Phenylamide Fungicides Research Articles

Resistance in Peronospora viciae to phenylamide fungicides: reduced efficacy of seed treatments of pea ( Pisum sativum) and assessment of alternatives

Metalaxyl seed treatments failed to control downy mildew (caused by Peronospora viciae) in New Zealand pea crops ( Pisum sativum), suggesting that resistance to the fungicide had developed after several years of use. Glasshouse experiments confirmed that seed treatments with metalaxyl or oxadixyl (systemic phenylamide fungicides) were ineffective for downy mildew control in young pea seedlings inoculated with field collections of P. viciae, and that seed treatments containing cymoxanil or fosetyl-Al protected seedlings from infection by metalaxyl-resistant collections. Laboratory bioassays determined that metalaxyl resistance was widespread and common in field collections of the pathogen in New Zealand. Estimated (R)-metalaxyl EC 50s for sensitive collections were <0.01 μg/ml, while those for resistant collections were 3–11 μg/ml. A field experiment confirmed that alternative fungicide seed treatments can be used for control of metalaxyl-resistant P. viciae. A seed treatment formulation containing cymoxanil (a metalaxyl alternative), (R)-metalaxyl and fludioxonil increased seedling establishment by 170% relative to untreated seed, gave complete control of downy mildew that was unaffected by (R)-metalaxyl used alone, and resulted in a 216% increase in grain yield. Similar effects on establishment and grain yield were recorded from a fosetyl-Al+thiabendazole+thiram seed treatment, although this treatment was not as effective for downy mildew control. These results demonstrate that metalaxyl-resistant P. viciae in pea crops can be controlled with alternative systemic fungicide seed treatments, and emphasise the value of seed treatments for ensuring good seedling establishment and subsequent crop production. Integrated approaches, incorporating appropriate cultural and chemical control strategies, and possibly use of disease-resistant cultivars, are recommended for management of phenylamide resistance in P. viciae and control of downy mildew of pea.

Multiple Loci Determining Insensitivity to Phenylamide Fungicides in Phytophthora infestans

ABSTRACT The diversity of mechanisms causing insensitivity to phenylamide fungicides in Phytophthora infestans was addressed by comparative genetic analyses of isolates from North America, Europe, and Mexico. Both semidominant major loci (MEX loci) and genes of minor effect were previously shown to determine insensitivity based on studies of isolates from Europe and Mexico. In this investigation, genetic analyses of three highly insensitive isolates from the United States and Canada revealed a similar pattern involving major and minor loci. However, MEX alleles in two Canadian isolates conferred higher levels of insensitivity than those examined previously, particularly in a heterozygous state. This suggested that not all MEX alleles in P. infestans were functionally equivalent. The chromosomal locations of the major insensitivity loci were also shown to vary in different isolates based on linkage analyses performed with the aid of DNA markers. The major determinant of insensitivity in the North American, Dutch, and Mexican isolates mapped to the same locus, which was named MEX1. In a British isolate, a different locus, dubbed MEX2, was implicated that mapped to the same linkage group as MEX1 but to a distinct site.

Sensitivity to the racemic mixture and isomeric forms of metalaxyl in Indian and European homothallic and heterothallic isolates of Peronospora parasitica in Brassica species

The production of oospores in monoculture (homothallism) and sensitivity to the phenylamide fungicide metalaxyl, in its racemic mixture form of (R)- and (S)-enantiomers (Ridomil-25WP) or the latest isomeric form (R)-enantiomer (Metalaxyl-M), was tested using 11 single-spore and 14 field isolates of Peronospora parasitica. Three isolates were from Brassica napus (UK), 13 from B. juncea (India), four from B. rapa (India), one from Eruca sativa (India) and four from B. oleracea (one from the UK, two from France and one from India). All isolates were found to be homothallic, as they produced oospores in monoculture, except P006 (a UK single-spore isolate derived from B. oleracea and previously known to be insensitive to Ridomil-25WP) which was heterothallic. All isolates were found to be sensitive to the two forms of the phenylamide fungicide metalaxyl (Ridomil-25WP and Metalaxyl-M) except P006 and FP13, a French isolate derived from B. oleracea in Brittany. These isolates were insensitive to both fungicides at the three concentrations used in this study (0.5, 5.0 and 50.0 μg/ml). To our knowledge, this is the first reported case of insensitivity to metalaxyl in P. parasitica from France. This study emphasises the importance of deploying sources of host resistance with fungicides in prolonging the effectiveness of both control procedures.

