Effect of fungicides on stages of the life cycle of Phytophthora cinnamomi

An assessment was made of the effects of several fungicides on the formation of zoosporangia, the release of zoospores from zoosporangia, zoospore motility, germ tube formation and mycelial growth, by Phytophthora cinnamomi Rands. Compounds active against mycelial growth (such as metalaxyl and furalaxyl), as well as those active against zoospores (such as captafol and zineb), could be detected by a lupin (Lupinus angustifolius L.) assay which has been developed. This assay may be useful as a screening procedure to detect new fungicides.     

Studies on metalaxyl-resistant Phytophthora infestans in potato crops in south-west Scotland

In 1981 metalaxyl-resistant Phytophthora infestans was found in 44% of the 63 crops examined in south-west Scotland. In 1982, when fungicides containing acylalanines (metalaxyl or ofurace) were not used in the area, it was detected in 15%. Metalaxyl-resistant strains were found in several crops which had been sprayed only with non-acylalanine fungicides or had received no fungicidal treatment. In neither year was resistance associated with a significant breakdown of disease control.
Metalaxyl-resistant isolates of the blight fungus were obtained from tubers from crops in which resistance was detected on the foliage.
In detached leaf experiments, resistant isolates retained their ability to grow on leaves treated with 100 μg/ml of metalaxyl after 52 weekly transfers on untreated leaves, and produced sporangia on leaves treated with up to 2000 μg/ml of metalaxyl or ofurace. In mixtures with sensitive sporangia the proportions remained stable after nine weekly transfers on untreated leaves. However, at concentrations as low as 50 μg/ml, metalaxyl and ofurace reduced the extent of colonization and the numbers of sporangia produced by resistant isolates by an average of 50%.     

Efficacy of phosphonate in controlling white powdery rot of fig caused by <Emphasis Type="Italic">Phytophthora palmivora</Emphasis>

Pre- and post-infection activities of phosphonate and fungicides (azoxystrobin, chlorothalonil, cyazofamid, copper hydroxide, and copper sulfate) against white powdery rot on fig leaves caused by Phytophthora palmivora were determined. Phosphonate and fungicides were applied at 3, 7, or 14 days before inoculation and 16, 24, or 40 h after inoculation. Phosphonate and four fungicides (except for chlorothalonil) had protectant activity for up to 14 days before inoculation. Protectant activity between phosphonate and the four fungicides did not differ significantly. These fungicides did not have a curative activity, whereas only phosphonate had a curative activity when applied 16 h after inoculation. The pre-infection activity of phosphonate was determined in fig fields. Phosphonate provided excellent protectant activity, and its activity did not differ significantly from that of the copper fungicides. The activity of phosphonate in the life cycle of P. palmivora was compared with that of the five fungicides. Half-maximal effective concentration of the inhibition of mycelial growth was low for phosphonate, chlorothalonil, and the two copper fungicides. Mycelial growth was least affected by azoxystrobin and cyazofamid, but n-propyl gallate, an alternative oxidase inhibitor, in combination with these fungicides, significantly inhibited mycelial growth. Sporangium formation was sensitive to all compounds. Germination of encysted zoospores was sensitive to azoxystrobin, cyazofamid, and chlorothalonil and least sensitive to phosphonate and the two copper fungicides. The activity of phosphonate on mycelial growth and sporulation of P. palmivora suggests it has potential for controlling the disease.     

Resistance to metalaxyl in isolates of the sunflower pathogen Plasmopara halstedii

Plasmopara halstedii isolates showing an atypical reaction to metalaxyl were collected in France, in 1995 and 1996, and tested in the laboratory for their level of sensitivity to this fungicide. Primary and secondary infections caused by one of these isolates were not controlled by the metalaxyl concentration registered for seed treatment. The EC₅₀ of this isolate was 12 800 mg a.i. kg^-1 compared with 22 mg a.i. kg^-1 for sensitive isolate, indicating a 582-fold decrease in sensitivity to the compound. There was no reduction in the agressiveness of the resistant isolate. Using other anti-oomycete fungicides, it appeared that propamocarb, contact fungicides (fluazinam, folpet, mancozeb) and the mixed formulations dimethomorph + mancozeb, cymoxanil + mancozeb and ofurace + folpet were effective against primary infections made with metalaxyl resistant and sensitive isolates, but not against secondary infections. Metalaxyl mixed with fluazinam, folpet or mancozeb was more effective against primary infections with the resistant isolate than metalaxyl alone. The EC₅₀ of five other isolates ranged from 5 800 to 32 900 mg a.i. kg^-1, indicating a variability in metalaxyl sensitivity of resistant sunflower downy mildew isolates. This is the first report of physiological resistance to metalaxyl in Plasmopara halstedii.     

