Identification and Expression of Secreted In Xylem Pathogenicity Genes in Fusarium oxysporum f. sp. pisi

Fusarium oxysporum is a soilborne fungal plant pathogen responsible for causing disease in many economically important crops with “special forms” (formae speciales) adapted to infect specific plant hosts. F. oxysporum f. sp. pisi (FOP) is the causal agent of Fusarium wilt disease of pea. It has been reported in every country where peas are grown commercially. Disease is generally controlled using resistant cultivars possessing single major gene resistance and therefore there is a constant risk of breakdown. The main aim of this work was to characterise F. oxysporum isolates collected from diseased peas in the United Kingdom as well as FOP isolates obtained from other researchers representing different races through sequencing of a housekeeping gene and the presence of Secreted In Xylem (SIX) genes, which have previously been associated with pathogenicity in other F. oxysporum f. spp. F. oxysporum isolates from diseased United Kingdom pea plants possessed none or just one or two known SIX genes with no consistent pattern of presence/absence, leading to the conclusion that they were foot-rot causing isolates rather than FOP. In contrast, FOP isolates had different complements of SIX genes with all those identified as race 1 containing SIX1, SIX6, SIX7, SIX9, SIX10, SIX11, SIX12, and SIX14. FOP isolates that were identified as belonging to race 2 through testing on differential pea cultivars, contained either SIX1, SIX6, SIX9, SIX13, SIX14 or SIX1, SIX6, SIX13. Significant upregulation of SIX genes was also observed in planta over the early stages of infection by different FOP races in pea roots. Race specific SIX gene profiling may therefore provide potential targets for molecular identification of FOP races but further research is needed to determine whether variation in complement of SIX genes in FOP race 2 isolates results in differences in virulence across a broader set of pea differential cultivars.

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Incidence of Fusarium oxysporum f. sp. lycopersici Races in Tomato in Sinaloa, Mexico

Plant Disease ◽

10.1094/pdis-04-10-0255 ◽

2010 ◽

Vol 94 (11) ◽

pp. 1376-1376 ◽

Cited By ~ 3

Author(s):

P. Sánchez-Peña ◽

S. O. Cauich-Pech ◽

J. Núñez-Farfán ◽

R. D. Núñez-Cebreros ◽

S. Hernández-Verdugo ◽

...

Keyword(s):

Fusarium Oxysporum ◽

Fusarium Wilt ◽

Vascular Bundles ◽

Potato Dextrose Broth ◽

Solanum Lycopersicum L ◽

Differential Cultivars ◽

Differential Cultivar ◽

Race 1 ◽

Production Area ◽

Race 2

Sinaloa State is the main producer of tomatoes (Solanum lycopersicum L) in Mexico where production attained 15,784 ha in 2008 (3). Fusarium wilt of tomato caused by Fusarium oxysporum f. sp. lycopersici (Sacc) Snyder & Hansen causes significant yield losses in Sinaloa each year (2). Three pathotypes or races of F. oxysporum f. sp. lycopersici have been described: races 1, 2, and 3 (1). The purpose of this study was to determine the races of F. oxysporum f. sp. lycopersici present in Sinaloa and distribution of these races in the region. F. oxysporum f. sp. lycopersici isolates were obtained from plants showing symptoms of yellowing and necrosis of vascular bundles. Plants were sampled from 50 fields throughout the production area in Sinaloa from November 2008 to March 2009. Four differential cultivars were used to identify the races of 26 F. oxysporum f. sp. lycopersici isolates collected across Sinaloa: Bonny Best (susceptible to all races), UC-82-L (susceptible to races 2 and 3), F1 MH-1 (susceptible to race 3), and IR-3 (resistant to all races). A microconidial suspension was prepared for each isolate (1 × 106 CFU/ml) from cultures grown in potato dextrose broth with constant agitation (110 rpm) at 28°C for 7 days. Five 25-day-old seedlings (three fully expanded true leaves) from each differential cultivar were immersed in the appropriate microconidial suspension for 10 min, then individually transplanted into a pot containing 1 kg of sterilized commercial potting mix, and grown in a growth chamber at 25 to 28°C and 60 to 75% relative humidity for 5weeks with 14-h light/10-h darkness. Control plants (root dipped for 10 min in sterile water) were grown similarly and remained asymptomatic. Susceptible cv. Bonny Best showed typical symptoms of Fusarium wilt including epinasty, yellowing, defoliation, vascular necrosis, and wilt. None of these plants survived 25 days postinoculation for any of the isolates. All UC-82-L plants inoculated with each of the 26 isolates died before 35 days, indicating that none of the isolates was of race 1. F1 MH-1 plants only survived inoculation with 3 of the 26 isolates (11.5%), indicating that the 23 isolates that killed these plants (88.5% of the 26 isolates) were of race 3, and only 3 isolates were of race 2. All IR-3 plants inoculated with the 26 isolates survived. The isolates showed variation in response to the differential cultivar UC-82L in duration from inoculation to when the plants died (variation in isolate aggressiveness). The three F. oxysporum f. sp. lycopersici race 2 isolates were restricted to the Culiacan Valley, whereas the 23 F. oxysporum f. sp. lycopersici race 3 isolates were widely distributed across Sinaloa. Koch's postulates were confirmed by reisolating the fungus from the roots and stem base of each dead, inoculated plant (4). This study provides baseline data for future surveys to monitor changes in distribution of F. oxysporum f. sp. lycopersici races in Sinaloa, Mexico. References: (1) G. Cai et al. Phytopathology 93:1014, 2003. (2) P. Sánchez-Peña. Programa de Fomento y Apoyo a Proyectos de Investigación (PROFAPI), Universidad Autónoma de Sinaloa, México, 2007. (3) Secretaría de Agricultura, Ganadería, Desarrollo Rural, Pesca y Alimentación, Servicio de Información Agroalimentaria y Pesquera, México. www.siap.gob.mx , 2009. (4) B. A. Summerell et al. Plant Dis. 87:117, 2003.

