Fitness of three Fusarium pathogens of wheat

Crown rot and head blight of wheat are caused by the same Fusarium species. To better understand their biology, this study has compared 30 isolates of the three dominant species using 13 pathogenic and saprophytic fitness measures including aggressiveness for the two diseases, saprophytic growth and fecundity and deoxynivalenol (DON) production from saprophytic colonization of grain and straw. Pathogenic fitness was generally linked to DON production in infected tissue. The superior crown rot fitness of Fusarium pseudograminearum was linked to high DON production in the stem base tissue, while Fusarium culmorum and Fusarium graminearum had superior head blight fitness with high DON production in grains. Within each species, some isolates had similar aggressiveness for both diseases but differed in DON production in infected tissue to indicate that more than one mechanism controlled aggressiveness. All three species produced more DON when infecting living host tissue compared with saprophytic colonization of grain or straw, but there were significant links between these saprophytic fitness components and aggressiveness. As necrotrophic pathogens spend a part of their life cycle on dead organic matter, saprophytic fitness is an important component of their overall fitness. Any management strategy must target weaknesses in both pathogenic fitness and saprophytic fitness.     

Pathogenic Variation of Fusarium Isolates Associated with Head Blight of Wheat in Australia

This paper examines the level of pathogenic diversity in Australian Fusarium pseudograminearum and Fusarium graminearum isolates for head blight from the assessment of 51 wheat germplasm lines, barley, triticale, rye, maize and sorghum plants. A set of nine putative wheat differentials were selected and assessed with 10 F. graminearum and 12 F. pseudograminearum isolates. Isolates of both species were pathogenic on all the wheat germplasm lines, barley triticale and rye. The isolates differed largely in a quantitative way with only small differential effects and were statistically demarcated into three pathogenicity groups: low, intermediate and high. Such distribution patterns suggest that wheat germplasm lines employ different resistance mechanisms to each group of isolates and the three pathogenicity groups may have different mechanisms controlling pathogenicity. The aggressiveness of F. graminearum and F. pseudograminearum isolates on the wheat germplasm lines were marginally correlated (r = 0.40). Durum wheats were ranked as the most susceptible while Sumai 3, Ituo Komugi, Sotome A, Sotome and Nobeokabouzu komugi were consistently grouped as resistant by both species. These findings reiterate the need to consider pathogen variability in the screening, selection and improvement of resistance to head blight in wheat.     

Entomogenous Fusarium species

Fusarium species are known for their abundance in nature and their diverse associations with both living and dead plants and animals. Among animals Fusarium is found primarily in relationship with insects. This literature review of the past 50 years includes both non-pathogenic and pathogenic relationships between Fusarium and insects. Special attention is given to the host range, particularly between plant- and insect-hosts, and to the possible microbial potential of the fungus to control insect pests. Correct classification of this fungus has been difficult because of its diverse and non-uniform morphological features. However, by now a usable and reliable taxonomic system has been developed. The fungus can be easily cultured and mass produced. Among the non-pathogenic associations mutualism and allotrophy are found between Fusarium and wood-inhabiting and flour beetles, respectively, enhancing development and production of beetle larvae. Some insects contribute to the dispersal of the fungus in the environment by means of spore passage through their guts. Plant-pathogenic Fusarium species gain access to host tissue by plant-feeding insects. A large number of Fusarium spp. are entomopathogenic; some are weak, facultative pathogens, especially of the lepidopteran and coleopteran orders, and they will colonize their dead hosts as saprophytes. In a few cases pathogenicity to both plant and insect by one isolate was found. Strong pathogens were reported primarily from homopterans and dipterans from field observations of natural mortalities as well as from pathogenicity tests. Potential Fusarium isolates which cause high insect mortalities also show high host specificity and no damage to crop plants. The question of host invasion has been addressed by few investigators. Entrance of the fungus via the oral route, oviposition tubes, wounds, or ectoparasitic activity, were stated, but no claim for penetration of the insect cuticle. Mycotoxins, such as trichothecenes (T-2) and other secondary metabolites, contributed to mortalities of termites, mealworms, flour beetles, maize borers and blow flies, while zearalenone (F-2) exhibited a beneficial effect on egg production in flour beetles and a detrimental effect on fecundity in mammals. Studies on adverse effects of the fungus on beneficial organisms (including mammals and plants) revealed that both harmful as well as safe Fusarium isolates exist in nature. Highly host-specific and strongly entomopathogenic Fusarium isolates should be more extensively studied and tested for their possible use in biological control.     

