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This chapter covers the following: variability in virulence and pathotypes within Melampsora larici-populina on poplar; variability in the pathogenicity of M. larici-populina, M. allii-populina and M. medusae and its effects on host resistance; effects of the breakdown of complete resistance; adaptation of poplar cultivation to the evolution of pathotype populations; and evaluation of resistance in clones considering the diversity in virulence within M. allii-populina and M. larici-populina.

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.

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.

There are eight Melampsora species infecting poplars in Europe (Pinon, 1973; Cellerino, 1999). Three of them, M. laricipopulina, M. allii-populina, and M. medusae are pathogenic on the poplars of the sections Aigeiros and Tacamahaca, i.e. P. nigra, P. deltoides, P. trichocarpa, and their interspecific hybrids. Most of the commercial poplar cultivation in Europe is made of P. × euramericana (P. deltoides × P. nigra) and P. × interamericana (P. trichocarpa × P. deltoides) hybrids. Rust was the most damaging disease of poplars in the past decade (Chapter 12, this volume). Five other Melampsora species, M. pinitorqua, M. larici-tremulae, M. rostrupii, M. magnusiana, and M. pulcherrima, are pathogenic on species of the Populus (formerly Leuce) section, i.e. P. alba, P. tremula, and their hybrids. Although these five species are difficult to distinguish, since they have in common evenly echinulated urediniospores, Pinon (1973) proposed a diagnosis key based on the morphology of the urediniospores and the paraphyses. Some authors follow the proposal of Wilson and Henderson (1966) to adopt M. populnea as a