Metalaxyl Sensitivity Research Articles

First Report of Phytophthora infestans Genotype US23 Causing Late Blight in Canada.

Late blight is caused by the oomycete Phytophthora infestans (Mont.) de Bary and is one of the most devastating diseases of potato and tomato. Late blight occurs in all major potato- and tomato-growing regions of Canada. Its incidence in North America increased during 2009 and 2010 (2). Foliar disease symptoms appeared earlier than usual (June rather than July) and coincided with the identification of several new P. infestans genotypes in the United States, each with unique characteristics. Prior to 2007, isolates collected from potato and tomato crops were mainly US8 or US11 genotypes (1). However, P. infestans populations in the United States have recently experienced a major genetic evolution, producing isolates with unique genotypes and epidemiological characteristics in Florida and throughout the northeastern states (2). Recent discoveries of tomato transplants with late blight for sale at Canadian retail outlets prompted an examination of the genotypes inadvertently being distributed and causing disease in commercial production areas in Canada. Analysis of isolates of P. infestans from across Canada in 2010 identified the US23 genotype for the first time from each of the four western provinces (Manitoba, Saskatchewan, Alberta, and British Columbia) but not from eastern Canada. Allozyme banding patterns at the glucose phosphate isomerase (Gpi) locus indicated a 100/100 profile consistent with US6 and US23 genotypes (4). Mating type assays confirmed the isolates to be A1 and in vivo metalaxyl sensitivity was observed. Restriction fragment length polymorphic analysis of 50 isolates from western Canada with the multilocus RG57 sequence and EcoRI produced the DNA pattern 1, 2, 5, 6, 10, 13, 14, 17, 20, 21, 24, 24a, 25 that was indicative of US23 (3). The recently described P. infestans genotype US23 appears to be more aggressive on tomato, and although isolates were recovered from both tomato and potato, disease symptoms were often more severe on tomato. Results indicate that movement and evolution of new P. infestans genotypes have contributed to the increased incidence of late blight and that movement of the pathogen on retail plantlets nationally and internationally may provide an additional early season source of inoculum. A major concern is that the introduced new A1 populations in western Canada have established a dichotomy with the endogenous A2 populations in eastern Canada, increasing the potential for sexual recombination producing oospores and additional genotypes should these populations merge.

SPATIAL ANALYSIS OF PHYLOGENETIC AND PHENOTYPIC DIVERSITY IN PYTHIUM POPULATIONS FROM A SINGLE GREENHOUSE

Forty two isolates of Pythium spp. obtained from a single cucumber greenhouse from Batinah region in Oman were tested for their phylogenetic and phenotypic diversity. All isolates were identified to the species level using sequences of the Internal Transcribed Spacer (ITS) region of the ribosomal DNA. All isolates were found to be Pythium aphanidermatum. The length of the ITS region was 777 base pairs for 41 isolates and 778 bp for one isolate. Metalaxyl sensitivity tests showed high levels of variation in sensitivity to metalaxyl among Pythium isolates from the same greenhouse, where the EC 50 values ranged from 0.07 to over 1.80, with an average of 0.83 μg ml -1 . This appears to be the first report of existence of Pythium isolates with moderate levels of resistance to metalaxyl in Oman. Most isolates were found to be very aggressive on cucumber seedlings, causing over 70% mortality. This study reports phylogenetic and phenotypic diversity among Pythium isolates from the same greenhouse. Findings are discussed in terms of sources of these isolates and possible management.

Distribution, Pathotypes, and Metalaxyl Sensitivity of Phytophthora sojae from Heilongjiang and Fujian Provinces in China.