Response of endophytic bacterial communities in banana tissue culture plantlets to Fusarium wilt pathogen infection

Endophytic bacteria reside within plant hosts without having pathogenic effects, and various endophytes have been found to functionally benefit plant disease suppressive ability. In this study, the influence of banana plant stress on the endophytic bacterial communities, which was achieved by infection with the wilt pathogen Fusarium oxysporum f. sp. cubense, was examined by cultivation-independent denaturing gradient gel electrophoresis analysis of 16S ribosomal DNA directly amplified from plant tissue DNA. Community analysis clearly demonstrated increased bacterial diversity in pathogen-infected plantlets compared to that in control plantlets. By sequencing, bands most similar to species of Bacillus and Pseudomonas showed high density in the pathogen-treated pattern. In vitro screening of the isolates for antagonistic activity against Fusarium wilt pathogen acquired three strains of endophytic bacteria which were found to match those species that obviously increased in the pathogen infection process; moreover, the most inhibitive strain could also interiorly colonize plantlets and perform antagonism. The evidence obtained from this work showed that antagonistic endophytic bacteria could be induced by the appearance of a host fungal pathogen and further be an ideal biological control agent to use in banana Fusarium wilt disease protection.     

Changing fitness of a necrotrophic plant pathogen under increasing temperature

Warmer temperatures associated with climate change are expected to have a direct impact on plant pathogens, challenging crops and altering plant disease profiles in the future. In this study, we have investigated the effect of increasing temperature on the pathogenic fitness of Fusarium pseudograminearum, an important necrotrophic plant pathogen associated with crown rot disease of wheat in Australia. Eleven wheat lines with different levels of crown rot resistance were artificially inoculated with F. pseudograminearum and maintained at four diurnal temperatures 15/15°C, 20/15°C, 25/15°C and 28/15°C in a controlled glasshouse. To quantify the success of F. pseudograminearum three fitness measures, these being disease severity, pathogen biomass in stem base and flag leaf node, and deoxynivalenol (DON) in stem base and flag leaf node of mature plants were used. F. pseudograminearum showed superior overall fitness at 15/15°C, and this was reduced with increasing temperature. Pathogen fitness was significantly influenced by the level of crown rot resistance of wheat lines, but the influence of line declined with increasing temperature. Lines that exhibited superior crown rot resistance in the field were generally associated with reduced overall pathogen fitness. However, the relative performance of the wheat lines was dependent on the measure of pathogen fitness, and lines that were associated with one reduced measure of pathogen fitness did not always reduce another. There was a strong correlation between DON in stem base tissue and disease severity, but length of browning was not a good predictor of Fusarium biomass in the stem base. We report that a combination of host resistance and rising temperature will reduce pathogen fitness under increasing temperature, but further studies combining the effect of rising CO₂ are essential for more realistic assessments.     

Production and fitness of Fusarium pseudograminearum inoculum at elevated carbon dioxide in FACE

