Phenylamide Fungicides Research Articles

Outbreak of a New Phytophthora sp. Associated with Severe Decline of Almond Trees in Eastern Spain

Since 2007, a decline of young almond trees (Prunus dulcis) has been observed in different field-grown nurseries in Valencia (east-central Spain). Early symptoms in affected trees included chlorosis, wilting, cankers, and profuse stem gumming. A Phytophthora sp. was consistently isolated from cankers, roots, and soil of affected trees. It was a heterothallic species with amphigynous and/or paragynous antheridia, and its morphological features did not conform to any of the described Phytophthora species. Pathogenicity was proved by artificial inoculation, completing Koch's postulates. All isolates were sensitive to the phenylamide fungicides metalaxyl and mefenoxam. Amplification and sequencing of the internal transcribed spacer (ITS) region, translation elongation factor 1 alpha gene (EF-1α), the β-tubulin (β-tub) gene, and the region containing the mitochondrial cytochrome c oxidase subunit 1 (cox1) gene fragment identified the species as Phytophthora taxon "niederhauserii", and phylogenetic analyses placed it in Phytophthora Clade 7b.

Dissipation of Mefenoxam Residue in Watermelon and Soil Under Field Conditions

Mefenoxam is the systemic phenylamide fungicide, which is widely used in controlling phytophthora disease of tomato, cucumber, pepper and watermelon, etc. The dissipation behaviour of mefenoxam residues in watermelons and soils was studied. The whole watermelon, melon flesh and soil matrices of mefenoxam were analyzed by GC-NPD. At three different spiking levels mean recoveries and relative standard deviation from spiked samples in six replicated experiments for each matrix were in the range 89.6-98.2% and 1.5-8.1%, respectively. Under field conditions, mefenoxam dissipation rate was found to be faster in the whole watermelons than in the soils. The results showed that the half lives in whole watermelon and soil from Beijing were 3.9 and 10.0 days, respectively, and the half lives in whole watermelon and soil from Shanxi were 3.7 and 28.4 days.

Mutagenesis of Phytophthora infestans for Resistance Against Carboxylic Acid Amide and Phenylamide Fungicides

The carboxylic acid amide (CAA) fungicides mandipropamid, dimethomorph, iprovalicarb, and the phenylamide fungicide mefenoxam (MFX, the active enantiomer of metalaxyl) are anti-oomycete fungicides effective against downy mildews and late blight. Resistance against MFX was reported in nature in several oomycetes including Phytophthora infestans and Plasmopara viticola, whereas resistance against CAAs was reported in P. viticola but not in P. infestans. In this study the mutability of P. infestans for resistance against CAAs and MFX (as a control) was explored under laboratory conditions. UV light or chemical mutagens (e.g., ethyl methan sulfonate [EMS]) were applied to sporangia, and the emergence of mutants resistant to CAAs or MFX, or with altered mating type, was followed. Many mutants resistant to CAAs developed at generation 0 after mutagenesis, but all showed erratic, instable resistance in planta, diminishing after 1 to 8 asexual infection cycles, and failed to grow on CAA-amended medium. In contrast, 19 mutants resistant to MFX were obtained: 6 with UV irradiation (in isolates 28 or 96) and 13 with EMS (in isolates 408, 409, and 410). In three experiments, a shift in mating type, from A1 to A2, was detected. To elucidate whether or not resistance to CAAs is recessive and therefore might emerge only after sexual recombination, A1 and A2 mutants were crossed and the F1 and F2 progeny isolates were tested for resistance. Offspring isolates segregated for resistance to MFX, with resistant isolates maintaining stable resistance in vitro and in planta, whereas all progeny isolates failed to show stable resistance to CAAs in planta or in vitro. The data suggest that P. infestans could be artificially mutated for resistance against MFX, but not against CAAs.

First Report of Mefenoxam-Resistant Isolates of Phytophthora capsici from Lima Bean Pods in the Mid-Atlantic Region.

