Pathogen Aggressiveness Research Articles

Density-related effects on the infectivity and aggressiveness of a sterilising smut in a wild population of Digitaria sanguinalis.

Understanding host-pathogen evolutionary dynamics needs characterisation and quantification of processes occurring at many spatiotemporal scales. With this aim, the effects of smut on a naturally infected population of the summer annual Digitaria sanguinalis were followed for 4years in an uncropped field. The main purpose of the study was to quantify the effects of within-population density on the infectivity and the aggressiveness of the pathogen in a range of densities that occurred naturally. The infectivity-related variable measured was the proportion of smutted plants at the end of each growing season; proportions were analysed using a generalised linear model with a binomial distribution considering the year, the density and their interaction as effects. The aggressiveness-related variables chosen were the number of smutted inflorescences per plant and per area, obtained over the last 2years; they were analysed by means of ancova considering disease status (seeded or smutted), year, density and all the interactions between them. Although the disease is monocyclic, results showed clearly that infectivity increased with plant density. The number of inflorescences per plant was 1.5 times higher in smutted plants than in healthy plants throughout the range of densities. This variable declined when density increased, but as the infectivity increased at a higher rate, the aggressiveness also increased with density. The surprising results on infectivity are discussed in the context of current knowledge of plant-pathogen interaction dynamics, as well as neighbour effects on pathogen aggressiveness. Moreover, the results could be useful to develop weed biological control strategies.

Genetic Structure and Aggressiveness of Rhizoctonia solani AG1‐IA, the Cause of Sheath Blight of Rice in Southern China

AbstractOne hundred and eighty isolates of Rhizoctonia solani AG1‐IA, the causal agent of rice sheath blight, were obtained from six locations in southern China. The genetic structure of R. solani isolates was investigated using random amplified polymorphic DNA (RAPD) markers, and a considerable genetic variation among R. solani isolates was observed. Most of the genetic diversity was distributed within populations, rather than among them. The distribution pattern of the genetic variation of R. solani appears to be the result of high gene flow (Nm) and low‐genetic differentiation among populations. The aggressiveness of R. solani was visually assessed by rice seedlings of five different cultivars in the glasshouse. All isolates tested were found to induce significantly different levels of disease severity, reflecting considerable variation in aggressiveness. The isolates were divided into highly virulent, moderately virulent and weakly virulent groups, and the moderately virulent isolates were dominant in R. solani population. No significant correlation was observed among the genetic similarity, pathogenic aggressiveness and geographical origins of the isolates. Information obtained from this study may be useful for breeding for improved resistance to sheath blight.

Aggressiveness, diversity and distribution of Alternaria brassicae isolates infecting oilseed Brassica in India

Alternaria brassicae (Berk.) Sacc., a necrotrophic fungus devastating oilseed Brassicacrops in India, causes up to 47% reduction in seed yield. Morphological characteristics of different isolates revealed variation in growth, shape and pigmentation of colony, conidial measurements and number of septa. Conidial length varied from 106.7 to 285.9 µm, width 33.5 to 57.0 µm and beak length 41.4 to 180.0 µm. Number of horizontal septa varied from 3.2 to 8.0 and vertical 0.3 to 1.4. Different synthetic media showed profound variation in mycelial growth of A. brassicae isolates and the poor sporulation indicated that the fungus requires some organic sources of nutrition for better growth and sporulation. The degree of sporulation of A. brassicae isolates is a function of nutrition proved for the first time. Per cent inhibition of mycelial growth showed diverge among A. brassicae isolates, which may be due to the variation towards fungicidal sensitivity among isolates. Pathogen aggressiveness study demonstrated the existence of considerable variation in tolerance ofBrassica species to A. brassicae, which is proved with the location specific disease severity. Key words: Variability, Alternaria brassicae, fungicides sensitivity, aggressiveness,Brassica.

Variation in aggressiveness among isolates of Cercospora zeae-maydis inlow-, medium- and high-altitude maize agro-ecologies of Tanzania

The objective of this study was to assess the aggressiveness and adaptation of Cercospora zeae-maydis the causal of grey leaf spot of maize in low-, mid- and high-altitude maize agro-ecologies. Field and laboratory studies were carried out to assess components of resistance, pathogen aggressiveness, sporulation, culture variations and morphological features of C. zeae-maydis isolates. Pathogen aggressiveness and morphological features differed among 30 isolates of C. zeae-maydis collected at different altitudes (p ≤ 0.05). Conidiophores per stroma between isolates ranged from 3.3 to 9.2 μ, conidiophore length 22.4–93.2 μ and colony growth rate ranged from 1.2 to 3.9 mm−1. High-altitude isolates grew 0.25 mm per day faster (9.3%), formed 1.1 cm longer lesions (25.9%), produced 1.1 × 104 more spores per cm2 of the lesion (13.6%) and formed symptoms 1 day earlier than low-altitude isolates. Prevalence of high numbers of aggressive isolates in low-altitude agro-ecologies indicates increased adaptation of C. zeae-maydis to warmer conditions.

