Objective 10 - Kim Hammond-Kosack (RRes)
Take-all disease (RRes)
Background
The classic black roots and white heads caused by the take-all fungus Gaeumannomyces graminis var tritici (Ggt) are usually minimal in first wheat crops grown after ‘clean’ break-crops (ie in the absence of susceptible grass weeds or volunteers), but second and third wheat crops can be severely affected. For many decades wheat breeders searched for germplasm which was either resistant or even less severely affected by the disease. The last efforts began at the PBI in Cambridge in the mid-1980s (just prior to privatisation) and were unsuccessful (Scott and Hollins, 1988, PBI annual report). Fortunately, the chemical industry had greater success and two products are now available on the market as seed dressings. These both give an intermediate level of disease control accompanied by modest but profitable increase in yield, approximately 1 tonnes/ha. These chemistries are marketed under the trade names of Latitude (Silthiofam) and Jockey Flexi and Galmano (Fluquinconazole). More recently, application of the fungicides Azoxystrobin (Amistar) and Fluoxastrobin at the T1 stage has also been shown to lower take-all severity to some extent. However, despite these control measures severe take-all can still occur, particularly in high disease pressure years. Severe take-all causes the plant to die prematurely and as a consequence the crop does not make full use of the nitrogen available. Recent studies within WGIN 1 has shown that upto three times more mineral N is left in the soil where severe take-all has occurred compared to areas where the disease was less severe. The distribution of the soil N varies depending on rainfall. In wet summers the N is pushed futher down the soil profile and the potential for leaching into the waterways is high causing pollution problems. Rotation practices change frequently according to the profitability of the break-crops. In the late 1990’s there was a move back to short rotations, where wheat was the dominant crop. However, even a shorter rotation of alternating wheat with oats, oilseed rape or potatoes is in operation on some farms. Overall, this has put more crops at risk from take-all disease, even first wheat crop in some situations, as was the case this year. Defra’s current policies aim to try to maintain a high diversity the crops grown on agricultural land. However, for the reasons stated above take-all disease can still remain a problem even in short diverse crop rotations. If the project described below is successful and wheat resistance or semi-resistant to take-all infection and / or inoculum build start to become available this would probably increase the practice of continuous cereal production (ie. reduced crop diversity in the landscape) . But with careful crop management, if these two distinct types of new genotypes are used in concert, then the take-all decline phenomena could potentially be accelerated into the second wheat cropping year thereby increasing overall farm profibtability whilst at the same time reducing the risk of a disease crop leaving a high environmental footprint.Objectives
1. To identify wheat germplasm resistance to Take-all2. Genetic analysis of resistance to take-all in hexaploid and non-hexaploid wheats.
3. Introgress resistance to take-all from different non-hexaploid wheats.
4. Identification and characterisation of hexaploid wheat germplasm which reduce take-all inoculum build up (TAB) in soil.
5. To explore the genetic basis of take-all inoculum build up.
Work plan
i. Identification of wheat germplasm resistance to take-all
Hexaploid wheat. To screen the AE Watkins Collection and an ‘improved’ Gediflux Collection to identify potential additional sources of resistance. This will be done using four successive field trials on land which has already built up a high level of natural take-all inoculum in the soil and follow up pot tests. Included within the trial will be a series of both positive and negative controls (see WGIN Management Meeting ppt KHK Oct 2007 for full experimental design). In each year the number of genotypes screened will diminish as the most susceptible are eliminated. Both of these hexaploid collections will have been genotyped using DArT markers (see above - resources section). Therefore association genetics studies can be done in an attempt to locate regions of the genome influencing take-all infection, colonisation and disease formation.
ii. Genetic analysis of resistance to take-all
a. Hexaploid wheat. To create and then screen a double haploid population between the already identified resistance source variety X and a contrasting hexaploid wheat. The production of this DH population is already in progress. It will take two years to produce sufficient seed to do pot bioassay and then field tests.
