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.     

The distribution of Phratora vulgatissima (Coleoptera: Chrysomelidae) on cultivated willows in Britain and Ireland

Insect samples were collected from the canopy of 24 willow short rotation coppice (SRC) sites on farmland in Britain and Ireland in 1995. The blue willow beetle Phratora (= Phyllodecta) vulgatissima (L.) (Coleoptera: Chrysomelidae), previously identified as the main defoliating pest of this crop, was recorded from 13 of these at varying levels of abundance. Data describing aspects of the environment at each site were also collected. These data were used to create potential explanatory variables for a regression analysis of beetle abundance. This analysis indicated that P. vulgatissima were most likely to occur and were more abundant at older willow sites on clay soils and at sites that bad certain free-living willow species growing nearby. Within sites, most willow clones sampled contained P. vulgatissima, although two, Salix burjatica‘Germany’ (‘Aquatica Gigantea’) and Salix mollissima‘Q83’ (Salix triandra × Salix viminalis), were avoided. Chrysomelid pest avoidance and clonal resistance could form part of an integrated pest management strategy for SRC crops.     

Effect of Melampsora rust on the growth and development of Salix burjatica‘Korso’in Northern Ireland

The impact of Melampsora rust on the growth of Salix burjatica‘Korso’was assessed. Comparisons were made between untreated plants and those where the disease had been partially controlled using fungicides. Fungicides delayed the onset of the disease and its effects on growth. Leaf loss was reduced whilst shoot growth and dry matter yields increased. The effect of secondary organisms was also reduced resulting in lower levels of shoot death in the following season.     

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.     

Rust scorings in a plantation of Salix viminalis clones during ten consecutive years

Among pests and diseases occurring in coppiced willow plantations established to produce biofuel, Melampsora rust is the most widespread and severe. In the present study, the variation in rust attacks in a set of Salix viminalis clones was investigated. The aim was to look for trends possibly arising from changes in rust genotype composition, to get some idea about the risks of rust overcoming resistance bred into new willow varieties. Rust levels on 92 S. viminalis clones belonging to 18 different families were scored once each year during 1987–96 in a plantation in southern Sweden. Rust scorings in 1993 deviated most from the average scoring pattern of the clones. Furthermore there was a successive change, such that the longer the time interval between scorings the lower was the correlation between rust scores. Willow families were identified that deviated from the others over time. Various explanations for inconsistency in rust scores over time are discussed.     

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.     

Varietal mixture of poplar clones: Effects on infection by Melampsora larici-populina and on plant growth

The effects of poplar varietal mixture on infection levels caused by Melampsora larici-populina and on plant growth were studied during two consecutive growing seasons. The varietal mixture consisted of an intimate mixture of three poplar clones, Populus x interamericana‘Boelare’, ‘Rap’ and Populus candicans‘NNT’, expressing different race-specific resistances. Infection levels of M. larici-populina were assessed in monoclonal and polyclonal plots. Plant heights were determined at the end of each growing season. Infection levels in the growing season of the first year were not significantly different between monoclonal and polyclonal plots at the end of the epidemic. During the second growing season, a naturally occurring complex pathotype, able to infect clones ‘Boelare’ and ‘Rap’, invaded some plots prematurely and changed the composition of pathogen population. During this season, four infection assessments were conducted. Infection reduction in polyclonal plots compared with monoclonal plots was observed on ‘Boelare’ and P. candicans plants. Nevertheless, no significant difference was observed for growth between monoclonal and polyclonal plots. Biological factors which may be involved in the reduction of infection levels in polyclonal plots are dilution of susceptible individuals, physical barriers caused by resistant plants and cross-protection. A high auto-infection ratio may have partly hidden the positive effects of the clonal mixture.     