Endophytic fungi associated with Fallopia japonica (Polygonaceae) in Japan and their interactions with Puccinia polygoni-amphibii var. tovariae, a candidate for classical biological control

Fallopia japonica (Polygonaceae), or Japanese knotweed, is now spreading globally, causing serious problems in Europe and North America in both natural and urban habitats. There is an urgent need for alternative management solutions, and classical biological control, using coevolved natural enemies found in the native range, is currently being investigated. Here, we isolated fungal endophytes from F. japonica in Japan, its natural habitat, to find endophytes that might increase the virulence of a coevolved rust pathogen, Puccinia polygoni-amphibii var. tovariae. A total of 1581 fungal endophytes were recovered from F. japonica and classified into 15 taxa. Five genera (Colletotrichum, Pestalotiopsis, Phoma, Phomopsis, and Alternaria) were dominant as endophytes in F. japonica. A greenhouse study of the dominant endophyte–pathogen interactions revealed three types of reactions: suppressive, synergistic, and neutral. In particular, one Phomopsis isolate – closely related to Diaporthe medusaea, based on ITS sequences – promoted the pathogenic aggressiveness of P. polygoni-amphibii var. tovariae and, therefore, this interaction is potentially useful to increase the effectiveness of the rust fungus as a biological control agent of F. japonica in its invasive range.

Quantification of the aggressiveness of a foliar pathogen, Colletotrichum gloeosporioides, responsible for water yam (Dioscorea alata) anthracnose

In the absence of a gene-for-gene relationship between a pathogen and its host, knowledge about aggressiveness is crucial to characterize novel pathogen populations that potentially emerge in agricultural pathosystems. Information about pathogen aggressiveness is also critical when establishing representative panels of pathogen isolates to test host resistance and in mapping quantitative trait loci involved in the host resistance. In this study, we focused on the fungus C. gloeosporioides that causes necrosis on the aerial part of one of its host plants, Dioscorea alata, and identified the in vitro conditions required to assess fungal aggressiveness on this host. Our main purpose was to convert the necrosis area development into a unique index for quantifying pathogen aggressiveness. The ‘Ag’ index described here has two advantages. First, it integrates the variance of symptom evolution curves to estimate the lesion development rates (initial and secondary) and the maximal necrosis area. Secondly, the new index takes two different symptoms commonly observed when inoculating D. alata leaves with C. gloeosporioides into account, one correlated with high leaf colonisation efficiency and the other with low colonisation efficiency. The weights accorded to each symptom in the index were proportional to leaf colonisation efficiency. We propose a framework for the acquisition of this index that has been designed to be conveniently combined with the routine bioassays required to establish representative panels of pathogen isolates. The general framework for the construction of this index can be broadly applied to diseases with necrotic symptoms.

Aggressiveness of eight Didymella rabiei isolates from domesticated and wild chickpea native to Turkey and Israel, a case study

Ascochyta blight, caused by Didymella rabiei, affects both domesticated chickpea and its congeneric wild relatives. The aim of this study was to compare the aggressiveness of D. rabiei isolates from wild and domesticated Cicer spp. in Turkey and Israel on wild and domesticated hosts from both countries. A total of eight isolates of D. rabiei sampled from C. pinnatifidum, C. judaicum and C. arietinum in Turkey and Israel was tested on two domesticated chickpea cultivars and two wild Cicer accessions from Turkey and Israel. Using cross-inoculation experiments, we compared pathogen aggressiveness across the different pathogen and host origin combinations. Two measures of aggressiveness were used, incubation period and relative area under the disease progress curve. The eight tested isolates infected all of the host plants, but were more aggressive on their original hosts with one exception; Turkish domesticated isolates were less aggressive on their domesticated host in comparison to the aggressiveness of Israeli domesticated isolates on Turkish domesticated chickpea. C. judaicum plants were highly resistant against all of the isolates from different origins except for their own isolates. Regardless of the country of origin, the wild isolates were highly aggressive on domesticated chickpea while the domesticated isolates were less aggressive on the wild hosts compared with the wild isolates. These results suggest that the aggressiveness pattern of D. rabiei on different hosts could have been shaped by adaptation to the distinct ecological niches of wild vs. domesticated chickpea.

