Use Of Metalaxyl Research Articles

Possible Reasons Affecting Different Phytophthora infestans Populations in Tomato and Potato Isolates in Thailand

Late blight, caused by the oomycete Phytophthora infestans, is one of the most important diseases affecting tomato and potato production worldwide. In Thailand, the disease is widespread in the north and northeast, especially in the Chiang-Mai and Tak provinces. The mating type, metalaxyl sensitivity, mitochondrial DNA (mtDNA) haplotype, RG57 fingerprinting, and microsatellite were used to characterize the P. infestans populations. The study revealed that the P. infestans of tomato isolates in Thailand are of the same lineage as those from 1994 until 2002. The clonal lineages that were found in the potato populations have changed since 1994. The changes in P. infestans isolates in the potato populations have likely been the result of the import of seed potatoes to Thailand. Furthermore, the P. infestans populations in potatoes show resistance to metalaxyl, whereas those from tomato isolates show sensitivity to fungicides. The reasons for the different responses can be attributed to (i) the use of metalaxyl, (ii) the host preferences of P. infestans, and (iii) the migration of new genotypes from infected potato seeds.

Effect of Oxathiapiproline-Famoxadone fungicide and Trichoderma sp. on Control of Downy Mildew and the Growth of Maize

Abstract Downy mildew caused by Peronosclerospora spp. is a major limiting factor in maize production in Indonesia. Susceptible varieties can have yield losses of around 90 to 100%. The continuous use of metalaxyl has triggered resistance to downy mildew. Therefore, it is necessary to have other fungicides that can replace the use of metalaxyl. In this study, the oxathiapiproline- famoxadone was tested in combination with the fungus Trichoderma sp. as an alternative to metalaxyl. This study used a randomized complete block design. The treatments were arranged in a factorial with 2 factors. The first factor was fungicide (without/F0 and with fungicide/F1). The second factor was Trichoderma sp., i.e. T0 (without Trichoderma sp), T1 (Trichoderma sp. 106 spores/ml), T2 (Trichoderma sp. 107 spores/ ml), and T3 (Trichoderma sp. 108 spores/ml). The results showed that oxathiapiproline-famoxadone was able to suppress downy mildew, while the application of Trichoderma sp. did not affect the control of downy mildew and maize plant growth. Fungicide application and Trichoderma sp. 107 spores/ml suppressed disease severity at 7 days after inoculation with Peronosclerospora sp.

Use of metalaxyl against some soil plant pathogens of the class Peronosporomycetes - A review and two case studies

Upon its discovery and implementation in plant protection, metalaxyl became one of the most important fungicides against Phytophthora infestans, but its efficiency has also been proven against other soil pathogens of the class Peronosporomycetes. The most important genus – Phytophthora comprises more than 150 plant pathogens, which cause significant losses in crop production or damage to natural plant associations. Many species of related genera Pythium, Phytopythium and Globisporangium have a similar ability as the species of Phytophthora. Those pathogens are able to quickly spread in wet soils by actively movable zoospores or in the air by means of zoosporangia; they are able to persist in an environment for long periods once they are introduced into the locality, having durability from their resting structures (chlamydospores, hyphal swelling and oospores). Metalaxyl has proven to be very efficient against these pathogens. However, shortly after its release, the rapid development of resistance against this compound was recorded in many species of the class Peronosporomycetes. Such easily developed resistance is due to the monogenic nature of the resistance, which also determines any anti-resistant strategies. The solution of this issue rests in the cautious use of metalaxyl, with consideration given to these strategies, and should be based also on precise information about the environment and the present pathogenic agents.