Phytophthora capsici Leonian, the causal agent of lima bean pod rot, was first identified as a pathogen of lima bean in 2002 (1) and poses a new threat to lima bean (Phaseolus lunatus L.) production in the Mid-Atlantic Region. The phenylamide fungicide mefenoxam (Ridomil Gold; Syngenta Crop Protection) is widely used in the region for controlling foliar and soilborne diseases caused by Oomycetes. Isolates of P. capsici were collected from lima bean pods from production fields in Delaware, Maryland, and New Jersey from 1998 to 2004. These isolates originated from survey samples of lima bean fields for another pathogen, P. phaseoli, in 1999 and 2000 and diagnostic samples were submitted to the Plant Disease Clinic. Isolates were from lima bean, except for one from pepper (basal stem). Identification was made on the basis of morphometric characteristics. No known sensitive or insensitive isolates were included in the evaluation. Single zoospore cultures were evaluated for mefenoxam sensitivity on V8 agar plates amended with 100 ppm of mefenoxam, a previously tested concentration (2). Seven-millimeter-diameter agar plugs of each isolate were cut from the edge of actively expanding cultures of P. capsici with a cork borer and transferred to three V8 agar plates amended with mefenoxam and three unamended V8 plates. The plates were arranged in a completely randomized design and incubated at 25°C in the dark for 3 days. After incubation, colony growth was measured in millimeters and averaged for the three replicate plates of each isolate and percent growth relative to the unamended control was calculated. Mefenoxam sensitivity was assigned according to methods of Lamour et al. (2). The experiment was repeated once, and also run with a treatment of 200 ppm of mefenoxam. Of sixteen isolates screened, nine were rated as sensitive, four were intermediately resistant, and three were resistant. There was no difference between the 100 and 200 ppm results, except for a slight increase in sensitivity for one isolate. A subsequent experiment tested five isolates at concentrations of 1, 10, 100, and 1,000 ppm. Results were consistent with previous tests, with resistant isolates exhibiting some growth at the highest concentration of mefenoxam. One resistant isolate was from a field in Delaware previously cropped to slicing cucumbers with a history of mefenoxam applications. The second was from Caroline County, MD, which is heavily cropped to pickling cucumbers and likely to have been exposed to mefanoxam applications for the control of fruit rot; the origin of the third insensitive isolate from lima bean is unknown. Mefanoxam usage on lima bean is usually limited to one foliar application of mefenoxam+copper hydroxide to control downy mildew in the fall crop in wet seasons. This study indicates that mefenoxam resistance is present in populations of P. capsici in lima bean fields in the Mid-Atlantic Region, presumably as a result of mefenoxam applications to other vegetable crops, principally cucurbits, which are planted in rotation with lima beans or from nearby cucurbit fields. Implementing strategies to minimize fungicide resistance in other vegetables is important to slow resistance development associated with this emerging pathogen on lima beans. Lima bean pod rot continues to be seen sporadically each year in fields with a history of P. capsici and abundant rainfall or excessive irrigation.

Persistence of Phenylamide Insensitivity in Pseudoperonospora humuli.

Downy mildew, caused by Pseudoperonospora humuli, is an important disease of hop in most production regions in the northern hemisphere. Insensitivity to phenylamide fungicides was detected in isolates of P. humuli in production regions in Oregon and Idaho in 1992, and these fungicides since have been used on a limited basis. In this study, the prevalence of phenylamide insensitivity among isolates of P. humuli collected from 2005 to 2007 in the northwestern United States was quantified using a leaf disk assay with a discriminating dose (25 μg/ml) of either metalaxyl or mefenoxam with inoculum derived from 201 systemically infected diseased shoots (basal spikes) collected from 6, 10, and 11 hop yards in Idaho, Washington, and Oregon, respectively. A subset of 47 basal spike isolates and 42 monosporic isolates collected from two yards in Idaho and nine yards in Oregon during 2006 and 2007 were assayed using a dilution series of metalaxyl to determine the effective dose that inhibited 50% of the incidence of sporulation (ED50). Insensitivity to mefenoxam was detected in 31 of 74 (41.9%) basal spikes collected from 13 hop yards. Insensitivity to the related compound metalaxyl was detected in 52 of 80 (65%) spikes collected from nine hop yards, including four hop yards in Washington. Log ED50 values ranged from -2.25 to 2.67 for basal spike isolates and -2.27 to 2.98 for monosporic isolates and had a similar distribution. Log ED50 values for monosporic isolates and entire basal spike isolates were significantly associated. However, the slope of the regression line was less than 1, indicating that the log ED50 values obtained from entire basal spike isolates were greater than the corresponding log ED50 values obtained from a monosporic isolate obtained from that spike. This research suggests that insensitivity to phenylamide fungicides is a stable phenotype in P. humuli. Management of downy mildew should rely on measures other than these fungicides in yards or regions where phenylamide insensitivity is prevalent, which is now known to include certain hop yards in Idaho, Oregon, and Washington.

