Disease tolerance in a crop species may be defined as the ability of a plant to produce a greater yield than would normally be expected from the observed disease (Newton and Thomas, 1994). It is rarely a priority objective in breeding for resistance to fungal foliar diseases, particularly in cereals where there is considered to be a close association between observed disease and yield loss, especially under high input management.

Relationship between yield loss and area under the disease progress curve for spring barley cultivars.

Many data can be found from individual trials which indicate disease tolerance in cereals but these data are often inconsistent among trials indicating genotype by environment interaction in this character. The character can be quantified and whilst there is evidence for a heritable component of disease tolerance in spring barley to powdery mildew (Blumeria graminis f.sp. hordei) (Newton and Thomas, 1994) its genetic basis is difficult to define. However, no account is taken of non-symptomatic infection of pathogens, and we now know this can be very extensive. Furthermore there are many other microbes causing plant responses above or below ground that may impact yield.


Ecological tolerance in barley
Crop protection against fungal diseases is normally achieved through fungicides that are highly effective at killing most fungi and are toxic to some non-target organisms. The non-targets that populate the leaf surface can include many fungal and bacterial species that may be beneficial, benign or pathogenic. Inevitably many become part of the disease complex and may enhance symptoms such as with Septoria tritici on wheat. Others may reduce symptoms, thereby offering the potential for biocontrol such as against powdery mildew. Therefore the non-target effects of crop protection treatments should be considered in order to enhance biocontrol organisms and inhibit pathogenicity “helper” microbes.

Figure: Bacterial biofilms and cultural microbes on barley leaf surface

Crop cultivar, agronomy, location, weather and many other factors affect the leaf microbial community composition. Crop protection treatments superimpose changes on top of these either by differentially killing components using conventional agrochemicals, and/or priming or inducing the plant’s own defence mechanisms using resistance elicitors or plant activators which will also differentially inhibit different microbes.
Epiphytes and endophytes (i.e. microbes living on or in the leaf respectively) are not modified similarly based on the applied treatment. We have carried out trials to find out how treatments could challenge leaf-associated micro-organisms to disadvantage pathogens, focussing on rhynchosporium on barley.


Key papers
Newton AC, Thomas WTB, 1994. Detection of tolerance of barley cultivars to infection by powdery mildew (Erysiphe graminis f.sp. hordei). Euphytica 75, 179-187.
Newton AC, Thomas WTB, Guy DC, Gaunt R, 1998. The interaction of fertiliser treatment with tolerance to powdery mildew in spring barley. Field Crops Research 55, 45-56.
Newton, A.C., Gravouil, C., Fountaine, J.M. 2010. Managing the ecology of foliar pathogens: ecological tolerance in crops. Annals of Applied Biology 157, 343-359.