Phytophthora sojae causes root and stem rot, one of the most devastating diseases of soybean worldwide. In Heilongjiang and Fujian provinces in China, serious cases of Phytophthora stem and root rot have occurred and caused heavy losses in the past several years. To determine the current population status of this pathogen, we investigated the pathogen's distribution, pathotypes, and metalaxyl sensitivity in both provinces. P. sojae was baited and isolated from 258 soil samples in both provinces using the soybean leaf bait method. The pathotypes of all isolates were characterized on 13 differential soybean cultivars using the hypocotyl slit inoculation method, and the sensitivity of all isolates to metalaxyl was tested in vitro. In all, 75 isolates were recovered from 75 fields in 33 counties; of these, 31 counties were in Heilongjiang Province and 2 counties were in Fujian Province. Thirty-five new pathotypes were identified compared with the previously defined races. Less than 5% of the isolates were virulent to cultivars with individual Rps genes 1a, 1c, or 1k. No metalaxyl-resistant isolates were found; the half maximal effective concentration values of all isolates ranged from 0.04 to 0.22 μg ml-1. These results suggest that effective management of the disease in both provinces can be accomplished through the use of resistant cultivars with Rps genes 1a, 1c, or 1k and the fungicide metalaxyl.

AGGRESSIVENESS AND SENSITIVITY TO METALAXYL OF PHYTOPHTHORA INFESTANS ISOLATES FROM CAMEROON

Isolates of Phytophthora infestans were obtained from diverse hosts (potato, tomato and garden huckleberry) in Cameroon and assessed for aggressiveness and sensitivity to metalaxyl in replicated in vitro experiments. Aggressiveness tests were conducted on detached leaves and metalaxyl sensitivity was determined on leaf disks. Inoculated leaves and disks were incubated in Petri dishes in temperature chambers at 18°C for 7-8 days, 12h daylight. The P. infestans isolates derived from potato and tomato were more aggressive than those isolated from huckleberry. The average lesion area on inoculated leaves ranged from 17 to 41%, 13 to 38% and 5 to 16% on potato, tomato and huckleberry, respectively. Variation in the sensitivity of P. infestans isolates to metalaxyl was recorded, depending on the geographic and host origins of the isolates. Resistance to metalaxyl was detected in 49.0 and 50.3% of the isolates in 2001 and 2002, respectively. Resistance to metalaxyl was greatest on isolates derived from tomato and least on those obtained from huckleberry. We discussed how assessment of metalaxyl resistance and aggressiveness among pathogen isolates would be useful for effective management of P. infestans in Cameroon.

Phenotypic and Genotypic Characterization ofPhytophthora infestansIsolates from China

A total of 288 (202 from potato and 86 from tomato) isolates of Phytophthora infestans were collected from 1998 to 2007 in China. The isolates were characterized based on mating type, in vitro metalaxyl sensitivity, virulence on potato differentials, allozymes of glucose-6-phosphate isomerase (Gpi), peptidase (Pep), and mitochondrial DNA (mtDNA) haplotype and examined by DNA-based simple sequence repeat (SSR) and random amplified polymorphic DNA (RAPD) fingerprinting. The majority (283 of 288) of the isolates were of the A1 mating type, the other three were the A2 mating type and two were the A1A2 mating type. Resistance to metalaxyl was frequently observed, with 248 (86.1%) resistant, 21 (7.3%) intermediate and 19 (6.6%) sensitive isolates identified. Virulence was assessed for 125 isolates on a set of 11 potato differentials and 61 races were detected. Most isolates were virulent on the differential genotype with gene R3, and all known virulence genes were found, with race 3.4.7.11 being the most common. This pattern did not appear to be associated with geographic origin, sample type, mating type or metalaxyl sensitivity. The dominant banding patterns for Gpi were 100/100/111 (176 isolates) and 100/100 (109 isolates), but genotypes 86/100 and 100/111 were also identified. All isolates tested were homozygous (100/100) at the Pep locus. The majority (205 of 288) of isolates tested was of mtDNA haplotype IIb, 76 were haplotype IIa and seven were the rare Ib haplotype. The genetic diversity of 60 representative isolates from China was assayed by two types of molecular markers, RAPD and SSR. A high level of polymorphism was found. The results demonstrated the diverse phenotypic and genotypic structure of the current populations of P. infestans in China.