Occurrence in Italy and characteristics of lettuce downy mildew (Bremia lactucae) resistant to phenylamide fungicides

Control of lettuce downy mildew (Bremia lactucae) with phenylamide fungicides has failed in some intensive lettuce-producing areas in Northern Italy since Spring 1993. Before then, these chemicals and particularly metalaxyl, provided the best disease control. The sensitivity of Bremia lactucae isolates collected in such areas to metalaxyl was evaluated in the laboratory. These strains grew and sporulated profusely on lettuce seedlings treated with 100 and 200 ppm of metalaxyl, whereas sensitive control strains were completely inhibited when treated with fungicide concentrations ranging from 0.5‐1 ppm. Thus in Italy occurrence of resistance to phenylamide fungicides in Bremia lactucaehas also been demonstrated, as in almost all the countries where these chemicals were previously authorised. Subsequently, a demonstration of occurrence of resistance was made and the virulence pattern of several strains (resistant and sensitive to metalaxyl) was characterised using differential NL series containing the 13 DM resistant genes or R factors. The results suggest the occurrence of a new pathotype in Italy different from all the 16 NL Bremia lactucaeraces studied.

Genetic Analysis of Metalaxyl Insensitivity Loci in Phytophthora infestans Using Linked DNA Markers

ABSTRACT Previous studies indicated that incompletely dominant loci determine insensitivity by oomycetes to phenylamide fungicides such as metalaxyl. To compare the bases of insensitivity in different strains of the late blight pathogen, Phytophthora infestans, crosses were performed between sensitive isolates and isolates from Mexico, the Netherlands, and the United Kingdom that displayed varying levels of insensitivity. Segregation analyses indicated that metalaxyl insensitivity was determined primarily by one locus in each isolate, and that two of the isolates were heterozygous and the other homozygous for the insensitive allele. Metalaxyl insensitivity was also affected by the segregation of additional loci of minor effect. DNA markers linked to insensitivity were obtained by bulked segregant analysis using random amplified polymorphic DNA (RAPD) markers and the Dutch and Mexican crosses. By studying the linkage relationships between these markers and the insensitivity in each cross by RAPD or restriction fragment length polymorphism analysis, it appeared that the same chromosomal locus conferred insensitivity in the Mexican and Dutch isolates. However, a gene at a different chromosomal position was responsible for insensitivity in the British isolate.

Aggressiveness and other factors relating to displacement of populations of Phytophthora infestans in England and Wales

Mating type, in vitro sensitivity to the phenylamide fungicide metalaxyl, and mitochondrial (mtDNA) haplotype were determined in some or all of 618 isolates of Phytophthora infestans from the years between 1978 and 1995. A2 mating type occurred infrequently in most but not all years and insensitivity to metalaxyl increased over time. After 1982, the mtDNA Ib haplotype was largely replaced (except for one isolate in 1986 and one in 1995) by two new haplotypes, Ia and IIa. Type Ia was much more common than type IIa. Approximately one quarter of these isolates (165) were compared using two components of fitness associated with aggressiveness (infection frequency × number of sporangia per lesion) on detached leaves of cultivars Maris Piper, Cara and Stirling, which were chosen as exhibiting increasing levels of race non-specific resistance. Isolates were compared with three ‘standard’ isolates of low, intermediate or high aggressiveness, and the data standardised for comparison between experiments. On cvs. Cara and Stirling, but not on Maris Piper, mtDNA Ia and IIa haplotypes were more aggressive than type Ib in several separate experiments. Similarly, metalaxyl sensitive phenotypes were more aggressive than insensitive phenotypes on Cara and Stirling but not on Maris Piper. The displacement of mtDNA type Ib by types Ia and IIa over this period may have been a result of the lower aggressiveness and lack of complete insensitivity to metalaxyl in type Ib isolates.