Rising atmospheric carbon dioxide (CO₂) concentration is increasingly affecting food production but how plant diseases will influence production and quality of food under rising CO₂ is not well understood. With increased plant biomass at high CO₂ the stubble‐borne fungal pathogen Fusarium pseudograminearum causing crown rot (CR) of wheat may become more severe. We have studied inoculum production by Fusarium using fungal biomass per unit wheat stubble, stem browning from CR and the saprophytic fitness of Fusarium strains isolated from two wheat varieties grown in 2007 and 2008 at ambient and elevated CO₂ in free‐air CO₂ enrichment (FACE) with or without irrigation and once in a controlled environment. Fungal biomass, determined using primers for fungal ribosomal 18s and the TRI5 gene, increased significantly at elevated CO₂ in two of the three studies. Stem browning increased significantly at elevated CO₂ in the 2007 FACE study. At elevated CO₂ increased stem browning was not influenced by irrigation in a susceptible variety but in a resistant variety stem browning increased by 68% without irrigation. Wheat variety was significant in regression models explaining stem browning and Fusarium biomass but pathogen biomass at the two CO₂ levels was not significantly linked to stem browning. Fusarium isolates from ambient and elevated CO₂ did not differ significantly in their saprophytic fitness measured by the rate of colonization of wheat straw. We show that under elevated CO₂Fusarium inoculum in stubbles will be amplified from increased crop and pathogen biomass while unimpeded saprophytic fitness will retain its effectiveness. If resistant varieties cannot completely stop infection, Fusarium will rapidly colonize stubble to further increase inoculum once the crop is harvested. Research should move beyond documenting the influence of elevated CO₂ to developing disease management strategies from improved knowledge of pathogen biology and host resistance under rising CO₂.     

Fusarium solani species complex isolates conspecific with Fusarium solani f. sp. cucurbitae race 2 from naturally infected human and plant tissue and environmental sources are equally virulent on plants, grow at 37 degrees C and are interfertile

In a previous taxonomic study based on multilocus sequencing of Fusarium from clinical specimens and hospital environments, the most common lineage was Fusarium solani species complex group 1 (FSSC 1) which is conspecific with F. solani f. sp. cucurbitae race 2, a pathogen of cucurbit fruits. The aims of our study were to determine if clinical and environmental isolates of FSSC 1 are plant pathogens and members of the same biological species as cucurbit isolates, and to determine if all isolates can germinate, grow and sporulate at 37 degrees C. Isolates from the different sources did not differ in virulence on zucchini fruits. All FSSC 1 isolates were pathogenic and produced more rot than FSSC isolates from plant hosts other than cucurbits. Both mating types were found among isolates from each of the sources, and all isolates were sexually compatible with cucurbit isolates. All isolates germinated, grew and sporulated at 37 degrees C. This is the first report in which plant pathogenicity has been verified for a collection of human clinical isolates. Our data are consistent with the hypothesis that all FSSC 1 isolates, regardless of source, are a single biological species, equally virulent plant pathogens and tolerant of the human body temperature.     

Be flexible and adapt easily—The great role of plasticity relative to genetic variation for aggressiveness of Fusarium culmorum isolates

The phenotypic variation in an array of pathogen isolates in natural environments can be partitioned into genotypic variation and environmental plasticity. The present study uses a mixed-model approach to partition the relative contribution of both factors among isolates of Fusarium culmorum from natural field populations in various environments. Twenty-eight and 38 isolates from an international collection were phenotyped for aggressiveness and deoxynivalenol (DON) accumulation across two locations during the years 2015 and 2016, respectively, on four winter type cereals as hosts: bread wheat, durum wheat, triticale and rye, thus providing 16 environments. Aggressiveness, measured as Fusarium head blight (FHB) severity, was assessed by visually rating the symptoms of all isolates on infected hosts, and for 10 isolates, additionally the mycotoxin deoxynivalenol (DON) was measured in the grain after harvest. Despite significant genotypic variation among the isolates, the interactions with years and locations explained the largest proportion of variance which disentangled the overwhelming role of plasticity. Host-by-isolate interaction was not significant and no significant (p < .001) change in the ranking of isolates from one host to another was detected. As the main factor of plasticity was isolate-by-year interaction, this implies that seasonal changes might be an important evolutionary driver in F. culmorum populations.     