Magnaporthe oryzae Populations Adapted to Finger Millet and Rice Exhibit Distinctive Patterns of Genetic Diversity, Sexuality and Host Interaction

In this study, host-specific forms of the blast pathogen Magnaporthe oryzae in sub-Saharan Africa (SSA) were characterised from distinct cropping locations using a combination of molecular and biological assays. Finger millet blast populations in East Africa revealed a continuous genetic variation pattern and lack of clonal lineages, with a wide range of haplotypes. M. oryzae populations lacked the grasshopper (grh) element (96%) and appeared distinct to those in Asia. An overall near equal distribution (47-53%) of the mating types MAT1-1 and MAT1-2, high fertility status (84-89%) and the dominance of hermaphrodites (64%) suggest a strong sexual reproductive potential. Differences in pathogen aggressiveness and lack of cultivar incompatibility suggest the importance of quantitative resistance. Rice blast populations in West Africa showed a typical lineage-based structure. Among the nine lineages identified, three comprised ~90% of the isolates. Skewed distribution of the mating types MAT1-1 (29%) and MAT1-2 (71%) was accompanied by low fertility. Clear differences in cultivar compatibility within and between lineages suggest R gene-mediated interactions. Distinctive patterns of genetic diversity, sexual reproductive potential and pathogenicity suggest adaptive divergence of host-specific forms of M. oryzae populations linked to crop domestication and agricultural intensification.

Evaluation of host susceptibility, pathogen aggressiveness and sporangial survival in soil as factors affecting incidence of potato tuber infection by Phytophthora infestans in Ecuador

Incidence of potato tuber infection by Phytophthora infestans is low in Ecuador. Factors considered to potentially affect the incidence of tuber infection include pathogen aggressiveness, host resistance, direct suppression from biological and chemical characteristics of soil acting on pathogen propagules, and exclusion resulting from soil structure and high ridging. In this study, we tested the hypothesis that low incidence could be due to reduced pathogen aggressiveness and/or low host susceptibility by comparing several pathogen isolates and commonly grown potato cultivars from Ecuador with isolates and cultivars from Europe, where tuber blight is known to be a problem. Additionally, in Ecuador, whole tubers and slices of common varieties were inoculated with local isolates of P. infestans to test for potential infection under Ecuadorian conditions. All isolates, regardless of origin, caused tuber infection. The aggressiveness of isolates varied, but this was both between and among Ecuadorian and Swedish isolates and it was not possible to establish a clear difference in the degree of infection based on isolate origin, or origin of potato variety. In general, we found no evidence to suggest that low aggressiveness of the pathogen or extreme resistance of the host explains low incidence of tuber blight in Ecuador. Therefore, we conclude that low incidence of tuber blight in Ecuador is probably caused by soil factors. Furthermore, exclusion due to soil structure and high hilling may play an important role as a preliminary soil infectivity study demonstrated that P. infestans sporangia were infective in six Ecuadorian field soils for at least 15 days.

Biochemical Evaluation of Resistance Responses of Potato to Different Isolates of Alternaria solani

The resistance phenotypes of nine potato cultivars to five isolates of Alternaria solani, causal agent of early blight, were studied after inoculation and growth under greenhouse conditions. We identified potato cultivars with both susceptible and resistant phenotypes as well as A. solani isolates with varying degrees of aggressiveness. Two potato cultivars and two pathogen isolates were selected for biochemical analysis of phenol production and peroxidase activity after inoculation. Phenol compounds were evaluated 2, 4, 6, and 8 days after inoculation, while peroxidase activities were monitored daily for 10 days. Native polyacrylamide electrophoresis was used to identify one protein with peroxidase activity in extracts taken 6 days after inoculation. Significantly higher peroxidase activity as well as total phenol content in potato was correlated with resistance in the Iranian potato cultivar Diamond. Variability of responses within the same cultivar to different isolates of A. solani suggests genotypic diversity between isolates that results in phenotypic diversity for pathogen aggressiveness.