The effect of phenylamide fungicides on the control of potato late‐blight (Phytophthora infestans) in England and Wales from 1978 to 1992

During the period 1978–1992, phenylamide fungicides in co‐formulation with the dithiocarbamate fungicide mancozeb were tested for the control of potato late‐blight in 51 separate field experiments in England and Wales. Whilst there was a general trend over all the experiments which indicated that foliage blight was less severe where the phenylamide + dithiocarbamate mixture had been used, the benefit was more marked in some than in others. Despite the detection of phenylamide resistance, at some sites the mixture gave better control of foliage blight than the dithiocarbamate alone. At one site where continuous data were available, the additional benefit of the phenylamide compound for control of foliage infection was lost after 1986 coinciding with a rise in phenylamide resistance from 31.7% in 1986 to 81% in 1987. Where foliar blight epidemics occurred, yield responses to fungicide programmes compared with unsprayed controls ranged from 0 to +118.5% with a mean response of +30.2%. This is equivalent to 30.8 and 12.92 t/ha respectively. Standardized yield differences were calculated to allow inter‐trial and inter‐year comparisons and showed no benefit from the phenylamide fungicide applied at 14‐day intervals in 33 out of 38 experiments where foliar blight epidemics occurred. At one site, Cusum analysis of standardized yield differences following treatment with the phenylamide + dithiocarbamate mixture and the dithiocarbamate alone showed a mean benefit from the phenylamide mixture of 2.23 t/ha during the period 1978 to 1986. From 1987 to 1992, the yield benefit dropped to a mean level of 0.68 t/ha a decrease of 69.5%. In experiments where blight was not recorded, fungicide treatments had no deleterious effect on yields. Over all the trials, there was no effect of fungicide treatment on the incidence of tuber blight at harvest.

Widespread Distribution and Probable Origin of Resistance to Metalaxyl in Clonal Genotypes ofPhytophthora infestansin the United States and Western Canada

The sensitivity of 251 isolates of Phytophthora infestans to the phenylamide fungicide metalaxyl was assessed by an in vitro radial growth assay on fungicide-amended agar media. Isolates were collected from many regions of North America from 1987 through 1993, including 15 states in the United States and British Columbia, Canada. A small sample of isolates from Europe and Israel was included for comparison. Isolates that grew less than 40% of the control on media containing 5 μg of metalaxyl per ml were considered sensitive; all other isolates were scored as resistant. Field trials and floating leaf-disk assays were used to confirm the accuracy of the amended-agar technique. All isolates collected from 1987 through 1989 were sensitive. Metalaxyl-resistant isolates were detected in 13 of 15 states and in British Columbia during 1992 and 1993. With one exception, sensitivity to metalaxyl was absolutely correlated with clonal lineage as determined by mating type, allozyme genotype, and DNA fingerprint analysis. The US-I clonal lineage, present in the United States and Canada for many years, was uniformly sensitive. In contrast, all isolates with the recently immigrated US-7 and US-8 clonal genotypes were resistant, even those obtained from fields with no history of metalaxyl application. All US-6 isolates collected since 1990 were resistant, but one sensitive US-6 isolate was collected in California in 1989. The cause of this polymorphism within US-6 could not be determined. Metalaxyl resistance was unimodally distributed within clonal lineages and limited to those that were recently immigrated. This strongly supports the hypothesis that resistance in the United States and Canada originated by migration, rather than by mutation and selection after migration. In contrast, evidence for selection of metalaxyl-resistant mutants within clonal lineages was detected among the limited sample of isolates from Europe and Israel.