Inheritance of resistance to carboxylic acid amide (CAA) fungicides in Plasmopara viticola

Mandipropamid is a new mandelic acid amide fungicide expressing high activity against foliar infecting oomycetes, including the grapevine downy mildew, Plasmopara viticola. Because cross‐resistance with the valinamide fungicides iprovalicarb and benthiavalicarb and the cinnamic acid amide fungicides dimethomorph and flumorph was postulated, all five compounds are classified as carboxylic acid amide (CAA) fungicides. To support this classification, cross‐resistance among these compounds with field isolates and the segregation of resistance in F1 and F2 progeny of P. viticola were evaluated. A bimodal distribution of sensitivity in field isolates and cross‐resistance among all CAAs for the vast majority of isolates were detected. Crosses between sensitive (s) and CAA‐resistant (r) isolates of opposite mating types, P1 and P2, yielded abundant oospores. All F1‐progeny isolates were sensitive to CAAs (s:r segregation 1:0), whereas in F2 progeny segregation of about 9:1 (s:r) was observed suggesting that resistance to CAA fungicides is controlled by two recessive nuclear genes. Mating type segregated in a ratio P1:P2 of c. 2:1 in F1 and 1:1 in F2 progeny. In the same crosses, resistance to the phenylamide fungicide mefenoxam segregated in a ratio of c. 1:3:2 (sensitive:intermediate:resistant), reflecting the monogenic, semidominant nature of resistance. The risk of resistance in P. viticola was classified as high for phenylamide and moderate for CAA fungicides. This is the first report on the inheritance of phenotypic traits in P. viticola.

In vitro somatic growth and reproduction of phenylamide‐resistant and ‐sensitive isolates of Phytophthora erythroseptica from infected potato tubers in Idaho

Pink rot of potato, most commonly caused by Phytophthora erythroseptica, is a major field and post‐harvest problem in southern Idaho, USA, particularly since 1998 when isolates resistant to the phenylamide fungicide metalaxyl‐M (mefenoxam) were detected. Isolates of P. erythroseptica were collected from infected tubers in 2001 and 2002 from six Idaho counties and tested for resistance to metalaxyl‐M on amended agar. Metalaxyl‐M resistant (MR) and metalaxyl‐M‐sensitive (MS) isolates were identified in six counties; 160 isolates were highly resistant, seven moderately resistant and 57 sensitive to metalaxyl‐M with mean EC50 values of 182, 23 and 0·5 mg L−1 ai metalaxyl‐M, respectively. Mycelial growth rates and oospore production in agar were assessed for 20 MS and 20 MR isolates at 10, 15, 20, 25 and 30°C. Growth rates of MR isolates were between 2·5 and 3·1 times greater (P < 0·05) than those of MS isolates at 10, 15, 20 and 25°C, and oospore production was between 6·8 and 20·5 times greater (P < 0·0001) for MR than for MS isolates at the same temperatures. Colony growth in V8 broth at 18°C was greater for MR than MS isolates (P < 0·0032). However, zoospore production at 18°C was greater for MS than for MR isolates (P < 0·0109), and zoospore production mm−1 of colony circumference was also greater for MS than for MR isolates, 14 191 and 9959, respectively (P = 0·0109). Sexual reproduction of MR isolates in nature may be greater than MS isolates, but MS isolates may be more asexually fit based on the fitness parameters studied.