Antifungal activity of metalaxyl and furalaxyl

The systemic acylalanines metalaxyl and furalaxyl proved in vitro highly selective to fungi of the order of the Peronosporales, the former being somewhat more active than the latter. Various other fungal, and bacterial, species did not show sensitivity to the toxicants. Metalaxyl was strikingly more active on Pythium splendens and Phytophthora cinnamomi at low than at high temperature. With Phytophthora species metalaxyl displayed improved activity in combination with sodium dodecylsulphate and Triton X-100. Neither the composition of the medium nor its pH influenced activity. None of many compounds tested could antagonize their effect. The fungitoxicity caused by metalaxyl in the gas phase was strikingly better than that of furalaxyl against P. splendens. The toxicants had little effect on germination of the hyphal swellings of P. splendens, but had significant influence on the subsequent hyphal development. This included reduced growth, abnormal variation in the fungal thickness, and inhibition of the formation of hyphal swellings. Metalaxyl and furalaxyl were fungistatic at minimal inhibitory concentration (MIC) values during the first 24 hr. Thereafter, the toxicants became fungicidal. Metalaxyl and furalaxyl did not inhibit respiration of P. splendens at MIC values or somewhat higher concentrations. Neither did metalaxyl inhibit the [¹⁴C]carbon dioxide evolution from externally supplied radiolabeled glucose.     

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

Mutants 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.     

Natural occurrence and distribution of stem cankers caused by <Emphasis Type="Italic">Phytophthora megakarya</Emphasis> and <Emphasis Type="Italic">Phytophthora palmivora</Emphasis>on cocoa

Epidemiological studies were conducted in five cocoa growing districts in the Eastern Region of Ghana solely infected by Phytophthora palmivora and five districts in the Ashanti and Brong Ahafo Regions prevalently infected by Phytophthora megakarya to determine the natural incidence, the vertical distribution on trees and the probable sources of stem canker infections, and to isolate and identify the causal pathogens. The incidence of canker in the solely P. palmivora infected area was higher (between 0% and 16.0%) than in the area mainly infected with P. megakarya (0.5–8.0%). Differences were found in the natural height distribution of cankers in the two areas, whilst the areas solely infected with P. palmivora showed a near normal curve, those prevalently infected with P. megakarya were positively skewed. Most of the cankers caused by P. megakarya were found at the base or near the base of the tree trunks (1–40cm above ground level), while those of P. palmivora were concentrated between 41 and 100cm from the ground level. The majority (71.8%) of cankers in the solely P. palmivora infected area were cushion-borne, followed by 24.3% from unknown sources and only 3.9% from the soil. In contrast, a significantly large proportion (32.6%) of the cankers in the prevalently P. megakarya infected area were soil-borne, although cushion-borne cankers formed the majority (48.4%) due to the presence of P. palmivora infection whilst those of unknown sources constituted 19.0%. Phytophthora megakarya was frequently isolated from all the three sources of canker infections, indicating P. megakarya readily causes stem canker on cocoa. These results emphasise the importance of different reservoirs as sources of primary inoculum for diseases caused by the two Phytophthora species particularly pod rot infection on cocoa.     

