Jean Pinon

Partial resistance to Melampsora larici-populina leaf rust in hybrid poplars: Genetic variability in inoculated excised leaf disk bioassay and relationship with complete resistance

ABSTRACT 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.

Comparison of genetic and virulence diversity of Melampsora larici-populina populations on wild and cultivated poplar and influence of the alternate host

ABSTRACT The aims of this study were, first, to compare the genetic and virulence diversity between populations of the rust fungus Melampsora larici-populina on wild and cultivated poplar stands and, second, to investigate the influence of the presence of the alternate host of the pathogen, larch, on which its sexual reproduction occurs, on these diversities. Nine French M. larici-populina populations collected from poplar trees in autumn and four populations collected from larch trees during the following spring were analyzed using both virulence factors and neutral markers. In all, 30 pathotypes were identified within the 13 populations studied. The pathotypic structure clearly distinguished the cultivated stands with high richness and complexity from the wild stands with low richness and complexity. High linkage disequilibria between virulences indicated preferential virulence associations, probably due to selection by the host. In all, 19 random amplified polymorphic DNA (RAPD) markers were used, which revealed a very high genetic diversity in the 743 isolates analyzed. The nine populations from poplar appeared moderately differentiated, indicating long-distance gene flow, and no isolation by distance was found. Linkage disequilibria between RAPD markers generally were low, indicating frequent recombination, but they were not lower in populations located near larch, probably due to long-distance dispersal.

Structure of Melampsora larici-populina populations on wild and cultivated poplar

Populations of Melampsora larici-populina, causal agent of poplar leaf rust, have been collected during 1992–1994 in three types of sites: natural stands of Populus nigra in the south-east of France; cultivated poplar stands in the northern half of France, including mainly interspecific and exotic hybrids of poplar; and two overseas populations collected from Northern Ireland and Western USA. The pathotype of each rust isolate was determined by inoculating a differential set of poplar clones. The number of virulences per isolate and the frequency of each virulence in each population were compared, according to the sites and to the clones from which the isolates were collected.Several diversity indices were applied to the populations. The α and Simpson indices were retained since they described best the richness and the evenness, respectively, of the populations, and they were the least sensitive to sample size. The complexity of races was lower in the natural stands of P. nigra than in the cultivated poplar stands. Populations of M. larici-populina collected on Populus × euramericana ‘Robusta’ and P. nigra ‘italica’ (Lombardy poplar) presented a high richness and evenness, confirming the value of these clones for describing race populations. Populations from Western USA presented a very low diversity, which is in accordance with the recent introduction of the pathogen in North America. These results suggest that the race populations of M. larici-populina are mainly influenced by the structure of the host populations.

Isolation and structure elucidation of hymatoxins B - E and other phytotoxins from Hypoxylon mammatum fungal pathogen of leuce poplars

Abstract Two groups of phytotoxic substances were isolated from a culture filtrate of the plant pathogenic fungus Hypoxylon mammatum (Wahl.) Miller, responsible of the canker formation in aspen. The main group consists of a mixture of original diterpenes, hymatoxins, from which have been isolated hymatoxins A, B, C, D and E that belong to the pimarane class. The minor group of toxins is composed of isomeric 3,4,5- and 3,4,8-trihydroxynapthalenones. The structures of the isolated compounds were determined by spectroscopic methods.

Methods and progress in the conservation of elm genetic resources in Europe

Se discute el progreso realizado en la conservacion de los olmos europeos desde la primera conferencia Internacional del Olmo y los metodos complementarios de conservacion in situ y ex situ. El apoyo financiero de la Union Europea al proyecto RESGEN CT96-78 ha permitido coordinar y racionalizar la conservacion ex situ de los olmos. El proyecto, en el cual estan involucrados 17 instituciones participantes en nueve paises de Europa Occidental, tiene por objetivo una mejor evaluacion, conservacion y utilizacion de las colecciones actualmente existentes de clones nativos de olmo. Los principales logros son: el establecimiento de una base de datos comun de aproximadamente 2.000 clones; la caracterizacion de mas de 500 clones usando RAPD y marcadores moleculares PCR-RFLP de ADN cloroplastico; la finalizacion y racionalizacion de las colecciones existentes; el establecimiento a largo plazo de una coleccion central con 850 clones; la criopreservacion de un conjunto de 444 clones; y la identificacion de clones de interes para la mejora y para su uso en la restauracion de setos en campo. La red «Noble Hardwoods» del programa pan-europeo EUFORGEN agrupa a miembros representantes de 31 paises, y promueve la conservacion dinamica de los recursos geneticos de varios generos de arboles planifolios, incluido Ulmus spp.. Las estrategias para la conservacion del potencial adaptativo de los recursos de los olmos se definieron y se dieron a conocer entre forestales y conservacionistas a traves de folletos guia. Algunos paises han comenzado ya a implementar medidas de conservacion para U. laevis mediante el uso de la preservacion in situ y el establecimiento de huertos semilleros. Otros estan elaborando inventarios, o adquiriendo informacion genetica de poblaciones de interes.

North American populations of Entoleuca mammata are genetically more variable than populations in Europe

Entoleuca mammata (syn. Hypoxylon mammatum) is a damaging pathogen of Populus tremuloides and P. grandidentata in North America and P. tremula in Europe, where the fungus occurs only sporadically in alpine regions and Scandinavia. It has been hypothesized that E. mammata was introduced to Europe from North America. In this study, E. mammata isolates collected from Europe and North America were compared by a sequence analysis of two DNA markers derived from DNA fingerprints. The objective of the study was to elucidate the relationship between North American and European E. mammata populations by testing two hypotheses: (1) North American and European isolates are conspecific; and (2) the fungus was introduced between continents causing both a founder effect and a genetic bottleneck. North American populations were found to be more polymorphic, but no major phylogenetic differences between fungal isolates collected from different continents were found. This result combined with the historical observations of the disease in Europe implies that E. mammata was introduced to Europe several centuries ago.

Hymatoxins K and L, Novel Phytotoxins from Hypoxylon mammatum, Fungal Pathogen of Aspens

Abstract From the broth culture of the fungal pathogen Hypoxylon mammatum responsible for canker in aspen (Populus species) two new phytotoxins, hymatoxins K (2) and L (3), diterpenoid α-D-manno-pyranosides were isolated and their structures determined by mass spectrometry and 2D-NMR.