b. Diploid wheat. The generation of three mapping populations is currently in progress using accessions A and B using each other as well as a suitable susceptible parent with a contrasting genotype. Screening for take-all resistance will take place in the F3 generation. The F1s between accessions A and B will be tested in the pot assay to determine their combined resistance phenotype.
c. DArT marker maps will be developed for up to three mapping populations developed to explore the genetic basis of resistance to the take-all fungus. The final selection of the populations screened is dependent upon the phenotyping results obtained.
iii. Introgression into hexaploid wheat
The resistance source variety Y (a non-hexaploid wheat) is so far the best identified. F1 seed is currently setting, between variety Y and the resistant hexaploid wheat variety X as well as between variety Y and known susceptible hexaploid wheats. The resulting F1 populations will be tested in pot assays to determine their combined resistance phenotype. These F1 will then be used in a backcrossing introgression programme, once a linked marker(s) have been identified to the locus/loci controlling this important trait.
iv. Introgression from diploid wheat
Both resistant accessions A and B have been crossed to the highly fertile T. monococcum accession PI355520 which carries both crossibility genes. At RRes, we have already found this accession to generate fully fertile progeny when crossed to the hexaploid wheat Chinese Spring. This will be the starting point for marker assisted introgression, once results from the mapping experiment become available. Graham Moore at JIC already has identified closely linked markers to the crossibility genes and these will be used to maintain the both loci during introgression.
v. Identification and characterisation of hexaploid wheat germplasm which reduce take-all inoculum build up (TAB) in soil
Continuation of the diverity screen. In WGIN 1, the choice of genotypes screened for this trait was solely determined by the needs of the NUE trial. In the next research period, we plan to carry out a separate trial which will focus on the trait take-all inoculum build up (TAB). In addition those selected for the NUE diversity trait will also be explored at the 200kg/N rate only. In collaboration with the breeders we will select up to 30 genotypes to explore for variation in take-all inoculum build-up in a 1st wheat situation. These experiments will only be done at a single N rate, typically 150-200kg/N. Initially, the determination of the soil inoculum will be done using both the pot bioassay and the new Predicta B DNA based test. The latter is currently under evaluation using a HGCA-Bayer funded project. If the results of the Predicta B test correlate well with those of the pot bioassay, then by project year 3 the Predicta B test will be used because it is faster, cheaper and less time consuming. It is also planned that after each year’s trial, the entire site will be crossed drilled with between two and four contrasting current commercial varieties to explore the influence of the take-all build up on the yield of the following wheat crop. This will give the first indication of the cropping sequences which may lead to the lowest take-all disease problems in 2nd wheats.
In WGIN 1, the variety Paragon was not initially selected for the assessment for TAB. However in year 5 Paragon has been included for the 1st time. If Paragon is shown in replicate years to minimise TAB, then experiments will be planned from year 3 onwards to include selected lines from the Paragon gamma ray irradiated population developed at JIC in WGIN 1/2 to explore specific chromosome regions suspected of controlling TAB.
vi. To explore the genetic basis of take-all inoculum build up
The experiments done in WGIN I revealed that the varieties Avalon and Cadenza consisted produced contrasting levels of take-all inoculum build up. For example in 2007 Cadenza built up a low level of inoculum (approximately 35%) whereas Avalon built up high inoculum levels (> 80%). Therefore we plan to take advantage of the Avalon x Cadenza DH mapping population developed within WGIN I, to determine the genetic basis of this difference. A full five years of field trialling will be required because of the anticipated differences in the annual level of take-all build-up which are highly weather dependent. To relate the soil inoculum levels to infectivity levels in the follow crop, the entire trial site will be sown at the end of year1 to the susceptible cultivar Hereward and the levels of root disease will be determined at the end of the following field season. If a genetic marker / genetic markers can be found linked to this trait, then these will be of great value to the plant breeding and research communities.
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