Rice false smut pathogen: implications for mycotoxin contamination, current status, and future perspectives

Rice serves as a staple food across various continents worldwide. The rice plant faces significant threats from a range of fungal, bacterial, and viral pathogens. Among these, rice false smut disease (RFS) caused by Villosiclava virens is one of the devastating diseases in rice fields. This disease is widespread in major rice-growing regions such as China, Pakistan, Bangladesh, India, and others, leading to significant losses in rice plantations. Various toxins are produced during the infection of this disease in rice plants, impacting the fertilization process as well. This review paper lightens the disease cycle, plant immunity, and infection process during RFS. Mycotoxin production in RFS affects rice plants in multiple ways, although the exact phenomena are still unknown.

Germination and Invasion of Paraphoma radicina on Roots of a Susceptible and a Resistant Alfalfa Cultivar (Medicago sativa)

Alfalfa Paraphoma root rot (APRR) (Paraphoma radicina) is a recently described alfalfa disease widely distributed in China, first reported in 2020. So far, the resistance levels of 30 alfalfa cultivars to APRR have been characterized; however, the resistance mechanisms among these cultivars remain unknown. In the present study, the alfalfa resistance mechanisms against APRR were investigated by studying the difference of P. radicina infecting susceptible (Gibraltar) and resistant (Magnum II) alfalfa cultivars under the light microscope and scanning electronic microscope. The conidial germination and germ tube growth in the root exudates of different resistant cultivars were also compared. The results revealed that conidial germination, germ tube development, and P. radicina penetration into root tissues of resistant plants were delayed. In susceptible and resistant cultivars, P. radicina infected roots by penetrating epidermal cells and the intercellular space between epidermal cells. During the infection process, germ tubes penetrated the root surface directly or formed appressoria. However, the penetration percentage on the susceptible cultivar was significantly higher than on the resistant cultivar, irrespective of the infection route. Moreover, disintegrated conidia and germ tubes were observed on resistant cultivar roots at 48 h postinoculation. The conidial germination and germ tube growth in root exudates of susceptible cultivars were significantly higher than in resistant cultivars. The current findings implied that the alfalfa resistance mechanism might be related to root exudates. These findings could provide insights into the alfalfa resistance mechanism following P. radicina infection.

Insecticidal Effect of the Entomopathogenic Fungus Lecanicillium araneicola HK-1 in Aphis craccivora (Hemiptera: Aphididae)

Aphis craccivora (Hemiptera: Aphididae) is an important pest affecting various crops worldwide. However, only few studies have been conducted on the infection of A. craccivora by Lecanicillium and related insecticidal mechanisms. We investigated the infection process of A. craccivora by Lecanicillium araneicola HK-1 using fluorescence microscopy and scanning electron microscopy (SEM), and our results indicated that the conidia of strain HK-1 easily attached to the feet and dorsum of A. craccivora. The activities of chitinase and extracellular protease were induced in the aphid after treatment with HK-1. A bioassay on A. craccivora showed that the median lethal concentration (LC50) of the fungus crude extract was 24.00 mg mL-1 for 24 h of treatment. Additionally, the results showed that the crude extract disrupted the enzyme system of A. craccivora, inducing the inhibition of carboxylesterase (CarE) and the induction of glutathione S-transferase (GST) and acetylcholinesterase (AChE). Combining these results with those of a gas chromatography-mass spectrometry (GC-MS) analysis, it is suggested that p-cymene, hymecromone, 9,12-octadecadienoic acid (Z, Z) methyl ester, and 9,12-octadecadienoic acid (Z, Z) may be connected to the insecticidal effects we observed. This study provides a theoretical basis for the use of L. araneicola HK-1 as a potential biological control agent.

Visualization of the entire process of rice spikelet infection by Ustilaginoidea virens through nondestructive inoculation

Introduction

Rice false smut caused by Ustilaginoidea virens, is a destructive fungal disease encountered in many rice-producing areas worldwide. To determine the process by which U. virens infects rice spikelets in the field.

Methods

The green fluorescent protein-labeled U. virens was used as an inoculum to conduct artificial inoculation on rice at the booting stage via non-destructive panicle sheath instillation inoculation.

Results

The results showed that the conidia of U. virens germinated on the surface of rice glumes and produced hyphae, which clustered at the mouth of rice glumes and entered the glumes through the gap between the palea and lemma. The conidia of U. virens colonized in rice floral organs, which led to pollen abortion of rice. U. virens wrapped the whole rice floral organ, and the floral organ-hyphae complex gradually expanded to open the glumes to form a rice false smut ball, which was two to three times larger than that observed in normal rice.