Phenotypic and Genotypic Changes in the Phytophthora infestans Population in Taiwan – 1991 to 2006

AbstractLate blight, caused by Phytophthora infestans, is one of the most destructive diseases of tomato in Taiwan. A total of 655 isolates of P. infestans, including 29 isolates from potato, was collected from major tomato and potato production areas of Taiwan during 1991 to 2006. Isolates were characterized for their pathogenicity, mating type, in vitro metalaxyl sensitivity and molecular genotype (including allozyme pattern, mitochondrial genomic haplotype and DNA fingerprint) to monitor population changes in P. infestans. The population of P. infestans in Taiwan underwent a dramatic genetic shift in the 1997–1998 cool growing season. Isolates collected from tomato before 1997 were aggressive to tomato but not potato; most isolates obtained after 1998, were aggressive to both hosts. Metalaxyl sensitivity of isolates changed from sensitive/intermediate before 1997 to resistant since 1998. Similarly, the isolates obtained before 1997 were all US‐1 clonal lineage (including variants US‐1.1, US‐1.2, US‐1.3 and US‐1.4). During the 1997–1998 cool growing season, the US‐11 clonal lineage and the TW‐1 genotype appeared, possibly introduced on imported table potatoes. The US‐11 lineage spread rapidly and since 1999 has almost completely displaced the old population in Taiwan. Mating type determined by pairing with A1 and A2 reference isolages of P. parasitica, showed all isolates were of the A1 mating type, suggesting that the A2 mating type had not become established in Taiwan. The increasing percentage (up to 42.3% in 2006) of the US‐11 variants (including US‐11.l, US‐11.2, US‐11.3 and US‐11.4) implied that genomic diversity of the pathogen is changing quickly. Therefore, it is important to continuously monitor the population changes of P. infestans and develop an integrated management strategy for this disease.

PCR-based monitoring of recent isolates of tobacco blue mold from Europe reveals the presence of two genetically distinct phenotypes differing in fungicide sensitivity

Bioassays testing the fungicide sensitivity against metalaxyl of Peronospora tabacina isolates collected in German tobacco fields in 2005 revealed the presence of two phenotypes, resistant and sensitive. DNA fingerprints using SSR and minisatellite primers allowed separation of the samples into two groups. The differences in amplification patterns coincided with the sensitive and resistant reaction of the isolates in metalaxyl bioassays. New primers were developed which allowed PCR-based detection of P. tabacina and differentiation of the metalaxyl-sensitive and the metalaxyl-resistant phenotype, respectively. Screening of recent blue mold isolates from Germany and other European countries for metalaxyl sensitivity with leaf disk bioassays coincided completely with the PCR-based identification of the two phenotypes. In Germany, exclusively resistant isolates were found between 2002 and 2004. These still dominate. Since 2005 the co-occurrence of sensitive isolates has been shown. No similar monitoring has been done in any other European country. However, we found the resistant phenotype reaction in a French isolate of 2004 and in two out of three Italian isolates in 2007. Two isolates from Poland and Bulgaria in 2007 were sensitive to metalaxyl. For all 58 isolates tested since 2002 the metalaxyl bioassay-based resistance type and the PCR-based tests for sensitive and resistant genotype coincided. Using genotype-specific primers for future population studies may help to trace the sources of the pathogen from which yearly propagation starts.

Pathotypes, Distribution, and Metalaxyl Sensitivity of Phytophthora sojae from North Dakota

Phytophthora root rot, caused by Phytophthora sojae, is the most important disease of soybean (Glycine max) in North Dakota. Because of the expansion of soybean hectares and appearance of disease on cultivars with resistance genes, we investigated the pathotypes, distribution, and metalaxyl sensitivity of P. sojae in North Dakota. Soil from 347 soybean fields in 20 counties in eastern North Dakota was collected between 2002 and 2004, and P. sojae was baited from the soil with the susceptible cultivar McCall. The virulence phenotype of each isolate was determined on eight differentials, and all isolates were tested for sensitivity to metalaxyl incorporated into V8 agar. The pathogen was recovered from 80 fields located in five counties. Sixteen pathotypes, which included 14 known races and two previously reported pathotypes that had not been assigned a race, were identified out of 157 isolates. A single pathotype was recovered from 61 fields, 2 pathotypes from 14 fields, 3 pathotypes from 4 fields, and 4 pathotypes from 1 field. Pathotypes with virulence phenotypes 1a,1c,7 (race 4; 39%) and 1a,7 (race 3; 28%) were the most common, representing 67% of the total isolates. One or both of these pathotypes was found in 79% of the fields where P. sojae was recovered. Seven of the 157 isolates showed limited growth on metalaxyl after 14 days of incubation. In the past 10 years, the number of pathotypes of P. sojae in North Dakota has increased from 4 to 16, and pathotypes have developed that can attack the three most common resistance genes found in soybean cultivars for the region.