Efektivitas Isolat-Isolat Bacillus sebagai Pengendali Penyakit Bulai dan Pemacu Pertumbuhan Tanaman Jagung pada Kondisi Terkontrol

The main disease in maize is downy mildew caused by the fungus Peronosclerospora spp.. This pathogen can cause yield losses of up to 100%. Therefore, efforts to control this disease are continuously carried out, including technical culture, assembly of resistant plants, and use of synthetic fungicides. At the farm level, the use of metalaxyl, synthetic fungicides is the most common practice. On the other hand, it has been reported that some Peronosclerospora groups are starting to become resistant to metalaxyl. These problems lead to the need for alternative controls, for example with biological agents. Biological agents from bacterial groups have been developed to control plant disease, but for downy mildew is still limited. The purpose of this study is to screen and test Bacillus spp. ability to suppress downy mildew and promote the growth of maize. The assay was carried out on seeds by invitro to investigate growth-promoting reactions and also testing under controlled conditions in greenhouses to investigate the suppression ability of downy mildew disease development. The results showed B. polymyxa strain BP18, Bacillus subtilis strain BS41, Bacillus sp. strain BT1, and Bacillus sp. strains can stimulate the growth of corn seedlings and suppress downy mildew. The best isolate in suppressing downy mildew was Bacillus sp. strain BT1 with the smallest AUDPC value (3.94) and the highest protection index (82.71%). It is hoped that these results will find potential isolates and have the potential to be developed into biopesticides and biofertilizers.

Pesticide exposure and incident thyroid cancer among male pesticide applicators in agricultural health study

BackgroundMany pesticides are known to have thyroid-disrupting properties. However, few studies have evaluated the association between specific pesticide ingredients and risk of thyroid cancer. We investigated self-reported pesticide use and incident thyroid cancer in the Agricultural Health Study (AHS), a large cohort of occupationally-exposed male pesticide applicators. MethodsThe AHS is a prospective cohort of licensed pesticide applicators in Iowa and North Carolina. At enrollment (1993–1997) and follow-up (1999–2005), participants reported use of 50 pesticides. We characterized exposure as ever use (44 pesticides with ≥5 exposed cases) and by cumulative intensity-weighted lifetime days (22 pesticides with ≥10 exposed cases), a metric that accounts for factors that influence exposure. We estimated hazard ratios (HR) and 95% confidence intervals (CI) using Cox regression for incident thyroid (n = 85 cases) cancer among male participants using follow-up through 2014/2015. ResultsUse of the fungicide metalaxyl (HR = 2.03, CI:1.16–3.52) and the organochlorine insecticide lindane (HR = 1.74, CI:1.06–2.84) was associated with increased risk of thyroid cancer. The herbicide chlorimuron-ethyl was inversely associated with risk when we restricted to papillary thyroid cancer, the most common subtype (HR = 0.52, CI:0.28–0.96). High use of the insecticide carbaryl (>median intensity-weighted days) was inversely associated with thyroid cancer (HR = 0.20, CI:0.08–0.53, ptrend = 0.001). ConclusionsIn this large cohort study, we observed increased risk of thyroid cancer associated with use of metalaxyl and lindane, and an inverse association with carbaryl. More work is needed to understand the potential role of these chemicals in thyroid carcinogenesis.

Differential responses of two cyanobacterial species to R-metalaxyl toxicity: Growth, photosynthesis and antioxidant analyses

Metalaxyl is a broad-spectrum chiral fungicide that used for the protection of plants, however extensive use of metalaxyl resulted in serious environmental problems. Thus, a study on the detoxification mechanism in algae/cyanobacteria and their ability for phycoremediation is highly recommended. Here, we investigated the physiological and biochemical responses of two cyanobacterial species; Anabaena laxa and Nostoc muscorum to R-metalaxyl toxicity as well as their ability as phycoremediators. Two different levels of R-metalaxyl, at mild (10 mg/L) and high dose (25 mg/L), were applied for one-week. We found that A. laxa absorbed and accumulated more intracellular R-metalaxyl compared to N. muscorum. R-metalaxyl, which triggered a dose-based reduction in cell growth, photosynthetic pigment content, and photosynthetic key enzymes’ activities i.e., phosphoenolpyruvate carboxylase (PEPC) and ribulose‒1,5‒bisphosphate carboxylase/oxygenase (RuBisCo). These decreases were significantly less pronounced in A. laxa. On the other hand, R-metalaxyl significantly induced oxidative damage markers, e.g., H2O2 levels, lipid peroxidation (MDA), protein oxidation and NADPH oxidase activity. However, these increases were also lower in A. laxa compared to N. muscorum. To alleviate R-metalaxyl toxicity, A. laxa induced the polyphenols, flavonoids, tocopherols and glutathione (GSH) levels as well as peroxidase (POX), glutathione peroxidase (GPX), glutathione reductase (GR) and glutathione-s-transferase (GST) enzyme activities. On the contrary, the significant induction of antioxidants in N. muscorum was restricted to ascorbate, catalase (CAT) and ascorbate peroxidase (APX), dehydroascorbate reductase (DHAR) enzyme activities. Although A. laxa accumulated more R-metalaxyl, it experienced less stress due to subsequent induction of antioxidants. Therefore, A. laxa may be a promising R-metalaxyl phycoremediator. Our results provided basic data for understanding the ecotoxicology of R-metalaxyl contamination in aquatic habitats and the toxicity indices among cyanobacteria.

