Genetics and race variability of the lucerne-Colletotrichum trifolii pathosystem in Australia

1998 ◽  
Vol 49 (4) ◽  
pp. 713 ◽  
Author(s):  
J. M. Mackie ◽  
J. A. G. Irwin
Keyword(s):  
Random Amplified Polymorphic Dna ◽  
Single Gene ◽  
The United States ◽  
Recessive Trait ◽  
Colletotrichum Trifolii ◽  
The Us ◽  
Finger Printing ◽  
Race 1 ◽  
Race 2 ◽  
Condition C

The Australian-bred lucerne cultivars, Trifecta and Sequel, were found to possess useful levels of resistance to both Colletotrichum trifolii races 1 and 2. Race 2 has only been previously observed in the United States and surveys did not reveal its presence in Australia. Multilocus finger printing using random amplified polymorphic DNA (RAPDs) analysis revealed low diversity (<10% dissimilarity) within Australian C. trifolii collections, and between the Australian race 1 isolates and a US race 2 isolate. Studies on the inheritance of resistance to C. trifolii race 1 in individual clones from Trifecta and Sequel revealed the presence of 2 different genetic mechanisms. One inheritance was for resistance as a recessive trait, and the other indicated that resistance was dominant. The recessive system has never been previously reported, whereas in the US, 2 completely dominant and independent tetrasomic genes An1 and An2 have been reported to condition C. trifolii resistance. It was not possible to fit the observed segregations from our studies to a single-gene model. In contrast to US studies, clones of cv. Sequel exhibiting the recessive resistance reacted differently to spray and stem injection with C. trifolii inoculum, being resistant to the former and susceptible to the latter, providing additional evidence for the presence of a different genetic mechanism conditioning resistance to those previously reported in the US. As C. trifolii is one of the most serious diseases of lucerne worldwide, the future development of molecular markers closely linked to the dominant and recessive resistances identified in these studies, and the relationships between these resistances and An1 and An2 as determined by genetic mapping, appear to be useful areas of future study.

scholarly journals First Report of Race 2 of Colletotrichum trifolii Causing Anthracnose on Alfalfa (Medicago sativa) in Wisconsin

Plant Disease ◽  
2014 ◽  
Vol 98 (6) ◽  
pp. 843-843 ◽  
Author(s):  
D. A. Samac ◽  
S. Allen ◽  
D. Witte ◽  
D. Miller ◽  
J. Peterson
Keyword(s):  
United States ◽  
Medicago Sativa ◽  
The United States ◽  
Crown Rot ◽  
Growth Chamber ◽  
Its Sequence ◽  
Colletotrichum Trifolii ◽  
Differential Cultivars ◽  
Race 1 ◽  
Race 2

Anthracnose of alfalfa (Medicago sativa), caused by Colletotrichum trifolii, is widespread in the United States. In addition to loss of forage due to death of stems, the pathogen causes crown rot, reducing stand life and winter survival (2), making it one of the most serious diseases of alfalfa. Three physiological races have been described (2). Race 1 is reported to be the dominant race that is present wherever alfalfa is grown, while race 2 was reported in a limited area in the Mid-Atlantic states, and race 4 was found in Ohio (1). Conspicuous, straw-colored dead stems with a “shepherd's crook” wilt and large, sunken, diamond-shaped lesions with a dark border were observed in experimental plots and breeding nurseries of experimental lines in Clinton and West Salem, WI, in August 2011 and in West Salem, WI, in mid-August 2012. Acervuli with black setae and orange spore masses were observed in lesions placed in moist chambers for 2 days at room temperature with ambient room lighting. Conidia were germinated on 1% water agar and then single hyphae were transferred to potato dextrose agar (PDA) plates. DNA was extracted from pure cultures of strains DA-1 (Clinton, WI) and FGI-3 (West Salem, WI), the rDNA ITS1-5.8S-ITS2 region was amplified with primers ITS1 (5′-TCCGTAGGTGAACCTGCGG-3′) and ITS4 (5′-TCCTCCGCTTATTGATATGC-3′), the products sequenced directly, and the sequences compared to the ITS region of known race 1 and race 2 strains of C. trifolii. The sequences from DA-1 and FGI-3 were identical to the ITS sequence of C. trifolii 2sp2 (race 1; KF444778) and C. trifolii SB-2 (race 2; KF444779), but distinct from the ITS sequence of C. destructivum (JQ005764) and C. dematium (JX567507), which can cause anthracnose on alfalfa (1). Conidia from DA-1 and FGI-3 were harvested from 7-day-old cultures grown on PDA plates, diluted to 2 × 106 conidia/ml, and sprayed to runoff on 10-day-old growth chamber grown plants of three differential cultivars: Saranac (susceptible to races 1 and 2), Arc (resistant to race 1, susceptible to race 2), and Saranac AR (resistant to races 1 and 2). Plants were maintained at 100% relative humidity for 48 h and then grown in a growth chamber at 24°C with a 16-h photoperiod. Symptoms were rated at 14 days after inoculation. In the three repetitions of the experiment using 75 plants of each cultivar in each experiment, less than 10% of the Saranac and Arc plants survived, while survival of Saranac AR was 31 to 44%. The approximate expected survival of differential cultivars inoculated with race 1 is 1% for Saranac, 65 to 70% for Arc, and 45% for Saranac AR (2). Aggressiveness of race 2 strains on Saranac AR is variable, ranging from 12 to 68% plant survival (3). The susceptibility of Arc when inoculated with DA-1 and FGI-3 is consistent with the reaction to race 2 strains, indicating that both strains are race 2. The isolation of race 2 strains in major alfalfa growing regions in Wisconsin indicates that this physiological race is currently more widespread than previously observed. Although most modern alfalfa cultivars have resistance to race 1, few cultivars with resistance to race 2 are available. The occurrence of C. trifolii race 2 in the Midwest United States should be considered in alfalfa breeding programs when developing multi-pest resistant alfalfa cultivars. References: (1) J. J. Ariss and L. H. Rhodes. Plant Dis. 91:1362, 2007. (2) N. R. O'Neill. Plant Dis. 80:450, 1996. (3) N. R. O'Neill et al. Phytopathology 79:750, 1989.

