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.     

Intrapathotype diversity for aggressiveness and pathogen evolution in cultivar mixtures

ABSTRACT A model was developed and used to study the consequences of diversity for aggressiveness within pathotypes on pathogen evolution in two-component and four-component cultivar mixtures. It was assumed that, within a pathotype, a proportion of the isolates would have higher or lower spore efficacy than the average on a given host genetic background. Two situations were examined in which the pathogen can have either independent or negatively correlated values for spore efficacy on different cultivars. In the latter case, a pathogen genotype more aggressive than the average on a host genotype was always less aggressive on other host genotypes. In the simulations, isolates with greater aggressiveness relative to a host genotype were selected for and increased in frequency. However, because simple pathotypes always reproduced on the same host genotype whereas complex pathotypes were able to grow on several hosts, selection was faster for simple pathotypes. Pathotypes with two different levels of diversity for aggressiveness were compared with nondiversified pathotypes. In order to make comparisons, the effect of a 5 and 10% cost of virulence on the development of complex pathotypes was simulated. In general, increased diversity within pathotypes reduced the rate of increase of complex pathotypes in host mixtures, and this effect was stronger with greater frequencies of autodeposition of pathogen spores.     

Lateral roots of carrot have a low impact on alloinfections in cavity spot epidemic caused by <Emphasis Type="Italic">Pythium violae</Emphasis>

Carrot cavity spot, caused by a complex of Pythium species, is characterized by sunken elliptical lesions on the taproot. Recent epidemiological studies of P. violae have demonstrated the occurrence of both primary and secondary infections, with two types of secondary infection, autoinfection and alloinfection. Investigating the mechanisms underlying alloinfection and the role of carrot lateral roots, we asked whether direct physical root contact plays a role in alloinfection and whether root exudates enhance mycelial growth in soil alone. A rhizobox system was designed to differentiate the effects of each mechanism: a buffer zone created by nylon mesh was used to test the first mechanism, and young carrots with a root system similar to lateral roots were used to test the second. Alloinfections were generated in rhizoboxes via diseased taproots transplanted close to healthy, mature carrots. The nylon mesh had no significant effect on disease intensity (reflecting alloinfection), providing evidence that mycelial growth in soil contributed more to disease spread than did physical contact among roots. Nor did young carrots significantly affect alloinfection; thus root exudates had little effect on mycelial growth.     

Cultivar mixtures for the simultaneous management of multiple diseases: tan spot and leaf rust of wheat

ABSTRACT Because of differences in life histories between Puccinia triticina, a highly specialized, polycyclic, windborne pathogen with a shallow dispersal gradient, and Pyrenophora tritici-repentis, a residue-borne pathogen with a steep dispersal gradient, wheat mixtures are expected to be more effective at controlling leaf rust than tan spot. The objectives of this research were to determine the effect of two-cultivar mixtures with varying proportions and different pathogen resistance profiles on the severity of tan spot and leaf rust, to evaluate yield of the mixtures in the presence or absence of disease, and to directly compare the relative effectiveness of cultivar mixing for tan spot versus leaf rust. In a field experiment at two sites in Kansas over two growing seasons, winter wheat cvs. Jagger and 2145, which have differential resistance reactions to leaf rust and tan spot, each were planted in proportions of 0.25, 0.50, 0.75, and 1.00. Plots were inoculated with each pathogen alone, both pathogens, treated with a fungicide, or exposed to ambient conditions. For both diseases for all siteyears, severity decreased substantially on the susceptible cultivar as the proportion of that cultivar decreased in mixture. Mixtures were significantly more effective at reducing leaf rust than tan spot in three of four site-years. Mixtures generally yielded the same as the weighted mean of components in monoculture although, in two of three site-years, at least one fungicide-treated and one diseased mixture each yielded higher than expected values. Although this particular mixture produced only modest yield benefits, the potential for simultaneous reductions in tan spot and leaf rust was demonstrated.     