Late blight ( Phytophthora infestans) on tomato in the tropics

Of all the pathogens of economically important plant species, Phytophthora infestans (Mont.) de Bary can rightly claim its place as the most notorious. Its occurrence in Ireland during the last century caused both the potato famine of 1846-49 and the establishment ofplant pathology as a scientific discipline by Rev. M. J. Berkeley and others. Even today, late blight frequently decimates potato crops during wet summers, despite significant advances in disease prediction, crop protection (usually with phenyl amide or dithiocarbamate fungicides) and the introduction of more resistant cultivars. Although' late blight has traditionally been associated with potatoes, in tropical climates with periods of high rainfall the same pathogen can be a significant factor limiting yields from tomato crops. In many developing countries, tomato represents an important food crop, not only because of the high price it commands compared to staples such as rice, but also because it provides a valuable source of vitamins when fresh and improves the flavour of meals when cooked. The requirement of tomatoes for moderately low night-time temperatures for efficient fruit-set limits production in the lowland tropics to the cooler seasons. Consequently summer production is centred upon cooler upland areas (e.g. in the Cameron Highlands of Malaysia and the Central African massif), where high rainfall levels are optimal for late blight infection. In such areas, late blight represents a serious problem, since application of costly imported fungicide is often ineffective due to dilution by rainfall and the potential occurrence offungicide-resistant strains of the pathogen. Research at the University of Wales, Bangor being funded by the UK Government Overseas Development Administration and in collaboration with the Asian Vegetable Research and Development Centre (AVRDC) in Taiwan has two objectives. Firstly, we are identifying promising sources of resistance to late blight by screening material from tomato germplasm collections; promising accessions are currently being field-tested in Taiwan, Tanzania and Costa Rica prior to the initiation of a breeding programme. Secondly, we are examining levels of genetic variability within populations of the pathogen from tropical areas; it is vital to know the extent of geographical variation in the aggressiveness and virulence within pathogen populations to maximize the possibility that resistance factors introduced into planting material will provide durable disease protection over a wide geographical range. An important component of our research therefore involves the acquisition of samples of the pathogen from a range of countries, both temperate and tropical. These will permit testing the resistance of promising tomato accessions against pathogens from the countries where the resistant cultivars will ultimately be grown. A further reason for wishing to assess P. infestans populations is the increase over recent decades in the international potato trade. The concomitant migration of the fungus in diseased tubers is believed to have led to significant population changes. We hope that by obtaining isolates ofthe fungus from around the world it will be possible to more accurately monitor these changes. Wherever tomatoes are grown without irrigation and without heavy use of fungicides, it is likely that late blight is present unless conditions are very dry. The symptoms include blackening of the leaves (Fig Ia; often spreading lesions originating from the tip of the leaf) and death of whole branches of the plant (Fig Ib). Infected green fruit become marbled brown in appearance (Fig Ic) without becoming soft (not to be confused with nutritional disorders such as blossom end rot, which usually affect the area around the tip of the fruit). Sporulation of the fungus (only under

Laboratory production of oospores of Peronospora parasitica (crucifer downy mildew) and the recovery and characterization of sexual progeny from crosses between isolates with different host specificity

The production of viable oospores of Peronospora parasitica under laboratory conditions and the recovery of isolates (referred to as sexual progeny) from these oospore populations are described. Oospores were produced when isolates of opposite sexual compatibility type, specialized to the same or different Brassica species, were grown together in seedling cotyledons of a host line capable of supporting growth of both isolates. Recovery of sexual progeny from oospore populations produced from two out of four pairings between isolates specialized to the same host species (homologous pairings) proved relatively easy. On the basis of their characterization with respect to virulence, response to phenylamide fungicides, sexual compatibility type and isoenzyme polymorphisms, there was evidence that the sexual progeny from these homologous pairings could be of hybrid origin. For the first time in a member of the Peronosporaceae, it proved possible to recover and successfully characterize a few sexual progeny from pairings between isolates specialized to different host species (heterologous pairings). However, the majority of such isolates sporulated weakly and as a consequence proved difficult to maintain and were lost. Nevertheless, some evidence for the hybrid nature of progeny from heterologous pairings was obtained.

Effectiveness of Three Phenylamide Fungicides AgainstPhytophthora cryptogeaIsolated From Kiwi and Their Mobility in Soil

The mobility of phenylamide fungicides in sand and clay loam soil columns was demonstrated using Phytophthora cryptogea from kiwi to assess for the fungicides. Mobility appears to be a function of the chemical nature of the fungicide and soil texture. Regardless of the soil type, metalaxyl was highest, ofurace was intermediate, and oxadixyl was lowest in the degree of mobility observed. There was a tendency toward less mobility in clay loam soil than in sandy soil

Selection and characterization of Phytophthora parasitica mutants with ultraviolet‐induced resistance to dimethomorph or metalaxyl

AbstractMutants of Phytophthora parasitica Dast. more or less resistant to dimethomorph or metalaxyl were obtained by treating mycelium with ultraviolet radiation. Some metalaxyl‐resistant mutants exhibited levels of resistance (i.e. the ratio between the EC50 values for the mutant and the wild‐type strain) greater than 100, but those observed in the dimethomorph‐resistant mutants never exceeded 25. Most mutants retained their resistance after sub‐culturing on unamended agar medium. Single zoospore clones derived from a metalaxyl‐resistant and a dimethomorph‐resistant mutant, selected as amongst the most resistant, had levels of resistance similar to those of the parental strains. The metalaxyl‐resistant mutants were also resistant to the related phenylamide fungicides, furalaxyl and benalaxyl, but remained sensitive to dimethomorph. The dimethomorph‐resistant mutants were not resistant to phenylamide fungicides. The pathogenicity of some metalaxyl‐ or dimethomorph‐resistant mutants on tobacco leaves was similar to that of the wild‐type strains.