scholarly journals A New Race of Colletotrichum trifolii Identified on Alfalfa in Ohio

Plant Disease ◽  
2007 ◽  
Vol 91 (10) ◽  
pp. 1362-1362 ◽  
Author(s):  
J. J. Ariss ◽  
L. H. Rhodes
Keyword(s):  
United States ◽  
North American ◽  
The United States ◽  
Morphological Characters ◽  
First Report ◽  
Anthracnose Resistance ◽  
Colletotrichum Trifolii ◽  
Race 1 ◽  
Race 2 ◽  
Race 4

Anthracnose of alfalfa (Medicago sativa L.), caused by the fungus Colletotrichum trifolii Bain & Essary, was identified as a potential problem of alfalfa in the United States in the late 1960s. Races 1 and 2 are known in the United States, and recently, race 4 was described in Australia (1). An additional race, race 3, had previously been reported in the United States, but isolates of this proposed race were not preserved and its status as a distinct race of C. trifolii is unclear. In June 2003, an isolate of C. trifolii was collected in Columbus, OH from a 4-year-old alfalfa stand of cv. 520. The isolate was obtained from typical anthracnose stem lesions. Stem sections with lesions were scraped with a sterile inoculation loop, and conidia were directly streaked onto acidified potato dextrose agar. Individual colonies were transferred to half-strength oatmeal agar. Morphological characters (conidia, acervuli, and setae) of this isolate, designated OH-WA-520, were consistent with those of other C. trifolii isolates and clearly distinguishable from C. destructivum or other Colletotrichum spp. that occur on alfalfa. The alfalfa differential cultivars Arc (resistant to race 1 and susceptible to race 2 of C. trifolii), Saranac AR (resistant to race 1 and race 2), and Saranac (susceptible to both races) were inoculated with conidia of isolate OH-WA-520 per the North American Alfalfa Improvement Conference standard protocol for determining anthracnose resistance (2). Isolate OH-WA-520 was avirulent on Arc but virulent on Saranac AR and Saranac. In each of three repetitions of the protocol, more than 65% of Arc plants survived, while less than 18% of Saranac and Saranac AR plants survived. These results indicate a physiological race inconsistent in reaction with C. trifolii race 1 or race 2, but similar in reaction to race 4 isolates previously described only from Australia (1). To our knowledge, this is the first report of a C. trifolii isolate virulent on Saranac AR but avirulent on Arc. This is also the first report of C. trifolii race 4 in the United States. References: (1) J. M. Mackie et al. Aust. J. Agric. Res. 54:829, 2003. (2) N. R. O'Neill. Anthracnose resistance. Page D-1 in: Standard Tests to Characterize Alfalfa Cultivars. Online publication. North American Alfalfa Improvement Conference, Beltsville, MD, 1991.

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

Plant Disease ◽  
2014 ◽  
Vol 98 (10) ◽  
pp. 1326-1332 ◽  
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.

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

1999 ◽  
Vol 89 (2) ◽  
pp. 156-160 ◽  
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.

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

Plant Disease ◽  
2015 ◽  
Vol 99 (12) ◽  
pp. 1713-1720 ◽  
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.

