The main objective of this investigation was to test the 'always more aggressive' hypothesis, often advocated to explain lineage replacements in clonal populations of the potato late blight oomycete Phytophthora infestans. To this end, genotypic and pathogenicity data on 1274 French isolates of P. infestans, collected over the period 2001-2008, were analysed. Overall, the populations sampled showed limited genetic diversity, with four multilocus lineages (1_A1, 2_A1, 8_A1 and 13_A2) accounting for over 80% of the isolates collected. As in other West European countries, drastic changes in these dominant clonal lineages were observed over the course of the 8 years, particularly in the appearance and propagation of the clone 13_A2. However, invasiveness of clones was not associated with higher aggressiveness; on the contrary, dominant clones had generally low or moderate aggressiveness relative to others present at the same time within the same populations. This finding challenges the link between invasive behaviour and increased aggressiveness often assumed for this biotrophic pathogen, and could reflect the existence of a trade-off between intra-season and inter-season transmission. This would be consistent with the concept of inclusive fitness, which involves the abilities to both reproduce and survive.

The pathogenicity and ecology of some isolates representative of the four main Streptomyces species (S. scabies, S. europaeiscabiei, S. stelliscabiei and S. reticuliscabiei) identified as pathogenic to potato tubers were investigated. Three pathogenicity groups could be distinguished. Group 1 included all isolates of S. scabies, S. europaeiscabiei and S. stelliscabiei from common scab lesions of potato and other susceptible root crops. All these produced similar symptoms and were pathogenic to potato, carrot and radish. Group 2 included all isolates from S. reticuliscabiei netted scab lesions; they were pathogenic to both tubers and roots of only a few potato cultivars, and did not infect carrot or radish. Group 3 included three isolates of S. europaeiscabiei from netted scab lesions on cv. Bintje, which produced either common or netted scab symptoms depending on the potato cultivar or plant species. In an experiment on a few isolates from each of the three groups, held at various soil temperature regimes, the three from group 1 were most pathogenic at higher temperatures (20ЊC or 20/30ЊC), the two from group 2 were most pathogenic at a lower temperature (17ЊC). The group 3 isolate caused netted scab symptoms on susceptible cultivars at low temperatures (Յ 20ЊC) and deep-pitted lesions at higher temperatures. Since the groups identified differ in ecological requirements, it is important to adapt the control methods to the pathogenic species present in the soil.

The strict dependency of biotrophic pathogens upon living host tissue for multiplication and survival makes them particularly exposed to evolutionary trade-offs. Such trade-offs can occur both between life history traits directly involved in pathogenicity (e.g. fitness costs due to unnecessary virulence factors), or between traits involved in either pathogenicity (within-season fitness) or survival (between-season fitness). Both types of trade-offs should result in the limitation of maximum pathogenicity, and shape the invasive potential of pathogen genotypes. While strong theoretical developments have been made on evolutionary consequences, including recent work taking seasonality into account (i.e. periodic host absence and/or periodic sexual/asexual reproduction), experimental evidence to confirm theoretical predictions is still scarce. This paper will therefore attempt to illustrate the different kinds of trade-offs that can be measured, and their likely consequences, taking Phytophthora infestans (the cause of potato and tomato late blight) as a case study.

Common and netted scabs are two disfiguring bacterial diseases of potato tubers, caused by various groups of Streptomyces species. Common scab, caused primarily by Streptomyces scabies and Streptomyces europaeiscabiei , is characterized by more or less deep pustules on the tuber surface, while symptoms of netted scab, caused mainly by Streptomyces reticuliscabiei , are superficial, corky alterations of the tuber periderm. Some isolates of S. europaeiscabiei are able to induce both common and netted scab symptoms, and therefore constitute a third pathogenicity group. Like most bacterial diseases, potato scabs would be best controlled by using resistant cultivars. Repeated experiments with soil artificially infested with isolates of three species representative of the three pathogenicity groups showed the level and stability of cultivar resistance, as well as the existence of a range of aggressiveness among different isolates. The distribution of scab severity indexes recorded on a collection of 16 potato cultivars and 27 breeding clones grown in soil infested with common scab-inducing isolates was continuous, suggesting isolate nonspecific quantitative resistance. Least susceptible cultivars were Nicola, BF15, Sirtéma, and Charlotte, while Urgenta, Désirée, Ondine and Bintje were very susceptible. The same genotypes proved either highly susceptible (e.g. cvs Bintje, Désirée or Carmine) or highly resistant (e.g. cvs Charlotte, Sirtéma, Monalisa, BF15 or Belle de Fontenay) to isolates forming netted scab symptoms, suggesting isolate-specific qualitative resistance. The ability was confirmed of some isolates of S. europaeiscabiei to induce one or the other type of symptoms depending on cultivar and soil temperature.

SummaryA trade-off between pathogenicity and transmission is often postulated to explain the persistence of pathogens over time. If demonstrated, it would help to predict the evolution of pathogenicity across cropping seasons, and to develop sustainable control strategies from this prediction. Unfortunately, experimental demonstration of such trade-offs in agricultural plant pathogens remains elusive.We measured asexual transmission of Phytophthora infestans isolates differing in pathogenicity in two sets of artificial infection experiments under controlled, semi-outdoor conditions.Higher foliar pathogenicity decreased mean daughter tuber weight, increased infection severity in daughter tubers, and increased stem mortality before emergence. The most pathogenic isolates thus suffer a double penalty for asexual transmission: a lower survival probability within small and severely infected tubers; and a lower infection probability of neighbouring healthy plants due to fewer infected stems produced by surviving tubers. Moderate tuber resistance favoured transmission of the least pathogenic isolates, while high levels of resistance almost abolished transmission of all isolates.These data demonstrate a trade-off between foliar pathogenicity and asexual transmission over seasons in P. infestans, which should stabilise pathogenicity over time in the potato late blight pathosystem and possibly favour clone replacement by less pathogenic lineages after demographic bottlenecks.