Phenotypic markers of Phytophthora infestans

Phytophthora infestans is one of the most destructive pathogens of potato and causal agents of notorious disease late blight. Different chemicals are used to control the pathogen of late blight but the most commonly used is metalaxyl; its extensive use of has caused decreased sensitivity in the P. infestans population. The metalaxyl sensitivity of the Pakistani population of P. infestans is investigated in the present study. For this purpose, 178 isolates of P. infestans were obtained from the lesions of diseased potato leaves and stems, and samples were collected from the different potato-growing areas of Pakistan, where late blight is a problem. Sensitivity of the isolates of P. infestans was investigated by metalaxyl sensitivity test and with the help of test isolates were divided into three categories, i.e. sensitive, intermediate and resistant, based on their Co-efficient of mycelial growth inhibition (CMGI) values. During the study, highest percentage (50.17%) of resistant isolates was observed in the population of Punjab (zone 2), whereas the lower percentage (33.33%) was observed in the population of Swat valley (zone 6b). In the present study, it was discovered that P. infestans late blight-causing fungus has adopted more resistance against metalaxyl because of its wide use.

Characterisation ofPhytophthora infestanspopulations from Vietnam

In 2002–03, 597 Phytophthora infestans isolates were collected from potato and tomato crops in nine provinces of Vietnam. A representative set of isolates were characterised for mating type, metalaxyl sensitivity, virulence, mitochondrial DNA (mt DNA) haplotype and RG57-fingerprint. All of the 294 tested isolates were of mating type A1. Metalaxyl-resistant isolates were found on both the potato and tomato hosts, and dominated the population in Lam Dong province (south of Vietnam). Nine known virulence genes were found among 27 tested isolates, although virulence to resistance gene R5 was not found. Among the 95 isolates tested for mtDNA haplotype, all were Ib type. Of these, all 34 isolates fingerprinted showed US-1-like genotype. Mating type, mtDNA haplotype and RG57-fingerprint data from this study indicate that the P. infestans population on tomato and potato from Vietnam still consist of the ‘old’ population.

Genetic Diversity, Aggressiveness and Metalaxyl Sensitivity of Pythium spinosum Infecting Cucumber in Oman

AbstractA total of 24 isolates of Pythium spinosum from cucumber obtained from five regions in Oman were characterized for genetic diversity using amplified fragment length polymorphism (AFLP) fingerprinting and three isolates from the Netherlands, South Africa and Japan were included for comparison. Isolates from Oman were also characterized for aggressiveness on cucumber seedlings and sensitivity to metalaxyl. Identity of all isolates was confirmed using sequences of the internal transcribed spacer (ITS) region of the ribosomal DNA (rDNA), which showed more than 99% nucleotide similarity among all isolates. Using six primer‐pair combinations, AFLP fingerprinting resolved 295 AFLP markers of which 193 were polymorphic among isolates from other countries and only six were polymorphic among isolates of P. spinosum from Oman. Seven different AFLP phenotypes of P. spinosum were recovered in Oman; two of them were found to contain over 79% of isolates and one was recovered from all regions in Oman. Phenotypes from Oman showed very high (≥99%) levels of genetic similarity to each other compared to moderate (mean =53%) levels of genetic similarity with phenotypes from other countries. In addition, all isolates from Oman were found to be highly sensitive to metalaxyl and all were aggressive on cucumber seedlings at 25°C. The high genetic similarity among phenotypes of P. spinosum in Oman as well as recovering two major clones across regions may suggest that P. spinosum has been recently introduced in Oman via a common source.

