Dynamics of Mycosphaerella graminicola Populations in Response to Selection by Different Fungicides

摘要

The selection process exerted by the use of a fungicide is based either on quantitative or qualitative mechanisms resulting in a disruptive or continuous change of sensitivity, respectively, in the new populations. Resistance factors RF between sensitive and resistant individuals are high (KF>100) for the disruptive and rather small (RF>3<50) for the continuous change of sensitivity, hi this paper, changes in sensitivity of Mycosphaerella graminicola (anamorph Septoria tritici) populations over a period of several years to Qol and DMI fungicides will be described. Resistant isolates to Qol fungicides were first detected in 2002 simultaneously in five European countries. RF values were >100 resulting in a separation into two distinct sub-populations.The resistance mechanism is based on the G143A substitution in the cytochrome b gene, and cross-resistance exists between all Qol fungicides. A strong increase in the frequency of resistant isolates was observed during the season of 2003 in northern Europe reaching high levels in 2004. In the UK, resistance is now wide spread throughout the entire country, whereas in France and Germany, resistance levels are higher in the north than in the south. The change hi sensitivity to Qol fungicides results in areduced field performance of products when used as solo formulations. The sensitivity to DMI fungicides has decreased continuously in recent years resulting in a shift of sensitivity by a factor of about 6 between 1991 and 2004. No resistant isolates have been detected so far. Several resistance mechanisms may be involved for the reduced sensitivity including ABC transporters as well as over-expression and alteration of the cypSl target gene. As a consequence, cross-resistance exists amongst all representatives of the DMIs for the majority of the isolates. The sensitivity shifts are about the same for all triazoles. Field rates of triazoles should be kept high to give adequate disease control. Since no cross resistance exists between DMI, Qol and contact fungicides such as chlorothalonil, robust disease control and significant delay of the evolution of resistance can be expected if the three chemical classes are combined in the spray programme. The dynamics of resistance evolution and possible disease and resistance management strategies will be discussed.