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Races and Inoculum Density of Fusarium oxysporum f. sp. niveum in Commercial Watermelon Fields in Maryland and Delaware

Plant Disease ◽

10.1094/pdis.2003.87.6.692 ◽

2003 ◽

Vol 87 (6) ◽

pp. 692-698 ◽

Cited By ~ 27

Author(s):

X. G. Zhou ◽

K. L. Everts

Keyword(s):

Fusarium Oxysporum ◽

Fusarium Wilt ◽

Total Population ◽

Research Field ◽

Causal Agent ◽

Inoculum Density ◽

Differential Cultivars ◽

Race 1 ◽

Race 2 ◽

The Relationship

A survey was conducted to determine races and inoculum density of Fusarium oxysporum f. sp. niveum, the causal agent of Fusarium wilt of watermelon in Maryland and Delaware. Virulence on six differential cultivars was tested for each of 63 isolates of F. oxysporum f. sp. niveum, obtained from 25 commercial watermelon fields. Thirteen isolates (21%) were identified as race 0, 36 isolates (57%) as race 1, and 14 isolates (22%) as race 2. Races 0 and 1 were present in 12 (48%) and 10 (40%) of the fields, respectively. The highly aggressive race 2 was identified from five fields in two counties in Maryland and from one field in Delaware, representing 24% of the fields. Race 2 was copresent with one or two other races. Race 2 (19 isolates) predominated among the 25 isolates obtained from a research field in Maryland. Nineteen commercial fields had inoculum densities of F. oxysporum f. sp. niveum ranging from 100 to 1,200 CFU/g of soil at harvest. Within this range of inoculum densities, >20% incidence of wilt was observed when the susceptible watermelon cv. Sugar Baby was planted in samples of soil collected from these fields. The relationship (P < 0.0001) between inoculum density of F. oxysporum f. sp. niveum (X) and incidence of Fusarium wilt (Y) on Sugar Baby was best described using the monomolecular equation, Y = 1 - exp[-0.0013 (X + 166)]. The ratio of pathogenic to total population of F. oxysporum in the fields linearly increased with increasing inoculum density of F. oxysporum f. sp. niveum (R 2 = 0.4; P < 0.0009).