Genetic diversity, aggressiveness and metalaxyl sensitivity of Pythium aphanidermatum populations infecting cucumber in Oman

Seventy three isolates of Pythium aphanidermatum obtained from cucumber from four different regions of Oman and 16 isolates of muskmelon from the Batinah region in Oman were characterized for aggressiveness, sensitivity to metalaxyl and genetic diversity using AFLP fingerprinting. Twenty isolates of P. aphanidermatum from diverse hosts from different countries were also included in the study. Most isolates from Oman were found to be aggressive on cucumber seedlings and all were highly sensitive to metalaxyl (EC50 < 0·80 µg mL−1). Isolates from cucumber and muskmelon were as aggressive as each other on both hosts (P > 0·05), which implies a lack of host specialization in P. aphanidermatum on these two hosts in Oman. AFLP analysis of all isolates using four primer–pair combinations resolved 152 bands, of which 61 (~40%) were polymorphic. Isolates of P. aphanidermatum from Oman and other countries exhibited high genetic similarity (mean = 94·1%) and produced 59 different AFLP profiles. Analysis of molecular variance indicated that most AFLP variation among populations of P. aphanidermatum in Oman was associated with geographical regions (FST = 0·118; P < 0·0001), not hosts (FST = –0·004; P = 0·4323). These data were supported by the high rate of recovery (24%) of identical phenotypes between cucumber and muskmelon fields in the same region as compared to the low recovery (10%) across regions in Oman, which suggests more frequent movement of Pythium inoculum among muskmelon and cucumber fields in the same region compared to movement across geographically separated regions. However, recovering clones among regions and different countries may imply circulation of Pythium inoculum via common sources in Oman and also intercontinental spread of isolates.

Occurrence of Late Blight Caused by Phytophthora infestans on Potato and Tomato in Alaska.

Phytophthora infestans, causal agent of late blight, was included in a list of plant pathogens found in Alaska in 1934 (1). No notes of symptoms, extent of disease, or dates were recorded. The only reference to the location was given as Wrangell, a town in southeast Alaska with subsistence gardening. Neither P. infestans nor late blight was noted again in the state for another 59 years. Late blight first appeared in Alaska's major potato-growing region in south-central Alaska's Matanuska Valley in 1995. Subsequent outbreaks have been sporadic, occurring only in 1998, 2005, and 2006. Each of these outbreaks was identified from rapidly enlarging brown foliar lesions with branched sporangiophores and lemon-shaped sporangia (~25 × 30 μm). The 1995 and 1998 potato late blight outbreaks in Alaska were not sampled extensively nor have they previously been formally reported. We recovered single isolates of P. infestans from symptomatic potato foliage in the 1995 and 1998 outbreaks. In 2005, symptomatic foliage was collected from individual potato plants in 10 commercial fields and from tomato plants in greenhouses at two locations. Sporulating stem and leaf tissue were used to inoculate semiselective rye medium and 147 isolates from potato and six from tomato were recovered. The isolates from the 1995, 1998, and 2005 outbreaks were analyzed to determine genotype at the allozyme loci GPI and PEP (3), mitochondrial haplotype (4), mating type, and metalaxyl sensitivity (2). The 1998 and 2005 outbreaks were similar because both were caused by the relatively aggressive US-11 allozyme genotype and had significant economic impact for commercial potato growers. All 153 isolates from potato and tomato in 2005 displayed the same allozyme pattern as the US-11 genotype, possessed the IIB mitochondrial haplotype, and were mating type A1. Of the 16 isolates tested, all were determined to be metalaxyl resistant because isolates grown on 5 and 100 μg/ml metalaxyl exhibited growth greater than 40% of the unamended control. The 1995 outbreak was caused by the relatively rare US-7 genotype and started so late during the season that economic impact was minimal. Similarly, the 2006 outbreak was noted from only one commercial potato field at the time of harvest in September 2006. However, the genotype of the 2006 isolate has not been determined because the patch was destroyed before adequate samples could be collected. Because the disease occurs so sporadically in Alaska, fungicides are not routinely in use, but it is unlikely that the pathogen has persisted locally between outbreaks. The source of P. infestans is unknown for each of the occurrences in Alaska. However, possible routes include seed potatoes for home gardens or commercial farms, tomato transplants, and retail vegetables shipped to Alaska from out of state. While potato is Alaska's main vegetable crop, there are less than 405 ha (1,000 acres) of potatoes planted in the state, with the majority planted in the Matanuska Valley. To our knowledge, this is the first formal report of P. infestans on both tomato and commercial potato in Alaska.

