Mate Finding, Sexual Spore Production, and the Spread of Fungal Plant Parasites

Spatial spread of infectious diseases with conditional vector preferences

We explore the spatial spread of vector-borne infections with conditional vector preferences, meaning that vectors do not visit hosts at random. Vectors may be differentially attracted toward infected and uninfected hosts depending on whether they carry the pathogen or not. The model is expressed as a system of partial differential equations with vector diffusion. We first study the non-spatial model. We show that conditional vector preferences alone (in the absence of any epidemiological feedback on their population dynamics) may result in bistability between the disease-free equilibrium and an endemic equilibrium. A backward bifurcation may allow the disease to persist even though its basic reproductive number is less than one. Bistability can occur only if both infected and uninfected vectors prefer uninfected hosts. Back to the model with diffusion, we show that bistability in the local dynamics may generate travelling waves with either positive or negative spreading speeds, meaning that the disease either invades or retreats into space. In the monostable case, we show that the disease spreading speed depends on the preference of uninfected vectors for infected hosts, but also on the preference of infected vectors for uninfected hosts under some circumstances (when the spreading speed is not linearly determined). We discuss the implications of our results for vector-borne plant diseases, which are the main source of evidence for conditional vector preferences so far.

Central American and Caribbean population history of the Pseudocercospora fijiensis fungus responsible for the latest worldwide pandemics on banana

Among the emerging fungal diseases threatening food security, the Pseudocercospora fijiensis fungus causing black leaf streak disease of banana is one of the most marked examples of a recent worldwide pandemic on a major crop. We assessed how this pathogen spread throughout the latest invaded region, i.e. Central America and the Caribbean. We retraced its population history combining detailed monitoring information on disease outbreaks and population genetic analyses based on large-scale sampling of P. fijiensis isolates from 121 locations throughout the region. The results first suggested that sexual reproduction was not lost during the P. fijiensis expansion, even in the insular Caribbean context, and a high level of genotypic diversity was maintained in all the populations studied. The population genetic structure of P. fijiensis and historical data showed that two disease waves swept northward and southward in all banana-producing countries in the study area from an initial entry point in Honduras, probably mainly through gradual stepwise spore dispersal. Serial founder events accompanying the northern and southern waves led to the establishment of two different genetic groups. A different population structure was detected on the latest invaded islands (Martinique, Dominica and Guadeloupe), revealing multiple introductions and admixture events that may have been partly due to human activities. The results of this study highlight the need to step up surveillance to limit the spread of other known emerging diseases of banana spread mainly by humans, but also to curb gene flow between established pathogen populations which could increase their evolutionary potential.

Black Sigatoka in bananas: Ecoclimatic suitability and disease pressure assessments

Black leaf streak disease, or black Sigatoka, is caused by the fungus Pseudocercospora fijiensis, and has been identified as a major constraint to global production of banana and plantain. We fitted a climatic niche model (CLIMEX) for P. fijiensis to gain an understanding of the patterns of climate suitability, and hence hazard from this disease. We then calibrated the climate suitability patterns against the results of an expert elicitation of disease pressure patterns. We found a moderately strong non-linear relationship between modelled climate suitability for P.°fijiensis and the expert ratings for disease pressure. The strength of the relationship provides a cross-validation between the CLIMEX model and the expert elicitation process. The bulk of global banana production experiences high potential threat from P. fijiensis, and the higher yielding areas for banana and plantain production are at greatest threat. By explicitly considering the role of irrigation we have been able to identify how strategic irrigation could be used to support banana production in areas that are at low risk from P. fijiensis.

Sexual Reproduction in the Fungal Foliar Pathogen Zymoseptoria tritici Is Driven by Antagonistic Density Dependence Mechanisms

This study provides empirical evidence for antagonistic density dependence mechanisms driving sexual reproduction in the wheat fungal pathogen Zymoseptoria tritici. Biparental crosses with 12 increasing inoculum concentrations, in controlled conditions, showed that sexual reproduction in Z. tritici was impacted by an Allee effect due to mate limitation and a competition with asexual multiplication for resource allocation. The highest number of ascospores discharged was reached at intermediate inoculum concentrations (from 5 × 10⁴ conidia mL^-1 to 10⁶ conidia mL^-1). Consistent with these results for controlled co-inoculation, we found that the intensity of sexual reproduction varied with both cropping period and the vertical position of the host tissues in the field, with a maximum between 25 and 35 cm above the ground. An optimal lesion density (disease severity of 30 to 45%) maximizing offspring (ascospores) number was established, and its eco-evolutionary consequences are considered here. Two ecological mechanisms may be involved: competition for resources between the two modes of reproduction (decrease in the host resources available for sexual reproduction due to their prior use in asexual multiplication), and competitive disequilibrium between the two parental isolates, due to differential interaction dynamics with the host, for example, leading to an imbalance between mating types. A conceptual model based on these results suggests that sexual reproduction plays a key role in the evolution of pathogenicity traits, including virulence and aggressiveness. Ecological knowledge about the determinants of sexual reproduction in Z. tritici may, therefore, open up new perspectives for the management of other fungal foliar pathogens with dual modes of reproduction.