Sensibilidad a metalaxyl y potencial patogénico de Phytophthora hydropathica aislada de canales de irrigación del Valle de Culiacán

Phytopthora hydropathica es una especie con escasos reportes relacionados con su capacidad de infectar cultivos agrícolas de manera natural, por lo cual se puede inferir que no ha estado en contacto con agentes químicos; por ello, el objetivo del presente trabajo fue determinar la patogenicidad y la sensibilidad a metalaxyl de 18 cepas de P. hydropathica provenientes de canales de irrigación en Culiacán, Sinaloa. La evaluación de patogenicidad de los aislados se realizó en hojas de calabacita y pepino y en frutos de calabacita, pepino y tomate. La resistencia a metalaxyl se evaluó in vitro en medio PDA adicionado con metalaxyl. Los aislados de P. hydropathica ocasionaron síntomas de necrosis y ablandamiento en frutos y síntomas de necrosis en hojas de las especies de plantas evaluadas; no se encontraron cepas resistentes, siete cepas presentaron sensibilidad intermedia y el resto fueron susceptibles a metalaxyl. La CE50 de los aislados fluctuó entre 0.000013 mg L-1 a 1 mg L-1. Se puede concluir que el uso de metalaxyl sería efectivo en el control de algún brote de enfermedad ocasionado por P. hydropathica.

Simultaneous determination and risk assessment of metalaxyl and azoxystrobin in potato by liquid chromatography with tandem mass spectrometry.

A liquid chromatography with tandem mass spectrometry method was developed and validated to simultaneously determine metalaxyl and azoxystrobin in soil, potato, and potato foliage samples. The samples were extracted by 20mL of acetonitrile and purified with dispersive solid-phase extraction using octadecyl silane as sorbent. The method showed good linearity (determination coefficients ≥ 0.9926) for metalaxyl (2.5-500ng/mL) and azoxystrobin (5-1000ng/mL). The limits of detection and quantification for both fungicides were 1.5-20μg/kg. The average recoveries in soil, potato, and potato foliage were 83.07-92.87% for metalaxyl and 82.71-98.53% for azoxystrobin. The intra- and inter-day relative standard deviations were all less than 9%. The method was successfully applied on the residual analysis of metalaxyl and azoxystrobin in field trial samples. The results showed that the concentrations of metalaxyl and azoxystrobin in potato samples collected from Guizhou and Hunan were below 50 and 100μg/kg (maximum residue limit set by China), respectively, at 5days after the last application. When following the recommended application manual, metalaxyl and azoxystrobin do not present health concerns to the population because the risk quotients are far below 100%. All the above data could help and promote the safe and proper use of metalaxyl and azoxystrobin in potato.