Tracking the Evolutionary History of the Potato Late Blight Pathogen with Historical Collections

in the mid 19th century in Ireland led to what isknown as the Irish potato famine. Approximately 1.5 millionpeople died from starvation and another 1.5 million weredisplaced and forced to emigrate from Ireland to otherregions of the world. The disease was first observed onpotatoes in the United States in 1843 in areas around theports of Philadelphia and New York (Peterson, Campbell, G Peterson, Campbell, & Griffith, 1992). Thepainstaking work of J. Teschemacher in the United States,M.J. Berkeley in Great Britain, Montagne in France, andlater DeBary in Germany, clearly elucidated that a fungus-like organism was responsible for late blight (Berkeley, 1846,DeBary, 1876; Peterson, Campbell, & Griffith, 1992).Research by early mycologists who studied the late blightpathogen was some of the first to document that fungi werecapable of causing plant disease and laid the groundwork forthe disciplines of microbiology and plant pathology(Berkeley, 1846, DeBary, 1876).Late blight has become an increasingly important diseaseworldwide in recent years, more than 150 years after thegreat famine (Smart and Fry, 2001). Since susceptiblepotatoes are widely planted in many areas of the world, thecrop cannot be grown without frequent fungicideapplication. The disease has reached epidemic proportions inNorth America, Russia, and Europe due to the developmentof resistance to the phenylamide fungicide metalaxyl inpopulations of the pathogen and the widespread occurrenceof new and more virulent genotypes (Drenth et al, 1994;Kadish and Cohen, 1995). Some strains including the Ib orUS-1 clonal lineage are sensitive to the fungicide metalaxyland mefenoxam, while many other strains have developedresistance to these fungicides.More fungicides are used onpotato worldwide, than any other food crop, largely becauseof late blight. The cost of fungicide use by US potato growersexceeds 3 billion dollars annually. The disease can bedevastating in the developing world where fungicides areoften not affordable or available.

Enantioselective degradation kinetics of metalaxyl in rabbits

Metalaxyl [methyl- N-(2′-methoxyacetyl)- N-(2,6-dimethylphenyl)- d, l- alaninate] is a potent phenylamide fungicide. The (−)-( R)-isomer accounts for most of the fungicidal activity. A possible stereo and/or enantioselective kinetics of metalaxyl in rabbits was investigated by intravenous injection. The concentrations of (−)-( R)- and (+)-( S)-metalaxyl in plasma, liver, and kidney tissue were determined by HPLC with a cellulose–Tris-(3,5-dimethylphenylcarbamate)-based chiral stationary phase and gas chromatography-mass spectroscopy. After intravenous administration of racemic metalaxyl (40 mg/kg), the (+)-( S)-enantiomer levels in plasma, liver, and kidney decreased more rapidly than the (−)-( R)-isomer. The area ratio of the (−)-( R)-/(+)-( S)-enantiomer under the concentration–time curve (AUC 0 → ∞ ) in plasma after drug application was 1.62. The total plasma clearance value of the (+)-( S)-enantiomer was 1.53 and higher than that of the (−)-( R)-enantiomer. The [ R]/[ S] ratio in plasma was >1 for standard rac-metalaxyl at each time point. The other pharmacokinetic parameters of the enantiomers were also different. The results indicate substantial stereoselectivity in the degradation of metalaxyl enantiomers in rabbits.

Reaction of field populations of sunflower downy mildew (Plasmopara halstedii) to metalaxyl and mefenoxam

Seed treatment with phenylamide fungicides (metalaxyl and mefenoxam) provides an effective chemical control of sunflower downy mildew caused by the oomycete Plasmopara halstedii. Resistance of the fungus to metalaxyl has been reported in Spain and differences of disease incidence (DI) caused by field populations (FPs) have been observed. The sensitivity of several FPs to seed treatment with metalaxyl was compared. Different groups of FPs were discriminated on the basis of DI caused on sunflower plants. None of the FPs was completely controlled by the fungicide at the dose used. In addition, the efficacy of metalaxyl and mefenoxam against FPs resistant to metalaxyl were evaluated. Three genotypes of sunflower were treated with the fungicides at two different doses. Neither metalaxyl or mefenoxam nor doses had a significant effect on the DI caused by the FPs, suggesting the absence of effect of the double quantity of active enantiomer provided by mefenoxam as compared with metalaxyl. Nevertheless, significant differences on DI were observed among genotypes. The different DIs could be related to: a) different germination rates and, consequently, size of the radicle in the moment of inoculation, b) higher amount of fungicide provided to the confectionery genotype, which has a larger size of seed.

