Multiple diseases, host resistance and the role of variety mixtures for disease control in organically grown spring barley

Summary

The presence of more than one disease at the same time is common in farmers’ fields. The use of disease resistant varieties is one of the most important disease management components in organic farming. While many modern spring barley cultivars possessing good resistance to individual diseases are available today, cultivars resistant to multiple diseases are still scarce because breeding for multiple disease resistance is a difficult and lengthy task. Mixing cultivars possessing high levels of resistance to individual diseases may be a good option to control multiple diseases in ecological farming. Research is underway in Denmark in which the effects of resistance properties of spring barley varieties on the development of two important foliar diseases, leaf scald (Rhynchosporium secalis) and net blotch (Pyrenophora teres [conidial stage Drechslera teres]), are being studied. The first years’ (2002) results showed that occurrence of both diseases in combination led to less-than-additive effects on total disease severity and crop health, as compared to single-disease scenarios. This indicates antagonism between the disease organisms and has direct implications for disease-yield loss relationships and yield loss appraisal. There were hints for higher-than-additive effects of variety mixtures in reducing disease severity and improving overall crop performance.
The study will be expanded to include additional varieties as well as aspects regarding the time of establishment and amount of initial inoculum of the two pathogens. The data will be used to develop a mathematical simulation model for the development of the two competing diseases as affected by resistance properties of varieties, and to derive decision aids for optimising choice of variety with optimal resistance properties to reduce development of disease complexes.