Evolution of a pathogen population in host mixtures: simple race–complex race competition

The evolution of the genetic structure of a pathogen population was studied in a varietal mixture with an epidemic simulator based on the model EPIMUL. The pathogen population was composed of simple races able to develop on only one component of the mixture and a complex race which developed on all mixture components. The effects on the simple race–complex race competition of a cost of virulence, of density dependence and of differential adaptation were studied. The selection for simple or complex races in the pathogen population did not depend on initial race frequencies. For a given multiplication rate, complex race frequency increased faster when the spore dispersal gradient was shallow, when distribution of initial disease was generalized, when amount of initial disease was reduced and when the number of mixture components was increased. This was attributed to a better efficacy of the mixture in controlling simple races, resulting in a higher relative fitness of the complex race. For measured values of density dependence or differential adaptation effects, the complex race was at a higher frequency after a mean number of pathogen cycles between 2.5 and 5. The effect of the cost of virulence was stronger and, in certain situations, could result in selection for simple races. In the conditions of our simulations and with the effects tested, stabilization of the pathogen population in host mixtures was unlikely to occur. However, more information is needed concerning the rate at which complex races could evolve and how quickly mixture resistance could be eroded.     

Effect of wheat cultivar mixtures on populations of Puccinia striiformis races*

This study quantifies the frequency of simple and complex races (races that can infect two or more components) of Puccinia striiformis in mixtures of wheat cultivars possessing different race-specific resistance genes. Treatments were designed so that the complex race changed depending on the host mixture, thus enabling us to observe the influence of pathogen complexity in different genetic backgrounds. Six cultivar mixtures and one pure stand of winter wheat were inoculated with three races of P. striiformis at two locations for two seasons. Potted plants of three winter wheat cultivars, each susceptible to one of the three races of the pathogen, were used to sample the pathogen during the field epidemics. Disease incidence on the differential cultivars was used to calculate the proportion of the three races in each treatment. The specific cultivars included in the mixtures influenced the frequencies of the three races. Increasing the number of virulent races in a mixture reduced the frequency of the complex race relative to the other two races. The results suggest that genetic background of the pathogen race, host composition, and interaction among pathogen races may be as important as cost of virulence in determining race frequencies in mixtures.     

The role of aggressiveness and competition in the selection of Phytophthora infestans populations

Selection within populations of Phytophthora infestans was investigated by comparing the aggressiveness of single‐lesion isolates on detached leaflets of four potato cultivars with differing levels of race‐nonspecific resistance to P. infestans. The isolates included 23 representative of Northern Ireland genotypes from the early 2000s, used to inoculate previously reported field trials on competitive selection (2003–2005), plus 12 isolates recovered from the 2003 trial. The cultivars were those planted in the previous trials: Atlantic (blight‐susceptible) and Santé, Milagro and Stirling (partially resistant). Very highly significant variation for latent period, infection frequency and lesion area was found between genotypes and cultivars; differences between genotypes were more marked on the more resistant cultivars, but no one genotype was the most aggressive across all. Detached leaflets were also inoculated with mixtures of isolates from each genotype group at three sporangial concentrations: differences in aggressiveness between genotypes were more apparent at lower concentrations and on the more resistant cultivars. Genotype groups that were the most aggressive on the more resistant cultivars tended to be those selected by the same cultivars in the field. A mixture of all isolates of all genotypes was used to inoculate detached leaflets of the same cultivars. With one exception, single spore isolates recovered from any one leaflet belonged to a single genotype, but different genotypes were recovered from different cultivars. Phytophthora infestans isolates from Northern Ireland showed significant variation for foliar aggressiveness, and pathogen genotypes exhibited differential aggressiveness to partially resistant cultivars and interacted competitively in genotype selection.     