Discussion

Panicle sheath instillation inoculation was shown to be a non-destructive inoculation method that could simulate the natural infection of U. virens in the field. The entire infection process of U. virens was visualized, providing a theoretical reference for formulating strategies to control rice false smut in the field.

Insights into Grape Ripe Rot: A Focus on the Colletotrichum gloeosporioides Species Complex and Its Management Strategies

Grape ripe rot, which is predominantly caused by the Colletotrichum species, presents a growing threat to global grape cultivation. This threat is amplified by the increasing populations of the Colletotrichum species in response to warmer climates. In this review, we investigate the wide-ranging spectrum of grape ripe rot, specifically highlighting the role and characteristics of the C. gloeosporioides species complex (CGSC). We incorporate this understanding as we explore the diverse symptoms that lead to infected grapevines, their intricate life cycle and epidemiology, and the escalating prevalence of C. viniferum in Asia and globally. Furthermore, we delve into numerous disease management strategies, both conventional and emerging, such as prevention and mitigation measures. These strategies include the examination of host resistances, beneficial cultivation practices, sanitation measures, microbiome health maintenance, fungicide choice and resistance, as well as integrated management approaches. This review seeks to enhance our understanding of this globally significant disease, aspiring to assist in the development and improvement of effective prevention and control strategies.

Comparative Transcriptomics of Fusarium graminearum and Magnaporthe oryzae Spore Germination Leading up To Infection

For fungal plant pathogens, the germinating spore provides the first interaction with the host. Spore germlings move across the plant surface and use diverse penetration strategies for ingress into plant surfaces. Penetration strategies include pressurized melanized appressoria, which facilitate physically punching through the plant cuticle, and nonmelanized appressoria, which penetrate with the help of enzymes or cuticular damage to breach the plant surface. Two well-studied plant pathogens, Fusarium graminearum and Magnaporthe oryzae, are typical of these two modes of penetration. We applied comparative transcriptomics to Fusarium graminearum and Magnaporthe oryzae to characterize the genetic programming of the early host-pathogen interface. Four sequential stages of development following spore localization on the plant surface, from spore swelling to appressorium formation, were sampled for each species on culture medium and on barley sheaths, and transcriptomic analyses were performed. Gene expression in the prepenetration stages in both species and under both conditions was similar. In contrast, gene expression in the final stage was strongly influenced by the environment. Appressorium formation involved the greatest number of differentially expressed genes. Laser-dissection microscopy was used to perform detailed transcriptomics of initial infection points by F. graminearum. These analyses revealed new and important aspects of early fungal ingress in this species. Expression of the trichothecene genes involved in biosynthesis of deoxynivalenol by F. graminearum implies that toxisomes are not fully functional until after penetration and indicates that deoxynivalenol is not essential for penetration under our conditions. The use of comparative gene expression of divergent fungi promises to advance highly effective targets for antifungal strategies. IMPORTANCE Fusarium graminearum and Magnaporthe oryzae are two of the most important pathogens of cereal grains worldwide. Despite years of research, strong host resistance has not been identified for F. graminearum, so other methods of control are essential. The pathogen takes advantage of multiple entry points to infect the host, including breaches in the florets due to senescence of flower parts and penetration of the weakened trichome bases to breach the epidermis. In contrast, M. oryzae directly punctures leaves that it infects, and resistant cultivars have been characterized. The threat of either pathogen causing a major disease outbreak is ever present. Comparative transcriptomics demonstrated its potential to reveal novel and effective disease prevention strategies that affect the initial stages of disease. Shedding light on the basis of this diversity of infection strategies will result in development of increasingly specific control strategies.

