Distribution of streptomycin-resistant strainsof Erwinia amylovora in Israel and occurrence of blossom blight in the autumn

Following failure in control of fire blight with streptomycin, the distribution of streptomycin-resistant strains ofErwinia amylovora in Israel was surveyed. During 1994–1997 109 pear, apple, loquat and quince orchards were monitored. Streptomycin-resistant strains ofE. amylovora were recovered from flowers and from infected branches collected at 18 locations in the Sharon, Galilee and Golan Heights regions. In the Sharon region all the isolated strains ofE. amylovora were streptomycin-resistant, whereas in the Galilee and Golan Heights, resistant as well as sensitiveE. amylovora strains were recovered at different locations. In the southern coastal plain no resistance could be detected. Streptomycin-resistant strains ofE. amylovora did not hybridize with the DNA probe SMP3, and resistance could not be transferred by mating to a sensitive strain, suggesting that streptomycin resistance in Israel is not plasmid-mediated. Fire blight symptoms were observed, for the first time, on pear blossoms during the autumn of 1994. A high population of 2x 10⁶-6x 10⁷ CFU/flower in the autumn of 1995 and of 1996 was correlated with the appearance of blossom blight symptoms.     

Efficacy of oxolinic acid and other bactericides in suppression ofErwinia amylovora in pear orchards in Israel

The efficacy of oxolinic acid (at 200 and 300 μg a.i./l) and of several antibiotic compounds (streptomycin sulfate at 100 μg a.i./l, glycocide B at 700 μg a.i./l, kasugamycin at 80 μg a.i./l and gentamicin sulfate at 30 and 60 μg a.i./l) againstErwinia amylovora, the causal agent of fire blight in pears, was evaluated in 43 orchard experiments in 1997–2000 in Israel. In addition to the above orchard experiments, the efficacy of the bactericides was tested in live experiments with artificial inoculation. Natural fire blight symptoms were observed in 16 of the 43 experiments; in 13 of them, disease intensity and its distribution among the experimental plots provided a basis for data analysis, leading to reliable conclusions concerning the efficacy of the tested bactericides. Oxolinic acid at 300 μg a.i./l was highly effective againstE. amylovora and reduced disease severity significantly in all experiments, as compared with the untreated plots; however, a concentration of 200 μg a.i./l was not effective in some cases. Among the tested antibiotics, only gentamicin sulfate was as effective as oxolinic acid. Results of the artificial inoculation experiments corroborated those obtained in the naturally infected orchards. The pre-infection activity of oxolinic acid was determined on blossom clusters that were sprayed with the bactericide before inoculation. Control efficacy on blossom clusters sprayed 1–4 days before inoculation ranged from 68% to 80%, a level which did not differ significantly from that observed on blossom clusters sprayed on the day of inoculation (80% control). The postinfection activity of oxolinic acid was determined on blossom clusters that were sprayed with the compound after inoculation. Oxolinic acid was as effective when applied 1 or 2 days after inoculation as when it was applied on the day of inoculation; however, application of the bactericide 3 days after inoculation no longer resulted in significant disease suppression. Oxolinic acid has been used commercially in Israel since 1998 with appreciable success. Contribution No. 533-00 from the Agricultural Research Organization     

Erwinia pyrifoliae, an Erwinia species different from Erwinia amylovora, causes a necrotic disease of Asian pear trees

Bacteria from necrotic branches of Asian pear trees ( Pyrus pyrifolia ) in Korea were consistently isolated as white colonies on nutrient agar and formed mucoid, slightly yellow colonies on a minimal medium with copper sulphate. Isolates with this colony morphology were studied in a series of microbiological, molecular and pathological tests. Most isolates allowed the verification of Koch's postulate on P. pyrifolia seedlings and on slices from immature pear ( Pyrus communis ) fruits and were also positive in hypersensitivity tests on tobacco leaves. They showed characteristics common to species in the genus Erwinia , but were different from Erwinia amylovora , the agent of fire blight. A relationship between the novel pathogen and E. amylovora was found in microbiological and serological tests. Both organisms had similar but not identical protein patterns in 2-D gel electrophoresis, and in growth morphology the new pathogen produced colonies on MM2 Cu medium that were mucoid and slightly yellow, compared with the clearly yellow colonies of E. amylovora . No similarity was found in the plasmid profiles, and consequently no PCR signal was obtained with primers from the E. amylovora plasmid pEA29. REP-PCR also produced bands differing for the two organisms.     