Screening of certain barley lines for resistance to root rot disease caused by Fusarium graminearum

This investigation was conducted in 2005/2006 and 2006/2007 to test 235 barley lines plus two varieties Giza 127 and Giza 128 for resistance and susceptibility to Fusarium graminearum. All screened barley lines showed varied significant degrees of infestation to root rot pathogen. A screening system is described for identifying barley lines which are effective in controlling resistant or susceptible lines. By detecting small but consistent differences in root rot severity, the bioassay proved effective in large-scale screening for partial resistance: already 335 barley lines and two varieties have been screened. We found five groups (7.12%), 22 barley lines and both varieties are resistant (R) (8.31%); 28 barley lines are moderately resistant (MR) (19.29%); 65 barley lines are moderately susceptible (MS) (27.89%); 94 barley lines are susceptible (S) and (37.39%) 126 barley lines are highly susceptible (HS). The high degree of precision makes this an invaluable tool in the understanding of pathogen aggressiveness, host specialisation and parasitic fitness. Disease scale was strongly negative and had moderate correlation with germination (−0.309** and −0.649**) under normal and disease treatment. The correlation between yield and normal and disease treatment during two seasons was strong and negative (−0.834** and −0.847**, respectively were detected).

Plant-pathogen interactions and elevated CO2: morphological changes in favour of pathogens.

Crop losses caused by pests and weeds have been estimated at 42% worldwide, with plant pathogens responsible for almost $10 billion worth of damage in the USA in 1994 alone. Elevated carbon dioxide [ECO2] and associated climate change have the potential to accelerate plant pathogen evolution, which may, in turn, affect virulence. Plant–pathogen interactions under increasing CO2 concentrations have the potential to disrupt both agricultural and natural systems severely, yet the lack of experimental data and the subsequent ability to predict future outcomes constitutes a fundamental knowledge gap. Furthermore, nothing is known about the mechanistic bases of increasing pathogen agressiveness. In the absence of information on crop species, it is shown here that plant pathogen (Erysiphe cichoracearum) aggressiveness is increased under ECO2, together with changes in the leaf epidermal characteristics of the model plant Arabidopsis thaliana L. Stomatal density, guard cell length, and trichome numbers on leaves developing post-infection are increased under ECO2 in direct contrast to non-infected responses. As many plant pathogens utilize epidermal features for successful infection, these responses provide a positive feedback mechanism facilitating an enhanced susceptibility of newly developed leaves to further pathogen attack. Furthermore, a screen of resistant and susceptible ecotypes suggest inherent differences in epidermal responses to ECO2.

Aggressiveness and its role in the adaptation of plant pathogens

Aggressiveness, the quantitative component of pathogenicity, and its role in the adaptation of plant pathogens are still insufficiently investigated. Using mainly examples of biotrophic and necrotrophic fungal pathogens of cereals and Phytophthora infestans on potato, the empirical knowledge on the nature of aggressiveness components and their evolution in response to host and environment is reviewed. Means of measuring aggressiveness components are considered, as well as the sources of environmental variance in these traits. The adaptive potential of aggressiveness components is evaluated by reviewing evidence for their heritability, as well as for constraints on their evolution, including differential interactions between host and pathogen genotypes and trade‐offs between components of pathogenicity. Adaptations of pathogen aggressiveness components to host and environment are analysed, showing that: (i) selection for aggressiveness in pathogen populations can be mediated by climatic parameters; (ii) global population changes or remarkable population structures may be explained by variation in aggressiveness; and (iii) selection for quantitative traits can influence pathogen evolution in agricultural pathosystems and can result in differential adaptation to host cultivars, sometimes leading to erosion of quantitative resistance. Possible links with concepts in evolutionary ecology are suggested.

Quantitative evolution of aggressiveness of powdery mildew under two‐cultivar barley mixtures

The effects of cultivar mixtures on the evolution of aggressiveness of barley powdery mildew (Blumeria graminis f.sp. hordei) were modelled. It was found that the rate and direction of evolution of pathogen aggressiveness in a race‐non‐specific system and value at equilibrium, depends on initial resistance levels, proportions of component cultivars, autodeposition rates, the relative magnitude of the benefit of autoinfection, and the cost of alloinfection of spores. In the model, mixing cultivars in any proportions tended to reduce the aggressiveness of pathogens at equilibrium compared with pure stands, but this effect decreased when two mixture components were extremely unbalanced in proportion. Under low and medium autodeposition rates, the best control of the evolution of the pathogen was achieved by mixing two components in roughly equal proportions. The magnitude of aggressiveness at equilibrium increased as autodeposition rates increased. Though the level of initial resistance of mixture components did not have an impact on the aggressiveness of pathogens at equilibrium, it strongly influenced the transient values of aggressiveness and therefore the total amount of disease caused over an evolutionary period. The cost to the pathogen of alloinfection and benefit of autoinfection per se did not affect the final level of aggressiveness, but did affect the time to reach equilibrium. However, the ratio of the cost to the benefit did influence the final aggressiveness of the pathogen.