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First Report of the Occurrence of Fusarium oxysporum f. sp. niveum Race 2 in Commercial Watermelon Production Areas of Maryland and Delaware

Plant Disease ◽

10.1094/pdis.2001.85.12.1291c ◽

2001 ◽

Vol 85 (12) ◽

pp. 1291-1291 ◽

Cited By ~ 12

Author(s):

X. G. Zhou ◽

K. L. Everts

Keyword(s):

Fusarium Oxysporum ◽

Growth Medium ◽

Citrullus Lanatus ◽

Culture Collection ◽

Peat Moss ◽

Mineral Salts ◽

First Report ◽

Differential Cultivars ◽

Race 1 ◽

Race 2

Resistance to race 1 of Fusarium oxysporum f. sp. niveum (E.F. Sm.) W.C. Snyder & H.N. Hans) is the most widely used tool for management of Fusarium wilt of watermelon (Citrullus lanatus (Thunb.) Matsum. and Nakai). However, this resistance is ineffective against the more aggressive F. oxysporum f. sp. niveum race 2. Race 2 was first identified in Israel in 1973 (2) and was subsequently reported in Texas (1981), Oklahoma (1988), and Florida (1989) (1). From July through September 2000 in Maryland and Delaware, 123 isolates of F. oxysporum f. sp. niveum were collected from wilted plants or plants with vascular discoloration from commercial production fields with moderate to severe wilt severity. Race determination was performed in the greenhouse on five race-differential cultivars. Differential cvs. Sugar Baby, Crimson Sweet, Charleston Gray, Allsweet, and Dixielee range from susceptible to highly resistant (in the order listed) to race 1, but all are susceptible to race 2. Seeds were planted in pots containing pasteurized vermiculite and peat moss (2:3 mixture) growth medium infested with 1 × 105 conidia per g of growth medium. Conidia were harvested from 5- to 6-day-old cultures of F. oxysporum f. sp. niveum grown in a liquid mineral salts medium (2). Control pots were treated with water or diluted liquid medium. Pots were maintained in a greenhouse at 19 to 28°C. Three replicate pots containing seven plants each were used for each isolate-cultivar combination. Race 0 obtained from B. D. Bruton (USDA-ARS, Lane, OK), and races 1 and 2 obtained from the American Type Culture Collection (Manassas, VA) were included in the tests for comparative purposes. Eleven isolates from Maryland and three isolates from Delaware consistently caused 60 to 100% wilt or mortality of all cultivars, with a mean of more than 75% wilt. Percent wilt of cvs. Sugar Baby and Dixielee to races 0, 1, and 2, and the 14 isolates from Maryland and Delaware was 78, 76, 100, and 95 to 100% and 0, 14, 59, and 63 to 93%, respectively. All control plants remained healthy. In each of the three replicated tests, these 14 isolates caused an equal or greater incidence of wilt as the reference race 2 isolate, and cvs. Dixielee and Allsweet were susceptible to these isolates. Therefore, these isolates were identified as F. oxysporum f. sp. niveum race 2. This is the first report of F. oxsporum f. sp. niveum race 2 occurring in Maryland and Delaware. References: (1) R. D. Martyn and B. D. Bruton. HortScience 24:696, 1989. (2) D. Netzer. Phytoparasitica 4:131, 1976.

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Multiple Evolutionary Trajectories Have Led to the Emergence of Races in Fusarium oxysporum f. sp. lycopersici

Applied and Environmental Microbiology ◽

10.1128/aem.02548-16 ◽

2016 ◽

Vol 83 (4) ◽

Cited By ~ 15

Author(s):

V. Chellappan Biju ◽

Like Fokkens ◽

Petra M. Houterman ◽

Martijn Rep ◽

Ben J. C. Cornelissen

Keyword(s):

