Resistance metalaxyl

When metalaxyl was used, necrotic reactions in susceptible crop cultivars similar to those of resistant ones, as well as phytoalexin accumulation, have been reported (12, 13, 21, 37, 41, 45). Here the use of metalaxyl-resistant strains could also clarify the signifi-... 

Cohen, Y. Samoucha, Y. (1984) Cross-Resistance to Four Systemic Fungicides in Metalaxyl-Resistant Strains of Phytopthora infes-tans and Pseudoperonospora cubensis. Plant Disease 68, 137-139. 

Case Histories. The evolution of metalaxyl resistance in the Bremia lactucae population in the United Kingdom and in the Phytophthora Lnfestans population in Northern Europe illustrate many of the principles discussed in this chapter. Mutation to resistance in the B. lactucae population is apparently a very rare event. Metalaxyl was used very effectively for five years (and maintained very low populations) before resistance became a problem (11). However, once established, selection was extremely strong for the resistant type and it quickly dominated B. lactucae populations in the lettuce growing areas of the United Kingdom. 

In the United States where metalaxyl resistance is not yet known in the P. Infestans population, it is important to maintain an integrated approach to disease management. Metalaxyl should be... 

Selection Experiments. The initial low frequency of fungicide resistance alleles in natural populations may be increased to detectable levels by selection. In growth room experiments with populations of P. infestans, initially inoculated to give a frequency of metalaxyl resistance at 1 x 10, Staub and Sozzi... 

Fisher, D.J., and Hayes, A.L. 1984. Studies of mechanism of metalaxyl fungitoxicity and resistance to metalaxyl. Crop. Prot. 3, 177-185. 

Phenylamides bind to an as yet unidentified component of this complex. The phenylamide binding site in resistant strains has a very low affinity for metalaxyl, whereas values of the dissociation constant are in the order of 0.1-0.2 pM for phenylamide-sensitive strains. Apparently a change at the phenylamide receptor is responsible for resistance. 

In phenylamide-resistant strains, phenylamides are not able to inhibit ribosomal RNA synthesis, as is indicated by the inability of metalaxyl to interfere with uridine incorporation in a phenylamide-resistant strain of P. megasperma f. sp. megasperma obtained in the laboratory after mutagenic treatment (14) and a phenylamide-resistant strain of Phytophthora infestans (Table III) originating from a potato field (15). 

Likewise, endogenous polymerase activity of isolated nuclei from these strains is significantly less sensitive to phenylamides than that of nuclei from sensitive strains (Table IV). This indicates that in both species a change in the target site of metalaxyl is responsible for resistance. Apparently both mutagen induced... 

Table III. Effect of Metalaxyl (1 /ig/ml) on Incorporation of Uridine by Mycelium of a Fhenylamide-sensitive (S) and a Phenylamide-resistant (R) Strain of P. megasperma f. sp. medicaginis and P. infestans...

Figure 4. Scatchard Plot of Binding Data for [ H]Metalaxyl by Cell-Free Mycelial Extracts of a Phenylamide-Sensitive (S) and a Phenylamide-Resistant (R) Strain of P. infescans.

The production of fit laboratory-induced resistant mutant can give some positive indication of the likely development of resistance, as for example in the control of Phytophthora megasperma f.sp. medicaginis by metalaxyl (6). Such mutants cannot be produced in relation to durable fungicides such as... 

Introduce a prepack of metalaxyl and mancozeb to limit the development of resistance. 

Continue to promote the alternation of metalaxyl with a non-cross-resistant Pythium fungicide. Ciba-Geigy held a turf symposium on December 4, 1987. 

Since zoxamide has a different mode of action from other products used in the Oomycete market, there is no likelihood of cross-resistance to existing products such as metalaxyl, dimethomorph, cymoxanil or strobilurins. Consequently, zoxamide provides a unique tool for resistance management in the Oomycete fungicide market. 

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