Competition and interactions among stripe rust pathotypes in wheat-cultivar mixtures

A series of experiments was conducted with wheat stripe rust to analyse competition between simple and complex pathotypes in host mixtures. Two different pathotype combinations were tested, with different host components. Each combination included a complex (able to infect two host components) and two simple pathotypes. For one of the combinations, induced resistance was tested in a separate experiment as a possible interaction among pathotypes. Disease severity and pathotype frequencies were measured three times during the epidemic, on each host component grown in pure stands and in mixtures. In one of the experiments, pathotype frequencies were also measured within secondary foci. One of the complex pathotypes appeared to have a low fitness on one of the host components and did not significantly increase in frequency in host mixtures relative to pure stands. The average frequency of the other complex pathotype increased during the first epidemic cycles, but remained stable afterwards, below expected values. The results suggest that the development of complex pathotypes in host mixtures may be influenced by differential aggressiveness on the host components, by induced resistance and by random effects resulting from the formation of disease foci, and depends on pathogen autoinfection rate and dispersal mechanisms.     

Effect of coinoculation by Verticillium dahliae and Colletotrichum coccodes on disease symptoms and fungal colonization in four potato cultivars

Interactions between Verticillium dahliae and Colletotrichum coccodes , major causal agents of potato early dying (PED) syndrome, were studied in four potato cultivars that differ in their susceptibility to these pathogens . Aseptic plantlets of Nicola, Desiree, Alpha and Cara were inoculated with identical concentrations of each pathogen or with a mixture of the pathogens, and grown for 4 weeks in a monitored growth chamber. Coinoculation of Nicola with both pathogens caused more severe foliar disease symptoms and crown rot and greater C. coccodes colonization, than inoculation with each pathogen separately. Significant reductions in weight and height were also observed in plants coinoculated with both pathogens, as compared with plants inoculated with each pathogen separately or noninoculated plants. In Desiree, more roots were covered with C. coccodes sclerotia and disease symptoms were significantly more severe in plants inoculated with both pathogens together. However, plant weight and height were similar to those of plants inoculated with C. coccodes only. In Alpha, disease symptoms and levels of sclerotia in the roots were not affected by simultaneous inoculation with both pathogens. Weight and height of all plants were similar, whether inoculated with each pathogen separately or with both pathogens together. In Cara, plants inoculated with the mixture or either pathogen alone were smaller than the noninoculated control. Disease symptoms and occurrence of sclerotia were similar in plants inoculated with the combination and with a single pathogen. Compared with the effects of inoculation with either pathogen, simultaneous inoculation with both pathogens can, in some cultivars, increase the incidence of PED syndrome and thus severely decrease yields.     

Durability of resistance in lily to basal rot: evaluation of virulence and aggressiveness among isolates ofFusarium oxysporum f. sp.lilii

Prospects of durability of resistance in lily to basal rot have been evaluated by testing the virulence and aggressiveness of 31 isolates ofFusarium oxysporum f. sp.lilii towards a number of different resistance sources inLilium spp. Isolates differed strongly in aggressiveness as did species and cultivars ofLilium spp. in resistance. Significant interactions were observed between isolates of the pathogen and genotypes ofLilium spp., but the magnitude was very small compared to the main effects. The interactions were mainly due to a small group of isolates with low aggressiveness. It is argued that the interactions might be based on minor genes. No major break down of the resistance was found. For practical purposes it will be sufficient to use highly aggressive isolates in screening tests.     

Heterogeneity of peach rust disease progress within the tree canopy

Peach rust (Tranzschelia discolor) is a major foliar disease of peach that can cause severe defoliation. Few epidemiological studies have been conducted on peach rust, and there is no information concerning the spatial distribution of the disease within the canopy. This study aimed to characterise the onset and temporal progress of peach rust in three height strata in peach trees grown in two different orchards: one treated with fungicide and the second non-treated. Evaluations were conducted fortnightly between November and April for two consecutive years. Positional information for the disease within the canopy was obtained by assessing the incidence and severity of peach rust on leaves in both orchards. A linear mixed model with the position of shoots as the fixed effect and cultivars and blocks as random effects was adopted for the data analysis over both seasons, in the fungicide-treated orchard incidence and severity ranged from 60 to 90 % and 0.5 to 2 %, respectively, while in the non-treated orchard incidence and severity ranged from 90 to 100 % and 10 to 15 %, respectively. The area under the disease progress curve was greater for the upper shoots compared to the lower shoots, but the disease progress rates were similar. The logistic and exponential models best fitted the incidence and severity data, respectively. Urediniospores were detected by traps within the experimental area between late winter and early spring. Predominance of autoinfection or alloinfection is discussed.     

