Summary• In order to elucidate the genetic control of resistance to Melampsora laricipopulina leaf rust in hybrid poplars, a Populus deltoides × P. trichocarpa F 1 progeny was analysed for qualitative and quantitative rust resistances.• This progeny was evaluated for three components of quantitative resistance (latent period, uredinia number and uredinia size) to seven M. larici-populina strains in controlled conditions, and for one component of field susceptibility (rust colonization on the most infected leaf).• One qualitative resistance locus inherited from P. deltoides , R 1 , was localized on the genetic map. It segregates 1 : 1 in the F 1 progeny and is effective against four of the studied strains. QTL analysis was performed separately on R 1 and r 1 genotype subsets. An additional detection was conducted on the entire F 1 progeny for the three strains able to overcome R 1 and for MAX2. A total of nine QTLs were detected. Two had large, broad-spectrum effects. One ( R US ) is inherited from the P. trichocarpa parent; the other is inherited from P. deltoides and colocalized with R 1 . Seven QTLs had only limited and specific effects. Significant interaction effects were detected mainly between the two major QTLs.• Implications of these results for durable resistance breeding strategies, and possible benefits from the Populus genome sequence, are discussed.

Two genetic factors explain a significant proportion of the variability for quantitative resistance to Melampsora larici-populina leaf rust in a Populus deltoides x P. trichocarpa F(1) progeny. One is inherited from P. deltoides and is associated with a defeated qualitative resistance gene R(1), and the other, R(US), is inherited from P. trichocarpa. To assess the potential contribution of these two factors for durable resistance breeding, 284 genotypes from this F(1) progeny were studied in laboratory experiments with three M. larici-populina strains and in a field experiment under natural inoculum pressure. Results confirmed that both factors can have strong beneficial effects in the laboratory. These effects were strain specific, thus impairing their chances for durability. However, association of both factors led to synergistic effects in most situations. In accordance with good field-laboratory relationships, especially those involving uredinia-size laboratory measurements, field effects of both resistance factors were significant. R(US) led to a significant reduction of rust colonization on the most infected leaf in the field, and its effect was significant both in the presence and the absence of R(1). In contrast, the presence of R(1) was useful in the field only when R(US) was absent. The nature of the genetic relationship between both factors remains unknown, but benefits from their association should be quantified over a longer period to evaluate potential adaptation of the pathogen.

Inoculated excised leaf disk technique allows decomposition of poplar partial resistance to Melampsora larici-populina leaf rust into key epidemiological components such as latent period (LP), uredinia number (UN), uredinia size (US), and spore production (SP) for a given M. larici-populina strain under controlled environmental conditions. Three hundred thirty-six genotypes from an interspecific Populus deltoides x P. trichocarpa F(1) progeny segregating for complete resistance to M. larici-populina strain 93ID6 were inoculated with M. larici-populina strain 93CV1. This strain was able to infect the whole family, except few probable recombinants. LP, final UN, and final US after one infectious cycle proved to be relevant complementary descriptors of partial resistance. Area under the disease progress curve and other parameters of uredinia appearance dynamics did not yield additional information. Indirect assessment of SP by US scoring was reliable and easy to access compared with direct spore counting. UN was the only trait for which a doubling of the inoculum pressure level had a significant effect, leading to greater differentiation between genotypes. Consistent with previous studies is the clear relationship between presence of complete resistance against M. larici-populina strain 93ID6 and higher partial resistance to M. larici-populina strain 93CV1 (32% longer LP, 76% smaller UN, and 34% smaller US). In the subpopulation compatible with 93ID6, bimodal distribution of genotypic means for US suggested implication of a major gene inherited from the P. trichocarpa parent. Residual variation was noted for the three epidemiological components, suggesting that additional genes might condition these quantitative traits.

A scarcity of favourable habitats and introgression from exotic cultivars are two major threats to black poplars (Populus nigra L.) in Europe. Natural vegetative propagation contributes to maintenance of the species in areas where seedling recruitment is limited. Exhaustive sampling of all mature trees in a natural P. nigra stand (413 individuals at recorded positions), genotyping at 11 SSR loci, and a standardized analysis framework resulted in a precise description of clonality in terms of (a) frequency, (b) spatial growth form, and (c) impacts on the overall spatial genetic structure (SGS). The high proportion of replicated genotypes detected resulted in a genotypic richness (R) of 0.47. Up to 18 ramets were found per multilocus lineage (MLL), but 95% of MLLs contained fewer than five ramets (Pareto index β=1.07). No significant difference in vegetative propagation potential was found between genders. Uneven spatial distribution of ramets, with clustering of clonal ramets (aggregation index A c =0.62) and near-zero intermingling between MLLs (clonal dominance index D c = 0.99), resulted in a 'phalanx' clonal growth form, explaining most of the SGS observed over short distances (0-20 m, Sp=0.0324). Although they did not exhibit the typical columnar shape of the Lombardy poplar (P. nigra var. italica), five trees were found to be probable F 1 hybrids of this old and widely distributed cultivar.

Since the early 1990s, ash dieback due to the invasive ascomycete Hymenoscyphus fraxineus is threatening Fraxinus excelsior in most of its natural range. Previous studies reported significant levels of genetic variability in susceptibility in F. excelsior either in field or inoculation experiments. The present study was based on a field experiment planted in 1995, 15 years before onset of the disease. Crown and collar status were monitored on 777 trees from 23 open-pollinated progenies originating from three French provenances. Health status was modeled using a Bayesian approach where spatiotemporal effects were explicitly taken into account. Moderate narrow-sense heritability was found for crown dieback (h = 0.42). This study is first to show that resistance at the collar level is also heritable (h = 0.49 for collar lesions prevalence and h = 0.42 for their severity) and that there is significant genetic correlation (r = 0.40) between the severities of crown and collar symptoms. There was no evidence for differences between provenances. Family effects were detected, but computing individual breeding values showed that most of the genetic variation lies within families. In agreement with previous reports, early flushing correlates with healthier crown. Implications of these results in disease management and breeding are discussed.