Difolatan,a promising fungicide for control of the tapping panel disease Black Thread in Hevea brasiliensis

In field experiments for control of Black Thread, caused byPhytophthora palmivora, the mean damage done to the trees in the previous rainy season should be of the same level for all the treatments. Satisfactory results and better disease control were obtained with Difolatan in weekly applications when compared with some recommended fungicides for tapping panel diseases. No unfavorable side effects were observed on the latex crop or to the trees. Difolatan (active ingredient: N-(1,1,2,2-tetrachloroethylsulfenyl)-cis4-cyclohexene-1,2-dicarboximide) is known to be of low mammalian toxicity.Samenvatting In veldproeven ter bestrijding van de tapvlakziekte Streepjeskanker, veroorzaakt doorPhytophthora palmivora, dient de gemiddelde schade—aangericht in het voorafgaande regenseizoen—in alle behandelingen van hetzelfde niveau te zijn. Op deze wijze worden de infectie-kansen gelijkelijk verdeeld over de behandelingen, aangezien de oude schade bepalend is voor de mate van infectie. Een betere ziekte-bestrijding werd verkregen met het fungicide Difolatan dan met enkele bekende middelen ter bestrijding van tapvlakziekten. Bevredigende resultaten werden verkregen bij wekelijkse toediening. Behandelingen met Difolatan hebben waarschijnlijk geen onaanvaardbare nevenwerking op de bomen en de latex; het middel is weinig giftig.     

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.     

大豆疫霉菌部分生物学特性及其药剂筛选研究

采用大豆疫霉菌在不同条件下菌丝生长速度法研究了大豆疫霉菌的部分生物学特性,并应用杀菌剂室内生测、盆栽药剂防效对药效作了评价。研究结果表明,大豆疫霉菌营养生长的最适温度为25～30℃;最适pH为6;光暗交替有利于该菌营养体的生长,在Rye或CA培养基上生长最快。室内药效测定结果表明,烯酰吗啉EC₅₀为0.165 4μg/mL,抑菌效果最好,甲霜灵、甲霜灵.锰锌和氟吗啉.锰锌的EC_{50分别为0.261 00、.451 0和0.984 2μg/mL,效果次之。盆栽试验结果表明,几种药剂在活体条件下对大豆疫病的防治效果较好,并有较长持效期。}     

Assessment of sanitation and fungicide application directed at cocoa tree trunks for the control of <Emphasis Type="Italic">Phytophthora</Emphasis> black pod infections in pods growing in the canopy

Studies were conducted in two cocoa-growing areas of Ghana, one solely affected by Phytophthora palmivora and the other predominantly by Phytophthora megakarya, to determine the effectiveness of sanitation practices and fungicide application on tree trunks for the control of black pod disease in the canopy. Sanitation practices including weeding, pruning, thinning, shade reduction and removal of mummified pods were carried out prior to fungicide applications, and diseased pods were routinely removed at monthly intervals during harvesting. Three types of fungicides were used: systemic (Foli-R-Fos 400) applied as injection into the main trunks, semi-systemic (Ridomil 72 plus) and contact (Nordox 75, Kocide 101, Kocide DF, Blue Shield and Funguran-OH) applied as sprays onto pods on the main trunk. Sanitation combined with fungicide application on the trunk significantly reduced black pod disease incidence in the tree canopy. For fungicides applied as a spray, Ridomil 72 plus at 3.3 g l⁻¹ and Kocide DF at 10 g l⁻¹ and as injection, 40 ml Foli-R-Fos 400 injected twice a year, performed better than the other fungicide treatments. The position of pods significantly influenced the incidence of canopy black pod infection in the P. megakarya predominantly affected area but to a lesser extent in the P. palmivora solely affected area. However, no significant interactions were found between fungicide treatments and the position of pods on the tree in both disease areas. The determined trunk-canopy relationship in the development of black pod disease on cocoa can be used in disease control programmes to maximise the impact of sanitation practices, achieve judicious application of fungicides, thereby reducing the environmental impact of fungicides on the cocoa ecosystem, and ultimately increase the economic returns.     