Developing smarter host mixtures to control plant disease

Adaptation of plant pathogens to disease control measures (both chemical and genetic) is facilitated by the genetic uniformity underlying modern agroecosystems. One path to sustainable disease control lies in increasing genetic diversity at the field scale by using genetically diverse host mixtures. In this study, a robust population dynamics approach was used to model how host mixtures could improve disease control. It was found that when pathogens exhibit host specialization, the overall disease severity decreases with the number of components in the mixture; this finding makes it possible to determine an optimal number of components to use. In a simple case, where two host varieties are exposed to two host‐specialized pathogen species or strains, quantitative criteria for optimal mixing ratios are determined. Using these model outcomes, ways to optimize the use of host mixtures to decrease disease in agroecosystems are proposed.     

Evaluation of diagnostic and quantitative PCR for the identification and severity assessment of eyespot and sharp eyespot in winter wheat

Diagnostic and quantitative polymerase chain reaction (PCR) provided clarification of the causes of symptoms and the extent of infection by eyespot ( Tapesia spp.) and sharp eyespot ( Rhizoctonia cerealis ) on winter wheat at early growth stages. Disease assessments made before stem extension, when decisions to apply fungicides are usually made, often did not agree with the pathogen diagnoses using PCR, suggesting that such early visual diagnoses may be unreliable. Visual and PCR diagnoses made on stems in summer generally supported each other, but there were often discrepancies in relating disease severity to amounts of pathogen present when determined by regression analyses of incidence or severity of symptoms on amount of pathogen DNA. Mixed symptoms caused by different pathogens may sometimes have been confounded. Relationships between symptoms and DNA of eyespot pathogens were less clear on some cultivars, often those with least disease. Sharp eyespot symptoms had a stronger relationship to DNA of its pathogen. Significant regressions often accounted for a small percentage of the variance, suggesting either that pathogens not assayed were contributing to symptoms or that lesions were in some cases persisting longer into the season than pathogen DNA. The frequency of pathogen detection before stem extension was a poor predictor of the amounts of pathogen DNA measured later in the season.     

Durability of Resistance to Globodera pallida I. Changes in Pathogenicity, Virulence, and Aggressiveness During Reproduction on Partially Resistant Potato Cultivars

ABSTRACT We studied changes in pathogenicity, virulence, and aggressiveness of Globodera pallida populations over time. As a measure for pathogenicity, the reproduction factor on a partially resistant host was used; for aggressiveness, the reproduction factor on a susceptible reference host was used; and, for virulence, the ratio pathogenicity/aggressiveness was used. The G. pallida populations were reared in a glasshouse for four generations on potato cultivars with different levels of resistance. The cultivar Elkana did not increase pathogenicity significantly, but the more resistant cultivars Karakter and Darwina did. This increase in pathogenicity was caused by an increase in virulence, whereas aggressiveness generally was not altered significantly. The increase in virulence appeared to be caused by an enhanced ability of eggs to develop into cysts, and not by an increase in egg production per new cyst. The observed changes in virulence could be predicted reasonably well by a simple numerical model. The rate of selection depended strongly on the nematode population. Rearing a mix of two different populations on a susceptible host decreased the virulence strongly, as predicted by the Hardy-Weinberg equilibrium, but increased the aggressiveness because of heterosis.     

Stepwise Evolution of Races in Fusarium oxysporum f. sp. ciceris Inferred from Fingerprinting with Repetitive DNA Sequences