梨火疫病的分布,传播及检测技术研究进展

本文介绍了梨火疫病的危害,传播及目前世界分布;讨论了各种植物材料和包装物传病的危害性,评价了包括传统的和分子生物学方法在内的梨火疫病菌的各种检测方法及该病对我国的潜在威胁,为对该病的检疫检测提供理论依据。     

Influence of amylovoran production on virulence of Erwinia amylovora and a different amylovoran structure in E. amylovora isolates from Rubus

The amylovoran structures of five Erwinia amylovora isolates from Malaceae sp. and four isolates from Rubus sp. host plants were fully established, mainly by NMR. The structural data on one E. amylovora isolate from a Malaceae sp. host, which had been previously suggested by mass and NMR (Nimtz et al., 1996), were completed. E. amylovora strains infective on Malaceae sp. host plants had an amylovoran composed of pentasaccharide and 30–40% hexasaccharide repeating-substructures, whereas amylovoran from E. amylovora isolates from Rubus sp. host plants had only the pentasaccharide substructures. On the other hand, the exopolysaccharide (EPS) production differed in wild-type E. amylovora strains. Data on in vitro amylovoran production per cell could account for the differences in aggressiveness found in E. amylovora strains, as deduced from a pilot test with highly, moderately, and weakly aggressive strains. This correlation was confirmed with several other wild-type E. amylovora strains from different origin.     

Fire blight of pears in Israel: infection, prevalence, intensity and efficacy of management actions

The pear production area in Israel is 1500 ha, most of which(ca 1200 ha) is located in the northern part of the country. Fire blight (caused by the bacteriumErwinia amylovora (Burrill) Winslowet al.) was first observed in Israel in that region (in 1985) and the disease has prevailed there since then. In a comprehensive survey conducted in Israel in 1996–1999, data were collected and observations were made yearly in one-third to one-half of the pear production area. The aim was to document the prevalence and intensity of fire blight in commercial orchards and to use the data to evaluate the efficacy of management measures employed for its suppression. Regionwise, a severe fire blight epidemic developed in 1996, moderate epidemics developed in 1998 and 1999, and a mild epidemic developed in 1997. The intensity of fire blight in the preceding season in a specific orchard was more influential on current season severity in a season with a mild epidemic than in a season with a moderate epidemic. Analysis of disease onset records and weather data revealed that only a few (1– 3) infection episodes occurred in individual orchards each year. Comparison of fire blight intensity in orchard-plots treated before green tip with copper hydroxide with nontreated plots revealed that the treatment had no effect on disease intensity during bloom. The efficacy of bactericide sprays applied during bloom was not related to the number of sprays applied but to the timing of spraying. Adequate control was achieved in orchard-plots sprayed soon before or after the occurrence of infection episodes. Contribution no. 508/00 from the Inst. of Plant Protection, ARO, Bet Dagan, Israel.     

Extended longevity of Erwinia amylovora vectored by honeybees under in vitro conditions and its capacity for dissemination

Fire blight outbreaks in Korea were first reported in 2015. Regular outbreaks have occurred since, indicating a continuous cycle of the fire blight pathogen in Korea. We determined the role of Apis mellifera (honeybee) as a vector of Erwinia amylovora by verifying the following: (a) E. amylovora longevity in/on honeybees; (b) the most common body parts that carry the bacteria; (c) the rate of bacterial spread to healthy host organs; and (d) the relationship between dispersal of viable but nonculturable (VBNC) and virulent bacterial cells. E. amylovora survived for 15 days on the exterior of honeybee bodies and was most abundant on the abdomen in comparison to other areas such as the labellum, wings, and hind legs. In the digestive system of honeybees, E. amylovora survived for 7 days, and bacteria were found in faeces for 3 days after exposure. The bacteria are likely to be VBNC on honeybees. Honeybees that were contaminated with bacteria transferred E. amylovora to healthy immature apple fruit, shoots, and flowers for 10 days after exposure. E. amylovora was also transferred from inoculated plant parts to uncontaminated honeybees. In addition, bacteria moved from inoculated plant tissues to unexposed honeybees and then from these honeybees to healthy plant tissues. Therefore, E. amylovora can survive in/on honeybees for extended amounts of time, which contradicts previous reports. The bacteria moved to host tissues via honeybees, suggesting that honeybees are the vectors of E. amylovora and play a role in the development of new outbreaks of fire blight disease in the central regions of Korea.     