Transposable Elements ◽

Fusarium Oxysporum ◽

Resistance Genes ◽

Pcr Analysis ◽

Genomic Fragment ◽

Avirulence Genes ◽

Content Type ◽

Race 1 ◽

Tomato Cultivars ◽

Race 2

ABSTRACT Race 1 isolates of Fusarium oxysporum f. sp. lycopersici (FOL) are characterized by the presence of AVR1 in their genomes. The product of this gene, Avr1, triggers resistance in tomato cultivars carrying resistance gene I. In FOL race 2 and race 3 isolates, AVR1 is absent, and hence they are virulent on tomato cultivars carrying I. In this study, we analyzed an approximately 100-kb genomic fragment containing the AVR1 locus of FOL race 1 isolate 004 (FOL004) and compared it to the sequenced genome of FOL race 2 isolate 4287 (FOL4287). A genomic fragment of 31 kb containing AVR1 was found to be missing in FOL4287. Further analysis suggests that race 2 evolved from race 1 by deletion of this 31-kb fragment due to a recombination event between two transposable elements bordering the fragment. A worldwide collection of 71 FOL isolates representing races 1, 2, and 3, all known vegetative compatibility groups (VCGs), and five continents was subjected to PCR analysis of the AVR1 locus, including the two bordering transposable elements. Based on phylogenetic analysis using the EF1-α gene, five evolutionary lineages for FOL that correlate well with VCGs were identified. More importantly, we show that FOL races evolved in a stepwise manner within each VCG by the loss of function of avirulence genes in a number of alternative ways. IMPORTANCE Plant-pathogenic microorganisms frequently mutate to overcome disease resistance genes that have been introduced in crops. For the fungus Fusarium oxysporum f. sp. lycopersici, the causal agent of Fusarium wilt in tomato, we have identified the nature of the mutations that have led to the overcoming of the I and I-2 resistance genes in all five known clonal lineages, which include a newly discovered lineage. Five different deletion events, at least several of which are caused by recombination between transposable elements, have led to loss of AVR1 and overcoming of I. Two new events affecting AVR2 that led to overcoming of I-2 have been identified. We propose a reconstruction of the evolution of races in FOL, in which the same mutations in AVR2 and AVR3 have occurred in different lineages and the FOL pathogenicity chromosome has been transferred to new lineages several times.

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Suppression of Fusarium Wilt Caused by Fusarium oxysporum f. sp. niveum Race 2 on Grafted Triploid Watermelon

Plant Disease ◽

10.1094/pdis-01-14-0005-re ◽

2014 ◽

Vol 98 (10) ◽

pp. 1326-1332 ◽

Cited By ~ 13

Author(s):

Anthony P. Keinath ◽

Richard L. Hassell

Keyword(s):

Fusarium Oxysporum ◽

Fusarium Wilt ◽

Disease Incidence ◽

The United States ◽

Infested Soil ◽

Marketable Yield ◽

Race 1 ◽

Susceptible Control ◽

Triploid Watermelon ◽

Race 2

Fusarium wilt of watermelon, caused by the soilborne fungal pathogen Fusarium oxysporum f. sp. niveum race 2, is a serious, widespread disease present in major watermelon-growing regions of the United States and other countries. ‘Fascination,’ a high yielding triploid resistant to race 1, is grown in southeastern states in fields that contain a mixture of races 1 and 2. There is some benefit to using cultivars with race 1 resistance in such fields, even though Fascination is susceptible to Fusarium wilt caused by race 2. Experiments in 2012 and 2013 were done in fields infested primarily with race 2 and a mixture of races 1 and 2, respectively. Fascination was grafted onto four rootstock cultivars: bottle gourd (Lagenaria siceraria) ‘Macis’ and ‘Emphasis’ and interspecific hybrid squash (Cucurbita maxima× C. moschata) ‘Strong Tosa’ and ‘Carnivor.’ Nongrafted and self-grafted Fascination were used as susceptible control treatments. In both experiments, mean incidence of plants with symptoms of Fusarium wilt was ≥52% in the susceptible control treatments and ≤6% on the grafted rootstocks. Disease incidence did not differ between rootstock species or cultivars. In both years, Fascination grafted onto Strong Tosa and Macis produced more marketable-sized fruit than the susceptible control treatments. Grafted Emphasis and Carnivor also produced more fruit than the control treatments in 2012. The cucurbit rootstocks suppressed Fusarium wilt caused by race 2 and increased marketable yield of triploid watermelon grown in infested soil.

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Characterisation of Strains of Fusarium oxysporum f.sp. cubense by Production of Volatiles

Australian Journal of Botany ◽

10.1071/bt9910161 ◽

1991 ◽

Vol 39 (2) ◽

pp. 161 ◽

Cited By ~ 18

Author(s):

NY Moore ◽

PA Hargreaves ◽

KG Pegg ◽

JAG Irwin

Keyword(s):

Fusarium Oxysporum ◽

Vegetative Compatibility ◽

Vegetative Compatibility Group ◽

Compatibility Group ◽

Race 1 ◽

Race 2 ◽

Race 4

The production of volatiles on steamed rice by Australian isolates of Fusarium oxysporum f. sp. cubense correlated well with race and vegetative compatibility group (VCG). All race 4 isolates (VCGs 0120, 0129) produced distinctive volatile odours which gave characteristic gas chromatograms where the num- ber of peaks equated to VCG. Race 1 (VCGs 0124, 0125) and race 2 (VCG 0128) isolates, as well as non-pathogenic isolates of F. oxysporum from the banana rhizosphere, did not produce detectable volatiles and gave chromatograms without significant peaks.

