Monitoring sensitivity to the DMI group of fungicides in net blotch

Abstract

During the four years 1988-91 net blotch, caused by the fungus Pyrenophora teres, was the most damaging of the splash-borne diseases recorded in the CSL/ADAS National Surveys of Winter Barley Diseases in England and Wales. Despite the fact that many crops were treated with one or more fungicides, estimated losses of up to £7m are likely to have occurred.

Chemicals in the 'DMI' group (eg cyproconazole, flusilazole, flutriafol, propiconazole, prochloraz and triadimenol), either singly or as components of formulated mixtures, dominate fungicide use on winter barley but particularly so after flag leaf emergence; during the period 1987-91 up to 82% of crops surveyed received an application of at least one 'DMI' fungicide. More than 50% received an application after flag leaf tip emergence (GS37), almost invariably propiconazole. Most of the fungicides offering effective control of net blotch belong to the 'DMI' group and base-line data on current sensitivity is required in order to detect rapidly changes which may lead to resistance problems.

Leaves showing symptoms of net blotch from the 1988, 1989 and 1991 CSL/ADAS National Survey of Winter Barley Diseases were used to establish isolates of the fungus from all the cereal growing areas of England and Wales. In all, 264 samples provided 1485 isolates which were tested for in-vitro sensitivity to the DMI fungicides prochloraz and propiconazole. Concentrations of 0.1 and 1.0 ug ai (0.5 and 5.0 ug ai in 1989) prochloraz and 0.5 and 5.0 ug ai (0.1 and 1.0 ug ai in 1989) propiconazole per ml agar were used. Sensitivity varied greatly between samples and isolates, but showed no reduction in 1988 and 1989 below that found in preliminary studies at Long Ashton Research Station on isolates made in 1979 and 1980.

However, there appears to have been a slight reduction in sensitivity of the population obtained during 1991 compared to that obtained during 1988. In general. reductions in sensitivity were associated with previous use of a DMI during the growing season, and sensitivity to both DMIs was well correlated in all three years (0.46, 0.39 and 0.81 1988-91 respectively, P < 0.0001). The significance of this apparent reduction needs to be assessed on the basis of in-vivo and field crop sensitivity tests but it does not currently appear to cause disease control problems in the field. The results of this work indicate that it is important to continue to monitor for possible further shifts towards insensitivity to DMI fungicides in populations of Pyrenophora teres at appropriate intervals.