Resistance to acylalanines in Phytophthora infestans in The Netherlands1

In The Netherlands in 1980 a severe late-blight epidemic involving metalaxyl-resistant strains of Phytophthora infestans did considerable damage to the potato crop. As a consequence metalaxyl or metalaxyl-containing products were withdrawn from the Dutch fungicide market for the control of potato late blight. In 1981 the majority of the P. infestans isolates obtained from various parts of the country were sensitive to metalaxyl. In three areas where metalaxyl mixtures were used on a relatively large scale metalaxyl-resistant strains dominated the population. Incidental use of metalaxyl mixtures in 1982 and 1983 showed adequate late-blight control and, when in 1984 an early and in some areas severe epidemic developed, a metalaxyl/mancozeb mixture was reintroduced for curative application in combination with conventional fungicides. Although disease control was adequate, resistant strains were present in a small number of fields. Whether resistant strains will increase in frequency and threaten the usefulness of mixtures of acylalanines will heavily depend on how well potato farmers adopt strategies involving a limited use of mixtures of acylalanines in a spray schedule with conventional late-blight fungicides.     

GC-MS metabolite profiling of <Emphasis Type="Italic">Phytophthora infestans</Emphasis> resistant to metalaxyl

Phytophthora infestans is the most important potato pathogen worldwide. Various alternatives have been used to control the pathogen, including continuous applications of phenylamide fungicides which has caused a rapid development of resistance in populations of P. infestans. Despite the importance of the disease, metabolite profiling of fungicide-resistant P. infestans has not been reported. In vitro resistance of Phytophthora infestans isolates to metalaxyl was characterized and metabolic changes in resistant isolates were evaluated at low (0.5 mg/L) and high (100 mg/L) concentrations of the fungicide. About 70% of the isolates tested showed resistance to metalaxyl and a total of 49 metabolites were differently expressed in resistant isolates growing in the presence of the fungicide. Principal components analysis revealed a distinct metabolite profile of resistant isolates exposed to both low and high levels of metalaxyl. The main metabolites responsible for the clustering in both fungicide concentrations included fatty acids such as hexadecanoic and octadecanoic acids, sugars such as glucose and fructose, aminoacids such as proline and valine, and organic acids such as butanedioic and propanoic acids. Potential resistance-related metabolic pathways are mostly involved in the regulation of the pathogen’s membrane fluidity and included the fatty acid biosynthesis as well as the glycerophospholipid metabolism pathways. This is the first metabolomic-based characterization of fungicide resistance in plant pathogens.     

Preliminary evaluation of nine fungicides for control ofPhytophthora cactorum andP. citrophthora associated with crown rot in peach trees

Excised twig assay and excised stem inoculation were used to evaluate nine fungicides (metalaxyl, fosetyl-Al, copper hydroxide, copper sulfate, copper oxychloride, captan, quintozene, propamocarb and chlorothalonil) againstPhytophthora cactorum andP. citrophthora associated with crown rot in peach trees. Segments were soaked in fungicide solutions at different concentrations and then inserted vertically intoP. cactorum orP. citrophthora cultures growing on cornmeal agar plus antibiotics, or inoculated by inserting a mycelium-bearing agar plug directly into the cambium. Following incubation, the bark was scraped off and length of necrosis was measured. Metalaxyl was the only fungicide that inhibited canker development on segments at the manufacturer-recommended concentration. Fosetyl-Al, captan, copper hydroxide and copper sulfate inhibited canker development at 3, 4, 4 and 8 gl^-1, respectively. The other fungicides did not affect canker length significantly compared with non-treated twigs, with the exception of propamocarb, which reduced the development ofP. cactorum on excised stems. The tested methods enabled rapid and effective evaluation of a large number of chemicals to prevent crown rot diseases caused byPhytophthora in the laboratory. http://www.phytoparasitica.org posting Dec. 5, 2001.     

Uptake and distribution of [14C]metalaxyl by detached tobacco leaves

When the petioles of detached tobacco leaves (10–17 cm²) were incubated in aqueous solutions containing [¹⁴C]metalaxyl, uptake of the fungicide was dependent on the temperature and photoperiod. Detached leaves took up 78% more [¹⁴C]metalaxyl at 26°C than at 16°C. The rate of uptake in the light at 21°C was linear, but after an additional 20h in the dark, there was only twice as much fungicide in the leaves. Different sized leaves contained the same amount of fungicide per cm² area. Uptake by detached leaves of the ¹⁴C-labelled anilide lactones ofurace and RE-26940 [2-methoxy-N-(tetrahydro-2-oxo-3-thienyl)acet-2′,6′-xylidide] was similar to that of metalaxyl. At the concentration of metalaxyl (66 ng ml^?1) that controlled blue mould (Peronospora tabacina) on detached tobacco leaves, the amount of fungicide in the leaves was found to be 7.25 ng. Autoradiography showed that the distribution of [14C]metalaxyl in detached leaves after incubation for 23h was uniform, although higher concentrations of the label were present in the smaller veins of the leaves.     