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Characterization of a Regional Population of Fusarium oxysporum f. sp. niveum by Race, Cross Pathogenicity, and Vegetative Compatibility

Phytopathology ◽

10.1094/phyto-97-4-0461 ◽

2007 ◽

Vol 97 (4) ◽

pp. 461-469 ◽

Cited By ~ 16

Author(s):

X. G. Zhou ◽

K. L. Everts

Keyword(s):

Fusarium Oxysporum ◽

Vegetative Compatibility ◽

Infested Soil ◽

Vegetative Compatibility Group ◽

Atlantic Region ◽

Compatibility Group ◽

Race 1 ◽

Regional Population ◽

Race 2

Eighty-eight isolates of Fusarium oxysporum f. sp. niveum, collected from wilted watermelon plants and infested soil in Maryland and Dela-ware, were characterized by cross pathogenicity to muskmelon, race, and vegetative compatibility. Four isolates (4.5%) were moderately pathogenic to ≥2 of 18 muskmelon cultivars in a greenhouse test, and one representative isolate also was slightly pathogenic in field microplots. The four isolates all were designated as race 2, and were in vegetative compatibility group (VCG) 0082. Of the 74 isolates to which a VCG could be assigned, 41 were in VCG 0080, the VCG distributed most widely; 27 were in VCG 0082, and were distributed in half of the 20 watermelon fields surveyed; and 6 were in the newly described VCG 0083, and were restricted to three fields. Among the isolates in VCG 0080, 8 were designated as race 0, 21 as race 1, and 12 as race 2. Of the isolates in VCG 0082, 6 were designated as race 0, 11 as race 1, and 10 as race 2. All isolates in VCG 0083 were designated as race 2. Isolates from more than one race within the same VCG or isolates from more than one VCG were recovered from single plants and fields. No differences in aggressiveness on differential watermelon cultivars were observed among isolates from different VCGs of the same race. A diverse association between virulence and VCG throughout the Mid-Atlantic region suggests that the pathotypes of F. oxysporum f. sp. niveum may be of local origin or at least long existent in the region.

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Biological and Molecular Characterization of Fusarium oxysporum f. sp. lycopersici Divides Race 1 Isolates into Separate Virulence Groups

Phytopathology ◽

10.1094/phyto.1999.89.2.156 ◽

1999 ◽

Vol 89 (2) ◽

pp. 156-160 ◽

Cited By ~ 79

Author(s):

Jurriaan J. Mes ◽

Emma A. Weststeijn ◽

Frits Herlaar ◽

Joep J. M. Lambalk ◽

Jelle Wijbrandi ◽

...

Keyword(s):

Fusarium Oxysporum ◽

Random Amplified Polymorphic Dna ◽

Vegetative Compatibility ◽

Avirulence Gene ◽

Resistance Response ◽

Vegetative Compatibility Groups ◽

Small Set ◽

Race 1 ◽

Race 2 ◽

Susceptible Tomato

A collection of race 1 and race 2 isolates of Fusarium oxysporum f. sp. lycopersici was screened for vegetative compatibility and characterized by random amplified polymorphic DNA (RAPD) analysis to establish the identity and genetic diversity of the isolates. Comparison of RAPD profiles revealed two main groups that coincide with vegetative compatibility groups (VCGs). In addition, several single-member VCGs were identified that could not be grouped in one of the two main RAPD clusters. This suggests that F. oxysporum f. sp. lycopersici is a polyphyletic taxon. To assign avirulence genotypes to race 1 isolates, they were tested for their virulence on a small set of tomato lines (Lycopersicon esculentum), including line OT364. This line was selected because it shows resistance to race 2 isolates but, unlike most other race 2-resistant lines, susceptibility to race 1 isolates. To exclude the influence of other components than those related to the race-specific resistance response, we tested the virulence of race 1 isolates on a susceptible tomato that has become race 2 resistant by introduction of an I-2 transgene. The results show that both line OT364 and the transgenic line were significantly affected by four race 1 isolates, but not by seven other race 1 isolates nor by any race 2 isolates. This allowed a subdivision of race 1 isolates based on the presence or absence of an avirulence gene corresponding to the I-2 resistance gene. The data presented here support a gene-for-gene relationship for the interaction between F. oxysporum f. sp. lycopersici and its host tomato.

