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QLK5-1999-01585
Integrated, non-fungicidal control of melampsora rusts in renewable energy willow plantations |
| Contract No: | QLK5-1999-01585 |
| Source: | Final Report 2005 |
Our studies concentrated on key elements of the complex rust/willow pathosystem in western, northern and Central Europe in order that an effective and sustainable management system could he developed. The achievements from this work have well exceeded original expectations in many aspects, such as molecular characterization of rust species/forms, pathogen gene flow and life-cycle, relationships in epidemiological parameters, characterization of rust resistance, effects of host mixture on disease and yields. The findings from this study have profound impacts on our understanding of the rust/willow pathosystem and directly contribute to implementing integrated, non-fungicidal control of the most serious disease in the most important energy crop in Europe.
Genetic variation, gene flow and epidemiology of Melampsora
Several species of Melampsora are common in Central, northern and western Europe. For biomass willows, M larici-epitea is the most important pathogen species. Within M larici-epitea, three formae speciales, larici-epitea typica, larici-retusae and larici-daphnoides, are most prevalent on willows for hiomass. The same situation exists in Central and northern Europe.
Melampsora capracarum is predominant on S. caprea and S. caprea hybrids; it infects both leaves and stems and, when severe causes defoliation and stem die-back. SIF was found to cause severe infection on S. viminalis Bowles Hybrid' but its distribution appears to be restricted to the British Isles.
Melampsoralarici-epitea, M. capraearum, SIF and M. epiphylla share a common ancestral lineage. The consistency in ribosomal DNA sequence in species/forms provides promising ground for the development of species-specific PCR primers to identify Melampsora species on Salicaceae. This approach can be particularly useful in identifying those species or forms which are morphologically indistinguishable but phylogenetically distant.
The major pathotypes the formae speciales LET, LR and LD of M. larici-epitea are genetically differentiated.
Drastic breakdown of rust resistance is often caused by the migration of new virulent pathotvpes. Different sources of inoculum may contribute to the initial new pathotype population. As the pathogen is capable of actively undergoing a sexual life-cycle, the original differences become less and the gene flow in the rust may prevent local populations evolve separately.
The rusts with shortened life-cycle, such as SIF on S. viminalis may overwinter as uredinial stage and re-infect the telial hosts in next growing season to continue their existence. In contrast typical life-cycle of the heteroecious, marcrocyclic rusts involves over-wintering as teliospores and producing basidiospores in the next spring to infect theft alterate hosts. For willow rusts, the ability to infect willow buds/stems is essential in over-wintering as uredinial stage.
It appears that some populations of the rust on S. viminalis in Northern parts may over-winter as uredinial stage or as telial stage but they are genetically highly homogenous. There is no evidence the rust on S. viminalis in northern and Central Europe over-winters as uredinial stage.
Disease development and yields in host genotype mixtures
The disease progress curves of the varieties growing as mixtures are shallower, that is the disease progresses more slowly in mixtures. This illustrates that diverse, intra-species mixtures of Salix spp. and varieties help to reduce the impact of rust disease, both in terms of delayed disease onset and final disease levels.
There was striking evidence of the mixture effects on biomass yields when the dry matter yield per unit area in mixtures plots was compared to an equivalent area of the components grown in monoculture, hi all cases the dry matter yields were higher for the mixtures plots. If the yields from mono plots were set as lUO% then the yields in mixtures were 141% to 157% with regards to that from mono plots.
Survival rates of different clones vary in mixtures. There was a trend that the highest differences in survival rate occur at the highest densities. The range of differences between the individual mono-plots was smaller than that for the various mixtures.
The impact of mixture plots composed of a single species in reducing disease levels on susceptible clones was limited and somewhat variable. However, results did indicate that there were important interactions occurring within the mixtures which reduced the disease on susceptible clones.
A straight S. viminalis mixture can function as a mixture in reducing disease and stimulating yield. However, more diverse mixtures comprising intra- as well as inter-species genotypes are to be recommended for planting.
Potential of biocontrol with the mycoparasite Sphaerellopsis filum
The issue of host specificity bears great significance in biologicai control as more virulent strains of S filum would be more effective in suppressing rust disease development The results from the present study clearly demonstrated S filum varies in its pathogenicity to willow rust.
Relatively small number of S filum spores are required to establish infection of rust pustules and under suitable conditions, S filum produces a large number of spores (a pycoidium may produce more than 1000 conidia per day). There was a close correlation between S filum inoculum and percentage infection of rust uredinia / the number of pycnidia. Such information can be useful in estimating the biocontrol efficacy of S filum in plantations.
There is considerable genetic variation in S filum and the fungus on Salicacea does not have a clonal lineage.
In willow plantations, asexually cycling S filum conidia are responsible for epidemics of S filum on rusts in the growing season.
To date, it is not certain whether the field populations of S. filum occurring on willow Melampsora undergo sexual life-cycle.
Overwintering as sexual teliospores are obligator in the LET4 rust examined. The main sources of the primary inoculum in western Europe appear to be newly recombined rust genotypes which are produced on the alternate host larch.
In Central Europe, the onset and the severity of rust disease could be assigned to several different patterns. Between the years there were slight differences in the clones assigned to different patterns.
Rust resistance in willow
Profiling host/pathogen reactions using a digital caniera greatly improved efficiency and accuracy of disease assessments. Using this method, a large number of samples can be processed in a single inoculation experiment and the reactions on leaf discs can be quantified. Furthermore, a range of resistance reactions, which could not be adequately recorded previously, such as discolouring and hypersensitivity, can be accurately profiled. This procedure has the potential to significantly improve the efficiency of characterisation of host resistance / pathogen virulence in studies of various rust diseases.
Willow rusts are highly efficient in infecting their hosts and capable producing a large number of spores (a pustule may produce up to 3000 spores per day).
Irrespective of the willow clone/rust isolate combinations, reasonable correlation was obtained between spore production and log pustule area. Therefore the pustule area measurement can be used to quantify the disease and pathogen spore production.
Genotypes in S. viminalis, the most important species for biomass in Europe, are generally lack of strong resistance to rust. In contrast, several Far-Eastern species related to the species are highly resistant. It suggests that there is a great potential to breed novel varieties from Sect. Vimen by incorporating resistance genes from different species.
There was a good correlation between the results from leaf disc tests and that from field disease assessments. It suggests that leaf disc inoculation test remains an effective means of characterising host resistance/pathogen virulence in Melampsora on poplar and willow because of its efficiency and simplicity.
Caution should be given that rust resistance shown in one geographical region may not be effective in another geographical region. In the future, testing novel clones for resistance against relevant rust populations before large scale release would be important in evaluating possible risks of rust outbreaks as a result of introduction of rust from different geographical regions.
In the cross between S. viminlais and S. schwerinii, the majority or the progeny were more resistani compared with their parents. indicating that the resistance may have somehow dominant effects. It is possible that the genes for resistance may be linked.
For low-input crop systems such SRC willow, one of the most effective options for disease management is to deploy resistance genes against pathogens. Breeding for resistance against diseases and pests is an immediate, yet vitally important task. Our studies on rust resistance in willows have clearly shown that natural sources of resistance have great potential for use in breeding against Melampsora rusts.
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by CPL Press:
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