Role of a 49 kDa Trypanosoma cruzi Mucin-Associated Surface Protein (MASP49) during the Infection Process and Identification of a Mammalian Cell Surface Receptor

Trypanosoma cruzi is the etiologic agent of Chagas disease, a parasitic disease of great medical importance on the American continent. Trypomastigote infection's initial step in a mammalian host is vital for the parasite's life cycle. A trypomastigote's surface presents many molecules, some of which have been proposed to be involved in the infection process, including a glycoprotein family called mucin-associated surface proteins (MASPs). This work describes a 49-kDa molecule (MASP49) that belongs to this family and is expressed mainly on the surfaces of amastigotes and trypomastigotes but can be found in extracts and the membrane-enriched fractions of epimastigotes. This protein is partially GPI-anchored to the surface and has a role during the internalization process, since its blockade with specific antibodies decreases parasite entry into Vero cells by 62%. This work shows that MASP49 binds to peritoneal macrophages and rat cardiomyocytes, undergoes glycosylation via galactose N-acetylgalactosamine, and can attach to the macrophage murine C-type lectin receptor (mMGL). These results suggest that MASP49 can be considered a virulence factor in T. cruzi, and a better understanding of its role in the infection process is necessary.

Hormonal and Metabolomic Responses of Dendrobium catenatum to Infection with the Southern Blight Pathogen Sclerotium delphinii

Southern blight caused by Sclerotium delphinii has a devastating effect on Dendrobium catenatum (an extremely valuable medicinal and food homologous Orchidaceae plant). However, the mechanisms underlying S. delphinii infection and D. catenatum response are far from known. Here, we investigated the infection process and mode of S. delphinii through microscopic observations of detached leaves and living plantlets and further explored the hormonal and metabolomic responses of D. catenatum during S. delphinii infection by using the widely targeted metabolome method. The results showed that S. delphinii infection involves two stages: a contact phase (12 to 16 h after inoculation) and a penetration stage (20 h after inoculation). S. delphinii hyphae could penetrate leaves directly (via swollen hyphae and the formation of an infection cushion) or indirectly (via stomatal penetration), causing water-soaked lesions on leaves within 24 to 28 h after inoculation and expanded thereafter. The content of jasmonates increased after the hyphal contact and remained at high levels during S. delphinii infection, whereas the ethylene precursor (1-aminocyclopropanecarboxylic acid) accumulated significantly after penetration. Furthermore, metabolites of the phenylpropanoid and flavonoid pathways were enriched after pathogen penetration, whereas several amino acids accumulated in significant amounts at the late stage of infection. Moreover, some other associated metabolites were significantly altered during pathogen infection. Therefore, the jasmonate, phenylpropanoid, flavonoid, and amino acid pathways could play crucial roles in D. catenatum resistance to S. delphinii infection. This study provides insight into the prevention and control of southern blight disease of D. catenatum.

Time-series gene expression patterns and their characteristics of Beauveria bassiana in the process of infecting pest insects

Beauveria bassiana has been widely used as an important biological control fungus for agricultural and forest pests, and clarifying the interaction mechanism between B. bassiana and its host will help to better exert the efficacy of the mycoinsecticide. Here, we proposed a novel pattern analysis (PA) method for analyzing time-series data and applied it to a transcriptomic data set of B. bassiana infecting Galleria mellonella. We screened out 14 patterns including 868 genes, which had some characteristics that were not inferior to differentially expressed genes (DEGs). Compared with the previous analysis of this data set, we had three novel discoveries during B. bassiana infection, including overall downregulation of gene expression, the more critical first 24 h, and enrichment of regulatory functions of downregulated genes. Our new PA method promises to be an important complement to DEGs analysis for time-series transcriptomic data, and our findings enrich our knowledge of molecular mechanisms of fungal-host interactions.

Establishment of an Artificial Inoculation Method of Ustilaginoidea virens Without Damaging the Rice Panicle Sheaths

Rice false smut (RFS) is a destructive disease of rice worldwide caused by Ustilaginoidea virens. Nevertheless, there is a lack of efficient and stable artificial inoculation method to simulate the natural infection of U. virens, which is an important factor limiting further research on the pathogen. The purpose of this study was to establish an artificial inoculation method, which can simulate the natural infection process of U. virens without destroying the panicle sheath structure of rice. In this research, rice plants were inoculated by soaking roots at the seedling stage, spraying at the tillering stage, injecting at the booting stage, and again spraying at the flowering stage to determine the appropriate artificial inoculation time. Meanwhile, the panicle sheath instillation method and the injection inoculation method were compared. The results show that stages 6 to 8 of young panicle differentiation are an important period for U. virens infection. There were no significant differences in the mean rates of infected panicles, mean rates of infected grains, and maximum infected grains per panicle between the two inoculation methods. However, the frequency of RFS ball occurrence at the upper part of the panicles was significantly higher on the spikelets inoculated by the injection method than that of spikelets inoculated by natural infection and panicle sheath instillation. Therefore, panicle sheath instillation method was more similar to the natural infection of U. virens in the field. This research exhibited an innovative artificial inoculation method for identification of U. virens pathogenicity and evaluation of rice resistance against RFS.

