Fungicides modes of action

Figure 4.6 Glycerophospholipid biosynthesis and fungicide mode of action (solid bars indicate sites of inhibition)...

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. 

In conclusion, available data suggest that cymoxanil has an unknown fungicidal mode of action and might also induce some host plant defense responses (similar hypersensitive effects are known from many fungicides, triazoles, car-boxin, strobilurins, etc.). 

Hassall, K.A. (1990). A readable student text, particularly useful for the metabolism and mode of action of herbicides and fungicides. 

The need to develop and use chiral chromatographic techniques to resolve racemates in pesticide residues will be driven by new hazard and risk assessments undertaken using data from differential metabolism studies. The molecular structures of many pesticides incorporate chiral centers and, in some cases, the activity differs between enantiomers. Consequently, in recent years manufacturers have introduced resolved enantiomers to provide pesticides of higher activity per unit mass applied. For example, the fungicide metalaxyl is a racemic mix of R- and 5-enantiomers, both having the same mode of action but differing considerably in effectiveness. The -enantiomer is the most effective and is marketed as a separate product metalaxyl-M. In future, it will not be satisfactory to rely on hazard/risk assessments based on data from metabolism studies of racemic mixes. The metabolism studies will need to be undertaken on one, or more, of the resolved enantiomers. 

There are few commercial fungicides that have glycerophospholipid biosynthesis inhibition as their mode of action. The validated targets are phosphatidylcholine synthesis and phosphatidylinositol synthesis. 

The first phosphatidylcholine inhibitors were amongst the earliest modern commercial fungicides to be discovered. At the time of discovery, the mode of action was unknown. 

Several compounds may interfere with nucleic acid metabolism but commonly their effects are secondary to their primary mode of action, for example, the benzimidazoles. Compounds that inhibit nucleic acid biosynthesis directly are either phenylamides, pyrimidines or hydroxy-pyrimidines. Recently, the phenoxyquinolines were identified as exhibiting a novel mode of action in purine biosynthesis and are potentially useful fungicides. 

DNA Synthesis. Hymexazol is the only fungicide that has an implicated mode of action against DNA synthesis. It has broad spectrum activity and is used commercially in several crops, including rice and vegetables. 

There are two commercial fungicides, the antibiotics blasticidin S and kasugamycin, that act via the inhibition of protein biosynthesis (Figure 4.19). Blasticidin S is a fermentation product obtained from cultures of Streptomyces griseochromogenes, and has specific activity in the control of P. oryzae, similar to kasugamycin, a secondary metabolite of S. kasugaensis. However, much of the earlier work on mode of action was carried out using another antibiotic, cycloheximide. 

Many commercial fungicides have unknown or indistinct modes of action. This in no way detracts from their commercial potential but does delay the development of particular chemistries through directed synthesis. 

Quinoxyfen (DE-795). Quinoxyfen (DE-795), announced by Dow AgroScience in 1996 (Figure 4.35), is a specific powdery mildew fungicide with particular utility in cereals. The mode of action is unknown but the chemistry is close to LY214352, a known inhibitor of pyrimidine synthesis. Quinoxyfen and RNA-synthesis fungicides also inhibit appressorium formation. 

E. Ammermann, G. Lorenz and K. Schelberger, BAS 490F - A broad spectrum fungicide with a new mode of action , Proceedings of the British Crop Protection Conference - Pests and Diseases , 1992, Vol. 1,403-410. 

Fungicides with specific modes of action place a strong selective pressure on sensitive fungal populations. The main reason for resistance is a change at the active site resulting from mutation and the subsequent selection of fit, insensitive individuals. 

This book examines the chemistry and mode of action of herbicides, insecticides, fungicides and plant growth regulators. It follows crop protection strategies from early discoveries to the present day, emphasising the biochemical targets of the compounds discussed. 

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