Wheat lines exhibiting variation in tolerance of Septoria tritici blotch differentiated by grain source limitation

Septoria tritici blotch (STB) is the most damaging disease of wheat crops in Europe. Because of the partial nature of genotypic resistance or the increasing resistance against fungicides, the tolerance, i.e. maintaining yield in the presence of expressed disease, is a relevant alternative. Tolerance is generally estimated through the yield loss per unit of source reduction, contrasts of tolerance between genotypes have been observed previously suggesting that either increasing the source availability or improving the use of stored assimilate could improve tolerance. This paper aims at developing a source/sink approach to understand the tolerance mechanism and identifying potential traits to increase tolerance of STB. A field experiment was designed to explore the relation between tolerance of STB and source/sink balance. Based on six wheat genotypes contrasting for tolerance exposed to natural STB epidemics, late nitrogen fertilization and a 50% spikelet removal were applied to change the source/sink balance. The tolerance of genotypes was quantitatively estimated over three additional field experiments. We found that STB tolerance was correlated with traits of healthy crops (high individual grain weight and high green leaf lamina area as the proportion of leaf 3). The spikelet removal revealed a highly variable degree of source limitation for grain filling amongst the six genotypes. Thus, we proposed an easily calculated index that highly correlated positively with the labor intensive estimation of STB tolerance. Finally, potential yield and tolerance were not correlated, which suggests that breeding for yield performance and tolerance could be possible

Predictability of wheat growth and yield in light limited conditions

In seeking better predictions of grain yield under light-limited conditions, shading was applied to field-grown winter wheat cv. Slejpner during each of five consecutive phases (canopy expansion, ear expansion, pre-flowering, grain expansion and grain filling). Absolute measures were taken of solar radiation and its effects on growth in three seasons, at a site where water and nutrient supplies were not limiting. Replicate mobile shades automatically occluded 0. 80 of incident light when mean total solar radiation exceeded 250 Jm2 per s. Mean effects over seasons of shading on incident total solar radiation were x 296, x 139, x 78, x 157 and x 357 MJm2 for the five phases respectively, and corresponding effects on shoot dry weight were x 236, x 184, x 58, x 122 and x 105 gm2 . Estimated efficiency of radiation use after flowering was 1.2 gMJ unshaded, tending to increase with shading. Shading in all phases reduced grain dry matter yield: mean effects over seasons were x 106, x 64, x 61, x 93 and x 281 gm2 for the five consecutive shading periods. Shading from GS31–39 increased mean maximum area of the two top leaves from 2700 to 3100 mm2 per leaf but, with fewer stems,canopy size remained unaffected. This and the next shading, from GS39–55, reduced specific leaf weight from 42 gm2 by 4 and 3 gm2 respectively, but effects on shoot dry weight were largely due to stem and ear. By flowering, stem weights, and especially their reserves of water-soluble carbohydrates, had partially recovered. Effects on yield of shading from GS31–39 were explained by a reduction in grainsm2 of 3070 from 26 109. Shading from GS39–55 reduced grainsm2 by 4211 due to fewer grains per ear, whilst mean weight per grain increased in compensation. Shading from GS55–61 decreased grainsear by 2.5. Shading from GS61–71 decreased ear growth and reduced stem weight, and at harvest resulted in 4. 3 less grainsear. Effects of the final shading from GS71–87 were fully explained by a reduction in mean dry weightgrain of 10. 3 mg. Except for shading from GS71–87, source- and sink-based explanations of grain yield both proved feasible, within the precision of the measurements. Constraints to accurate comparison of source- and sink-based approaches are identified, and the implications for yield forecasting are discussed

A foliar disease model for use in wheat disease management decision support systems.

A model of winter wheat foliar disease is described, parameterised and tested for Septoria tritici (leaf blotch), Puccinia striiformis (yellow rust), Erysiphe graminis (powdery mildew) and Puccinia triticina (brown rust). The model estimates diseaseinduced green area loss, and can be coupled with a wheat canopy model, in order to estimate remaining light intercepting green tissue, and hence the capacity for resource capture. The model differs from those reported by other workers in three respects. Firstly, variables (such as weather, host resistance and inoculum pressure) which affect disease risk are integrated in their effect on disease progress. The agronomic and meteorological data called for are restricted to those commonly available to growers by their own observations and from meteorological service networks. Secondly, field observations during the growing season can be used both to correct current estimates of disease severity and modify parameters which determine predicted severity. Thirdly, pathogen growth and symptom expression are modeled to allow the effects of fungicides to be accounted for as protectant activity (reducing infections which occur postapplication) and eradicant activity (reducing growth of pre-symptomatic infections). The model was tested against data from a wide range of sites and varieties, and was shown to predict the expected level of disease sufficiently accurately to support fungicide treatment decisions

Nitrogen per unit leaf area affects the upper asymptote of Puccinia striiformis f.sp. tritici epidemics in winter wheat

Field trials tested which components of epidemic development of Puccinia striiformis, the cause of yellow rust, were affected by nitrogen (N) fertilizer applied to winter wheat. Both timing and amount of N were varied to affect canopy size and leaf N content, and to provide a supply of mobile N to the pathogen, by causing fresh N uptake after leaf expansion was complete. No N was applied to control plots. A logistic disease-progress function was fitted to disease-severity data, which were assessed in absolute units. Leaf area and specific leaf N (g N per m2 leaf tissue) were quantified. Large and highly significant effects of N on the upper asymptotes, or ‘carrying capacities' (c) were found. Effects on rates and points of inflection of the epidemics were not significant. Early N resulted in larger shoot numbers and leaf area, but disease was also more severe, so that by grain filling, the remaining green leaf areas were larger without N than with N. Later N treatments did not increase canopy size, but did increase symptom area compared with the control. These effects differ from the concept that N affects disease as a result of its effect on canopy growth, and therefore canopy microclimate, and suggest instead a substrate effect. Linear regression revealed that 51% of the observed differences in c were explained by variation in specific leaf N, suggesting that growth of the rust fungus may depend directly on particular components of total leaf N