Phenotypic and genetic variation in Finnish potato‐late blight populations, 1997–2000

This study was conducted on potato late blight samples collected for monitoring early attacks in Finland from 1997 to 2000. Most of the 1726 Phytophthora infestans isolates were collected soon after the onset of the epidemics. Most of the isolates were tested for mating type as well as metalaxyl and propamocarb hydrochloride (HCl) sensitivity, while virulence on potato R‐gene differentials and mitochondrial DNA (mtDNA) haplotype were determined for a subset of the isolates. In half of the fields from which more than one isolate was tested, both mating types were detected, indicating strong potential for sexual reproduction. The mating types coexisted more often in organic fields or gardens than in conventional fields. The proportion of A1 mating type decreased from approximately 80% (1997 and 1998) to 22% by the year 2000. Simultaneously, the proportion of isolates resistant to metalaxyl decreased from approximately 40% to 16%. Resistance to metalaxyl was confined to the IIa mtDNA haplotype and clearly associated with the A1 mating type, as resistance was 10 times more common among A1 isolates than among A2 isolates. Resistance to metalaxyl therefore probably derives from common descent from an isolate with the IIa haplotype. Most of the regional variation in metalaxyl sensitivity was also linked to mating type, as both metalaxyl resistance and the A1 mating type were most common in the north and south‐west of Finland. In contrast to metalaxyl, only three propamocarb‐HCl‐insensitive isolates were found, but propamocarb‐HCl sensitivity decreased significantly during the study.

Genetic Structure of the Population of Phytophthora infestans Attacking Solanum ochranthum in the Highlands of Ecuador

Thirty-nine isolates of Phytophthora infestans were collected from the wild host Solanum ochranthum in the highland tropics of Ecuador and characterized with a set of phenotypic and molecular markers (mating type, metalaxyl sensitivity, the allozyme loci Gpi, and Pep, mitochondrial DNA haplotype, RFLP, and SSR), as well as for pathogenicity on various hosts. Three groups of isolates (A, B, and C) were identified based on their multilocus genotypes and variable abilities to cause disease on different hosts. Group A had a combination of alleles for the Gpi (86/100), Pep (96/100) and mtDNA (Ia) loci, as well as an RFLP fingerprint, that have not been reported for P. infestans in Ecuador, or elsewhere. Group B shares many marker characteristics with the US-1 lineage described in Ecuador on tomato, pear melon (S. muricatum), and S. caripense, but has SSR alleles not present in typical US-1 isolates. Group C for all markers tested is identical to the EC-1 lineage described on cultivated and wild potatoes in Ecuador. All isolates from S. ochranthum were able to re-infect their host of origin in the detached leaf assay; however, we did not draw clear conclusions as to the relative aggressiveness of the three groups on this host. Isolates of group A were the most specialized and were generally non-pathogenic or weakly pathogenic on all hosts other than S. ochranthum. Groups B and C infected tuber-bearing hosts, including the cultivated potato but were generally non-pathogenic on other non-tuber bearing hosts. Solanum ochranthum was infected by isolates coming from tuber-bearing Solanum hosts (i.e., the EC-1 lineage of P. infestans) and some US-1 isolates from non-tuber bearing hosts. Thus, in nature this species might be a potential reservoir of inoculum of different pathogen populations able to infect the cultivated hosts potato, tomato and pear melon (S.␣muricatum). Unlike potato and tomato in Ecuador, each of which is primarily attacked by a highly specialized pathogen population, S. ochranthum appears to harbour at least three pathogen groups of␣different genetic make-up. The unresolved issue of potential host specificity in isolates found on S.␣ochranthum could complicate efforts to use this species in tomato improvement.

