The virulence of Finnish Pyrenophora teres f. teres isolates and its implications for resistance breeding

Summary

In Finland, barley, Hordeum vulgare L., covers 50 % of the total acreage devoted to cereal cultivation. The most common disease of barley in Finland is net blotch, a foliar disease caused by the ascomycete Pyrenophora teres Drechsler. Disease resistance based on plant genes is an environmentally friendly and economical way to manage plant diseases caused by biotic stresses. Development of a disease resistance breeding programme is dependent on knowledge of the pathogen. In addition to information on the epidemiology and virulence of a pathogen, knowledge on how the pathogen evolves and the nature of the risks that might arise in the future are essential issues that need to be taken into account to achieve the final breeding aims.
The main objectives of this study were to establish reliable and efficient testing methods for Pyrenophora teres f. teres virulence screening, and to understand the role of virulence of Pyrenophora teres f. teres in Finland from a disease resistance breeding point of view. The virulence of P. teres was studied by testing 239 Finnish P. teres f. teres isolates collected between 1994 – 2007 originating from 19 locations, and 200 P. teres progeny isolates originating from artificially produced P. teres matings.
According to the results of this study, screening for P. teres f. teres isolates on barley seedlings under greenhouse conditions is a feasible and cost efficient method to describe the virulence spectrum of the pathogen. However, the environmental conditions of temperature, light and humidity, need to be stable to achieve reliable and comparable results between different studies. Inoculum concentration and the seedling leaf used to gauge virulence had significant effects. Barley grain size, morphological traits of P. teres isolates, spore production and growth rate on agar did not affect the expression of virulence. A common barley differential set to characterize the P. teres virulence was developed and is recommended to be used globally: c-8755, c-20019, CI 5791, CI 9825, Canadian Lakeshore, Harbin, Prior, Skiff, and Harrington.
The virulence spectrum of Finnish P. teres f. teres isolates collected in 1994-2007 was constant both within and between the years. The results indicated differences in the pathogen’s aggressiveness and in barley genotypes resistance. However, differences in virulence were rarely significant. No virulent reactions were recorded on barley genotypes CI 5791 and CI 9819. Unlike in laboratory conditions, no indications of changes in virulence caused by the sexual reproduction have been observed in Finnish barley fields.
In Finland, durable net blotch resistance has been achieved by introducing resistance from other barley varieties using traditional crossing methods, including wide crossing, and testing the breeding material at early generations at several sites under natural infection pressure. Novel resistance is available, which is recommended to minimize the risk of selection of virulent isolates and breakdown of currently deployed resistance. Barley genotypes c-8755, CI 9825 and CI 5791 are potential resistance sources to be used in Finnish barley. They differed in their reaction to the P. teres f. teres isolates collected globally and all of them showed excellent resistance to Finnish P. teres f. teres isolates.