Propagule Pressure Build-Up by the Invasive Hymenoscyphus fraxineus Following Its Introduction to an Ash Forest Inhabited by the Native Hymenoscyphus albidus

Dieback of European ash, caused by the ascomycete Hymenoscyphus fraxineus originating from Asia, has rapidly spread across Europe, and is threatening this keystone tree at a continental scale. High propagule pressure is characteristic to invasive species. Consistently, the enormous production of windborne ascospores by H. fraxineus in an ash forest with epidemic level of disease obviously facilitates its invasiveness and long distance spread. To understand the rate of build-up of propagule pressure by this pathogen following its local introduction, during 2011-2017 we monitored its sporulation at a newly infested ash stand in south-western Norway characterized with mild winters and cool summers. We also monitored the propagule pressure by Hymenoscyphus albidus, a non-pathogenic native species that competes for the same sporulation niche with H. fraxineus. During the monitoring period, crown condition of ash trees had impaired, and 20% of the dominant trees were severely damaged in 2017. H. fraxineus showed an exponential increase in spore production between 2012 and 2015, followed by drastic decline in 2016 and 2017. During 2011-2013, the two Hymenoscyphus species showed similar sporulation level, but thereafter spores of H. albidus were no longer detected. The data suggest that following local introduction, the population of H. fraxineus reaches rapidly an exponential growth stage if the local weather conditions are favorable for ascomata maturation across years. In the North Atlantic climate, summer temperatures critically influence the pathogen infection pressure, warm summers allowing the population to grow according to its biotic potential, whereas cold summers can cause a drastic decline in propagule pressure.

Fungal diversity and seasonal succession in ash leaves infected by the invasive ascomycete Hymenoscyphus fraxineus

High biodiversity is regarded as a barrier against biological invasions. We hypothesized that the invasion success of the pathogenic ascomycete Hymenoscyphus fraxineus threatening common ash in Europe relates to differences in dispersal and colonization success between the invader and the diverse native competitors. Ash leaf mycobiome was monitored by high-throughput sequencing of the fungal internal transcribed spacer region (ITS) and quantitative PCR profiling of H. fraxineus DNA. Initiation of ascospore production by H. fraxineus after overwintering was followed by pathogen accumulation in asymptomatic leaves. The induction of necrotic leaf lesions coincided with escalation of H. fraxineus DNA levels and changes in proportion of biotrophs, followed by an increase of ubiquitous endophytes with pathogenic potential. H. fraxineus uses high propagule pressure to establish in leaves as quiescent thalli that switch to pathogenic mode once these thalli reach a certain threshold - the massive feedback from the saprophytic phase enables this fungus to challenge host defenses and the resident competitors in mid-season when their density in host tissues is still low. Despite the general correspondence between the ITS-1 and ITS-2 datasets, marker biases were observed, which suggests that multiple barcodes provide better overall representation of mycobiomes.

Mate Limitation in Fungal Plant Parasites Can Lead to Cyclic Epidemics in Perennial Host Populations

Fungal plant parasites represent a growing concern for biodiversity and food security. Most ascomycete species are capable of producing different types of infectious spores both asexually and sexually. Yet the contributions of both types of spores to epidemiological dynamics have still to been fully researched. Here we studied the effect of mate limitation in parasites which perform both sexual and asexual reproduction in the same host. Since mate limitation implies positive density dependence at low population density, we modeled the dynamics of such species with both density-dependent (sexual) and density-independent (asexual) transmission rates. A first simple SIR model incorporating these two types of transmission from the infected compartment, suggested that combining sexual and asexual spore production can generate persistently cyclic epidemics in a significant part of the parameter space. It was then confirmed that cyclic persistence could occur in realistic situations by parameterizing a more detailed model fitting the biology of the Black Sigatoka disease of banana, for which literature data are available. We discuss the implications of these results for research on and management of Sigatoka diseases of banana.