The Northern Ireland Phytophthora infestans population 1998–2002 characterized by genotypic and phenotypic markers

A total of 204 isolates of Phytophthora infestans from Northern Ireland, almost all from commercial potato crops, were collected over 5 years (1998–2002). Phenotypic diversity was assessed using mating type and metalaxyl resistance; genotypic diversity was assessed using two allozyme loci (glucose‐6‐phosphate isomerase, Gpi, and peptidase, Pep), mitochondrial DNA haplotype and the multilocus RFLP probe RG57. All isolates were A1 mating type and Gpi 100/100. The majority were Pep 100/100, but four Pep 83/100 and six Pep 96/100 isolates were identified. Three mtDNA haplotypes were detected; haplotype IIa was the most common, but each year up to 2001 its frequency declined, with a concomitant increase in Ia isolates. Three isolates had the rare haplotype IIb, but this was only detected in 1998. Metalaxyl resistance and mtDNA haplotype were markedly associated: most haplotype Ia isolates were metalaxyl‐resistant, whereas haplotype IIa was more commonly associated with metalaxyl sensitivity. Analysis of a subsample of 91 isolates revealed nine RG57 genotypes, three associated exclusively with haplotype IIa and six exclusively with haplotype Ia. The most common RG57 genotype (51% of isolates) comprised both metalaxyl‐resistant and ‐sensitive haplotype IIa isolates. However, of haplotype Ia isolates, all metalaxyl‐resistant phenotypes belonged to one of four RG57 types, one of which was the second most frequent overall (29% of isolates), while all metalaxyl‐sensitive isolates belonged to one of two other types. The P. infestans population in Northern Ireland appears to consist of a limited number of clones related to, but differentiated from, the populations in mainland Britain and elsewhere in Europe.

Sensitivity of Brazilian Isolates of Phytophthora infestans to Commonly Used Fungicides in Tomato and Potato Crops.

Isolates of the US-1 and BR-1 clonal lineages of Phytophthora infestans, collected from tomato and potato fields of two main producing regions of Brazil (south and southeast), were tested for sensitivity to the systemic fungicide metalaxyl, plus the three protectant fungicides mancozeb, chlorothalonil, and cymoxanil. For metalaxyl, one agar test and two leaf-disc tests were carried out. For all tests, metalaxyl insensitive (I), intermediately insensitive (II), and sensitive (S) isolates were detected. There was no association among metalaxyl sensitivity and region, lineage, or host. In the agar test, 35.0% of 210 isolates were classified as I, 36.0% as II, and 29.0% as S. US-1 isolates were classified as 36.1% I, 30.6% II, and 33.3% S. BR-1 isolates were 33.3% I, 23.8% II, and 42.9% S. In leaf-disc test 1 (240 isolates tested), 24.3% were classified as I, 35.0% as II, and 40.7% as S. Isolates of US-1 were 21.0% I, 39.8% II, and 39.2% S, whereas BR-1 isolates were 36.0% I, 20.0% II, and 44.0% S. In leaf-disc test 2 (96 isolates tested), which was based on an effective dose for 50% sporulation inhibition (ED50), most isolates were either I (44.8%) or II (51.0%), and only three (4.2%) were S. The US-1 isolates were 46.0% I, 51.0% II, and 3.0% S. Isolates of the BR-1 lineage were 45.0% I, 52.0% II, and 3.0% S. For mancozeb, the ED50 for 53 of 59 isolates was below 1.0 μg/ml. No isolate grew on medium amended with more than 100 μg of chlorothalonil/ml and the ED50 for 38 of 50 isolates was below 1.0 μg/ml. For cymoxanil, the ED50 for all 47 isolates tested was below 1 μg/ml. There is no evidence of resistance of resistance of P. infestans to protectant fungicides commonly used in Brazil.

