Resistance phenylamides

Table IV. Elements of Phenylamide Anti-Resistance Strategy After Urech and Staub (5)...

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...

Table IV. Effect of Phenylamides (1 pg/ml) on Endogenous RNA Polymerase Activity of Nuclei isolated from a Phenylamide-Sensitive (S) and a Phenylamide-Resistant (R) Strain of...

These experiments clearly show that phenylamide resistance in both a laboratory mutant of P. megasperma f. sp. medicaginis and a field isolate of P. infestans is due to a change at the target site. 

Phenylamide Resistance Research and Management Goals and Their Implementation in North America... 

Fungicides that interfere with nucleic acid synthesis are an important group of phenylamides that have kept a strong position in the Oomycete market despite major resistance problems (Table 12.6). Fungicides inhibiting adenosine-... 

Although phenylamides are considered to bear a high intrinsic resistance risk [33], they have failed to fully eliminate the sensitive sub-populations from nature even after 25 years of intensive use [17]. The proportion of resistant isolates in P. infestans and P. viticola fluctuates from year to year and also within the season. It increases within a season, more rapidly in fields treated with phenylamides than in untreated fields, starts to decline at the end of the season and is significantly lower at the beginning of the next season compared with the proportion at the end of the previous year [23]. 

A wide range of fungicides are available, both protectants (e.g. dithiocarbamates or oiganotins) and systemics, e.g. phenylamide group. Due to resistance problems, care should be taken with chemical choice and mixtures... 

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