ABSTRACT Plant pathogens often exhibit variation in virulence, the ability to cause disease on host plants with specific resistance, evident from the diversity of races observed within pathogen species. The evolution of races in asexual fungal pathogens has been hypothesized to occur in a stepwise fashion, in which mutations to virulence accumulate sequentially in clonal lineages, resulting in races capable of overcoming multiple host plant resistance genes or multiple resistant cultivars. In this study, we demonstrate a simple stepwise pattern of race evolution in Fusarium oxysporum f. sp. ciceris, the fungus that causes Fusarium wilt of chickpeas. The inferred intraspecific phylogeny of races in this fungus, based on DNA fingerprinting with repetitive sequences, shows that each of the eight races forms a monophyletic lineage. By mapping virulence to each differential cultivar (used for defining races) onto the inferred phylogeny, we show that virulence has been acquired in a simple stepwise pattern, with few parallel gains or losses. Such a clear pattern of stepwise evolution of races, to our knowledge, has not been demonstrated previously for other pathogens based on analyses of field populations. We speculate that in other systems the stepwise pattern is obscured by parallel gains or losses of virulence caused by higher mutation rates and selection by widespread deployment of resistant cultivars. Although chickpea cultivars resistant to Fusarium wilt are available, their deployment has not been extensive and the stepwise acquisition of virulence is still clearly evident.     

Durability of Resistance and Cost of Virulence

A seasonal model, where a growing season is defined as the time between sowing and harvest and alternates with an inter-crop period, was derived to study the effects of the ‘cost of virulence’ and cropping ratio on durability of resistance. We assumed a single strain of virulent pathogen, a single strain of avirulent pathogen and two cultivars (one resistant and one susceptible) and studied two measures of durability of resistance (‘take-over time’ and ‘usefulness time’). Take-over time is defined as the time needed for the virulent strain of the pathogen to reach a preset threshold and predominate over the previous pathogen population. Usefulness time is the time needed before the estimated gain in green canopy area duration per plant through the use of the resistant cultivar becomes negligible. The model suggested that, although it could take several seasons before the virulent strain of the pathogen predominated over the previous pathogen population, the usefulness time of the resistant cultivar was always much shorter. Furthermore, increasing selection for the virulent strain of the pathogen (through increasing the cropping ratio of the resistant cultivar) caused the virulent strain of the pathogen to invade the system more rapidly. Cost of virulence, reflecting differences in pathogen infection rates between the four possible combinations of cultivar/pathogen strain, significantly affected durability of resistance, with the dynamics of the virulent and avirulent strains ranging from a case where the virulent strain of the pathogen died out to a case where the virulent strain of the pathogen invaded the resident pathogen population. An intermediate state, where the system reached equilibrium and the virulent strain of the pathogen neither became predominant nor died out, was defined as ‘coexistence’ of both strains of the pathogen. Occurrence of coexistence was directly related to the cost of virulence since it did not occur when virulence of the pathogen did not have a fitness cost. Two methods to include cost of virulence in the model gave similar results in relation to the two measures of durability of resistance studied.     

Cultivar mixture effects on disease and yield remain despite diversity in wheat height and earliness

Cultivar mixtures can stabilize yield and reduce pathogen spread in plant populations. A field experiment was performed to determine (a) whether a large difference between the cultivars in the mixture (e.g., plant height or earliness) would have an impact on mixture performance, and (b) whether such differences would modify the classical rules for mixture design. Mixtures were constituted from wheat cultivars with diversity for many traits, including plant height, flowering date, disease resistance, and yield potential. The field experiment was conducted over 3 years, testing each year 72–90 mixtures of two, four, or eight cultivars, and their corresponding pure stands. Disease severity and yield of cultivar mixtures were strongly related to the mean values of the component cultivars in pure stands. Despite the considerable diversity of the mixtures tested, the classic rules (e.g., proportion of susceptible cultivars) already tested in mixtures with similar height and earliness were effective for decreasing disease severity. Agronomic heterogeneity for traits such as plant height, yield potential, or earliness of the cultivars in mixtures did not have a negative impact on disease severity and yield relative to pure stands. Increasing the number of cultivars in the mixture from two to eight had no impact on the mean disease severity and yield of the mixtures, but reduced the variability of disease severity and yield in the mixture relative to pure stands. These results suggest that it may be possible to increase within-field wheat diversity by combining more contrasting cultivars in mixtures than was previously thought.     