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Characterization of Pathogenic Races of Fusarium oxysporum f. sp. melonis Causing Fusarium Wilt of Melon in New York

Plant Disease ◽

10.1094/pdis.1997.81.6.592 ◽

1997 ◽

Vol 81 (6) ◽

pp. 592-596 ◽

Cited By ~ 20

Author(s):

T. L. Zuniga ◽

T. A. Zitter ◽

T. R. Gordon ◽

D. T. Schroeder ◽

D. Okamoto

Keyword(s):

New York ◽

Fusarium Oxysporum ◽

Nuclear Dna ◽

New York State ◽

Washington County ◽

Length Polymorphisms ◽

Close Relationship ◽

Compatibility Group ◽

Race 1 ◽

Race 2

Forty-six isolates of Fusarium oxysporum f. sp. melonis obtained from soil samples throughout melon-producing areas in New York State were identified on the basis of pathogenicity and colony morphology. Physiological races 1 and 2 were identified by their reaction on a set of differential melon cultivars. Race 1 was widely distributed, occurring in six of the seven New York counties surveyed. Twenty-seven of the 28 race 1 isolates were associated with vegetative compatibility group (VCG) 0134, whereas one was incompatible with all known VCGs of F. oxysporum f. sp. melonis. Twelve out of 18 race 2 isolates were associated with VCG 0131, and occurred in four counties in eastern and western New York. Five isolates of race 2, associated with VCG 0130, were recovered from a farm in Washington County, as was a single race 2 isolate which was incompatible with all known VCGs of F. oxysporum f. sp. melonis. Restriction fragment length polymorphisms in the nuclear DNA revealed variability among the isolates examined, but race 1/VCG 0134 isolates from New York and Maryland were identical or nearly so, as were race 2/VCG 0131 isolates from the two states. These findings suggest a close relationship between the populations of F. oxysporum f. sp. melonis in New York and Maryland. Race 2 isolates were more virulent than race 1 isolates, based on the number of days to first symptoms and death of melon seedlings.

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Nondefoliating and Defoliating Strains from Cotton Correlate with Races 1 and 2 of Verticillium dahliae

Plant Disease ◽

10.1094/pdis-03-15-0261-re ◽

2015 ◽

Vol 99 (12) ◽

pp. 1713-1720 ◽

Cited By ~ 21

Author(s):

Xiao-Ping Hu ◽

Suraj Gurung ◽

Dylan P. G. Short ◽

German V. Sandoya ◽

Wen-Jing Shang ◽

...

Keyword(s):

Verticillium Wilt ◽

Verticillium Dahliae ◽

Reference Strain ◽

The United States ◽

Specific Primers ◽

Differential Cultivars ◽

Race 1 ◽

Polymerase Chain ◽

Race 2 ◽

Important Disease

Verticillium wilt, caused by Verticillium dahliae, is an important disease of cotton worldwide. Isolates of V. dahliae can be characterized as race 1 or race 2 based on the responses of differential cultivars of tomato and lettuce, or as defoliating or nondefoliating based on symptom expression in cotton. To investigate the frequency and distribution of races and defoliation phenotypes of cotton-associated V. dahliae, 317 isolates from China, Israel, Turkey, and the United States were tested by polymerase chain reaction (PCR) using defoliating, nondefoliating, and race 1- and race 2-specific primers DF/DR, NDF/NDR, VdAve1F/VdAve1R, and VdR2F/VdR2R, respectively. Of the total, 97.2% of isolates genotyped as defoliating were also characterized as race 2, while 90.8% of isolates genotyped as nondefoliating were also genotyped as race 1. To verify these results, three cotton cultivars—‘FM 2484B2F’ (highly resistant), ‘98M-2983’ (highly susceptible), and ‘CA4002’ (partially resistant)—used as differentials were each inoculated with 10 isolates characterized by PCR: six defoliating/race 2 strains (GH1005, GH1021, HN, XJ2008, XJ592, and reference strain Ls17) and four nondefoliating/race 1 strains (GH1015, GH1016, GH1020, and reference strain Ls16). All defoliating/race 2 isolates except for Ls17 caused defoliation on 98M-2983 and CA4002. Isolate Ls17 caused defoliation on 98M-2983 only. The nondefoliating/race 1 isolates caused Verticillium wilt symptoms devoid of defoliation on 98M-2983. The greenhouse assays confirmed the molecular identification of race and defoliation phenotype. Although the existence of races has not been previously established among V. dahliae isolates from cotton, the long-established nondefoliating and defoliating population structure corresponded with V. dahliae races 1 and 2, respectively.

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