Development of PCR assay based on ITS2 rDNA polymorphism for the detection and differentiation of Fusarium sporotrichioides

A polymerase chain reaction assay was developed for detection of Fusarium sporotrichioides, a plant pathogen in many parts of the world. Based on small nucleotide differences in ITS2 (Internal Transcribed Spacer) rDNA of our local isolate of F. sporotrichioides (Accession No. AY510069) and other isolates found in NCBI/GeneBank database, species specific primer FspITS2K was selected. Primer pair FspITS2K and P28SL amplified a fragment of 288 bp containing a portion of ITS2 and 28S rDNA of all the F. sporotrichioides isolates tested, originated from different hosts and regions of the world but did not amplify any other species of Fusarium and plant's DNA. To use the PCR assay in seed health testing, a protocol was setup for the rapid and effective preparations of fungal DNA from wheat seeds. The method developed may be useful for the rapid detection and identification of F. sporotrichioides both from culture and from plant tissue.     

Impact of aggressiveness of <Emphasis Type="Italic">Fusarium graminearum</Emphasis> and <Emphasis Type="Italic">F. culmorum</Emphasis> isolates on yield parameters and mycotoxin production in wheat

Plant-associated isolates from Fusarium graminearum and F. culmorum were inoculated on wheat in field experiments in 2007 and 2008 to ascertain their influence on fungal colonization of the ears, as well as mycotoxin contamination (deoxynivalenol, DON; nivalenol, NIV; zearalenone, ZEA) and yield parameters in the mature crop after inoculation with or without irrigation. The isolates were assigned to four different groups of aggressiveness on the basis of pathogenic symptom development and mycotoxin production in vitro. Increased levels of trichothecene-producing Fusarium DNA in the ears indicated a successful inoculation of the plants, which resulted in increased DON content in the wheat kernels in 2007. Dry conditions at anthesis markedly suppressed fungal colonization as well as mycotoxin accumulation. However, due to precipitation during the ripening period, yield and thousand-kernel weight were similar whether or not irrigation was applied at the time of inoculation. The level of aggressiveness among the isolates as determined in vitro was not reflected in the field experiment. The activity of the extracellular invertase in developing ears increased as a plant response to pathogen infection, especially when the plants were irrigated at the time of inoculation. In 2008, the Fusarium inoculation of wheat heads did not cause fungal growth and mycotoxin contamination in the grain, because of the dry weather conditions that occurred over the entire period of anthesis and ripening. The risk of future mycotoxin contamination in grains was discussed based on climate change prognosis.     

In vitro assessment of Fusarium head blight spp. on wheat cultivars

Aggressiveness in four isolates of Fusarium head blight (FHB) species (F. culmorum, F. solani, F. verticillioides and F. equiesti) was studied in vitro on six wheat cultivars using a modified Petri-dish test. Results showed differences between cultivars inoculated with FHB isolates and control for three aggressiveness criteria: germination rate reduction, standardised area under disease progress curve (AUDPC_standard), and coleoptile length reduction. Regarding AUDPC_standard and Petri-dish aggressiveness index, significant differences were detected among fungal isolates. The other two aggressiveness criteria: germination rate reduction and coleoptile length reduction did not distinguish between FHB isolates. The Petri-dish test was repeatable and stable method to assess aggressiveness of four FHB species for all tested wheat cultivars. The current study confirmed the suitability of in vitro modified Petri-dish method to be used as fast and reliable test to analyse aggressiveness in FHB species.     

Molecular Genetic Diversity and Variation for Aggressiveness in Populations of Fusarium graminearum and Fusarium culmorum Sampled from Wheat Fields in Different Countries