Effect of plantation design on stem‐infecting form of rust in willow biomass coppice

Planting of genotype mixtures is a major strategy for the non‐chemical control of willow rust in short‐rotation coppice willows grown for biomass. In 1997 and 1998, the relative severity and spatiotemporal distribution of stem canker damage by Melampsora spp. on Salix viminalis `Bowles Hybrid' in a field trial containing monocultures and two design mixtures (random or regular) of three or five genotypes were assessed. Sphaerellopsis filum, a hyperparasite of Melampsora spp. was detected in all plots during the summer (1998) between the time of the two canker assessments. The percentage of rust pustules with this hyperparasite was initially greater in monoculture plantings than in mixtures in June, but by July, more than 92% of pustules were parasitized, irrespective of plantation design. The stem rust assessments showed that, in general, there were fewer cankers per stool in plots containing mixtures of willow genotypes compared with monoculture plantings. The distribution of rust cankers was different between monocultures and willow mixture designs, with differences between years for monocultures.     

Production losses due to a summer frost in a Salix viminalis short‐rotation forest in southern Sweden

The effect of a single night frost during late spring on the current year production of two clones of Salix viminalis was determined in two monoclonal stands that displayed a gradient from virtually undamaged plants, shielded from outgoing radiation by a neighbouring spruce forest, to heavily damaged plants in the unshielded parts. Compared with the undamaged stand parts, frost caused production losses of 47% and 60% in the two clones during the first year of the second cutting cycle. The clone‐specific allometric relationship between shoot dry weight and diameter was changed by the frost in one clone, showing lower shoot weights in damaged shoots compared with undamaged shoots of the same diameter. In the other clone, allometric relations were not affected by the frost. The relative variation in weight between the stools did not differ systematically with frost damage intensity, and therefore it could not be concluded that frost in monoclonal stands would enforce a competitive hierarchy. It was inferred that the single night frost indirectly can have a negative effect on the production of the stands during the years to come.     

两种柳树无性系幼苗生长和光合特性研究

为进一步比较不同柳树无性系幼苗生长及光合特性的差异,选择优良柳树无性系,以垂爆109柳SalixבChuibao 109’和旱快柳Salix matsudana var.anshanensis为试材,采用Hoagland营养液水培方法,研究正常情况下垂爆109柳和旱快柳幼苗生长性状和光合特性。结果表明:培养45d后,与旱快柳相比垂爆109柳幼苗的叶片数、叶长、根长、根数、基径无显著差异;垂爆109柳的叶宽、叶间距、新枝长、枝条鲜质量和干质量,根鲜质量和干质量,都明显高于旱快柳(p<0.01)。垂爆109柳和旱快柳的净光合速率有明显的差异,净光合速率值在不同光照强度下均表现为垂爆109柳>旱快柳。垂爆109柳的光饱和点和光补偿点都低于旱快柳,表明旱快柳具有较强的光强利用能力,强光照射对旱快柳的抑制作用较低;而垂爆109柳利用弱光的能力较强。     

Variation in leaf phenolics of field‐cultivated willow (Salix myrsinifolia) clones in relation to occurrence of Melampsora rust

Concentrations of potential antifungal phenolics in field‐cultivated willow (Salix myrsinifolia) clones were analysed during three growing seasons, and correlated to the occurrence of Melampsora rust. Consistent relationships between phenolics and rust were not found across the experimental years. There was significant clonal and temporal variation in phenolic content and rust frequency. Levels of some phenolics varied considerably within a sequence of four full‐grown leaves, but the variation in rust occurrence within the same leaf sequence was nonsignificant. The results suggest that the possible association between willow phenolics and rusts is not straightforward, and emphasize the importance of long‐term studies to investigate the chemical basis of willow rust interactions.     

Inter‐ and intraspecific variation and genotype × site interaction in Salix Alba L., S. Dasyclados Wimm. and S. viminalis L.

Three test series were planted on sites selected for their contrasting properties to study the genotype × site interaction. Various clones of Salix alba (8 clones in the 1980 series), S. dasyclados (8 clones in 1982 series) and S. viminalis (6, 9, 13 clones in the various series) were included. Growth capacity, survival and infection by rust fungi were analysed. Interspecific variation was seen at some of the sites in the 1982 series but it was pronounced in the 1980 series where 5. viminalis outgrew 5. alba at all sites. Intraspecific variation could be seen for growth capacity, survival and rust resistance. A genotype × site interaction was detected for all variables studied. Some of the interaction, especially at the species level, can be explained by the variation in susceptibility to rust fungi and preference for soil types. In the breeding work with Salix the genotype × site interaction must be considered.     

