A new species of Melampsora rust on Salix elbursensis from Iran

A rust fungus was found causing stem cankers on 1‐ to 5‐year‐old stems of Salix elbursensis in the north west of Iran. The rust also forms uredinia on leaves and flowers of the host willow. Light and scanning electron microscopy revealed that the new rust is morphologically distinct from several Melampsora species occurring on the willows taxonomically close to S. elbursensis, but indistinguishable from Melampsora larici‐epitea. Examination of the internal transcribed spacer (ITS) region of the ribosomal DNA suggested that the rust fungus is phylogenetically close to Melampsora allii‐populina and Melampsora pruinosae on Populus spp. Based on both the morphological characteristics and the ITS sequence data, the rust is described as a new species –Melampsora iranica sp. nov.     

Infection of Populus alba var. hickeliana by Melampsora medusae Thüm.

The first record of infection of Populus alba var. hickeliana by the American rust, Melampsora medusae Thum. is reported. The morphology of uredospores of rust on P. alba is compared with that of other Melampsora spp. infecting white poplars. The implications of these observations for the widespread establishment in the future of white × black hybrid poplars are discussed.     

Black canker and leaf spot of Salix in New Zealand caused by Glomerella miyabeana (Colletotrichum gloeosporioides)

Two new diseases have been observed attacking willows in New Zealand. In the first instance black stem cankers formed on S. matsudana × S. alba × S. alba cultivars, eventually killing them. In the second instance foliage of S. gooddingii and S. amygdaloides was attacked and premature defoliation occurred. Morphological studies and host inoculations established that Glomerella miyabeana (Colletotrichum gloeosporioides anamorph) was the causal agent of both diseases. Foliage of cultivars exhibiting stem cankering was not attacked nor were stems of cultivars with foliar lesions. Electron microscope studies showed that conidia of Colletotrichum gloeosporioides formed phialidically within acervuli and comdial dimensions varied significantly with host.     

Life cycle of Melampsora larici‐epitea on short‐rotation cultures of Salix burjatica

The host alternation of Melampsora larici‐epitea occurring on Salix burjatica in experimental short‐rotation cultures in Finland was studied by inoculation tests. Larix sibirica was tested as the most probable alternate host. It was inoculated with rust basidiospores, and the life cycle of the pathogen was succesfully completed by inoculating the willow with aeciospores developed on larch needles.     

Host range and pathogenicity studies of Marssonina castagnei to poplars

Laboratory studies investigating the host range and pathogenicity of Marssonina castagnei (Desm. + Mont.)Magn. to poplars established that this species was not specifically pathogenic to P. alba but also to cultivars of the Sections Aigeiros, Tacamahaca, and inter sectional hybrids Aigciros × Tacamahaca, Leuce × Aigeiros and Lcucc × Tacamahaca. The potential threat of this pathogen could be reduced by routine screening of new selections for resistance.     

Host range and pathogenicity studies of Marsonnina brunnea to poplars

Laboratory studies of Marssonina brunnea (Ell. + Ev.) Magn. established the wide host range and strong pathogenicity of this pathogen to poplars. New Zealand isolates were uniformly pathogenic whereas gross differences in pathogenicity were observed between international isolates. The adoption of a standard inoculation technique for screening poplars for Marssonina resistance is urged.     

Genetic relationships among genetically distinct forms of Melampsora larici‐epitea and related species based on AFLP data

Amplified Fragment Length Polymorphism (AFLP) was used to study genetic relationship among leaf rust fungi (Melampsora spp.) occurring on Salix species in short‐rotation coppice. Special interest was paid to a newly identified rust fungus found on S. viminalis in southern Sweden, morphologically similar to M. larici‐epitea but with a distinct DNA profile. Genetic distances among 40 Melampsora isolates collected from S. viminalis, S. dasyclados, S. viminalis x dasyclados, S. daphnoides, S. acutifolia and M. larici‐populina were calculated based on 101 AFLP markers. Neighbour‐joining analysis revealed the presence of six clusters, which corresponded exactly to predefined groups, namely three formae speciales of M. larici‐epitea, a stem‐infecting form of Melampsora on S. viminalis, the newly identified Melampsora on S. viminalis and a group consisting of two isolates from the poplar rust M. larici‐populina. All six clusters were well supported by bootstrap analysis (84 to 100% support). The newly identified Melampsora on S. viminalis was indicated to be genetically separated from M. larici‐epitea as well as from the stem‐infecting form of Melampsora.     

Temperature sensitivity of qualitative race-cultivar interactions in Melampsora medusae Thüm. and Populus species

Six mono-uredospore isolates (races) of Melampsora medusae Thiim. produced qualitatively distinct reactions, in vitro, when incubated at 15 °C on leaf disks of certain cultivars of Populus deltoides Marsh. The infection types in some race-cultivar combinations were very temperature sensitive with less distinct reactions when incubated at 20 or 25°C. The significance of differential race-cultivar-temperature interaction in the epidemiology of M. medusae is discussed.     