Developed by the Phenylamide Fungicide Resistance Action Committee (PA-FRAC) of GIFAP

Methods are presented for monitoring the sensitivity to phenylamide fungicides of plant pathogenic fungi belonging to the Peronosporales, e.g. Phytophthora infestans, Plasmopara viticola, Peronospora tabacina, Pseudoperonospora cubensis and soil-borne Phytophthora and Pythium species. The methods are intended for practical use in the implementation of anti-resistance strategies. Methods for sterol demethylation inhibitors (DMIs), benzimidazoles and dicarboximides have already been published in Bulletin OEPP/EPPO Bulletin 21, 291–354 (1991)

1. Monitoring pathogen sensitivity to phenylamide fungicides: principles and interpretation

1. Monitoring pathogen sensitivity to phenylamide fungicides: principles and interpretation

Control of onion downy mildew in the presence of phenylamide-resistant strains of Peronospora destructor (Berk.) Caspary

Two isolates of Peronospora destructor differed in their sensitivities to phenylamide fungicides. An isolate (3014) from the Lockyer Valley did not respond to increasing concentrations of metalaxyl, benalaxyl and oxadixyl between 0.1 and 200 mg a.i./L. An isolate (2967) from Bundaberg was sensitive to these fungicides, with EC50 values (concentrations which reduce disease severity by 50%) of 1.6, 6.0 and 12.1 mg a.i./L, respectively. Isolates 3014 and 2967 were both sensitive to dimethomorph, with EC50 values of 4.3 and 1.4 mg a.i./L, respectively. Field trials compared the efficacy of fungicides for the control of onion downy mildew in the Lockyer Valley in 1989 and 1990. In both trials, treatments containing mancozeb gave the best control, resulting in yield increases of 60% in 1990. There was no difference from the check treatment in either disease severity or yield when metalaxyl was applied, suggesting that phenylamide-resistant strains were prevalent. Other fungicide treatments which did not significantly reduce the severity of downy mildew in the 1990 trial included copper hydroxide, chlorothalonil, fosetyl-Al, and propamocarb. The experimental compound dimethomorph, although less effective than mancozeb, significantly reduced disease severity and increased yield by 24%.

Synergistic interactions between phenylamide fungicides and respiration inhibitors in controlling phenylamide-resistant populations ofPhytophthora infestans on potato tuber disks

The efficacy of mixtures of respiration inhibitors and phenylamide fungicides (oxadixyl and metalaxyl) in controlling late blight was investigated using potato tuber disks. Results showed that uncoupling agents(e.g. DNP), inhibitors of ATP formation(e.g. oligomycin), ionophores(e.g, valinomycin) and inhibitors that block specific carriers in the electron transport chain(e.g. rotenone, antimycin A) increased significantly the control efficacy of both metalaxyl and oxadixyl towards populations ofPhytophthora infestons when applied to tuber slices pretreated with sublethal doses of both fungicides. When applied alone at the same doses, respiration inhibitors did not inhibit the development of the late blight fungus. Increased control efficacy of mixtures was not noted against phenylamide-sensitive populations of the fungus.

Evaluation of Phosphonate, Fosetyl-Al and Several Phenylamide Fungicides for Post-Infection Control of Grapevine Downy Mildew Caused by Plasmopara Viticola.

Post-infection efficacy of phosphonate for the control of grapevine downy mildew was tested in two glasshouse and one field experiment on Vitis vinifera cultivars. Significant inhibition of sporulation occurred when phosphonate was applied up to 17 days after inoculation of leaves with Plasmopara viticola. In a field experiment it reduced disease incidence, hastened the necrosis of oilspots, and delayed the appearance of oilspots of P. viticola. In two experiments, fosetyl-Al, metalaxyl, benalaxyl, and ofurace similarly inhibited sporulation and reduced disease incidence. This was the first in vivo demonstration of the control of P. viticola by phosphonate, which is now registered as the first systemic fungicide with symplastic ambimobility for P. viticola in Australia. It has potential for use in anti-resistance strategies as an alternative to metalaxyl and the other phenylamides. All fungicides tested have the capacity to reduce the rate of development of epidemics of P. viticola.