Fusarium langsethiae pathogenicity and aggressiveness towards oats and wheat in wounded and unwounded in vitro detached leaf assays

In vitro detached leaf assays involving artificial inoculation of wounded and unwounded oat and wheat leaves were used to investigate the potential pathogenicity and aggressiveness of F. langsethiae, which was linked recently to the production of type A trichothecenes, HT-2 and T-2 in cereals in Europe. In the first two experiments, two assays compared disease development by F. langsethiae with known fusarium head blight pathogen species each used as a composited inoculum (mixture of isolates) at 10°C and 20°C and found all fungal species to be pathogenic to oat and wheat leaves in the wounded leaf assay. In the unwounded leaf assay, F. langsethiae was not pathogenic to wheat leaves. Furthermore, there were highly significant differences in the aggressiveness of pathogens as measured by lesion length (P < 0.001). In the second two experiments, pathogenicity of individual F. langsethiae isolates previously used in the composite inoculum was investigated on three oat and three wheat varieties. The wounded leaf assay showed that all isolates were pathogenic to all oat and wheat varieties but only pathogenic towards oat varieties in the unwounded assay. Highly significant differences (P < 0.001) in lesion length were found between cereal varieties as well as between isolates in the wounded assay. Significant differences in lesion lengths (P = 0.014) were also observed between isolates in the unwounded assay. Results from the detached leaf assays suggest that F. langsethiae is a pathogen of wheat and oats and may have developed some host preference towards oats.

Differentiation of Bean pod mottle virus isolates based upon host symptoms

Accessions from Glycine, Phaseolus, and Vigna genera were screened for their reactions to different subgroups of isolates of Bean pod mottle virus (BPMV) in order to establish a differential host system. Screening results indicated that the BPMV isolates differed in pathogenic aggressiveness but not in virulence. No major resistance genes were found in soybean (Glycine max) or G. soja since all screened accessions showed mosaic or necrotic symptoms to BPMV inoculation. However, these accessions expressed differences in severity of symptoms when challenged by various BPMV isolates. The inoculation of G. tomentella accessions did not result in mosaic symptoms, and some accessions did not support systemic infection of some of the isolates. Resistance, presented as a hypersensitive reaction, was observed in some of Phaseolus and Vigna genotypes, and resistant response or susceptibility was stable to all the isolates used in the screening. In conclusion, the selected G. soja genotypes PI 407019, PI 464889A, and PI 464928, and ‘Amsoy 71’ soybean may help to separate severe (reassortant) from mild isolates of BPMV based upon their phenotypic reactions.

Variability in Morphology and Aggressiveness Among North American Vegetative Compatibility Groups of Colletotrichum coccodes

North American isolates of Colletotrichum coccodes, representing six vegetative compatibility groups (NA-VCG), were used to study morphological and pathogenic variability. The objective was to determine if variability in conidial and microsclerotial size was related to pathogenicity. Significant differences were detected in length, width, and length/width ratios of conidia as well as in the length and width of microsclerotia among the NA-VCGs. The longest and widest conidia were produced by isolates belonging to NA-VCG1 and the largest microsclerotia were produced by isolates of NA-VCG2. Conidial and microsclerotial lengths and widths also were affected significantly by type of growth medium. There was no relationship between the size of conidia and the size of microsclerotia among the NA-VCGs studied. Conidial and microsclerotial size may affect inoculum potential and survival as isolates of NA-VCG2 have been demonstrated to occur more frequently than other NA-VCGs. Aggressiveness of 17 isolates of C. coccodes representing six NA-VCG's was studied on three potato cultivars using foliar and root inoculation methods. C. coccodes infection reduced tuber weight in all cultivars with both inoculation methods although tuber weight reductions were significantly higher following root inoculations than foliar inoculations. Pathogenic aggressiveness varied among NA-VCGs. Isolates belonging to NA-VCG2 and 3 were the least aggressive on potato foliage and isolates of NA-VCG1, 2, 3, 4, and 5 produced higher microsclerotial density on all three cultivars compared with isolates of NA-VCG6. Across inoculation methods, isolates of C. coccodes belonging to NA-VCG2 and 6 were the most aggressive based on reductions in tuber weight. Umatilla Russet was the most susceptible cultivar to C. coccodes compared to other cultivars regardless of inoculation method. These results demonstrate variability in morphology and pathogenic aggressiveness among the NA-VCGs of C. coccodes but these traits are not related.

