Assessment of resistance and virulence in the pathosystem sugar beet (Beta vulgaris)/powdery mildew (Erysiphe betae) — development of basics for an effective powdery mildew resistance breeding

Abstract

Erysiphe betae, the pathogen causing sugar beet powdery mildew, represents an increasing risk to world sugar beet production, resulting in yield losses of up to 30%. So far, Erysiphe betae and the specific host-pathogen interaction of powdery mildew and sugar beet (Beta vulgaris) received little attention. Therefore, the aim of this study was to analyse the presence and frequency of virulence in European powdery mildew populations and to characterize the host-pathogen interaction more precisely. Hence, eight different sugar beet varieties were used to test eight different powdery mildew populations sampled all over Europe. Among all beet varieties and breeding lines, powdery mildew populations showed significantly different disease severities and thus different aggressiveness, which indicates a coexistence of different pathotypes. In addition leaf age seemed to be relevant for pathogen spread. For an establishment of convenient test methods, it was necessary to analyse epidemiological parameters under controlled conditions in order to derive optimal conditions for climate chamber and in vitro leaf disc assay. The most favourable temperature during incubation and fructification was between 20–25°C at a relative humidity of 50%. To provide a comprehensive characterisation of the powdery mildew situation, field experiments were conducted under natural infection conditions over a period of two years. Eighteen sugar beet varieties, with different susceptibility towards powdery mildew, were tested to deliver a basis for calculation of a yield-loss relation. A significant correlation with the formula y = 94.75 — 0.0021 · (R2 = 0,098**) was determined. To examine the informational value of collected data the two test methods (leaf disc assay, and fied trials) were compared and a significant correlation (R = 0,955*) between disease assessment in field trials and leaf disc assay data was observed. Furthermore, durability of existing resistances and fungicide sensitivity of the pathogen were tested to provide information for an effective resistance management. Differences between efficiency of the tested fungicides were statstically not significant. There was no evidence for fungicide resistance so far.