|Take-all: new possibilities for progress?
- a workshop at Fitzwilliam College, 24 February, 2006
See attached list for attendees.
Notes from final discussion.
Importance of take-all
BWB (British Wheat Breeders) consider it an important ‘intransigent’ disease which has been neglected and urgently needs a programme of basic and applied research.
Most of the discussion focussed on control approaches which might be delivered ultimately through plant breeding. However, it was recognised that:
Fungicide treatments have provided a useful level of control, but current treatments need to be used in conjunction with other control methods if second wheats are to be economically viable. Development of enhanced active substances was the province of the agrochemical industry.
Introduced biocontrol agents had not, so far, produced reliable control in the field, despite prolonged efforts by international research groups.
Enhancing naturally occurring antagonism through, for example, interventions in the inter-crop period might provide a viable approach.
Approaches to mitigating effects of the disease through the host could be broadly divided into avoidance of effects and increasing host resistance.
The most obvious avoidance strategy is not to grow wheat, which then poses the question “why is a second wheat grown?” The most likely market is for feed/bioethanol where bulk and hence profitability is the key driver.
Strategies might therefore be
breed for maximum yield in the presence of take-all
breed for low input but maximum margin/profitability
change crop species. If the required output is starch; could substitute maize or other immune or less susceptible crop (e.g.? triticale, rye or oats). However, the economics of these crops are currently marginal.
Strategies to avoid effects of take-all
Tolerance of disease. Assuming the current WIGIN experiments confirm some varieties as poor second wheats, identifying traits associated with poor performance, and hence the desirable alternatives, is important. Mapping should follow, with the possibility of stacking good traits if more than one mechanism is identified to give enhanced tolerance. Examples of traits suggested include root density profile (without high tiller death), and associations with other important characters of wheat such as the 1B/1R translocation, Pch1, dwarfing genes, major alien introgressions.
Primary infection is critical for epidemic development. Varietal differences in primary and secondary infection might be exploited. There may be a trade-off between low root density to minimise primary infection and the high root length density required to tolerate infection.
Hydroxamic acids. The glucoside DIMBOA is preformed in roots of maize and wheat and released into the rhizosphere. This compound has been shown to inhibit Ggt in culture so might have a role in disease suppression. Do wheat varieties differ in the rate/timing/amount produced?
Ability to utilise manganese. Considered an important characteristic of ‘good’ varieties in Indiana. Anacdotal evidence suggest UK varieties differ in tolerance to manganese deficiency (commonly seen on high organic content soils) and tall varieties are said to perform better than short ones. Observational evidence suggests Consort is intolerant of deficiency.
Strategies to increase resistance:
Transfer resistance from related species:
Rye has good resistance but this has not been demonstrated in wheat/rye addition lines. However, wheat/rye substitutions have not been comprehensively assessed. If resistance is associated with one or two chromosomes then prospects for transfer are good.
Aegilops squarrosa: Chromosome 6D in wheat enhances resistance. D genome transfers to wheat should readily occur. Other accessions of Ae. sqarrosa might have further useful genes. Stacking, with marker assisted selection should be possible.
Other: Agropyron (Elymus) caninum and Hayaldia (Dasapyrum)villosa have both been reported with resistance. Transfer to wheat will be more difficult due to genome divergence. Confirmation of resistance in a source stock is an essential prerequisite.
Oat resistance: Resistance of oats to Ggt is almost complete. However, Gga isolates exist which can severely damage oats but distribution evidence suggests these to be competitatively unfit. The extent to which exposure of the oat resistance would increase these is unclear but it might be possible to modify the genome sector of oats responsible for the avenacinase mediated resistance (appro 600kb region) to use this resistance in wheat.
Population dynamics: Interactions of varieties and rotations with the population structure of Ggt need to be better understood.
Assessment of resistance:
Methods used to test for resistance are frequently inadequate. Much work has been done with seedlings grown for limited times in soil, sand or other inert media after, often heavy, inoculation. Such tests might not effectively demonstrate the ‘field value’ of the material and will rarely be suitable for the single plant tests needed for introgression studies. Methods need to be devised to more accurately identify resistance and distinguish between escape, resistance and disease dilution.
Resistance breakdown/pathogen evolution
The take-all pathogen has evolved in mixed swards where microbial antagonisms are extensive. The view was expressed that this may have resulted in limited pathogen variation within the fungus minimizing the danger of evolution/change of the pathogen to overcome new resistance introduced to wheat.
Pathogen variation/inoculum survival
Pathogen variation nevertheless exists but at the level of host species (var. avenae, and also to rye), sensitivity to silthifam and epidemic stage. It is not yet clear to what extent, if any, these overlap. The var. avenae strain is seldom found on wheat whereas the rye-attacking strain is frequently isolated from wheat roots. This strain does not appear to jeopardize the resistance of rye in agriculture. Survival of the fungus is poor without the host (wheat, barley or some grasses), and is least under bare fallow. It may be possible to manipulate the intercrop period to minimise viable inoculum carryover.
Sequence the Ggt genome:
This is considered worthwhile as it may lead to new understanding and possibilities for control. An expression of support for a US consortium was underway.
Funding sources/proposal strategies
Options include DEFRA/Sustainable Arable LINK, BBSRC including the link with INRA and, importantly, international collaboration opportunities.
Positioning of a project would be critical to get funds from a particular source. For example, using take-all as a model system of a root pathogen or using a particular character/trait to investigate a host/pathogen interaction.
List of participants
University of Nottingham
University of Cambridge
University of Leicester
University of Cambridge
Not present: Thomas Jollife, Roger Williams