Modification of the existing maximum residue levels for metalaxyl in various crops

In accordance with Article 6 of Regulation (EC) No 396/2005, the evaluating Member State (EMS), Spain, received an application from the company Industrias Químicas del Vallés to modify the existing maximum residue levels (MRLs) for the active substance metalaxyl in oranges, grapefruits, strawberries, Brussels sprouts and the group ‘spinaches and similar leaves’. Spain proposed to maintain the MRL in strawberries at the level of 0.5 mg/kg and to raise the existing EU MRLs for the other crop commodities. According to EFSA, the submitted residue trials are sufficient to derive MRL proposals for all crops under consideration. In addition, as the residues transferred in citrus pomace used to fed livestock result in an increase in animal burdens, MRLs were proposed for animal matrices considering two possible enforcement residue definitions. Based on the risk assessment results, EFSA concludes that the proposed use of metalaxyl on oranges, grapefruits, strawberries, Brussels sprouts and the group ‘spinaches and similar leaves’ will not result in a consumer exposure exceeding the toxicological reference values and therefore is unlikely to pose a consumer health risk.

Persistence of metalaxyl residues on tomato fruit using high performance liquid chromatography and QuEChERS methodology

Metalaxyl is a widely used fungicide around the world. Very limited data have been reported concerning the dissipation and residue of metalaxyl in agricultural products. Residues and dissipation rate of metalaxyl were estimated in tomato fruit by HPLC following single application of the fungicide at 262.5g a.i. ha−1. The average initial deposit of metalaxyl on tomato fruits was found to be 2.39mgkg−1. Residues of metalaxyl dissipated below the maximum residue limit (MRL) of 0.5mgkg−1 in 7days. Half-life (t1/2) for degradation of metalaxyl on tomato fruit was observed to be 1.81days. A waiting period of 7days is suggested for safe consumption of tomato. The results would be useful for the safe use of metalaxyl and to prevent any health problem to customers.

Sensitivities of Phytophthora infestans to Metalaxyl, Cymoxanil, and Dimethomorph

The isolates of Phytophthora infestans on tomato in the Guangxi Zhuang Autonomous Region, China, were determined for the sensitivities to metalaxyl, cymoxanil and dimethomorph to give the basic information for integrating disease management. Sensitivities were tested by measuring the radial growth on agar medium amended with fungicide, compared with the floating-leaf-disk method. 239 isolates were collected from eight tomato growing areas during 2000-2006. The testing results indicated that the frequencies of sensitive, intermediate, and resistant isolates to metalaxyl were 42.26, 35.98, and 23.53%, respectively. Variations in sensitivities amongst isolates from different areas or different years were very high for metalaxyl. All isolates from Tianlin and Wuxuan were sensitive to metalaxyl, but the metalaxyl-resistant isolates predominated in Tianyang, with the frequency of 51.35%. The EC 50 values of certain isolates from Tianyang were higher than 500 μg mL −1 and their resistance levels were over 100 000 folds. Cymoxanil has been used for nearly 10 years in Guangxi, and dimethomorph has been used for 5-6 years. However, there was no decrease in sensitivity of P. infestans populations and the sensitivities of the pathogen were nearly normally distributed. Hence, their mean EC 50 value [cymoxanil (0.1647 ± 0.0255) μg mL −1, dimethomorph (0.0970 ± 0.0052) μg mL −1] could be used as the baseline sensitivities for monitoring the field resistance development. The comparison with the floating-leaf-disk method indicates that both the techniques provided equivalent results. These studies suggested that metalaxyl can be continuously applied in Tianlin, Wuxuan, and Nanning due to the resistant isolates that have not been found, while for those areas with resistant isolate, the use of metalaxyl should be reduced or alternated, and cymoxanil or dimethomorph was recommended for controlling late blight disease of tomato.

Phenotypic variation in Nordic populations of Phytophthora infestans in 2003

A total of 743 single‐lesion isolates of Phytophthora infestans were collected in summer 2003 from Denmark, Finland, Norway and Sweden. Most of the isolates were tested for mating type, and subsets were tested for sensitivity to fungicides and virulence (host specific pathogenicity). Approximately 60% of the isolates were A1 mating type in each country. Both mating types were present in 40% of the fields where more than one isolate was tested, indicating strong potential for sexual reproduction. The proportion of metalaxyl‐resistant isolates dropped to under 15% from the 60% observed in the early 1990s in Norway and Finland, possibly due to lower selection pressure because of decreased use of metalaxyl. Propamocarb‐HCl sensitivity remained unchanged in the Nordic countries compared to the situation in 1997–2000 in Finland. Four isolates collected from Finland and Sweden were able to sporulate in the presence of this fungicide at a concentration of 1000 mg L−1. In Norway and Finland the frequencies of virulence factors and pathotypes remained nearly unchanged since the 1990s, but the mean number of virulence factors per isolate increased from 5·6 to 6·3. In Denmark and Sweden virulence factors 2 and especially 6 were more common than in Norway and Finland. In addition, in the Swedish population the frequencies of pathotypes were quite even while in other countries pathotype 1,3,4,7,10,11 was most prevalent.