Resistance to acylalanine fungicides in Phytophthora megasperma f.sp. medicaginis

Isolates ofPhytophthora megasperma f. sp.medicaginis resistant to the systemic fungicide metalaxyl were obtained by mycelial adaptation and by mass selection from zoospores either untreated or treated with UV irradiation or with the chemical mutagenN-methyl-N-nitro-N-nitrosoguanidine. In an assay with one-week-old lucerne seedlings all of 19 spontaneously obtained isolates showed a relatively low degree of resistance and were less virulent than the original isolate. Only one of these isolates showed resistance in vivo in this test. From 176 highly resistant isolates obtained after mutagenic treatment, 81 were as virulent as the orginal isolate and 33 of the latter displayed a considerable degree of resistance in vivo.Metalaxyl at a concentration of a.i. in soil of 20 mg.l^–1 failed to control root rot of 7–8 week old lucerne plants inoculated with mycelial fragments of virulent resistant isolates. Under similar conditions root rot caused by the original isolate was completely prevented even at a concentration of a.i. of 2.5 mg.l^–1.The resistance to metalaxyl appeared to be highly stable since virulent and resistant strains did not loose their resistance after 12 infection cycles in a seedling assay in the absence of the fungicide, neither did mixed populations of a resistant and a sensitive isolate under similar conditions.The likelihood of development of resistance to acylalanine fungicides under practical conditions is discussed.Samenvatting VanPhytophthora megasperma f. sp.medicagninis, een wortelpathogeen van luzerne, konden tegen metalaxyl resistente isolaten worden verkregen na adaptatie van mycelium en selectie uit, zoösporen die al dan niet behandeld waren met het chemisch mutagensN-methyl-N-nitro-N-nitrosoguanidine. Alle 19 spontaan verkregen isolaten vertoonden een lage graad van resistentie en hun virulentie in een zaailingentest was lager dan van het oorspronkelijke isolaat. Slechts één van deze isolaten vertoonde resistentie in vivo. Bij toetsing van 176 isolaten met een hoge graad van resistentie die na een mutagene behandeling waren verkregen bleek dat 81 isolaten even virulent waren als het oorspronkelijke isolaat en dat er 33 een aanzienlijke graad van resistentie in vivo te zien gaven.Metalaxyl in een a.i. concentratie van 20 mg.l^–1 grond kon het optreden van wortelrot bij 7–8 weken oude luzerne planten niet verhinderen, wanneer deze werden geinoculeerd met gefragmenteerd mycelium van resistente isolaten. Onder dezelfde omstandigheden kon het oorspronkelijke isolaat reeds volledig worden bestreden bij een a.i.-dosering van 2.5 mg.l^–1.Resistentie tegen metalaxyl bleek een zeer stabiele eigenschap te zijn omdat bij een viertal getoetste stammen de resistentie niet verdween na 12 infectiecycli in een zaalingentest bij afwezigheid van het fungicide.Onder deze omstandigheden bleek ook dat de resistente stammen niet verdwenen uit mengpopulaties van een gevoelige en een resistente stam hetgeen erop wijst dat de virulentie van de resistente isolaten van hetzelfde niveau is als dat van het oorspronkelijke isolaat.Op grond van de gevonden resultaten mag de mogelijkhei tot resistentieontwikkeling tegen acylalanine fungiciden in de praktijk aanwezig worden geacht.     