Pathogenicity and metalaxyl sensitivity of Phytophthora infestans isolates obtained from garden huckleberry, potato and tomato in Cameroon

Late blight is a significant disease of solanaceous crops worldwide. Knowledge of pathogenicity of isolates and metalaxyl sensitivity is important for effective management of late blight. Solanaceous crops were surveyed in the highlands of west and northwest provinces of Cameroon during 2001 and 2002. A total of 233 isolates of Phytophthora infestans were collected, of which 53 were from huckleberry, 104 from potato, and 76 were from tomato. The pathogenicity of the isolates were conducted on detached leaves of huckleberry, potato and tomato and, metalaxyl sensitivity was assayed on leaf disks at 18 °C. The results of pathogenicity tests showed that the potato, tomato and huckleberry isolates infected their primary hosts of origin as well as the other two plant hosts. The isolates of P. infestans from huckleberry were more aggressive on potato and tomato than on huckleberry. The potato isolates were most pathogenic on potato and least pathogenic on huckleberry. The tomato isolates were equally aggressive on potato and tomato, and significantly less pathogenic on huckleberry. Sensitivity of P. infestans isolates to metalaxyl varied with the geographic location where the isolates were collected and source of isolation (primary hosts). Among the 233 isolates tested for metalaxyl sensitivity, 49% were metalaxyl resistant (MR) in 2001 and 51% in 2002. The least percentage of isolates (34%) obtained huckleberry, were MR. The aggressiveness of the isolates were increased with their corresponding resistance to metalaxyl. The cross-infectivity of the isolates and their increased metalaxyl resistance suggests that integrated management options are essential for effective disease control.

Recent occurrence of Phytophthora infestans US-11 as the cause of severe late blight on potato and tomato in Taiwan

A severe outbreak of late blight on potato occurred in central Taiwan in the winter of 1997. Subsequently, the disease spread to potato and tomato fields over the entire island within 2 months. Since that time, late blight has become endemic and poses a significant threat to potato and tomato production throughout Taiwan. To understand why the causal agent Phytophthora infestans has caused such severe epidemics of late blight in Taiwan, diseased plant samples were collected from the major potato- and tomato-growing areas of Taiwan from the 1997 outbreak to 2002, and compared with isolates collected prior to the 1997 outbreak. Among the 11 isolates collected prior to the 1997 outbreak, all were of the US-1 lineage, with the following characteristics: A1 mating type, metalaxyl sensitivity, Gpi genotype of 86/100, Pep genotype of 92/100 or 100/100, mtDNA haplotype of Ib, and DNA fingerprint pattern of US-1 (n = 7), US-1.2 (n = 1), or US-1.3 (n = 3). In contrast, 83 isolates collected from the 1997 outbreak through 2002 displayed moderate to high levels of metalaxyl insensitivity, Gpi genotype of 100/100/111, Pep genotype of 100/100, mtDNA haplotype of IIb, and DNA fingerprint pattern of US-11 or its variants. These results suggested that, during 1997–1998, the US-1 population of P. infestans was rapidly displaced by the US-11 clonal lineage, possibly introduced into Taiwan with latently infected potato tubers from overseas.

Characterization of Pythium spp. from Three Ohio Fields for Pathogenicity on Corn and Soybean and Metalaxyl Sensitivity

Pythium spp. were baited with corn and soybean seed from soils collected from three locations in Ohio where soybean and corn stand establishment was a concern. Five species, P. catenulatum, P. irregulare, P. paroecandrum, P. splendens, and P. torulosum, were recovered and a subset of these isolates was then tested for pathogenicity on corn and soybean seed and sensitivity to the seed treatment fungicide metalaxyl. There was a range of both pathogenicity and sensitivity to metalaxyl within and among the Pythium spp. recovered from the three locations. A more thorough evaluation of the Pythium populations that exist in grain production fields in the north central region of the US is needed to facilitate the development and deployment of broader based seed treatment products. Accepted for publication 12 January 2004. Published 2 February 2004.