubation time were detected. Despite variability, the laser microdissection process proved substantially more time-efficient, requiring only 30 s-2 min per inoculation. In Experiment 3, the influence of inoculum source was assessed, with success rates ranging from 16.7% to 19.4% across three P. brassicae pathotypes (3A, 3D, 3H). No significant pathotype effect was detected. Overall, laser microdissection offers a precise and time-efficient alternative for single-spore isolation. While further optimisation is required to improve consistency and recovery rates, this technique shows strong potential for downstream applications in P. brassicae research.
Araceae includes numerous flowering plant species that are cultivated as ornamentals in gardens, pots or by the cut-flower industry. Relatively little has been published on the fungal pathogens attacking these ornamentals, particularly in Brazil. Here we report the results of a study on Colletotrichum spp. found in association with anthracnose and leaf spot symptoms on Aglaonema commutatum, Anthurium andraeanum, Dieffenbachia amoena, Philodendron imbe and Philodendron tripartitum. Nine isolates of Colletotrichum were obtained from diseased tissues. A polyphasic approach was used to identify these isolates as four novel species: Colletotrichum anthuriicola sp. nov., C. araceiphilum sp. nov., C. imbeicola sp. nov. and C. philodendrifolium sp. nov. Cross-inoculation assays were conducted using the plant host species listed above, as well as Epipremnum aureum, and showed that all isolates were pathogenic to at least three of the inoculated aroid species. These findings expand the current knowledge of the genus Colletotrichum and its association with members of the Araceae and highlight the need to consider individual ornamental aroids as potential inoculum reservoirs for infection of other cultivated members, given the low host specificity of aroid-associated Colletotrichum species.
Powdery mildew is a major foliar disease affecting Cucurbita moschata, caused by obligate biotrophic fungi of the order Erysiphales. Despite its economic importance, research on its causal agents remains limited due to the challenges associated with maintaining viable isolates for experimentation. Traditional isolation and conservation rely on the Pathogen-Leaf system (Path-I), which has been widely used but presents limitations in sustaining isolates over time. In this study, we developed and evaluated an alternative system, namely the Pathogen-Seedling (Path-II), designed to optimise pathogen isolation and maintenance. Path-II proved to be the most effective method, ensuring higher recovery rates and prolonged viability of powdery mildew isolates compared to Path-I. Furthermore, long-term conservation methods described in the literature, including tissue-free approaches, were unsuccessful in maintaining viable isolates. To confirm taxonomic identity, we combined morphological and molecular approaches, optimising a direct-PCR protocol targeting the ITS region of rDNA. Both techniques consistently identified Podosphaera xanthii as the predominant causal agent among the isolates obtained from C. moschata in this study. By simplifying pathogen isolation and maintenance, this study enhances the feasibility of powdery mildew research and provides valuable tools for breeding programmes focused on disease resistance.