Infection Process and Genome Assembly Provide Insights into the Pathogenic Mechanism of Destructive Mycoparasite Calcarisporium cordycipiticola with Host Specificity

Calcarisporium cordycipiticola is the pathogen in the white mildew disease of Cordyceps militaris, one of the popular mushrooms. This disease frequently occurs and there is no effective method for disease prevention and control. In the present study, C. militaris is found to be the only host of C. cordycipiticola, indicating strict host specificity. The infection process was monitored by fluorescent labeling and scanning and transmission electron microscopes. C. cordycipiticola can invade into the gaps among hyphae of the fruiting bodies of the host and fill them gradually. It can degrade the hyphae of the host by both direct contact and noncontact. The parasitism is initially biotrophic, and then necrotrophic as mycoparasitic interaction progresses. The approximate chromosome-level genome assembly of C. cordycipiticola yielded an N50 length of 5.45 Mbp and a total size of 34.51 Mbp, encoding 10,443 proteins. Phylogenomic analysis revealed that C. cordycipiticola is phylogenetically close to its specific host, C. militaris. A comparative genomic analysis showed that the number of CAZymes of C. cordycipiticola was much less than in other mycoparasites, which might be attributed to its host specificity. Secondary metabolite cluster analysis disclosed the great biosynthetic capabilities and potential mycotoxin production capability. This study provides insights into the potential pathogenesis and interaction between mycoparasite and its host.

Studies on the control effect of Bacillus subtilis on wheat powdery mildew

BACKGROUND

Wheat powdery mildew is a worldwide fungal disease and one of the main diseases harming wheat production. Bacillus subtilis is a vital biocontrol bacteria with broad-spectrum antimicrobial activity. In this study, we systematically studied the control effect of B. subtilis on wheat powdery mildew.

RESULTS

The control efficiency of 4 × 10⁵  CFU ml^-1 B. subtilis on wheat leaves was 71.75% in vitro and 70.31% in a pot experiment. Application of 4 × 10⁵  CFU ml^-1 B. subtilis significantly inhibited spore germination (spore germination rate of 22.23%) and increased appressorium deformity (appressorium deformity rate of 69.33%). This was significantly different from the results in the sterile water treatment. Through transcriptome sequencing analysis, we found that differentially expressed genes were mainly enriched in the biosynthesis and metabolism of amino acids (including phenylalanine), carbon metabolism, the pentose phosphate pathway and other pathways. In particular, the plant hormone signal pathway gene nonexpressor of pathogenesis-related genes 1 (NPR1) was significantly upregulated.

CONCLUSION

B. subtilis at concentrations of 4 × 10⁵  CFU ml^-1 had a significant control effect on wheat powdery mildew and can inhibit germination of the conidial germ tubes and the normal development of appressorium. B. subtilis may induce disease resistance in wheat to control wheat powdery mildew, and this effect is related to the salicylic acid-dependent signal pathway. © 2021 Society of Chemical Industry.

Characterization of Pestalotiopsis sp. causing gray leaf spot in coconut (Cocos nucifera L.) in Bangladesh

Coconut (Cocos nucifera L.) is one of the most important fruit trees in Bangladesh. This tree is susceptible to various pathogens. Among them, a fungus was consistently isolated from gray leaf spot symptom in coconut. This study aimed to isolate, characterize, and find the management strategies of the causal fungus of gray leaf spot disease in coconut. Both morphological and molecular characters identified the pathogen as Pestalotiopsis sp. for the first time in Bangladesh. Artificial inoculation of this fungus showed symptoms similar to those previously observed in the field. Cross-inoculation test suggests that Pestalotiopsis sp. has a wide host range. The infection process of Pestalotiopsis sp. started at 2 h after inoculation (hai) with the formation of germ tube followed by the formation of infection hyphae, which penetrated directly into the host at 6 hai. Gray leaf spot symptom was developed at 120 hai. Numerous conidia developed from the acervuli at 168 hai. These conidia acted as the source of inocula for secondary infection. The optimum temperature for the growth of Pestalotiopsis sp. was 25°C, however, the growth of Pestalotiopsis sp. ceased at 15°C and 35°C. This pathogen was completely inhibited by Autostin 50 WDG (carbendazim) at 100 ppm. Trichoderma viride (Pb-7) was found as the potential biocontrol agent against Pestalotiopsis sp. These findings could contribute to describing the disease cycle and epidemiology of Pestalotiopsis sp. that would ultimately require to undertake effective control measures against this pathogen.

