The First Record of a North American Poplar Leaf Rust Fungus, Melampsora medusae, in China

A wide range of species and hybrids of black and balsam poplars or cottonwoods (Populus L., sections Aigeiros and Tacamahaca) grow naturally, or have been introduced to grow in plantations in China. Many species of Melampsora can cause poplar leaf rust in China, and their distributions and host specificities are not entirely known. This study was prompted by the new susceptibility of a previously resistant cultivar, cv. ‘Zhonghua hongye’ of Populus deltoides (section Aigeiros), as well as by the need to know more about the broader context of poplar leaf rust in China. Rust surveys from 2015 through 2018 in Shaanxi, Sichuan, Gansu, Henan, Shanxi, Qinghai, Beijing, and Inner Mongolia revealed some samples with urediniospores with the echinulation pattern of M. medusae. The morphological characteristics of urediniospores and teliospores from poplar species of the region were further examined with light and scanning electron microscopy. Phylogenetic analysis based on sequences of the rDNA ITS region (ITS1, 5.8S rRNA gene, and ITS2) and the nuclear large subunit rDNA (D1/D2) was used to further confirm morphology-based identification. Based on combined analyses, five of the fifteen fully characterized samples were identified as Melampsora medusae: one from Shaanxi and four from Sichuan. Two of the five were from Populus deltoides cv. ‘Zhonghua hongye’. Three others were identified on Populus szechuanica, P. simonii, and P. yunnanensis. Additional samples of M. medusae were collected in Shaanxi in 2017 and 2018, and from Henan in 2015 through 2018. Altogether these findings show that this introduced pathogen is widespread and persistent from year to year in China. This is the first report of this North American poplar leaf rust species, Melampsora medusae, in China. It has previously been reported outside North America in Argentina, Europe, Australia, New Zealand, Japan, and Russia.

Specificity of SSR loci for Melampsora species on poplars

Two rust fungi, Melampsora larici populina and Melampsora medusae are common in all poplar growing regions worlwide. M. larici populina is native to Eurasia, while M. medusae is endemic to eastern Unated States on Populus deltoides. Poplar leaf rust (Melampsora sp.) is widely spread disease in our country, and can cause significant growing problems. Race that prevails in our growing region is M. Larici populina. This work initiated the research of population composition, i.e. qualitative participation of Melampsora races in popoluation of black poplars hybrid progenies using molecular methods. When five SSR markers were used the results showed that M. larici populina was found in three tested clones, while only M. medusae was determined in clone I-214.

Genetic Diversity in Poplar Leaf Rust (Melampsora medusae f. sp. deltoidae) in the Zones of Host Sympatry and Allopatry

Poplar leaf rust caused by Melampsora medusae f. sp. deltoidae is a widespread disease in North America, where epidemics occur within zones of sympatry and allopatry of telial hosts (Populus spp.) and aecial hosts (Larix spp.). To test the hypothesis that epidemics originate in the zone of sympatry where the rust can complete its life cycle, populations in sympatry and allopatry were analyzed with single-strand conformational polymorphism for codominant detection of alleles directly from uredinia. More alleles were detected in rust populations in the zone of host sympatry than in allopatry. Almost all alleles found in the zone of allopatry were a subset of the allelic diversity present in the zone of host sympatry. Distance analyses clustered populations according to geographic origin, but not sampling year or type of stand (plantation or natural stands). Large differences in allelic and genotypic frequency were observed between years in allopatry but not in sympatry, suggesting new colonizations in allopatric populations. Our results point to a dynamic and complex pattern of inoculum dissemination in polar leaf rust. The hypothesis most consistent with our results is that populations in sympatry represent a source of inoculum for epidemics, with some annual recolonization in allopatry, possibly via intermediate population jumps.

Genetic diversity of poplar leaf rust populations in the north-central United States

Uredinia of poplar leaf rust, Melampsora medusae Thüm f.sp. deltoides, appeared in late July 1998, June 1999, and August 2000 in a cottonwood (Populus deltoides Bartr.) stand in Ames, Iowa. Seedlings of the alternate host (eastern larch, Larix laricina (Du Roi) K. Koch) set out at the site in spring 2000 formed aecia 3 months before uredinia appeared. Using three PCR-based microsatellite markers, the aecial population was genetically diverse and in Hardy–Weinberg equilibrium, typical of a population that had gone through sexual reproduction. Uredinia populations in 1998–2000 had lower levels of gene diversities (from 0.58 to 0.71) and were not in Hardy–Weinberg equilibrium. Of nine populations in Minnesota, Iowa, and Missouri during the early stage of the 1999 epidemic, the two populations with the highest gene diversity, the highest level of heterozygosity, and the greatest number of unique genotypes were within the natural geographic range of larch. However, the southernmost population in Missouri was also in Hardy–Weinberg equilibrium. Epidemics started sooner in Iowa than in Minnesota, which contradicts the hypothesis that epidemics begin in northern regions where the alternate hosts naturally overlap. Epidemics appeared to commence independently in the various locations, perhaps focused around areas where ornamental larch is in proximity to P. deltoides.