Notes on the role of water potential in the pathogenesis of fire blight,caused by Erwinia amylovora

The rate of multiplication of fire blight causing bacteriumErwinia amylovora (Burrill) Winslow et al. depends on the availability of water. Water availability can be quantified by means of the parameter water potential. The relationship between water potential and relative multiplication rate ofE. amylovora was derived from experiments of L. Shaw (1935). This relationship appears to be applicable toE. amylovora in plant tissues and in nectar of flowers.Multiplication and expansion ofE. amylovora in a restricted space, e.g. an intercellular hole, creates a pressure, which may cause schizogenic cavities in soft tissue. Strong tissue, however, may be able to resist this multiplication pressure of the bacteria, so that symptom progression can be prevented. A hypothesis is formulated on how the multiplication pressure may be quantified by means of the parameter water potential. Expansion of bacterial ooze may alo be due to absorbtion of water without increase of dry weight (e.g. a daily cycle of shrinkage and expansion). This expansion may give rise to a swelling pressure, which again may be quantified by means of the parameter water potential.Samenvatting De vermenigvuldigingssnelheid van de bacterie die bacterievuur veroorzaakt (Erwinia amylovora (Burrill) Winslow et al.,), hangt af van de beschikbaarheid van water. De beschikbaarheid van water kan worden gekwantificeerd met de parameter waterpotentiaal. De relatie tussen waterpotentiaal en relatieve vermenigvuldigingssnelheid vanE. amylovora werd afgeleid uit experimenten van L. Shaw (1935, Cornell University Agricultural Experiment Station, Ithaca. Memoir 181). Deze relatie kan zowel worden toegepast op de pathogenese in planteweefsel als op de epifytische ontwikkeling in nektar.Vermenigvuldiging vanE. amylovora in een beperkte ruimte, bijvoorbeeld in een intercellulaire holte, creëert een druk, die tot scheuren van zacht weefsel kan leiden. Sterk weefsel kan de vermenigvuldigingsdruk van de bacteriën vermoedelijk wel weerstaan, zodat uitbreiding wordt verhinderd. In een hypothese wordt beschreven hoe de vermenigvuldigingsdruk zou kunnen worden gekwantificeerd met behulp van de parameter waterpotentiaal. Wateropname door bacterieslijm zonder toename van het drooggewicht (bijvoorbeeld een dagelijkse gang van krimpen en zwellen) kan ook leiden tot een druk. Deze druk kan eveneens worden berekend met de parameter waterpotentiaal.     

Potential and limits of acylcyclohexanediones for the control of blossom blight in apple and pear caused by Erwinia amylovora

Growth-regulating acylcyclohexanediones such as prohexadione-calcium and trinexapac-ethyl have been shown to be effective in controlling fire blight infections on shoots. Since blossoms represent the primary site of infection for the fire blight pathogen, Erwinia amylovora , trinexapac-ethyl and prohexadione-calcium were evaluated for their ability to reduce fire blight infection on apple and pear flowers. Field experiments and experiments under controlled conditions were conducted on apple flowers for 4 years. A reduction of up to 50% of blossom blight was observed in treated plants. In addition, treatment with trinexapac-ethyl reduced up to the 77% the percentage of fireblight-affected flowers from which disease progressed into shoots. On pear, numbers of flower infections were reduced by a quarter and flower infections leading to diseased shoots was reduced by up to 50%. Mechanisms underlying diseased reduction following treatment with the two acylcyclohexanediones was studied using a confocal laser scanning microscope combined with a gpf -labelled strain of E. amylovora . These non-invasive techniques demonstrated bacterial migration was reduced by up to 60 and 66% in apple and pear xylem, respectively.     

Multiplication of Erwinia amylovora in fruit-trees. 2. A simulation study on sensitivity to weather

The sensitivity of an output variable of a model to changes of an input parameter value can be analyzed in various ways. Some methods of sensitivity analysis are described, and applied to a simulation model which has daily minimum and maximum temperatures (T _min andT _max, respectively), and daily global radiation as input parameters, and standardized relative multiplication rate ofErwinia amylovora in shoots in fruit-trees, averaged over a 24 hours' period, as output variable. Values of the input parameters were obtained from a weather station near Wageningen, the Netherlands, and refer to the second half of June, 1974–1988.According to the model, the output variable was twice as sensitive toT _max as toT _min. Because of this difference in sensitivity, and because the standard deviation ofT _max was larger than that ofT _min, the variation of the output variable due toT _max was three times larger than that due toT _min. The sensitivity to daily global radiation was negligible when the soil was moist.     