Fusarium graminearum and Fusarium culmorum are the major pathogenic organisms causing head blight in small-grain cereals. Natural epidemics may result in severe yield losses, reduction in quality, and contamination of the grain by mycotoxins. The genetic diversity of four field populations of F. graminearum from Germany, Hungary, and Canada, and one population of F. culmorum from Russia was investigated by polymerase chain reaction (PCR)-based fingerprinting. Additionally, a world-wide collection and two of the F. graminearum populations were analysed for their aggressiveness on young plants of winter rye in the greenhouse. The number of isolates analysed per population varied from 25 to 70. Significant quantitative variation for aggressiveness was observed within each of the individual field populations amounting to the same range as the world-wide collection. Abundant variation within populations was also revealed by DNA markers. The F. graminearum populations from Hungary and Winnipeg displayed the least genotypic diversity, the two German F. graminearum populations and the Russian F. culmorum population were highly diverse. Population diversity, however, followed no spatial pattern among samples within a German field for aggressiveness or molecular markers. For F. graminearum , sexual recombination is the most likely explanation for the large genetic diversity within field populations. Asexual and/or parasexual recombination, and balancing selection caused by the periodic alternation between the saprophytic and parasitic phase might play an additional role and account for the variation within the F. culmorum population. For improving Fusarium resistance, several resistance genes of different sources should be combined to avoid an unspecific adaptation of the genetically variable pathogen to an increased resistance level.     

Fusarium culmorum Infection of Barley Seedlings: Correlation between Aggressiveness and Deoxynivalenol Content

Fusarium culmorum is a serious plant pathogen, especially on cereals. The production of deoxynivalenol (DON) by F. culmorum is believed to play a role in pathogenesis. This relationship has been almost exclusively studied in connection with head blight. The present paper reports the first finding of DON in cereal seedlings infected with F. culmorum . A pathogenicity test was performed, including 70 isolates of this pathogen from different sites within northern and central Europe. All isolates caused disease on barley seedlings. For 15 isolates with varying aggressiveness, the DON content in the 19-day-old-barley seedlings was determined. There was a significant correlation between DON concentration and disease index. The aggressiveness of two outlying isolates with very low DON production is discussed. The results indicate that for F. culmorum isolates of the DON chemotype, production of this toxin influences the aggressiveness of the isolates towards barley seedlings.     

Migrate or evolve: options for plant pathogens under climate change

Findings on climate change influence on plant pathogens are often inconsistent and context dependent. Knowledge of pathogens affecting agricultural crops and natural plant communities remains fragmented along disciplinary lines. By broadening the perspective beyond agriculture, this review integrates cross‐disciplinary knowledge to show that at scales relevant to climate change, accelerated evolution and changing geographic distribution will be the main implications for pathogens. New races may evolve rapidly under elevated temperature and CO₂, as evolutionary forces act on massive pathogen populations boosted by a combination of increased fecundity and infection cycles under favourable microclimate within enlarged canopy. Changing geographic distribution will bring together diverse lineages/genotypes that do not share common ecological niche, potentially increasing pathogen diversity. However, the uncertainty of model predictions and a lack of synthesis of fragmented knowledge remain as major deficiencies in knowledge. The review contends that the failure to consider scale and human intervention through new technology are major sources of uncertainty. Recognizing that improved biophysical models alone will not reduce uncertainty, it proposes a generic framework to increase focus and outlines ways to integrate biophysical elements and technology change with human intervention scenarios to minimize uncertainty. To synthesize knowledge of pathogen biology and life history, the review borrows the concept of ‘fitness’ from population biology as a comprehensive measure of pathogen strengths and vulnerabilities, and explores the implications of pathogen mode of nutrition to fitness and its interactions with plants suffering chronic abiotic stress under climate change. Current and future disease management options can then be judged for their ability to impair pathogenic and saprophytic fitness. The review pinpoints improving confidence in model prediction by minimizing uncertainty, developing management strategies to reduce overall pathogen fitness, and finding new sources of data to trawl for climate signatures on pathogens as important challenges for future research.     