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.     

Rust resistance in Salix induced by inoculations with avirulent and virulent isolates of Melampsora larici‐epitea

Six Salix clones and four Melampsora larici‐epitea isolates were used in two leaf‐disc inoculation experiments to determine whether disease severity was affected by the presence of both virulent and avirulent rust genotypes. In the first experiment, an equal amount of urediniospores of a virulent isolate and an avirulent isolate was applied simultaneously using four levels of spore suspension. In the second, the willows were inoculated with one isolate first and then, after 3 days, with another using two spore concentrations. In the first experiment, overall rust spore production was reduced by 48.6% in inoculations with mixed inocula compared with that in the inoculations with single virulent isolates. In the second experiment, 20 of the 36 combinations involving pre‐inoculations with avirulent isolates showed significant reduction in spore production. The suppressive effects on rust sporulation were more obvious at the higher inoculum densities and on the clones S. × calodendron and S. × mollissima‘Q83’.     

Intra-annual growth and mortality of four <Emphasis Type="Italic">Populus</Emphasis> clones in pure and mixed plantings

Intra-annual growth rates were assessed during 3 years for four Populus clones grown in pure and mixed clonal stands at square spacings of 0.5, 1.0 and 1.5 m in western Washington, USA. Height growth rate peaked in August, except at the 0.5-m spacing where it peaked in July and June in years 2 and 3, respectively. Diameter growth rate generally peaked in June. Seasonal growth patterns in mixed clonal stands, relative to those in pure clonal stands, varied by clone. Following early season moisture stress in year 2, growth of a P. balsamifera ssp. trichocarpa clone declined significantly relative to three P. balsamifera ssp. trichocarpa × P. deltoides clones; this trend was most evident at the 0.5-m spacing. During this period of moisture stress, P. balsamifera ssp. trichocarpa had 11 and 36% mortality in pure and mixed stands, respectively, at the 0.5-m spacing; this clone had no mortality at wider spacings during the same period. Although height rankings among clones shifted during the first growing season, rankings at the end of year 1 were very similar to those at the end of year 3.     

Clonal variation in root suckering ability of hybrid aspen (Populus tremula L. × P. tremuloides Michx.)

Current information on the root sucker ability of hybrid aspen (Populus tremula L. × P. tremuloides Michx.) is in most cases based on clone mixtures. In this study, we could separate the performance among clones by using two experimental sites with monoclonal plots of the crossing. The natural regeneration was followed for 2 years after harvest of the formerly planted stands, clear-cut at 22 and 25 years of age, respectively. We found that clonal differences were present in the number of root suckers produced per area unit and their biomass production. However, all included clones produced sufficiently many root suckers for a successful regeneration. To receive a more complete evaluation of the root sucker dynamics for future selection on the clonal level, further studies are needed where vitality, self-thinning and effects of thinning measures are coupled to the goal of the aspen forestry.     

Detection and transport of soluble antigens of Erwinia salicis and their role in symptom expression of the Watermark disease

An inproved method for the detection of soluble antigens of Erwinia salicis in leaves of Salix alba by ELISA is described. These antigens play a role in symptom expression of the Watermark disease. The antigen concentration in the leaves fluctuates strongly during the growing season. Presence of E. salicis inside willow trees could be determined without disturbing both host and pathogen.     