Variation in pathogen aggressiveness within a metapopulation of the Cakile maritima–Alternaria brassicicola host–pathogen association

Variation in aggressiveness and its consequences for disease epidemiology were studied in the Cakile maritima–Alternaria brassicicola host–pathogen association. Variability in pathogen growth rates and spore production in vitro , as well as disease severity and lesion growth rate on C. maritima in glasshouse inoculation trials, were investigated . Substantial variation was found in growth rates among individual A. brassicicola isolates, as well as among pathogen populations. A significant trade-off also existed between growth and spore production, such that faster-growing isolates produced fewer spores per unit area. While there was little evidence for a link between growth in vitro and either disease severity or lesion development among fast- vs slow-growth isolate classes at the individual isolate level, the results suggest that variation in pathogen fitness components associated with aggressiveness may influence disease dynamics in nature. An analysis using an independent data set of disease prevalence in the associated host populations found a significant positive relationship between the average growth rate of pathogen populations in vitro and disease progress over the growing season in wild host populations. Trade-offs such as those demonstrated between growth rate and spore production may contribute to the maintenance of variation in quantitatively based host–pathogen interactions.     

Competition Effects Among Isolates of Fusarium culmorum Differing in Aggressiveness and Mycotoxin Production on Heads of Winter Rye

Fusarium culmorum is a phytopathogenic, toxigenic fungus causing seedling diseases, foot rot and head blight of cereals. For estimating competition effects in mixtures of two single-spore isolates, two winter rye single crosses were tested with either four isolates individually or four 1 : 1 mixtures of the same isolates in six field environments. Two isolates (FC46, FC64) were highly aggressive deoxynivalenol (DON) and 3-acetyl DON-producers, the other two (FC30, FC71) were medium aggressive nivalenol-producers. Rye heads were inoculated during flowering with conidia of pairs of isolates expressing similar (FC46 + FC64, FC30 + FC71) or contrary (FC46 + FC71, FC30 + FC64) levels of aggressiveness and similar or different concentrations and chemotypes of mycotoxins, respectively. Head blight rating and yield components relative to the non-inoculated plots were recorded as aggressiveness traits. Additionally, mycotoxin concentrations were measured in the rye grain. Random pathogen samples were re-isolated from heads at the onset of symptom development and analysed by molecular markers (RAPD–PCR) in one environment. Aggressiveness of the isolate mixtures was significantly lower than that of the isolates applied individually on both rye genotypes. Similarly, mycotoxin concentrations were significantly lower in the mixtures in seven out of eleven comparisons. Among the re-isolates, the component genotypes of a mixture significantly deviated from the inoculated 1 : 1 ratio when a particular isolate (FC46) was present in the mixture. This isolate displayed a superior competitive ability irrespective of the aggressiveness or mycotoxin profile of the mixing partner illustrating that pathogenic fitness is caused by additional factors that have not, as yet, been identified.     

Improved control of septoria tritici blotch in durum wheat using cultivar mixtures

Mixtures of cultivars with contrasting levels of resistance can suppress infectious diseases in wheat, as demonstrated in numerous field experiments. Most studies focus on airborne pathogens in bread wheat, while splash-dispersed pathogens have received less attention, and no studies have been conducted in durum wheat. We conducted a 2-year field experiment in Tunisia to evaluate the performance of cultivar mixtures with varying proportions of resistance (0%–100%) in controlling the polycyclic, splash-dispersed disease septoria tritici blotch (STB) in durum wheat. To measure STB severity, we used a high-throughput method based on digital image analysis of 3,074 infected leaves collected from 42 and 40 experimental plots during the first and second years, respectively. This allowed us to quantify pathogen reproduction on wheat leaves and to acquire a large data set that exceeds previous studies with respect to accuracy and precision. Our analyses show that introducing only 25% of a disease-resistant cultivar into a pure stand of a susceptible cultivar provides a substantial reduction of almost 50% in disease severity compared to the susceptible pure stand. However, incorporating two resistant cultivars instead of one did not further improve disease control, contrary to predictions of epidemiological theory. Susceptible cultivars can be agronomically superior to resistant cultivars or be better accepted by growers for other reasons. Hence, if mixtures with only a moderate proportion of the resistant cultivar provide a similar degree of disease control as resistant pure stands, as our analysis indicates, such mixtures are more likely to be accepted by growers.