Emerging Roles of Posttranslational Modifications in Plant-Pathogenic Fungi and Bacteria

Posttranslational modifications (PTMs) play crucial roles in regulating protein function and thereby control many cellular processes and biological phenotypes in both eukaryotes and prokaryotes. Several recent studies illustrate how plant fungal and bacterial pathogens use these PTMs to facilitate development, stress response, and host infection. In this review, we discuss PTMs that have key roles in the biological and infection processes of plant-pathogenic fungi and bacteria. The emerging roles of PTMs during pathogen-plant interactions are highlighted. We also summarize traditional tools and emerging proteomics approaches for PTM research. These discoveries open new avenues for investigating the fundamental infection mechanisms of plant pathogens and the discovery of novel strategies for plant disease control.

Diversity and Function of Appressoria

Endophytic, saprobic, and pathogenic fungi have evolved elaborate strategies to obtain nutrients from plants. Among the diverse plant-fungi interactions, the most crucial event is the attachment and penetration of the plant surface. Appressoria, specialized infection structures, have been evolved to facilitate this purpose. In this review, we describe the diversity of these appressoria and classify them into two main groups: single-celled appressoria (proto-appressoria, hyaline appressoria, melanized (dark) appressoria) and compound appressoria. The ultrastructure of appressoria, their initiation, their formation, and their function in fungi are discussed. We reviewed the molecular mechanisms regulating the formation and function of appressoria, their strategies to evade host defenses, and the related genomics and transcriptomics. The current review provides a foundation for comprehensive studies regarding their evolution and diversity in different fungal groups.

Activity of the Novel Fungicide Mefentrifluconazole Against Colletotrichum scovillei

The prevalence and destructiveness of anthracnose, caused by Colletotrichum scovillei, in pepper production regions seriously affects pepper yield and quality. Mefentrifluconazole, the first of the isopropanol-azole subgroup of triazole fungicides, was introduced for the control of pepper anthracnose. However, the growth characteristics of pepper fruit and rapid spread of anthracnose suggest that the fungicide application method must be optimized to enhance fungicide efficacy. The sensitivity of C. scovillei to mefentrifluconazole was determined by mycelial growth and germ tube elongation assays using 157 single-spore isolates with mean 50% effective concentration values of 0.462 ± 0.138 and 0.359 ± 0.263 mg/liter, respectively. The in vivo data also showed that mefentrifluconazole had favorable protective and curative effects against pepper anthracnose. Mefentrifluconazole significantly affected C. scovillei infection on pepper by reducing appressorium formation and sporulation, shriveling spores and germ tubes, and causing the abnormal development of appressoria and conidiophores. Mefentrifluconazole could move acropetally, horizontally, and basipetally in pepper plants. Compared with a knapsack sprayer, mefentrifluconazole applied by mist sprayer exhibited significantly better activity against pepper anthracnose. Additionally, as the spray volume increased from 45 to 150 liters/ha, the control efficacy of mefentrifluconazole first increased and then tended to be steady, with an optimal spray volume of 90 liters/ha. The difference in disease control efficacy was related to the deposition and droplet distribution of mefentrifluconazole on the pepper fruit. These results provide scientific guidance for the application of mefentrifluconazole in pepper fields and improved fungicide utilization.

Unraveling the Mode of Action of Cordyceps fumosorosea: Potential Biocontrol Agent against Plutella xylostella (Lepidoptera: Plutellidae)