Life-cycles and ecology of willow beetles on Salix viminalis in England

The life-cycles of three willow beetle pests, Phratora (= Phyllodecta) vulgatissima, Phratora (=Phyllodeeta) vitellinae and Galerucella lineola (Coleoptera, Chrysomelidae), were investigated during 1994–95 in a plantation of short-rotation coppiced willows (Salix viminalis cv. Bowles Hybrid) at Long Ashton, Bristol, UK. The P. vulgatissima had one generation during the year. Overwintered adults emerged from hibernation in April and after a short feeding period, copulation and egg-laying occurred. Larval stages were found from May–July. The new generation of adult beetles appeared in July–August and fed for a while before hibernating. Hibernating adults were found in hedgerows around the site and in the plantation on willow stools and ground vegetation. Dispersal of adult beetles between feeding and overwintering sites appeared to be fairly localized. Hence, crop management strategies that reduce the potential for overwintering in and around plantations may help to minimize spring re-invasion and damage. Although based on limited observations, the biology of P. vitellinae and G. lineola appeared similar to that of P. vulgatissima. However, there is evidence in the literature that both these species, unlike the latter, can have a partial second generation in some years. The potential effect of insect herbivory on the growth and biomass yield of S. viminalis cv. Bowles Hybrid was investigated experimentally by artificial hand-defoliation of pot-grown plants. Yield losses were strongly correlated with the amount and time of defoliation. Results indicated that even slight or moderate damage by insect herbivores could have a significant impact on the biomass productivity of coppiced willows.     

The significance of rotation periods for mycorrhiza formation in Short Rotation Coppice

Rotation periods control not only the above-ground growth but also the assimilate transfer to the root systems in Short Rotation Coppice (SRC). Since assimilates are needed for the nutrient supply of associated mycorrhizal fungi, their control by rotation period length seems most probable. One poplar (Populus nigra × maximowiczii cv. Max 4) and one willow clone (Salix viminalis clone 78–101) cultivated as SRC were investigated on their ectomycorrhiza formation in response to 15 years of continuous different rotation periods (three and six years) at the same test site in Northern Germany. On the poplar clone the frequency of ectomycorrhizae was significantly lower in 6-year than in 3-year rotation. On the willow clone frequency of ectomycorrhizae was not significantly affected, but the portion of dead fine roots was significantly higher in the 6-year than in the 3-year rotation in autumn. In both rotation systems, the frequency of ectomycorrhizal (EM) colonisation was significantly higher in autumn than in spring. Five EM morphotypes were found on the poplar and seven on the willow clone. EM morphotypes which were common on both clones were formed with two fungal partners of the Pezizales (Geopora cervina, Tuber rufum), one of the Agaricales (Laccaria sp.) and one of the Thelephorales (Thelephoraceae). In spring G. cervina constituted the largest part of all observed EM morphotypes on P. nigra × maximowiczii and S. viminalis. The results indicated a selective promotion of EM formation of some Pezizales (Tuber and Peziza spp.) and some Agaricales (Laccaria spp.) due to shorter rotations, and a selective promotion of other Agaricales (Inocybe sp.) and Boletales (Scleroderma spp.) due to longer rotations. This might allow selective manipulation of the mycorrhizal diversity by the selection of the rotation system. A future challenge will be to select which mycorrhizal diversity might be more advantageous for the vitality and biomass production of poplar and willow clones.     

Interactions between Melamspora larici‐epitea pathotypes and the mycoparasite Sphaerellopsis filum from willow rusts

Five pathotypes of the willow rust Melamspora larici‐epitea were inoculated with 12 isolates of Sphaerellopsis filum derived from Melampsora species/forms occurring on willows. On average, 20.5% uredinial pustules produced S. filum pycnidia and rust spore production was reduced by 38.4% on leaf discs inoculated with S. filum. Some rust isolates were more readily infected by S. filum than others while some S. filum isolates caused higher levels of infections than other S. filum isolates. In general, the suppressive effects of these S. filum isolates on rust spore production were similar on the majority of rust pathotypes tested. There appeared to be a positive link between the rust pustule area and the rate of infection by S. filum. Sphaerellopsis filum inoculum densities were positively correlated with the reduction in rust spore production but not with the number of rust pustules. Implications from the results were discussed in relation to the deployment of S. filum in biological control of willow rust in willow mixture plantations which harbour more diverse rust pathotypes compared with monocultures.