Multiplication of Erwinia amylovora in fruit-trees. 1. A simulation study on limitations imposed by temperature and water,weather and soil

Temperature and water may limit multiplication of the fire blight causing bacteriumErwinia amylovora in shoots of fruit-trees. To gain insight into these limitation under field conditions, two simulation models were designed: a short-term model for immediate effects of weather and soil water potential, and a long-term model for effects of rain and soil profiles on the limitations imposed by temperature and water on bacterial multiplication.In the Netherlands, in the month June, if the soil is moist and the weather average, water hardly limitsE. amylovora multiplication in shoots but, according to the short-term model, temperature reduces then the multiplication by about 60^. When the soil is dry and the potential transpiration rate of the trees high, water may limitE. amylovora multiplication in shoots considerably. According to the long-term model, rain has a delayed effect on multiplication. The effect of rain was simulated for three soil profiles representing three typical fruit growing areas in the Netherlands.     

Simulation of pressure caused by multiplication and swelling of Erwinia amylovora in intercellular space of host tissue

WhenErwinia amylovora grows, in an intercellular space of a host, and fills this space, further multiplication or swelling may create a pressure, and may cause tearing of host tissue. Theoretically, this bacterial pressure equals the actual water potential of the host tissue minus the water potential at which the bacterial biomass would completely fill the intercellular space, but without exerting pressure.Simulation runs indicate that, when the pressure increases, the extracellular slime ofE. amylovora shrinks by releasing water, thus allowing further production of bacterial dry matter. The slime remains around the bacterial cells as a dense substance, low in water content, having a strong capacity to swell when the pressure induces tearing apart of the host tissue. Simulation runs show that the pressure attains its highest values at evening and night.Some fire blight symptoms that illustrate the evidence of bacterial pressure are discussed.     

An amino acid substitution at position 205 of acetohydroxyacid synthase reduces fitness under optimal light in resistant populations of Solanum ptychanthum

The Ala₂₀₅Val substitution in acetohydroxyacid synthase confers herbicide resistance in Solanum ptychanthum , but the effect on fitness is unknown. This study aimed to compare the germination, growth, seed production and competitiveness of resistant (R) and susceptible (S) populations under various light conditions in the field and in a controlled environment. Under optimal light conditions of high irradiance and high red/far-red, the S populations produced more seeds compared with R populations, despite no difference in germination and above-ground vegetative productivity. Any reduction in light intensity and red/far-red diminished the reproductive ability of both R and S populations. This effect was more pronounced for S, so that, under sub-optimal conditions, there were no differences between the two types. Replacement series with three ratios of R:S (16:0, 8:8 and 0:16) showed no competitive difference between the two types. Under optimal light conditions, the S populations always had a higher reproductive output and fitness than the R populations. This would likely cause S individuals to dominate in the absence of herbicide selection pressure, if light competition is reduced.     

中国4省份葡萄霜霉病菌群体对烯酰吗啉的抗性及适合度

为明确我国山西、山东、河北和云南省葡萄霜霉病菌对烯酰吗啉的抗性以及田间病菌群体的适合度,本研究采用Taqman-MGB技术检测了2020年-2021年采自上述4省份292株葡萄霜霉病菌对烯酰吗啉的抗性频率;采用叶盘法测定了田间抗性菌株与敏感菌株的适合度。结果表明,上述4个地区的平均烯酰吗啉抗性频率及抗性等位基因频率分别为64.0%和74.7%,其中山东省蓬莱区最高(91.2%和96.3%),河北省廊坊市(73.8%和85.0%)和云南省宾川县(75.0%和82.6%)次之,山西省清徐县(16.7%和34.7%)最低。适合度分析发现,田间抗性菌株的产孢能力明显高于敏感菌株,但二者的复合适合度指数无显著差异。上述结果表明,4省份霜霉病菌群体均对烯酰吗啉产生了不同程度的抗性,田间抗性菌株与敏感菌株相比无明显适合度变化。     

Use of a diagnostic medium for<Emphasis Type="Italic">in situ</Emphasis> determination of the response of<Emphasis Type="Italic">Erwinia amylovora</Emphasis> strains to bactericides