Influence of metalaxyl- and mefenoxam-based fungicides on chemical and biochemical attributes of soil quality under field conditions in a southern humid forest zone of Cameroon

A field experiment was conducted to study the changes in the chemical and biochemical attributes of soil quality resulting from a single application of metalaxyl- and mefenoxam-based fungicides applied as their EC and WPC formulations at the commercially recommended application rate under tropical rainforest field conditions. No significant change in soil pH was observed with fungicide application. Microbial activity indices (available P, available N and specific enzymatic systems) were more sensitive indicators of change. Acid and alkaline phosphatase, and β-Glucosidase activities in soil were affected at different levels with application of fungicides. However, use of metalaxyl and mefenoxam is not likely to harm the soil quality, in general. Simple correlations done between physico-chemical and microbial properties did not produce any significant relationship.

First Report of Wilt and Stem Canker of Poinsettia (Euphorbia pulcherrima) Caused by Phytophthora nicotianae in Puerto Rico.

Poinsettias are grown extensively in greenhouses in Puerto Rico, a five million dollar industry during the winter season of 2005. More than 2,000 'Freedom' plants from an ornamental nursery near Aibonito exhibited severe wilt and dieback symptoms. The disease was found in 5 of 12 surveyed greenhouses where severity ranged from 50 to 100% and occurred regardless of the use of metalaxyl and mefenoxam. Symptoms during the growth phase included stunted plants, thin stems, chlorotic leaves, and brown roots. During flower bract development, symptoms consisted of leaf wilting often in sectors, stem canker, and purple-to-black lesions that extended from the stem to the petioles followed by soft rot. Isolations from symptomatic stem and root tissue were made on corn meal agar (CMA) (17 g/liter) amended with pimaricin (10 mg/liter), ampicillin (250 mg/liter), rifampicin (10 mg/liter), pentachloronitrobenzene (100 mg/liter), and hymexazol (50 mg/liter) (PARPH) (1). Pure cultures were obtained by hyphal-tip transfers onto CMA-PARPH. Colonies grown on acidified potato dextrose agar (APDA) at 25°C were arachnoid with abundant aerial mycelium and sporangia. Sporangia formed singly or in a loose sympodium on long sporangiophores and were papillate, mostly persistent, or caducous with short pedicel. The shape of sporangia varied from ellipsoidal to elongated ovoid, with a length/breath ratio of 16.5 × 14.2 μm. Large, spherical, terminal, and intercalary chlamydospores readily formed in APDA and PARPH. The isolate was identified as Phytophthora nicotianae on the basis of morphological and molecular characteristics. Molecular identification by sequence analysis of the internal transcribed spacer rDNA region confirmed the identity of the isolate P210.1 as P. nicotianae (GenBank Accession No. DQ485412). Pathogenicity tests were conducted with isolates P210.1 and IG grown on APDA for 7 days at 24°C. Isolates were then covered with sterile water for 48 h to induce sporangia and zoospore production at 24°C and continuous light. A suspension of 10 ml of approximately 120 sporangia ml-1 were added to pots containing 2.2 kg of a 1:1 mixture of field soil and soilless media (Promix) composed of peat moss vermiculite and perlite. One healthy poinsettia rooted stem and one 2-month-old plant were transplanted individually into the inoculated soil and pots were flooded with sterile water for 24 h. A noninoculated control was included, each isolate and control were replicated five times. First symptoms (7 to 10 days after inoculation) included brown, limited root growth and lower stem discoloration. Plants (30- to 45-day-old) show a decline in vigor and rapid death of branches that are similar symptoms to those observed on infected poinsettias in commercial greenhouses. P. nicotianae was reisolated from infected root, crown, and stem tissue on PARPH media. A strict sanitation program was recommended to prevent recontamination. To our knowledge, this is the first report of P. nicotianae on poinsettia in Puerto Rico. Reference: D. J. Mitchell and M. E. Kannwischer-Mitchel, Phytophthora. Page:31 in: Methods for Research on Soilborne Phytopathogenic Fungi. L. L. Singleton et al., eds. The American Phytopathological Society, St. Paul, MN, 1993.

