Vanessa E. McMillan PhD student
vanessa.mcmillan@rothamsted.ac.uk

Identification and characterisation of resistance to the take-all fungus in wheat

Take-all is an important root disease of wheat and is a major constraint on wheat productivity in the UK and around the world. The disease is caused by the soil dwelling ascomycete fungus Gaeumannomyces graminis var. tritici (Ggt). Black necrotic lesion form on the roots of take-all infected plants and when severe can cause stem base blackening resulting in premature ripening and stunting of the wheat crop (Figure 1).

Figure 1. Take-all symptoms in the field and the effect on root health and grain quality.

In the absence of a living host Ggt survives saprophytically on dead roots and stem bases and this forms the main source of inoculum for the next susceptible crop. Ggt is a poor saprophytic competitor so take-all inoculum levels decline rapidly in the absence of a living wheat crop. A first wheat usually has negligible take-all but can build-up sufficient inoculum in the soil during the year to cause severe disease in a second susceptible crop. 

In recent years as part of the defra funded core project within the Wheat Genetic Improvement Network (WGIN) and my initial PhD studies funded by the BBSRC and HGCA the build-up of inoculum under first wheats has been measured. This study revealed that consistent differences do exist between wheat varieties in their ability to build-up take-all inoculum in the soil when grown as first wheat crops (Figure 2). These results have recently been published in Plant Pathology (McMillan et al., 2011).

Figure 2. Percentage of roots infected in a bioassay using soil cores taken after harvest from plots containing different winter wheat cultivars grown as first wheats - 4 year means.

The majority of the wheat varieties used in the WGIN field trials were not currently grown wheat varieties and did not feature on the UK national and recommended winter wheat variety lists of the time. Information on more current wheat varieties is required and as part of my PhD I am investigating the take-all inoculum build-up (TAB) trait in varieties on the current HGCA (Home Grown Cereals Authority) recommended elite winter wheat variety list in the UK. Genetic and pedigree analyses of this trait are underway to identify the sources of the trait within the elite germplasm pool. Results from this work will help to give an indication of which varieties could be grown as a first wheat crop in order to reduce the risk of severe take-all in a second wheat crop.  For example, by growing a low TAB variety as the first wheat,  then the likelihood that the second crop will suffer significant economic losses as a result of take-all infection will be considerable less than if a  high TAB variety is grown as a first wheat (Figure 3).

Figure 3. How growing different wheat varieties in Year 1 could influence the disease outcome in the following crop. Growing a low building variety in Year 1 could reduce the risk of severe disease in the following wheat crop and so allow farmers to grow more profitable second wheat crops.

Within the WGIN project (objective 10) two major quantitative trait loci (QTLs) have been identified which confer the low TAB trait in the cultivar Cadenza. This is fully described in the pdf of the WGIN stakeholder newsletter (November 2010) available at the bottom of this webpage.

Wheat germplasm is also being assessed for tissue based resistance to take-all in pot bioassay tests and field trials. Epidemiology studies and microscopy analyses are being carried out to characterise wheat genotypes giving partial resistance to take-all within current wheat varieties, diploid wheat genotypes and older germplasm collections. The identification of resistant germplasm could help provide more durable disease control, significantly reduce yield losses due to take-all and give farmers more freedom in rotational cycles.

My project is funded by a CASE studentship from BBSRC with industrial support from HGCA.

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Publications

McMillan VE, Gutteridge RJ, and Hammond-Kosack KE (2011) Evidence that wheat cultivars differ in their ability to build-up inoculum of the take-all fungus, Gaeumannomyces graminis var. tritici, under a first wheat crop. Plant Pathology 60: 200-206.

Abstract
The effect of wheat cultivar on the build-up of take-all inoculum during a first wheat crop was measured after harvest using a soil core bioassay in field experiments over five growing seasons (2003–2008). Cultivar differences in individual years were explored by analysis of variance and a cross-season Residual Maximum Likelihood (REML) variance components analysis was used to compare differences in those cultivars present in all years. Differences between cultivars in the build-up of inoculum were close to or at significance in two of the five trial years (2004 P < 0.05; 2006 P < 0.07), and current commercially listed cultivars were represented at both extremes of the range. In 2007 and 2008, when environmental conditions were most favourable for inoculum build-up, differences were not significant (P < 0.3). In 2005 the presence of Phialophora spp. at the trial site restricted the build-up of take-all inoculum under all cultivars. The cross season REML variance components analysis detected significant differences (range: 3.4–47.8% roots infected in the soil core bioassay; P < 0.01) between the nine cultivars present in all years (excluding 2005). This is the first evidence of relatively consistent differences between hexaploid wheat cultivars in their interactions with the take-all fungus, and this could give an indication of those cultivars that could be grown as a first wheat crop, in order to reduce the risk of damaging take-all in a second wheat crop. This phenomenon has been named the take-all inoculum build-up (TAB) trait.

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Science Communication

WGIN stakeholders newsletter, November 2010

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