Erwinia amylovora, the causal agent of fire blight, is managed by application of bactericides to protect fruit tree blossoms from infection. Monitoring the response ofE. amylovora strains to bactericides is crucial for adequate disease management. The coliform agar medium produced by Merck was recently reported as an effective tool for rapid diagnosis ofE. amylovora (RD-medium). The objective of the present study was to examine the possibility of using the RD-medium forin situ determination of the response ofE. amylovora strains to oxolinic acid and streptomycin. The phenotypic response of 48E. amylovora strains isolated in 2002 to both bactericides was determined with the RD-medium and, for comparison, by a routine laboratory test. The results of 45 samples (93.7%) were in agreement with the findings of the routine laboratory test. Aχ ² test rejected the null hypothesis that the phenotypic characteristics as determined by the two respective methods differed significantly (P=0.389). Thein situ test was implemented on a national scale in 2003 and the results were in agreement with those obtained in laboratory tests, which suggests that this medium can be usedin situ for monitoring the appearance of resistance inE. amylovora populations. http://www.phytoparasitica.org posting Feb. 11, 2004.     

Negatively correlated cross-resistance to phenylcarbamate fungicides in benomyl-resistant Venturia inaequalis and V. pirina

Isolates ofVenturia inaequalis and ofV. pirina sensitive (S) or resistant (R) to benomyl were examined in vitro on media amended with two phenylcarbamate fungicides. There was a negatively correlated cross-resistance (NCCR) to both methyl N-(3,5-dichlorophenyl) carbamate (MDPC) and isopropyl N-(3,4-diethoxyphenyl) carbamate (NPC) in some benomylresistant isolates. InV. inaequalis, isolates with low benomyl resistance (LR) did not show NCCR to MDPC, whereas isolates with medium (MR), high (HR) and very high (VHR) resistance to benomyl were more sensitive to MDPC than were the benomyl-sensitive isolates. To NPC, MR and VHR isolates showed NCCR whereas LR and HR isolates reacted similarly as sensitive isolates. InV. pirina only HR and VHR isolates showed NCCR to MDPC. The VHR isolates were sensitive to NPC, whereas the reactions of S, LR, MR and HR to NPC were similar.Crosses between benomyl-sensitive and benomyl-resistantV. pirina as well as between different resistant isolates showed that NCCR is inheritable and controlled by a single Mendelian gene.Samenvatting Benomyl-gevoelige en-resistente isolaten vanVenturia inaequalis enV. pirina werden in vitro onderzocht op media met de fungiciden methyl N-(3,5-dichlorophenyl) carbamate (MDPC) en isopropyl N-(3,4-dichlorophenyl) carbamate (NPC). Een aantal benomyl-resistente isolaten van deze pathogeen bleken een negatief gecorreleerde kruisresistentie (NCCR) te vertonen ten opzichte van MDPC en NPC.Isolaten vanV. inaequalis met matige (MR), hoge (HR) en zeer hoge (VHR) benomyl-resistentie vertoonden NCCR. Ten opzichte van NPC vertoonden alleen MR en VHR isolaten NCCR, en niet de LR en HR isolaten. InV. pirina vertoonden HR en VHR isolaten NCCR ten opzichte van MDPC, maar alleen de VHR isolaten ten opzichte van NPC.Kruisingen tussen benomyl-gevoelige en-resistenteV. pririna, en tussen verschillende benomyl-resistente isolaten onderling, toonden aan dat NCCR erfelijk is en berust op een enkel gen.Contribution No. 1712-E, 1986 series, from the ARO.     

Variation in populations of Phytophthora infestans in Finland and Norway: mating type, metalaxyl resistance and virulence phenotype

Phytophthora infestans was isolated from potato leaves and tubers collected from different parts of Finland in 1990–96 and Norway in 1993–96. Isolates were assessed for mating type, resistance to metalaxyl and virulence phenotype. In Finland 15% of 200 isolates tested and in Norway 25% of 642 tested were A2 mating type. Differences in the A1/A2 ratio were evident among regions and A2 was not found in northern areas. In Finland the frequency of metalaxyl-resistant isolates, among 1834 tested, decreased from 59% in 1990–95 to 2% in 1996. In Norway 59% of 491 isolates tested were resistant to metalaxyl in 1996. Among 269 Finnish and 105 Norwegian isolates, all known virulence genes were found in both countries and race 1.3.4.7.10.11 was the most common (resistance gene R9 was not included in the differential set). Oospores were observed in potato leaves from three locations in the southern part of Norway. The implications of the 'new' populations in the Nordic countries are discussed.     