The entomopathogenic fungus, Cordyceps fumosorosea is a potential eco-friendly biocontrol agent. The present study revealed the entire course of infection of P. xylostella by C. fumosorosea with particular reference to cuticular penetration. Comparative studies on the infection of Plutella xylostella larvae by two strains of C. fumosorosea with different pathogenicity were carried out using light, scanning, and transmission electron microscopy. We found that C. fumosorosea tended to adhere to the cuticle surfaces containing protrusions. Although conidia of the lower pathogenic strain IFCF-D58 germinated, they failed to penetrate and complete the development cycle. In contrast, the higher pathogenic strain IFCF01 began to germinate within 4 h and attached to the cuticle by a thin mucilaginous matrix within 8 h post-inoculation. After 24 h post-inoculation, germ tubes and penetrating hyphae reached the cuticular epidermis and began to enter the haemocoel. Within 36 h post-inoculation, the hyphal bodies colonized the body cavity. Hyphae penetrated from inside to outside of the body after 48 h and sporulated the cadavers. After 72 h post-inoculation, numerous conidia emerged and the mycelial covered the entire cuticular surface. The two strains showed similarities in terms of conidial size and germination rate. However, IFCF-D58 exhibited significantly fewer appressoria and longer penetrating hyphae compared to the more infective IFCF01 on all surface topographies. The current pathogen invasion sequence of events suggested that the aggressive growth and propagation along with rapid and massive in vivo production of blastospores facilitate the conidia of IFCF01 to quickly overcome the diamondback moth's defense mechanism.

Screening and histopathological characterization of sunflower germplasm for resistance to Macrophomina phaseolina

Twenty-two sunflower germplasms were screened for resistance to Macrophomina phaseolina to select parental genetic resources useful for the development of charcoal rot-resistant sunflower cultivars. Potting soil inoculated with pathogen (10 mL pot^-1, 2 × 10⁵ sclerotia mL^-1) sown with sunflower seeds was examined for disease severity index (%), disease incidence (%), mortality (%), and growth inhibition index (%) 90 d after inoculation. None of the germplasm was disease-free; four were found to be resistant, five moderately resistant, six moderately susceptible, five susceptible, and two highly susceptible. All inoculated plants exhibited disease symptoms both externally and internally. Mild to severe symptoms included brown lesions on aboveground plant, pith disintegration in stem, and shredded appearance of tap root. Histopathological features exposed different colonization mechanism of the pathogen in the resistant and susceptible cultivars. Physical blockage, tissue disintegration, blackening and rupturing of cortical, pith and vascular regions by fungal mycelia, and sclerotia and pycnidia causing large spaces in the center of stem rendered it a hollow structure in all susceptible germplasm. However, stem and root tissues of the resistant germplasm indicated local infection restricted to few cells. This suggested expression of true resistance genes in resistant germplasm. Therefore, the sunflower lines resistant to the M. phaseolina infection are potential genetic resources for the development of quality sunflower cultivars resistant to charcoal rot disease.

Effect of Pyrisoxazole on Colletotrichum scovillei Infection and Anthracnose on Chili

Anthracnose caused by Colletotrichum scovillei is one of the most destructive diseases affecting chili production. Disease control mainly relies on conventional fungicides, and repeated exposure to single-site mode-of-action fungicides may pose a risk for the development of resistant isolates within the population. Our previous study suggested that pyrisoxazole has strong inhibitory activity against C. scovillei in vitro. However, the effects of pyrisoxazole on the C. scovillei infection process and the performance of pyrisoxazole in the field remain unclear. In this study, pyrisoxazole exhibited strong inhibitory activity against the mycelial growth, appressorium formation, and appressorium diameter of C. scovillei, with half maximal effective concentration values of 0.1986, 0.0147, and 0.0269 μg/ml, respectively, but had no effect on sporulation, even at the highest concentration of 1.6 μg/ml. The baseline sensitivity curves were unimodal with a long right-hand tail. The in vivo data showed that pyrisoxazole provided both preventive and curative activity against anthracnose on chili. Pyrisoxazole decreased the incidence of anthracnose and reduced disease progress. The results of electron microscopy showed that pyrisoxazole can affect the C. scovillei infection process by altering mycelial morphology, degrading conidia and germ tubes, suppressing conidial germination and appressorium formation, and enhancing conidiophore production. Pyrisoxazole can be used to effectively control anthracnose under field conditions and increase chili yield; moreover, no phytotoxicity symptoms were observed after treatment. These results provide new insight into the mechanisms by which pyrisoxazole controls disease and suggest that pyrisoxazole is a feasible alternative for the management of anthracnose in chili.