棉铃疫病菌对甲霜灵的抗药性风险研究

在实验室条件下,对分离自安徽省芜湖、无为、宣城、望江、肥东、寿县、滁州等7个地区的棉铃疫病菌(Phytophthora boehmeriae Sawada)对甲霜灵的抗药性进行了测定。结果显示,甲霜灵对各供试菌株的EC50分布为0.008 3～0.079 1μg／mL,平均为0.026 1 μg／mL,远小于1 μg／mL,且供试菌株在含甲霜灵质量浓度为1μg／mL的LBA平板上菌丝生长几乎完全被抑制,表明各供试菌株对甲霜灵十分敏感,但其敏感程度地区间存在较大差异。通过室内药剂直接诱变,获得抗甲霜灵菌株,经抗性测定结果表明,抗性菌株的抗性水平较其敏感亲本菌株高13 000倍以上,提示安徽省棉铃疫病菌对甲霜灵具有潜在的抗药性风险。     

PCR‐based identification of cacao black pod causal agents and identification of biological factors possibly contributing to Phytophthora megakarya's field dominance in West Africa

Among the Phytophthora species that cause black pod of cacao, P. megakarya is the most virulent, posing a serious threat to cacao production in Africa. Correct identification of the species causing the black pod and understanding the virulence factors involved are important for developing sustainable disease management strategies. A simple PCR‐based species identification method was developed using the species‐specific sequences in the ITS regions of the rRNA gene. A phylogenetic tree generated for 119 Phytophthora isolates, based on the 60S ribosomal protein L10 gene and rDNA sequence, verified the PCR‐based identification assay and showed high interspecific variation among the species causing black pod. Phytophthora megakarya isolates were uniformly virulent in an assay using susceptible cacao pod husks inoculated with zoospores, while the P. palmivora isolates showed greater divergence in virulence. The virulence of P. megakarya was associated with earlier production of sporangia and an accelerated induction of necrosis. While zoospore germ tubes of both species penetrated pods through stomata, only P. megakarya produced significant numbers of appressoria. A hypersensitive‐like response was observed when attached SCA‐6 pods were inoculated with P. palmivora. SCA‐6 pods became vulnerable to P. palmivora when wounded prior to zoospore inoculation. Phytophthora megakarya was more aggressive than P. palmivora on attached SCA‐6 pods, causing expanding necrotic lesions with or without wounding. Phytophthora megakarya is predominant in the Volta region of Ghana and it remains to be seen whether it can displace P. palmivora from cacao plantations of Ghana as it has in Nigeria and Cameroon.     

Control of potato late blight using controlled-release granules containing ofurace

In an attempt to reduce or eliminate the need for spraying to control potato late blight caused by Phytophthora infestans, investigations were made on the use of controlled-release granules containing the systemic fungicide ofurace. The granules when mixed with sand and buried in soil released the fungitoxicant for about 100 days. When the granules were applied in furrows at planting, protection of potato plants (cv. Maris Bard), assessed by inoculation of detached leaflets with P. infestans, lasted for between 85 and 100 days. Application of the granules when the plants were earthed up 48 days after planting did not result in better late-season protection, possibly due to poorer uptake of the fungitoxicant by the plants at this time. Chemical analysis of leaves from plants that had received in-furrow treatment indicated that ofurace at 0.2–0.5 μg g^?1 fresh weight was needed to confer protection from late blight.     

Fungicide resistance in soil-borne Phytophthora species1

Systemic fungicides such as the phenylamides and phosphonates have provided new opportunities for chemical control of soil-borne Phytophthora species. Potential problems in their use include biodegradation in soils, and the emergence of pathogen isolates resistant to these compounds. Resistance to phenylamides is well known in leaf-infecting Oomycetes, but less is known about the development and relative fitness of resistant isolates among populations of soil-borne Phytophthora species. Experiments with P. capsici and P. palmivora have shown that stable resistant and virulent isolates can be selected under laboratory conditions. Some mutants resistant to both metalaxyl and fosetyl-A1 have been recovered, with growth rates and sporulation equivalent to parental wild types. Selected mutants proved able to compete in vivo with fungicide-sensitive wild types in the absence of the compounds. More information is required to fully evaluate the implications of these results for chemical control of soil-borne Phytophthora species in the field. Résistance des Phytophthora du sol aux fongicides