Fungicides resistance mechanisms

Among the biochemical mechanisms of fungicide resistance are reduced permeability, metabolism (increased detoxification or decreased conversion to the toxic material), and reduced affinity of the target site for the toxin. 

A number of current commercial fungicides have been reported to enhance disease resistance in addition to direct fungitoxicity (103,105,107). Figure 3 presents structures of commercial pesticides reported to enhance endogenous plant disease resistance mechanisms. 

Considerable interest has developed in resistance management through the use of 1) compounds that are more active against fungicide-resistant biotypes than their sensitive counterparts (i.e., the resistant biotypes exhibit negatively-correlated cross resistance) and 2) compounds that interfere with the resistance mechanism (i.e., synergists). Both approaches to resistance control have been reviewed by De Waard (24,45). 

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. 

The contribution FRAC Mode of Action Classification and Resistance Risk of Fungicides (Chapter 12), starting with a short introduction on the history of fungicides, explains the importance of individual modes of action relative to their market penetration, their mechanisms and occurrence of resistance, the importance and occurrence of practical resistance on the management of fungicide resistance by FRAC (Fungicide Resistance Action Committee) and its recommendations to farmers related to the modes of action of the different fungicides. 

The factors described above are the result of both intrinsic properties of resistant isolates and the way in which the fungicides were used. Intrinsic properties of resistant isolates are strongly related to the biochemical mechanism that causes a reduced sensitivity. Several types of resistance mechanisms can be distinguished ... 

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