URM1-Mediated Ubiquitin-Like Modification Is Required for Oxidative Stress Adaptation During Infection of the Rice Blast Fungus

Ubiquitin is a small modifier protein which is usually conjugated to substrate proteins for degradation. In recent years, a number of ubiquitin-like proteins have been identified; however, their roles in eukaryotes are largely unknown. Here, we describe a ubiquitin-like protein URM1, and found it plays important roles in the development and infection process of the rice blast fungus, Magnaporthe oryzae. Targeted deletion of URM1 in M. oryzae resulted in slight reduction in vegetative growth and significant decrease in conidiation. More importantly, the Δurm1 mutant also showed evident reduction in virulence to host plants. Infection process observation demonstrated that the mutant was arrested in invasive growth and resulted in accumulation of massive host reactive oxygen species (ROS). Further, we found the Δurm1 mutant was sensitive to the cell wall disturbing reagents, thiol oxidizing agent diamide and rapamycin. We also showed that URM1-mediated modification was responsive to oxidative stresses, and the thioredoxin peroxidase Ahp1 was one of the important urmylation targets. These results suggested that URM1-mediated urmylation plays important roles in detoxification of host oxidative stress to facilitate invasive growth in M. oryzae.

The Epidemiology of Fusarium Wilt of Banana

Fusarium wilt of banana (also known as Panama disease) has been a problem in Australia since 1874. Race 1 of the pathogen (Fusarium oxysporum f. sp. cubense) is responsible for damage to 'Lady Finger' (AAB, Pome subgroup) and other less widely grown cultivars such as 'Ducasse' (Pisang Awak, ABB). Subtropical Race 4 (STR4) also affects these cultivars as well as Cavendish cultivars (AAA) in southern Queensland and northern New South Wales where cold temperature predisposition is involved. Tropical Race 4 (TR4) has led to the demise of the Cavendish industry in the Northern Territory, and its presence was confirmed in a North Queensland plantation in 2015, which warranted destruction of all banana plants on the property; as of this writing (April 2019), TR4 has spread to two adjacent properties. This review, which was commissioned by Biosecurity Queensland in response to the 2015 TR4 outbreak, considers the key epidemiological factors associated with the onset of a Fusarium wilt epidemic. Resistance to TR4, which is mediated by events following entry by the pathogen into the xylem, is not present in any commercially acceptable banana cultivar. Also, there is no effective chemical agent that can be used to manage the disease. Besides prevention, very early recognition and rapid containment of a disease outbreak are necessary to prevent epidemic development. A good understanding of the key factors responsible for disease development is required when devising practical protocols for the destruction of infected plants, treatment of surrounding infested soil, and reduction of inoculum in plant residues and soil.

Genome-Wide Analysis of Secondary Metabolite Gene Clusters in Ophiostoma ulmi and Ophiostoma novo-ulmi Reveals a Fujikurin-Like Gene Cluster with a Putative Role in Infection

The emergence of new microbial pathogens can result in destructive outbreaks, since their hosts have limited resistance and pathogens may be excessively aggressive. Described as the major ecological incident of the twentieth century, Dutch elm disease, caused by ascomycete fungi from the Ophiostoma genus, has caused a significant decline in elm tree populations (Ulmus sp.) in North America and Europe. Genome sequencing of the two main causative agents of Dutch elm disease (Ophiostoma ulmi and Ophiostoma novo-ulmi), along with closely related species with different lifestyles, allows for unique comparisons to be made to identify how pathogens and virulence determinants have emerged. Among several established virulence determinants, secondary metabolites (SMs) have been suggested to play significant roles during phytopathogen infection. Interestingly, the secondary metabolism of Dutch elm pathogens remains almost unexplored, and little is known about how SM biosynthetic genes are organized in these species. To better understand the metabolic potential of O. ulmi and O. novo-ulmi, we performed a deep survey and description of SM biosynthetic gene clusters (BGCs) in these species and assessed their conservation among eight species from the Ophiostomataceae family. Among 19 identified BGCs, a fujikurin-like gene cluster (OpPKS8) was unique to Dutch elm pathogens. Phylogenetic analysis revealed that orthologs for this gene cluster are widespread among phytopathogens and plant-associated fungi, suggesting that OpPKS8 may have been horizontally acquired by the Ophiostoma genus. Moreover, the detailed identification of several BGCs paves the way for future in-depth research and supports the potential impact of secondary metabolism on Ophiostoma genus’ lifestyle.