Phenotypic and genotypic diversity in Phytophthora infestans on potato in Great Britain, 1995–98

A total of 2691 single‐lesion isolates of Phytophthora infestans was established from samples of late‐blight disease from 354 commercial and garden/allotment sites in Scotland, England and Wales over four growing seasons, 1995–98. The A2 mating type was rare (3·0% of isolates) and was detected at only 34 sites. In vitro tests of sensitivity to the phenylamide fungicide metalaxyl showed that 316 sites yielded isolates with some insensitivity (resistant and/or intermediate); these were more often commercial sites than garden/allotment sites. Over the four seasons, the frequency of isolates with intermediate fungicide sensitivity increased, while the frequency of resistant phenotypes decreased. Resistant isolates were always of A1 mating type. A subset of 1459 isolates from 326 sites was analysed for molecular diversity. Mitochondrial DNA (mtDNA) haplotype Ia predominated (91·0% of isolates); haplotype IIa was present at 54 sites and both haplotypes at 33 sites. The multilocus RFLP probe RG57 detected 30 fingerprints. Four fingerprints were particularly common (RF002, RF006, RF039 and RF040) and 10 were unique to a single site in a single year. The three commonest fingerprints (RF039 > RF002 > RF006) were of A1 mating type and the fourth (RF040) was A2. RF002 isolates were resistant to the phenylamide metalaxyl and were more common in Scotland than in England and Wales. Small sample sizes limited the usefulness of estimates of diversity. Although approximately half of all sites appeared to be colonized by RF039 genotypes, some sites (both commercial and garden/allotment) showed a higher diversity, having both common and unique genotypes. The genotypic diversity within isolates collected from commercial sites and those from garden/allotment sites were similar. The contributions of sexual reproduction and alternatives to sex to the generation of variation are discussed.

Crosses between isolates of Phytophthora infestans from potato and tomato and characterization of F1 and F2 progeny for phenotypic and molecular markers

Nineteen crosses were carried out in vitro between seven and nine isolates of Phytophthora infestans originating from potato and tomato, respectively. Oospores were produced abundantly in all but two crosses, but oospore germination was generally low (a few per cent) and depended on the combination of parental isolates. The highest fertility in F1 progeny was observed when at least one parental isolate originated from tomato, and the lowest in crosses of isolates from potato; half the crosses were not fertile. Forty‐three F1 and 51 F2 single‐oospore progeny of a selected cross, along with the 16 field isolates, were analysed phenotypically and with molecular markers. Phenotypic characterization included mating type; sensitivity to the phenylamide fungicide metalaxyl‐M; specific virulence on potato R‐gene differentials; and aggressiveness (infected leaf area) on potato and tomato leaf discs (host preference). Isolates and progeny were also assessed for mitochondrial DNA haplotype with RFLP–PCR (restriction fragment length polymorphism–polymerase chain reaction), and characterized with AFLP (amplified fragment length polymorphism) and SSR (simple sequence repeats, microsatellites). Parental isolates were sensitive and resistant to metalaxyl‐M, whereas all F1 were intermediate phenotypes. In the F2 progeny, the majority of isolates (43 of 51) were intermediate in sensitivity and four each were sensitive and resistant to metalaxyl‐M, respectively. In both F1 and F2 progeny, four isolates emerged through selfing. The A1 : A2 ratio was 25 : 18 in F1 and 24 : 21 (plus six self‐fertiles) among the F2 progeny. Many F1 progeny isolates were highly aggressive on both hosts, but 15 and 23% of isolates preferred tomato and potato, respectively. Among F2 progeny, few highly aggressive isolates were recorded and a quarter of isolates lost pathogenicity almost completely. Isolates preferring tomato increased, and those preferring potato significantly decreased in the F2 progeny. Inheritance of mitochondrial haplotype in F1 progeny was uniparental and mostly (25 of 27) from one parent only. Six and four different SSR genotypes were identified in F1 and F2 progeny, respectively, of which two were identical to the parents. The two microsatellite loci, 4B and 4G, segregated in the ratios 15 : 22 : 2 and 22 : 17 in F1 and 24 : 17 and 26 : 15 in F2 progeny, respectively, while the majority of AFLP markers segregated in either a 1 : 0, 1 : 1, 3 : 1 or 1 : 2: 1 ratio. There was no obvious association between AFLP and SSR genotypes, nor between genotypic and phenotypic traits.