Intra- and inter-species variability of the aggressiveness in four Fusarium head blight species on durum wheat plants detected in an in vitro Petri-dish assay

Abstract

Aggressiveness assessment of Fusarium head blight (FHB) is crucial for explaining the interaction in FHB–wheat association. Currently, in vitro tools have proved to be very useful in identifying disease responses in wheat to FHB infection. To update our knowledge, intra- and inter-species variability of the aggressiveness was studied in 16 isolates of four FHB species (F. culmorum, F. solani, F. verticillioides and F. equiseti) using an in vitro Petri-dish test. Three aggressiveness criteria, germination rate reduction, standardised area under disease progress curve (AUDPC_standard) and coleoptile length reduction, were evaluated on a durum wheat cultivar showing a moderate level of quantitative resistance. Regarding AUDPC_standard, intra- and inter-species variability was detected. The other two aggressiveness criteria did not distinguish isolates within and among species. The three aggressiveness criteria were not significantly correlated. However, it was not possible to cluster the isolates based on their species origins because of similarity in pathogenic level among the 16 fungal isolates. The results provide evidence that there is a differential interaction between quantitative resistance genes in wheat plants and the 16 tested FHB isolates. The values of AUDPC_standard showed significant correlation with both: disease incidence and disease severity obtained under controlled conditions (r?=?0.627** and r?=?0.539*, respectively), and disease incidence generated by field conditions (r?=?0.525*). AUDPC_standard could be of potential use in evaluating the aggressiveness of FHB in adult wheat plants under controlled and field conditions.     

Pathogenicity of Fusarium graminearum Schwabe isolates from Poland towards seedlings of cereal species

Ten isolates ofFusarium graminearum Schwabe originating from diseased cereal plants and kernels were tested for pathogenicity to various cultivars of wheat, rye, triticale and oats. The isolates varied greatly in their pathogenicity to the seedlings of the species, and were most pathogenic to rye and triticale, less pathogenic to barley and wheat and least pathogenic to oats.     

Plant defense response against Fusarium oxysporum and strategies to develop tolerant genotypes in banana

Soil-borne fungal pathogen, Fusarium oxysporum causes major economic losses by inducing necrosis and wilting symptoms in many crop plants. Management of fusarium wilt is achieved mainly by the use of chemical fungicides which affect the soil health and their efficiency is often limited by pathogenic variability. Hence understanding the nature of interaction between pathogen and host may help to select and improve better cultivars. Current research evidences highlight the role of oxidative burst and antioxidant enzymes indicating that ROS act as an important signaling molecule in banana defense response against Fusarium oxysporum f.sp. cubense. The role of jasmonic acid signaling in plant defense against necrotrophic pathogens is well recognized. But recent studies show that the role of salicylic acid is complex and ambiguous against necrotrophic pathogens like Fusarium oxysporum, leading to many intriguing questions about its relationship between other signaling compounds. In case of banana, a major challenge is to identify specific receptors for effector proteins like SIX proteins and also the components of various signal transduction pathways. Significant progress has been made to uncover the role of defense genes but is limited to only model plants such as Arabidopsis and tomato. Keeping this in view, we review the host response, pathogen diversity, current understanding of biochemical and molecular changes that occur during host and pathogen interaction. Developing resistant cultivars through mutation, breeding, transgenic and cisgenic approaches have been discussed. This would help us to understand host defenses against Fusarium oxysporum and to formulate strategies to develop tolerant cultivars.     

Biotic relation between Fusarium oxysporum schlecht. and fungi isolated from the substrate of Stewartia pseudocamellia (max.)

The effect of the fungi community colonizing the substrate of Stewartia plants on the growth of Fusarium oxysporum Schlecht. was investigated. The soil samples from 2 years old Stewartia cuttings were taken for analysis in the second decade of October 2002 and 2003, when the symptoms of disease appeared on Stewartia plant. Fungi were isolated from the substrate using the sand method (Mańka K. 1974). Fusarium oxysporum were isolated from root system of Stewartia pseudocamellia plant. The isolates selected for the investigation proved pathogenic influence on Stewartia in an infection experiment. For estimation of biotic effect of saprobiotic fungi community on Stewartia pathogen F. oxysporum the biotic series method (Mańka K. 1974) was applied. The results showed that species of genera: Penicillium and Trichoderma were the most frequent in the community of fungi in the substrate. Both fungi communities of these fungi could not limit the growth of investigated pathogen F. oxysporum. It showed negative ABSTRACT biotic effect. The strongest antagonistic effect displayed Trichoderma viride and Trichoderma harzianum.