The use of metalaxyl and potassium phosphonate, mounds, and organic and plastic mulches, for the management of Phytophthora root rot of papaya in far northern Queensland

Options for the control of root rot of papaya caused by Phytophthora palmivora were evaluated in two field experiments in far northern Queensland, Australia. In the first experiment, foliar sprays of potassium phosphonate (10 g/L) applied fortnightly reduced the incidence of root rot by 47%. Metalaxyl granules (1.25 g/m2) applied every 6 weeks, and metalaxyl granules (1.25 g/m2) applied at transplanting combined with fortnightly foliar sprays of potassium phosphonate (10 g/L) also provided a level of control. Fortnightly foliar sprays of potassium phosphonate (5 g/L) and monthly foliar sprays of potassium phosphonate (5 and 10 g/L), and fortnightly and monthly soil drenches with potassium phosphonate (1 and 2 g/L) had no significant effect (P > 0.05) on the incidence of root rot. In the second experiment, growing papaya on 0.75 m mounds reduced the incidence of root rot by 38.4% and significantly increased fruit yield. Soil covers of 2 m wide plastic mulch and organic mulch in combination with 0.75 m mounds, further reduced plant losses by 20 and 10%, respectively. Plastic mulch on flat ground was as effective as the mounded treatments in reducing the incidence of root rot and increasing yield.

Management of Phytophthora pod rot disease on cocoa farms in Ghana

From 1991 to 1997, field observations on trials involving the use of metalaxyl and copper-based fungicides were made on farmers’ farms in four Phytophthora megakarya affected cocoa growing regions of Ghana to control Phytophthora pod rot disease. Data on farm management practices, cocoa and shade tree types and densities, plot sizes, yield, land tenure and labour arrangements for farm operations, disease incidence and profitability of disease control were collected. Lower disease incidence and higher yields were recorded on fungicide-treated plots than on the untreated plots. The profitability of fungicide application depended on the level of farm management, nature of land tenure and labour arrangements for farm operations. The challenges involved in conducting trials with active participation by farmers are discussed. The involvement of farmers in the development of disease control programmes is crucial for subsequent adoption of the technology.

Practical experience with antiresistance strategies in the fungicides control of potato late blight

Pathogens with rapid reproductive cycles, producing large number wind-spread dispersed infection propagules like sporangiospores, are more likely to pose problems of resistance than slowly reproducing pathogens, which are dispersed over only short distances. Phytophthora infestans, is highly adaptable pathogen capable of exerting high disease pressure on crop, and develop resistance to phenylamid fungicide methalaxyl in the potato crop in Europe (1981–1984). Metalaxyl was then reintroduced for use in mixtures coformulated with multisite compounds, and the proportion of resistance strains “subpopulations” rose again, and albeit to a level lower than at the outset. The use of metalaxyl on seed crops, with the possibility of resistance being carried over the next season. Current recommendations are to use alternative multiside fungicides on potato crops. Our results illustrate the importance of understanding pathogen epidemiology in the management of fungicide resistance.

Mefenoxam-Resistant Isolates of Pythium irregulare in an Ornamental Greenhouse in California.