Identification and Characterization of Pythium Species Associated with Greenhouse Floral Crops in Pennsylvania.

During 1996 to 2001, samples submitted to clinics from commercial greenhouses involved 11 species and two unidentified isolates of Pythium from 110 plant samples, five potting soil tests, and five tests of irrigation water. Pythium irregulare was found in 45% of the plant samples, four of the five water samples, and three of the five potting soils. Pythium aphanidermatum accounted for 29% of all plant but 77% of the poinsettia samples. The Pelargonium samples received were infected with P. aphanidermatum, P. dissotocum, P. heterothallicum, group F, P. irregulare, P. myriotylum, and P. ultimum. The base pair sequence of the ITS1, 5.8S, and ITS2 regions of ribosomal DNA effectively differentiated the species encountered. The ras-related protein gene sequence did not differentiate P. aphanidermatum, P. arrhenomanes, and P. deliense from one another. One isolate each of P. cylindrosporum, P. dissotocum, P. heterothallicum, P. splendens, and P. ultimum exhibited resistance to the phenylamide fungicide mefenoxam, an isomer of metalaxyl, while 38% of the P. aphanidermatum and 37% of the P. irregulare isolates were resistant.

Phenotypic and genotypic structure of Phytophthora infestans populations on potato and tomato in France and Switzerland

A total of 134 isolates of Phytophthora infestans were collected from potato and 42 from tomato fields in Switzerland and France in 1996 and 1997, and compared with isolates from other countries. The structure of the populations was analysed phenotypically and genotypically, and associated to geographical, seasonal and host origin. Phenotype characteristics of the isolates included mating type; sensitivity to phenylamide fungicides; virulence on potato differentials; and pathogenic fitness. Genotypes were assessed for mitochondrial DNA haplotype with restriction fragment length polymorphism‐polymerase chain reaction (RFLP‐PCR) as well as amplified fragment length polymorphism (AFLP) and simple sequence repeats or microsatellites (SSR). The majority (96%) of isolates originating from potato were the A1 mating type, whereas half the isolates collected from tomato were A2 mating type. Isolates with sensitive, intermediate and resistant responses to the phenylamide fungicide metalaxyl‐M were detected in the populations. Isolates from potato represented races with highly complex virulence spectra, whereas those from tomato belonged to simple races. The pathogenic fitness of isolates was highest on the host of origin, and was significantly reduced for isolates from potato on tomato. One of the four haplotypes, Ia, dominated the population (93% of isolates). Among isolates collected from potato, 15 different SSR genotypes were detected of which two, A‐03 and A‐06, dominated the population. From tomato, 11 SSR genotypes were found of which four, A‐03, B‐03, D‐03 and F‐01, formed the major proportion of the population. Every ninth and fourth isolate from potato and tomato, respectively, represented a different SSR genotype. Four genotypes were isolated from both hosts, whereas 11 and seven genotypes, respectively, originated exclusively from either potato or tomato. The SSR genotype D‐02, represented by the ‘old’ Ib haplotype, was still detected in a few isolates in the current population, and in older reference isolates from different countries. The SSR genotype was not associated with mating type or sensitivity to phenylamide fungicides. A total of 40 AFLP genotypes were distinguished among the isolates, every second isolate representing another genotype. The diverse phenotypic and genotypic structure of the current field populations in Europe suggests that they may have evolved from local processes including sexual recombination, host preference and selection rather than through long‐distance migration.

Sorptive behavior of the phenylamide fungicides, mefenoxam and metalaxyl, and their acid metabolite in typical Cameroonian and German soils.