Toxicity of propamocarb to the late blight fungus on potato

Propamocarb (Previcur-N; propyl-[3-dimethylamino-propyl] carbamate-monohydrochloride) was testedin vivo against 32 field isolates ofPhytophthora infestans from six countries. Fungicide dosages required to achieve 90% control of the blight ranged between 676 and 1530 ppm a.i. in potted potato (cv. ‘Alpha’) plants and between 1135 and 2648 ppm in potato tuber slices. Isolates from Israel were less sensitive to the fungicide than isolates from Europe or North America. Toxicity of propamocarb was not related to resistance or sensitivity to phenylamide fungicides (e.g. metalaxyl). Nevertheless, most metalaxyl-resistant isolates from Israel were less sensitive to propamocarb than most metalaxyl-sensitive isolates from this country. Monocyclic epidemics conducted with the 20 Israeli isolates in the field showed that 1081–2012 ppm of the fungicide was required to achieve 90% control of the disease. Laboratory experiments revealed that the fungicide was poorly active against sporangial germination and had a limited curative efficacy. It exhibited a translaminar translocation in leaves but a poor acropetal or basipetal systemicity from foliage. Propamocarb + mancozeb mixtures (1:1, v/w) were synergistically effective in controlling the blight. Growers in Israel use tank mixtures of propamocarb (Dynone) and mancozeb to combat late blight in potato fields where phenylamide-resistant isolates ofP. infestans are prevalent.

フェニルアマイド耐性キュウリベと病菌の発生

All of the ten isolates of Pseudoperonospora cubensis sampled in greenhouses at Asahi area, Chiba Prefecture, where the effectiveness of the application of phenylamide fungicides for the control of downy mildew of cucumber appeared to have declined since the spring of 1989, were found to be resistant to the fungicides at 100ppm concentration, in assay using the cucumber leaf disc method. Both the preventive and curative effect of the mixtures of phenylamide and mancozeb or copper compounds on downy mildew caused by the resistant isolates were found to decrease in experiments using potted cucumber plants.

Antifungal modes of action of metalaxyl, cyprofuram, benalaxyl and oxadixyl in phenylamide-sensitive and phenylamide-resistant strains of Phytophthora megasperma f. sp. medicaginis and Phytophthora infestans

The activity of the phenylamide fungicides metalaxyl, cyprofuram, benalaxyl and oxadixyl against phenylamide-sensitive and phenylamide-resistant strains of Phytophthora megasperma f. sp. medicaginis and Phytophthora infestans has been compared in several tests. Metalaxyl showed the highest activity amongst the four fungicides against mycelial growth of sensitive strains on agar media. Benalaxyl and cyprofuram showed the highest activity against resistant strains. RNA synthesis of sensitive strains, measured as [ 3H]uridine incorporation, was inhibited by about 80% ( P. megasperma) and by about 40% ( P. infestans) by all phenylamides at concentrations of 1 μg/ml. At higher concentrations inhibition remained at this level, indicating that only part of the RNA synthesis was inhibited. RNA synthesis of resistant strains was completely insensitive to metalaxyl and oxadixyl at concentrations as high as 200 μg/ml. Cyprofuram, however, at concentrations above 1 μg/ml ( P. megasperma) and 10 μg/ml ( P. infestans) was inhibitory and at 200 μg/ml this compound almost completely inhibited the phenylamide-sensitive part of the RNA synthesis of both strains. In contrast to the other phenylamides, benalaxyl affected [ 3H]uridine uptake into mycelium at concentrations higher than 10 μg/ml, and at 100 μg/ml inhibition of uptake was almost total with both sensitive and resistant strains of both Phytophthora species. Inhibition of [ 3H]uridine uptake obviously is a feature of a second mechanism of action of benalaxyl. Endogenous RNA polymerase activity of isolated nuclei from sensitive strains of P. megasperma was inhibited at phenylamide concentrations as low as 0·1 μg/ml whereas that from resistant strains was not affected at concentrations as high as 100 μg/ml. Similarly, endogenous nuclear RNA polymerase activity of sensitive P. infestans strains appeared to be more sensitive to the phenylamides than that of resistant P. infestans strains. This suggests that resistance that developed in field strains of P. infestans has a basis similar to that of mutagen-induced resistance in laboratory strains of P. megasperma. Tests of the fungicidal activity of the phenylamides determined in a lucerne seedling assay ( P. megasperma) and a detached potato leaf assay ( P. infestans) clearly differentiated between sensitive and resistant strains. Resistance levels to cyprofuram and benalaxyl were as high as those to metalaxyl and oxadixyl, indicating that the additional effect of a second mechanism of action of benalaxyl and the property of cyprofuram to inhibit RNA synthesis of resistant strains at higher concentrations are of limited practical importance for counteracting phenylamide resistance.