Development of a GFP-Expressing Aspergillus flavus Strain to Study Fungal Invasion, Colonization, and Resistance in Cottonseed

Cotton bolls were inoculated with a green fluorescent protein (GFP)-expressing Aspergillus flavus (strain 70) to monitor fungal growth, mode of entry, colonization of cottonseeds, and production of aflatoxins. The GFP strain and the wild-type did not differ significantly in pathogen aggressiveness as indicated by similar reductions in inoculated locule weight. GFP fluorescence was at least 10 times higher than the blue green yellow fluorescence (BGYF) produced in response to infection by A. flavus. The GFP produced by the strain made it possible to identify and monitor specific plant tissues colonized by the fungus. For example, the inner seed coat and cotyledon were colonized by the fungus within 72 h of inoculation and the mode of entry was invariably through the porous chalazal cap in intact seeds. The amount of GFP fluorescence was shown to be an indicator of fungal growth, colonization and, to some extent, aflatoxin production. The A. flavus strain expressing GFP should be very useful for rapidly identifying cotton lines with enhanced resistance to A. flavus colonization developed through genetic engineering or traditional plant breeding. In addition, development of GFP expressing A. flavus strain provides an easy and rapid assay procedure for studying the ecology, etiology, and epidemiology of cotton boll rot caused by A. flavus resulting in aflatoxin contamination.

Pathogen evolution across the agro-ecological interface: implications for disease management.

Infectious disease is a major causal factor in the demography of human, plant and animal populations. While it is generally accepted in medical, veterinary and agricultural contexts that variation in host resistance and pathogen virulence and aggressiveness is of central importance to understanding patterns of infection, there has been remarkably little effort to directly investigate causal links between population genetic structure and disease dynamics, and even less work on factors influencing host–pathogen coevolution. The lack of empirical evidence is particularly surprising, given the potential for such variation to not only affect disease dynamics and prevalence, but also when or where new diseases or pathotypes emerge. Increasingly, this lack of knowledge has led to calls for an integrated approach to disease management, incorporating both ecological and evolutionary processes. Here, we argue that plant pathogens occurring in agro-ecosystems represent one clear example where the application of evolutionary principles to disease management would be of great benefit, as well as providing model systems for advancing our ability to generalize about the long-term coevolutionary dynamics of host–pathogen systems. We suggest that this is particularly the case given that agro-ecological host–pathogen interactions represent a diversity of situations ranging from those that only involve agricultural crops through to those that also include weedy crop relatives or even unrelated native plant communities. We begin by examining some of the criteria that are important in determining involvement in agricultural pathogen evolution by noncrop plants. Throughout we use empirical examples to illustrate the fact that different processes may dominate in different systems, and suggest that consideration of life history and spatial structure are central to understanding dynamics and direction of the interaction. We then discuss the implications that such interactions have for disease management in agro-ecosystems and how we can influence those outcomes. Finally, we identify several major gaps where future research could increase our ability to utilize evolutionary principles in managing disease in agro-ecosystems.

Passage through alternative hosts changes the fitness of Fusarium graminearum and Fusarium pseudograminearum

Species of the necrotrophic fungal pathogen Fusarium that cause head blight and crown rot of cereals including wheat also infect a number of alternative host plants. This raises the prospect of more damaging pathogen strains originating and persisting as highly successful saprophytes on hosts other than wheat. The immediate impact on pathogenic (aggressiveness) and saprophytic (growth rate and fecundity) behaviour of six isolates with low, moderate or high initial aggressiveness was examined in two species of Fusarium after their passage through 10 alternative plant hosts. One passage through alternative hosts significantly reduced the pathogenic fitness of most isolates, but this change was not associated with a concomitant change in their overall saprophytic behaviour. The overall weak association between aggressiveness, fecundity and growth rate both before and after passage through the alternative hosts indicate that pathogenic and saprophytic fitness traits may be independently controlled in both Fusarium species. Thus, there was no trade-off between pathogenic and saprophytic fitness in these necrotrophic plant pathogens.