Laboratory soil sorption experiments were conducted on mefenoxam, formulated metalaxyl (F-metalaxyl), pure metalaxyl (P-metalaxyl) and metalaxyl acid metabolite to elucidate differences in their sorptive behaviour on typical Cameroonian forest soil (sand clay loam, pH 4.8 and 3.01% OC) and German soil (sandy loam, pH 7.2, 1.69% OC) using a batch equilibrium method. The data obtained on all test chemicals conformed to linear and Freundlich adsorption isotherms. The Langmuir equation failed to describe the sorption of the substances tested. All substances were adsorbed to a greater extent by the Cameroonian soil. The average percentage adsorptions for mefenoxam, F-metalaxyl, P-metalaxyl and the acid metabolite on the Cameroonian soil were 27.8%, 28.3%, 31.8% and 46.8% respectively while for the German soil they were 21.7%, 21.5%, 24.7% and 9.8% respectively. The KD and KF parameters and the Freundlich exponential term (1/n) were low, indicating that the interactions between soil particles and the fungicides were weak. The sorption parameters were lower in the German soil. P-metalaxyl exhibited a higher adsorption capacity than F-metalaxyl in both soils. Mefenoxam and F-metalaxyl exhibited similar sorption parameters in soils, whereas those of P-metalaxyl and acid metabolite differed. Differences observed in the adsorption between the two soils could be attributed to their properties. Desorption studies revealed that the adsorbed fungicides were not firmly retained by soil particles and their adsorption was reversible. Desorption of adsorbed mefenoxam, P-metalaxyl and of the acid metabolite from German soil was almost completely reversible with percentage desorption rates of more than 91.0%, whereas the rate for F-metalaxyl was 74.1%. All compounds exhibited some resistance to desorption from the Cameroonian soil, with percentage desorption rates less than 77.0%. Therefore if degradation in the soil is slow the fungicides described have a potential to leach to lower soil horizons.

Phytophthora infestans: populations, pathogenicity and phenylamides.

Isolates of Phytophthora infestans (Mont) de Bary (the potato and tomato late blight pathogen) resistant to phenylamides appeared in Europe and North America in the late 1970s and early 1990s respectively. Concurrent, but coincidentally, with both these events there were radical structural shifts in the pathogen populations as immigrant genotypes from Mexico displaced the indigenous populations. Both A1 and A2 mating type isolates are now present in blighted crops, permitting alternative inoculum via germinating sexually produced oospores to influence dynamics of late blight populations. Studies of inheritance of ploidy, host-specific pathogenicity, mating type and resistance to antibiotics and phenylamide fungicides have provided insight into mechanisms of variation in this potent pathogen.

Effects of the phenylamide fungicides, mefenoxam and metalaxyl, on the microbiological properties of a sandy loam and a sandy clay soil

The objective of the study was to compare ecotoxicological data obtained from laboratory experiments on the side-effects of three phenylamide fungicides, pure metalaxyl (racemic mixture of R- and S-enantiomers), formulated metalaxyl and mefenoxam (only active R-enantiomer) on the chemical and biochemical parameters of two soils of different type and origin. The purpose of the comparison was to determine to what extent mefenoxam, developed as alternative to metalaxyl, can affect the activity of soil micro-organisms and their processes, and to elucidate the differences between the effects of pure and formulated metalaxyl. The dynamics of the quantitative changes in biochemical parameters induced by the addition of these fungicides at their recommended field rate were determined in a sandy clay soil from Cameroon and a sandy loam soil from Germany, during a 120-day incubation experiment. The type of soil significantly influenced the effect of these fungicides on the soil parameters studied. Incorporation of these fungicides generally stimulated the activity of phosphatases and s-glucosidase, mineralization and the availability of N and most plant nutrients in soils. The activity of dehydrogenase and the availability of NO3– were generally adversely affected. Among the fungicides tested, the stimulation was more pronounced with mefenoxam followed by formulated metalaxyl.

