DMI resistance

DMI resistance may be governed by different resistance mechanisms co-existing in the same isolate. Molecular detection tools can help to find low frequencies of mutated individuals in populations. However, since resistance to DMIs may be a combined effect of several mechanisms, the molecular results have to be treated with care and bioassays may be always needed for verification. 

To allow for Inevitable differences In Infection levels between experiments, transfer was expressed relative to the incorporation of HJ methionine into trichloroacetic acid insoluble material (= protein), which provided a measure of mildew growth. This technique only accounted for radioactivity In conidla and surface mycelium, not haustorla, but significant differences In the transfer of triadlmenol were apparent between strains (Table III). These differences were not, however, correlated with triadlmenol sensitivity, so that if reduced accumulation is involved in DMI resistance in barley powdery mildew, it must not operate in all resistant strains. In the same experiments we looked at the possibility that detoxification of triadlmenol might account for resistance, but were unable to find any evidence of triadlmenol metabolites in the six strains examined. 

Sterol Changes and Resistance. DMI-resistant mutants of U. maydls lacking 14DM have been isolated (32). These mutants, which were initially selected for polyene resistance, have no detectable demethyl sterols and grow more slowly than wild-type... 

All available genetic evidence points, therefore, to more than one mechanism of resistance operating against DMI... 

Determine genetic control and biochemical modes of action/resistance for DMIs and dicarboximides. 

III. Determine Mechanisms of Resistance to DMIs and Possible Differences Among Compounds Within the Class Is cross-resistance automatic across all members of the DMI class, and if not, why not ... 

The fungicides are very versatile in the contol of fungi that have become resistant to the demethylase inhibitor (DMI) fungicides described later. They have surprisingly low mammalian toxicity, but as with many other respiratory poisons, they show some toxicity to fish and other aquatic organisms. They may also be toxic to earthworms. In fungi they inhibit spore germination. The structures show the natural products strobilurin B and azoxystrobin, which has been marketed since 1996. 

Besides the intrinsic properties of each resistance mechanism, the economic impact of Qol resistance on the Qol market is also governed by the availability of alternative fungicidal modes of action for disease control and resistance management in each crop. For example, the early and rapid development of resistance of cereal powdery mildew (Blumeria graminis) in European wheat and barley production from 1998 on had a remarkably low impact on Qol consumption. The broad disease spectrum controlled by Qols and the availability of several other modes of action for the control of powdery mildews in cereals such as DMIs, amines, cyprodinil, quinoxyfen, and metrafenone resulted in a nearly unchanged use frequency of Qols in European cereal production. 

However, in 2002, when G143A-based resistance was detected in Mycosphaerdla graminicola (anamorph Septoria tritici), only DMIs and chlorothalonil-based products were available to substitute Qols. Accordingly, because of the high economic importance of Septoria tritici in European wheat production, Qol applications dropped sharply after the rapid spread of this resistance in the years 2003 and 2004. 

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