Modelling binary mixtures of herbicides in populations resistant to one of the components: evaluation for resistance management

BACKGROUND: Herbicide mixtures are commonly proposed to delay the selection of herbicide resistance in susceptible populations (called the SM strategy). However, in practice, herbicide mixtures are often used when resistance to one of the two active ingredients has already been detected in the targeted population (called the RM strategy). It is doubtful whether such a practice can select against resistance, as the corresponding selection pressure is still exerted. As a consequence, the effect of mixtures on the evolution of an already detected resistance to one of the herbicides in the combination remains largely unexplored. In the present work, a simple model was developed to explore further the necessary and sufficient conditions under which a binary RM strategy might stabilise or even reduce resistance frequency. RESULTS: Covering the hypothetical largest range of parameters, 39% of 9000 random simulations attest that the RM strategy might theoretically reduce resistance frequency. When strong enough, high genetic cost of resistance, negative cross‐resistance between the herbicides associated in the mixture and reduced selection differential between resistant and susceptible plants can counterbalance the resistance advantage to one of the two applied herbicides. However, the required conditions for an RM strategy to ensure resistance containment in natural conditions seldom overlap with experimental parameter estimates given in the literature. CONCLUSION: It is concluded that the sufficient conditions for an RM strategy to be effective would rarely be encountered. As a consequence, the strategy of formulating mixtures with herbicides for which resistance has already been detected should be avoided. Copyright © 2008 Society of Chemical Industry     

Influence of competition and host plant resistance on selection in Phytophthora infestans populations in Michigan, USA and in Northern Ireland

Competition between genotypes of Phytophthora infestans was studied by inoculating potato cultivars with differing susceptibility to late blight in field experiments over three years in Northern Ireland, UK, and Michigan, USA. Multiple isolates of six genotype groups of P. infestans were chosen from the local populations in both N. Ireland and Michigan for inoculation of separate field trials planted in 2003, 2004 and 2005. Four cultivars were used in each trial; two (susceptible cv. Atlantic and the partially resistant cv. Stirling) were common to both locations, whereas the two additional cultivars (with partial resistance to late blight) were cvs Santé and Milagro in N. Ireland and cvs Pike and Jacqueline Lee in Michigan. Single-lesion isolates of P. infestans were obtained from leaves at 1% level of infection, characterized using pre-assigned markers and re-assigned to their respective genotype groups. Extreme selection occurred within the population of genotypes of P. infestans in N. Ireland in each year, with different genotype groups dominating the infection of different cultivars. Selection was observed on all cultivars tested, but was greatest on the more resistant cultivars. Over the 3 years, all of the 114 isolates obtained from cv. Milagro belonged to a single group, whereas among the 118 isolates from cv. Atlantic all six groups were represented. By contrast, in Michigan, the US-8 genotype dominated infection in all cultivars in each year; only 12 of 374 isolates characterized belonged to other genotypes (11 US-14 and a single US-10 isolate).     

Phenotypic and genotypic variation in Michigan populations of Phytophthora infestans from 2008 to 2010

Severe potato and tomato late blight epidemics in Michigan since 2008 prompted characterization of Phytophthora infestans isolates from the region. From eight counties in Michigan, 124 isolates were collected from infected potato and tomato plants from 2008 to 2010 and characterized using ‘classical’ markers and microsatellites. The classical markers included mating type, Gpi allozyme, mitochondrial DNA haplotype, sensitivity to metalaxyl‐M and tuber pathogenicity. All isolates from 2008 to 2010 were A2 mating type and Ia mtDNA haplotype (124 isolates), 105 isolates had Gpi profile 100/122, 17 isolates had the profile 100/100/111 and the remaining two isolates had 100/111/122. Sensitivity to metalaxyl‐M, expressed as EC₅₀ for mycelial growth in vitro, ranged from <0·1 to 91 μg mL^?1, where 95 and 96% of isolates were classified as either sensitive or intermediate in 2008–2009 and 2010 respectively. The metalaxyl‐M sensitivity and dominant Gpi profile were typical of clonal lineage US‐22, first isolated in 2008 in North America from tomato plants. Tuber pathogenicity, characterized as severity of tuber late blight, was also variable among isolates; however, isolates were less aggressive than previous genotypes present in Michigan, such as US‐8. Microsatellites (simple sequence repeats; SSRs) revealed a shift in the population, characterized by two clusters differentiated over time. These results suggested displacement of the US‐8 genotype by US‐22 from 2008 to 2010 in Michigan. Continuous tracking of changes within P. infestans populations provides evidence of genetic shifts due to migration, prompting modification of management strategies based on the phenotypic characteristics of causal genotypes.