The Spatiotemporal Genetic Structure of Phytophthora capsici in Michigan and Implications for Disease Management

Root, crown, and fruit rot caused by Phytophthora capsici Leonian is a limiting factor for the production of peppers, tomatoes, and cucurbit crops in Michigan and the United States. Like many species in the genus Phytophthora, P. capsici has the potential for rapid polycyclic disease development from a limited amount of initial inoculum (6). P. capsici produces caducous sporangia that can be spread by wind-blown rain or release 20 to 40 motile zoospores in the presence of free water. The polycyclic phase of disease development is thought to be driven primarily by asexual spore dispersal at a local scale (within and down rows). Sexual reproduction requires both the A1 and A2 compatibility types (CTs) and results in the production of thick-walled oospores. Oospores are thought to serve as the primary survival structure outside of host tissue. Recommended disease management strategies stress the importance of avoiding excess water in the plant rhizosphere by using well-drained fields, conservative irrigation, and planting on raised beds. Additional recommendations include rotation to nonsusceptible hosts for at least 2 years and the use of fungicides. The phenylamide fungicide (PAF) mefenoxam is a systemic compound with high activity against P. capsici and has been used by growers throughout the United States to control P. capsici. Insensitivity to PAF has been reported for a number of other oomycetous organisms (Bremia lactucae, P. infestans, and P. sojae, etc.) and appears to be conferred by a single incompletely dominant gene of major effect (1). Growers in Michigan practicing 2+-year rotation in well-drained fields using an array of fungicidal management tools have experienced significant losses to P. capsici. Michigan is the number one producer of cucumbers for pickling in the United States and it was at the request of grower groups associated with this industry that research into the epidemiology and reproductive biology of P. capsici on cucurbit hosts was initiated. Although many researchers cite oospores as the most likely propagule for survival outside of host tissue, there have been very few investigations specifically aimed at determining the impact of sexual reproduction in natural populations of P. capsici. Our hypothesis was that the sexual stage may play an important role not only in survival but also in the adaptation of P. capsici populations to environmental stresses (e.g., fungicides). Our goal was to perform a comprehensive investigation of the phenotypic and genetic diversity present in P. capsici populations from the major vegetable production regions of Michigan, with the implicit intention of addressing questions concerning epidemiology, reproductive biology, and the durability of currently recommended management strategies.

The effect of fungicides, irrigation and plant density on the development of Peronospora sparsa, the cause of downy mildew in rose and blackberry

SummaryDowny mildew of rose (Rosa spp.) and blackberry (Rubus fructicosus), caused by Peronospora sparsa, can cause significant losses in production. In a series of experiments, fungicides with different modes of action to the commonly used phenylamide‐based products were examined for efficacy on both crops. Cymoxanil + mancozeb + oxadixyl and fluazinam gave good downy mildew control on both rose and blackberry. On outdoor, container‐grown rose, high volume sprays of fosetyl‐aluminium were also effective, but on young micropropagated blackberry plants, application as a drench treatment was better than as a spray. Good control was also achieved on blackberry with chlorothalonil and with metalaxyl in formulation with either thiram or mancozeb. There was no evidence of loss of control with phenylamide fungicides due to resistance. Irrigation regimes were also examined on blackberry and sub‐irrigation of plants in propagation on a sand bed led to significantly lower disease levels than those found where plants had been irrigated by overhead sprinklers. Reducing the density of container‐grown rose plants had a measurable effect on the progress of downy mildew, though this was small and temporary when compared with the effect of fungicide treatment. Fungicide programmes which gave season‐long control of rose downy mildew resulted in significantly improved extension growth.

Recent developments in elucidating modes of resistance to phenylamide, DMI and strobilurin fungicides

Resistance risk assessment, analysis of resistance events and risk management are based on elements such as the mode of resistance in target organisms, similarity of inhibitors, detection methods for resistant individuals, and design and validation of product use strategies. The mode of resistance may be analysed in several steps including the molecular, genetic, biochemical, physiological, and population level. Historical and recent findings are described for phenylamide fungicides (PAFs), demethylation inhibitors (DMIs) in sterol biosynthesis and fungicides of the strobilurin-type action and resistance (STAR) group (Qo respiration inhibitors, QoIs) with regard to the site of resistance in target pathogens and the presence of resistant individuals in populations.