Physical Changes in Satsuma Mandarin Leaf after Infection of Elsinoë fawcettii Causing Citrus Scab Disease

Citrus scab disease is one of the destructive diseases that reduce the value of fruit for the fresh market. We analyzed the process of symptom development after infection with scab pathogen Elsinoë fawcettii in the susceptible satsuma mandarin leaves to observe the structural modification against pathogen. The cuticle and epidermal cells along with 3-5 layers of mesophyll tissue were degraded 1-2 days post inoculation. Surrounding peripheral cells of degraded tissues grew rapidly and then enveloped the necrotic area along with the growing conidia. Cross sections through the lesion revealed hyphal colonization in epidermis and mesophyll tissues. In response to the pathogen colonization, host cell walls were lignified, inner cells were rapidly compartmentalized and a semi-circular boundary was formed that separated the infected region from the non-infected region, and finally prevented the intercellular pathogen spread.

Spore Germination, Infection Structure Formation, and Colony Development of Erysiphe pulchra on Dogwood Leaves and Glass Slides

Spore germination, infection structure formation, and colony development of Erysiphe pulchra on glass slides and leaf disks of a susceptible flowering dogwood line were examined using light and scanning electron microscopes. On both glass slides and leaf disks, germination of conidia started within 2 h after inoculation (hai). One to four germ tubes grew from two poles of a conidium, one or two of the germ tubes formed initial appressoria, and only one of the germ tubes with initial appressoria formed secondary appressoria. However, formation of initial and secondary appressoria was delayed on glass slides (48 and 72 hai, respectively) compared with that on dogwood leaf disks (3 and 24 hai, respectively). Branching hyphae did not grow from germinated conidia on glass slides. However, on dogwood leaf disks, branched hyphae were observed 48 hai. In epidermal cells, the fungus formed compact and globose haustoria. Conidia formation on conidiophores started on leaf disks 7 days after inoculation.

Influence of Moisture and Temperature on Infection of Canada Thistle by Alternaria cirsinoxia

Relative humidity (RH), temperature, continuous leaf wetness, and intermittent leaf wetness were evaluated for their influence on conidial germination, appressoria formation, and infection of Canada thistle by Alternaria cirsinoxia. Conidia germinated and formed appressoria at 98% RH, but required at least 100% RH, and preferably free water, to penetrate leaves. In free water, conidia germinated equally well at 10, 15, 20, 25, or 30°C. Appressoria formation and leaf penetration also occurred at all temperatures evaluated from 10 to 30°C, with the highest values at 20 to 25°C and 20°C, respectively. Conidia required 8 h of continuous leaf wetness to establish visible symptoms of infection on Canada thistle. Exposure of conidia of A. cirsinoxia to up to five intermittent leaf wetness cycles (30 min wet/30 min dry) reduced germination, appressoria formation, and leaf penetration, but conidia remained infective after all cycles. Exposure to cycles of 4 h dew/20 h dry was most detrimental to infection, compared with a 72 h dry period or cycles of 1 h dew/23 h wet or 2 h dew/22 h wet, indicating greater sensitivity of more fully germinated conidia to drying. Such nonspecific temperature requirements and survival during repeated intermittent leaf wetness periods are beneficial characteristics for a bioherbicide. However, high moisture requirements for infection may limit the potential of using A. cirsinoxia to control Canada thistle in the semi-arid prairie provinces of Canada.

Colonization of tomato root by a non-pathogenic strain of Fusarium oxysporum

A strain of non-pathogenic Fusarium oxysporum Schlecht. emend, Snyd. & Hans. has been selected for its capacity to reduce the incidence of Fusarium wilt of tomato. Among the possible modes of action of this strain, competition with the pathogen for the colonization of the root surface and tissues has been proposed. In order to study the pattern of root colonization, young Lycopersicon etculentum Miller (tomato) plants grown in a nutrient solution were inoculated by a suspension of F. oxysporum microconidia and processed at time-intervals for microscopic observations. The fungal strain was transformed with the Gus reporter gene to facilitate the observations. Within 24 h of inoculation the root surface was colonized by a dense network of hyphae, with the exception of the apex, which was colonized only after 48 h. A few hyphae were observed penetrating into the epidermis, leading to the internal colonization of the root cortex. This colonization was always discontinuous, since defence reactions of the plant limited the extension of the fungus. The barrier formed by thickenings and coilings of the cell walls and hypertrophied cells was most frequently observed in the external cortex and, sometimes, deeper in the internal cortex, close to the vessels which were never colonized. Typical defence reactions such as wall appositions, intercellular plugging and intracellular osmiophilic deposits, were frequently observed. This is the first report, based on microscopic observations, of the capacity of a non-pathogenic strain of F. oxysporum to colonize roots of tomato.