Growing clonal mixtures of willow to reduce effect of Melampsora epitea var. epitea

Rust (Melampsora epitea var. epitea) is the single most limiting factor in the production of biomass from short rotation coppice willow. The use of fungicides is unacceptable for economic, environmental and practical reasons. Consequently, clonal mixtures were investigated as an alternative disease control strategy. At one of two sites Salix × dasyclados, Salix burjatica ‘Korso', S. burjatica‘Germany’, Salix viminalis‘Bowles Hybrid’ and S. viminalis‘683’ were included in the clonal mix. At the second site six clones were used. These were as above with the exception of S. burjatica‘Korso’ and including, Salix × calodendron, Salix mollissima-undulata‘SQ83’. At both sites all clones were also planted in monoclonal stands. Throughout each growing season disease levels were recorded on individual clones, both in monoclonal stands and mixtures. Fungicide treatments were included to compare their effectiveness to mixtures in reducing disease impact. Growing willow in mixed clonal stands delayed the onset of disease, slowed its build up and resulted in lower disease levels at the end of the growing season. These reductions in disease impact contributed to the increased yields obtained from clonal mixtures and were generally comparable to those achieved with fungicides.     

Short rotation coppice willow in Northern Ireland since 1973: development of the use of mixtures in the control of foliar rust (Melampsora spp.)

A review is given of the use of clonal mixtures in the development of disease control strategies in short rotation coppice willow in Northern Ireland. Salix burjatica‘Korso’ had been grown successfully for over 10 years when, in 1985, Melampsora epitea var. epitea caused serious problems. Although fungicides were effective, their use was not practical for environmental and economic reasons. Therefore, in 1987 large scale field experiments were initiated to investigate the use of clonal mixtures as a disease control strategy. Increased yields were consistently recorded from mixed stands when compared to either the mean yield of component clones or the individual yields of any of the component clones grown in monoculture. Part of this increased yield was due to a reduction in the impact of rust disease. Investigations are currently being conducted on the effect of number of clones within a mixture, the best clones to use and the optimum planting density. It is essential that there is a range of susceptibilities to M. epitea var. epitea pathotypes within the components of the mixture.     

Variability in pathogenicity of Melampsora allii-populina expressed on poplar cultivars

A collection of 42 isolates of Melampsora allii-populina was established. Fourteen isolates were collected from various poplar cultivars over several French localities and 28 isolates from the alternate hosts (Allium spp., Arum spp. and Muscari comosum) obtained from nature or after inoculation. These isolates were inoculated in the laboratory on a large range of poplar cultivars belonging to several species. No isolate proved pathogenic on all cultivars and many qualitative interactions were observed between isolates and cultivars. This is the first evidence for the existence of physiological races within this fungus. No link was found between pathogenicity on poplar and on the alternate hosts. Among the inoculated cultivars, at least three were susceptible to all isolates (Beaupre, Candicans and Robusta), whereas the following showed qualitative reactions to the inoculated isolates: Altichiero, Carpaccio, Cima, Fritzi Pauley, I 154, Isieres, Luisa Avanzo, NL 2842, Rap and Spijk. Race-specific resistance to M. allii-populina (i.e. resistance to some races of the pathogen, but not to the others) has been found in two North American species (Populus deltoides and Populus trichocarpa) which have never coevolved with this fungus. The results are discussed in comparison with Melampsora larici-populina.     

Studies of Marssonina castagnei in Australasia

Comparative studies of conidium morphology and host range and pathogenicity tests of Australian and New Zealand isolates of Marssonina castagnei in the laboratory established their equivalence which was indicative of a common origin. It is speculated that trans-Tasman dispersal is responsible for the introduction of M. castagnei into New Zealand and its widespread distribution. This is based on the prior recording of M. castagnei in Australia, the potential of M. castagnei conidia for long distance transport and the proven records of trans-Tasman dispersal of other pathogens. Laboratory inoculations and field observations showed P. alba cv. Silver poplar and its hybrids to be highly susceptible to M. castagnei. Populus alba X P. glandulosa hybrids and Italian selections of P. alba showed high field resistance.     

Genetics of field resistance to Melampsora in Salix viminalis

The inheritance of resistance to Melampsora. spp. on Salix viminalis, the variation in resistance over space and time, and rust infection rate in relation to the geographic origin of the host was studied. The analysis showed that there was considerable additive genetic variation for rust resistance, that the results did not have high repeatability over space and time, and that the material from southernmost Sweden was most sensitive to the rusts occurring in south Sweden.     