Root rot, caused by Pythium species, is a common malady in ornamental greenhouses in Monterey County, CA. In 2001, a root rot of Gerbera daisy (Gerbera jamesonii) and lisianthus (Eustoma grandiflora) caused plant losses in excess of 15 and 75%, respectively, in one greenhouse. Some plantings were total losses. Although mefenoxam was used repeatedly, no disease control was reported. P. irregulare was identified based on morphological structures produced on grass blades in water (1) and on the sequence of the rDNA internal transcribed spacer (ITS) region. Four isolates from each host were tested for sensitivity to mefenoxam in a laboratory bioassay. Using corn meal agar amended with mefenoxam at 0, 0.1, 1, 10, 50, or 100 μg a.i./ml, no inhibition of growth of any isolate occurred at concentrations of 10 μg/ml or less. At 50 and 100 μg/ml, radial growth of colonies was inhibited by approximately 20%. In contrast, 26 isolates of P. ultimum from various agricultural soils in California were completely inhibited by mefenoxam at 100 μg a.i./ml. At 0.1, 1, 10, and 50 μg/ml, growth of these isolates was inhibited by 33, 61, 78, and 96%, respectively. Each treatment was replicated three times, and the experiment was repeated with similar results. Mefenoxam was introduced in 1996 to replace metalaxyl. While metalaxyl contains a mixture of active and inactive enantiomers, mefenoxam contains only the active enantiomer. The intensive use of metalaxyl and mefenoxam in this greenhouse may have resulted in selection for resistant isolates. A resistant isolate (8-007 from Gerbera) has been submitted to the American Type Culture Collection: Manassas, VA. Reference: (1) A. J. van der Plaats-Niterink. Monograph of the Genus Pythium. Stud. Mycol. No. 21. Centraalbureau voor Schimmelcultures, Baarn, the Netherlands, 1981.

Effect of Metalaxyl Resistance and Cultivar Resistance on Control of Phytophthora nicotianae in Tobacco.

Phytophthora nicotianae is a devastating root and stem pathogen of tobacco (Nicotiana tabacum) in South Africa. Growers strive to control the resulting disease, known as black shank, with metalaxyl treatments and resistant cultivars. The aim of this study was to consider whether development of metalaxyl resistance in P. nicotianae has contributed to poor disease control and if recently developed cultivars with high levels of resistance require metalaxyl for effective control. One hundred and thirty-two isolates of P. nicotianae were screened for sensitivity to metalaxyl. P. nicotianae isolates from most tobacco farms were metalaxyl sensitive. Growth of most isolates was inhibited completely at 1.0 μg a.i./ml. However, isolates from the MKTV tobacco producing area showed EC50 values ranging from 1.02 μg a.i./ml to 3.57 μg a.i./ml. Twenty-one tobacco cultivars were planted and treated with and without metalaxyl in two different growing seasons to evaluate their resistance to P. nicotianae and the value of using metalaxyl. Hicks was the most susceptible cultivar. Vuma/3/46, LK30/40/60-1, and LK33/60 exhibited the greatest resistance to P. nicotianae. Use of metalaxyl in combination with moderately resistant cultivars such as NC60 × TL33 and LK10/80/60 effectively reduced black shank in the field. Resistant cultivars were healthy and no significant difference between metalaxyl treated and untreated plants was observed.

Two decades of monitoring irish populations ofPhytophthora infestans for phenylamide resistance

Between 1981 and 1998,Phytophthora infestans populations in 878 potato crops in Ireland were monitored for phenylamide resistance. The incidence of resistance was very high in 1981 following intensive use of metalaxyl for blight control from 1977 onwards. It decreased rapidly in the absence of phenylamide use between 1981 and 1984. Following the reintroduction of phenylamide based fungicides in 1985, there was a rapid increase in phenylamide resistance following the widespread curative use of these fungicides. From 1990 onwards there was greater adherence among growers to the use of phenylamides in accordance with an antiresistance strategy and resistance distribution again decreased. During the 18-year period of the study, resistance was always lower at the beginning of the season and increased as the season progressed. Following the reintroduction of the phenylamide based fungicides in 1985 there was a steady increase in phenylamide use up to 1986 followed by a continuous decrease in use up to 1990. Since then there has been a steady increase in phenylamide use. They are currently used on some 60% of crops in Ireland and are still providing acceptable levels of disease control.