Maximising disease escape, resistance and tolerance in wheat through genetic analysis and agronomy

Abstract

Three mechanisms can act in sequence to reduce yield loss caused by foliar diseases: escape inhibits spore transfer to the upper crop canopy, resistance reduces the capacity of spores which arrive on the upper leaves to infect and cause symptoms, and tolerance reduces the impact of symptoms on yield.  This research tested the extent to which escape, resistance and tolerance might be improved in order to contribute to a reduction in disease and yield loss, and hence reduced dependence on fungicides.

Disease escape: Near-isogenic lines (NILs) of wheat were used to test the effect of major genes on escape traits against Septoria tritici.  These lines differ in the presence or absence of single genes known to be of agronomic importance (dwarfing (Rht) genes and photoperiod insensitivity (Ppd) genes).  So the effects of those single genes on candidate traits which might affect disease escape, could be measured in lines which share a common level of disease susceptibility. Field experiments demonstrated that the benefit from escape can be equivalent to a well timed full rate application of triazole fungicide.  High levels of dwarfing from Rht3 and Rht12, cause increased spore transfer and lead to earlier epidemics.  Rht2 reduced height to an agronomically acceptable extent, whilst maintaining reasonable escape.  Rain shelter studies demonstrated that escape effects are robust, as they are expressed both in the presence and absence of rain-splash events which transfer spores.  A cross was made between parents selected to contrast for traits thought to confer escape (Avalon and Cadenza), in order to create a mapping population, comprising large numbers of true-breeding (doubled-haploid) lines that vary for the candidate traits.  Assessments demonstrated that improved escape could be achieved in taller plants, but, more importantly, some short lines demonstrated low disease. 

Disease resistance:  Most of the genetic resistance of wheat varieties against the important foliar diseases is partial and quantitative in effect.  The field expression of variety resistance is subject to considerable environmental variation.  This variation has implications for the assessment of resistance in breeding and variety evaluation, and for the exploitation of resistance by the industry.  Pathogen growth is dependent on access to nutrients - principally nitrogen (N) and carbon (C) - via the tissues of the host crop.  This project tested the extent to which variation in nutrient availability, due to differences in weather and agronomic inputs, might explain variation in the susceptibility of host tissues.  Foliar pathogens with contrasting modes of nutrition - biotrophy and necrotrophy - were used for the studies.  Variation in carbon (C) nutrition had only a small effect on epidemics.  N, varied by crop nutrition, caused large and consistent effects on both Phaeosphaeria  (Septoria) nodorum and Puccinia striiformis.  Specific leaf N (g m 2) explained 61% of variation in the potential size of the epidemic.  Clearly something other than availability of green leaf area was limiting disease, since low N leaves had significant green area remaining as disease progress slowed.  This is hopeful in terms of constraining disease through crop N nutrition because 1.5 g m 2 specific leaf N, which substantially limited disease carrying capacities, is sufficient for efficient leaf photosynthesis in UK conditions.  In laboratory experiments, P. nodorum levels increased with N supply, and ammonium nutrition caused more disease than nitrate.  Both effects were correlated with increases in leaf amino acid content.  In vitro cultures indicated that growth and sporulation of P. nodorum is promoted by certain amino acids (particularly asparagine) and inhibited by others. Surprisingly, N effects on P. nodorum were consistent with effects on P. striiformis.  Thus, similar effects on the important 'hemi-biotroph' S. tritici might be expected. 

Disease tolerance: Evidence was found for significant variation in tolerance between elite wheat genotypes.  There was an association between high attainable yield and greater loss per unit disease (intolerance), which may explain a proportion of the trend towards greater disease-induced losses - and hence greater fungicide requirement - in high yielding, modern wheats.  Direct selection for tolerance in the large number of lines handled in breeding programmes is impractical.  However, tolerant lines might ultimately be selected by phenotyping for tolerance traits (where these are visible and readily assessed) or by testing for genetic markers tightly linked to quantitative trait loci QTL.  NILs were available from the JIC collection to test the effect on tolerance of the major genetic changes in wheat over recent decades.  A doubled-haploid mapping population was derived from a cross between Cadenza x Lynx, known from previous work to contrast for traits thought to affect tolerance.  The results suggest that most of the variation in tolerance relates to variation in resource capture by the leaf canopy.  Other factors, such as stem carbohydrates and loss of green area per unit disease appear to be either neutral, deleterious or exhibit insufficient phenotypic variation to allow selection.  The aim now is to identify determining mechanisms and traits of tolerance which are compatible with efficient resource capture, by quantifying the effects of the more influential traits.