Comparing Chilean and Swedish rust populations: possible impact of long‐distance transport of Melampsora rust

As willow (Salix) cultivation grows globally, worldwide monitoring of pathotypes for Melampsora rust, the most serious disease in willow, becomes increasingly important. We compared the pathotype composition of the Melampsora rust population in Chile with that in Sweden and assessed inocula exchange for the rust between the continents. For pathotyping rust isolates, a willow differential was used (a standardized set of willow test clones) to obtain clone‐virulence patterns. These patterns were used to identify virulence components involved in determining which willow clones a given individual pathogen isolate (genotype) can successfully attack. Each virulence component detected earlier in Sweden's rust population was present in Chile's rust population. Using isoenzyme analysis to classify willow species in Chile, a low genetic variation of Salix viminalis (an introduced willow species) was found, compared with the endemic species Salix humboldtiana. Comparison of rust populations for the two countries supports the present hypothesis that intercontinental inocula exchange can be a significant determinant of local pathotype structure, and consequently can be important for willow‐resistance breeding.     

Effect of Melampsora medusae leaf rust infection on yield of hybrid poplars in the north-central United States

Melampsora medusae Thuem. is a hazard to cultivation and fiber production of hybrid poplars grown under short rotations, particularly when the alternate host is present. This rust can be controlled with biweekly fungicide applications but resistant clones are the best method of control.     

A review of exapted resistance to diseases of Populus

The cultivation of hybrid poplar in the Pacific North-west (PNW) has expanded in the last 15 years. Native Populus trichocarpa is susceptible to many pests and fungal pathogens but its interspecific hybrids were largely disease-free until 1991 when Melampsora medusae f. sp. deltoidae first appeared in the region. Genetic analyses of resistance in the PNW, and elsewhere, reveal a pattern whereby genes for resistance are inherited in interspecific hybrids from the exotic, noncoevolved Populus parent. With some pathogen/poplar hybrid combinations, this resistance segregates in the F₁ generation, but with others, only in the F₂. Such exapted resistance is now known to occur in P. trichocarpa (versus the exotic, eastern North American rust species, M. medusae f. sp. deltoidae), in Populus deltoides (versus the Eurasian rust, Melampsora larici-populina, the PNW population of Septoria populicola, and an undescribed PNW eriophyid mite), and in Populus maximowiczii (versus Melampsora occidentalis, and the PNW populations of Venturia populina, S. populicola, and Linospora tetraspora).     

Sensitivity to Cladosporium species of the reactions of cultivars of Populus X euramericana to races of Melampsora larici-populina

All five species of Cladosporium tested were found to be antagonistic to the production of uredinia by three races of Melampsora larici-populina on leaf disks cut from four cultivars of Populus × euramericana. The Cladosporium spp. also colonised the uredinia produced on the leaf disks. Of the species employed, C. tenuissimum was the most active antagonist and coloniser of the uredinia. The significant race × Cladosporium spp. interaction in the data analysis indicates a degree of physiological specialisation in the relationship between the races of the rust and species of Cladosporium. The possible epidemiological significance of the results is discussed.     

Development of Melampsora ciliata rust on nursery‐grown poplars in north‐western Himalayas

Development of leaf rust (Melampsora ciliata) in different species, hybrids and cultivars of poplar (Populus spp.) was studied in nursery‐grown plants. Five different criteria were used to assess the disease development. The mean disease index on 10 August was 9.4% which increased to 70.0% on 10 October. The lowest disease index (5.5%) was recorded in P. yunnanensis whereas the maximum was recorded in P. × euramericana‘Rubra‐Poiret’ (59.5%). The apparent infection rate per unit per day was highest in P. nigra × P. trichocarpa (9.7 × 10^–2) whereas the minimum occurred in P. maximowiczii × P. berolinensis‘Oxford’ (3.3 × 10^–2). The area under the disease progress curve was a maximum in P. × euramericana‘Rubra‐Poiret’ (26.7) and a minimum in P. yunnanensis (1.6). Complete defoliation by the first half of October occurred in P. nigra × P. trichocarpa, which also had the maximum apparent infection rate and area under the disease progress curve. Inoculum production was highest in P. nigra × P. trichocarpa and lowest in P. maximowiczii × P. berolinensis‘Oxford’.     

Temperature-light sensitivity of infection types expressed by cultivars of Populus deltoides Marsh. to races of Melampsora medusae Thüm.

Infection types were assessed when four cultivars of Populus deltoides Marsh. were inoculated with six races of Melampsora medusae Thüm. and incubated in combinations of light intensity (100, 500 and 1000 μEm^-2s^-1) and temperature (15 and 25°C). Resistance in the cultivars was always at a maximum when the race/cultivar complexes were incubated in the light intensity (1000 μ^-2s^-1)/high temperature (25°C) regime. Inereasing light intensity was generally associated with reduced infection type in the cultivars; the reduction was more pronounced in an incubation temperature of 25 than 15°C. The potential epidemiological significance of the results is discussed.