Vegetative compatibility within Fusarium oxysporum f.sp. niveum and its relationship to virulence, aggressiveness, and race

1990 ◽  
Vol 36 (5) ◽  
pp. 352-358 ◽  
Author(s):  
R. P. Larkin ◽  
D. L. Hopkins ◽  
F. N. Martin
Keyword(s):  
Fusarium Oxysporum ◽  
The United States ◽  
Soil Samples ◽  
Vegetative Compatibility ◽  
Vegetative Compatibility Groups ◽  
Nit Mutants ◽  
The World ◽  
Infested Field ◽  
Race 1 ◽  
Race 2

Over 250 isolates of Fusarium oxysporum collected from infected watermelon plants and soil samples from a pathogen-infested field, as well as known isolates of F. oxysporum f. sp. niveum imported from various locations around the world, were tested for pathogenicity on watermelon and used to determine vegetative compatibility groups (VCGs) within F. oxysporum f. sp. niveum. Vegetative compatibility was assessed on the basis of heterokaryon formation among nitrate-nonutilizing mutants. Race determinations were made by screening isolates on six different watermelon cultivars of varying resistance. All isolates of F. oxysporum f. sp. niveum belonged to one of three distinct VCGs, and were incompatible with isolates that were not pathogenic on watermelon. Isolates of F. oxysporum f. sp. niveum were subdivided into two races and there was a direct relationship between VCG and race. VCG 0080 consisted of race 1 isolates from five states of the United States, Taiwan, and Australia. VCG 0081 consisted solely of race 1 isolates from Florida. VCG 0082 was comprised solely of race 2 isolates, all of which were capable of causing severe wilt on all cultivars tested. Numerous Florida isolates were compatible with race 2 isolates from Texas and demonstrated comparable virulence on all cultivars, confirming the presence of race 2 in Florida. With F. oxysporum f. sp. niveum, vegetative compatibility can be utilized as an alternative or collaborative method to distinguish pathogenic from nonpathogenic strains of F. oxysporum and to differentiate subforma specialis virulence characteristics. Key words: fusarium wilt, nit mutants, watermelon.

Development of Colletotrichum trifolii races 1 and 2 on alfalfa clones resistant and susceptible to anthracnose

1988 ◽  
Vol 66 (1) ◽  
pp. 75-81 ◽  
Author(s):  
A. C. L. Churchill ◽  
C. J. Baker ◽  
N. R. O'Neill ◽  
J. H. Elgin Jr.
Keyword(s):  
Cell Wall ◽  
Epidermal Cell ◽  
Pore Formation ◽  
Epidermal Cells ◽  
Germ Pore ◽  
Colletotrichum Trifolii ◽  
Direct Penetration ◽  
Race 1 ◽  
Epidermal Cell Wall ◽  
Race 2

Resistant and susceptible alfalfa clones derived from the cultivar Arc were spray inoculated with conidia from race 1 or race 2 isolates of Colletotrichum trifolii Bain in compatible and incompatible combinations. No significant differences were found in the frequencies of formation of immature or mature appressoria or in germ-pore formation by either race of C. trifolii on resistant or susceptible plants. These results indicate that incompatibility is not associated with the failure of conidia to germinate or to form appressoria with germ pores. In a small number of observations, penetration pegs were observed in tissue of both resistant and susceptible plants. Colletotrichum trifolii initiated infections in alfalfa by direct penetration of the epidermis via a penetration peg from the appressorium. Although the fungus spread rapidly throughout susceptible hosts, we observed fungus penetration only of epidermal cells of resistant hosts. Therefore, it appears that expression of alfalfa resistance to C. trifolii occurs near the time of epidermal cell wall penetration.

Pathogenic specialisation within Colletotrichum trifolii in Australia, and lucerne cultivar reactions to all known Australian pathotypes

2003 ◽  
Vol 54 (9) ◽  
pp. 829 ◽  
Author(s):  
J. M. Mackie ◽  
J. M. Musial ◽  
N. R. O'Neill ◽  
J. A. G. Irwin
Keyword(s):  
North American ◽  
Crown Rot ◽  
Sources Of Resistance ◽  
Colletotrichum Trifolii ◽  
Race 1 ◽  
Host Origin ◽  
New Races ◽  
Race 2 ◽  
Race 4 ◽  
First Time

Anthracnose and crown rot, caused by Colletotrichum trifolii, are serious diseases of lucerne (Medicago sativa L.) in humid regions of the world. A race survey was conducted by inoculating individual lucerne clones (genotypes) with C. trifolii isolates collected from a range of Medicago hosts, locations, and years in south-eastern Queensland. This survey revealed for the first time in Australia the presence of race 2 (virulence on anthracnose resistance gene An1) and the first world report of race 4 (virulence on An2). A collection of North American race 1 and race 2 C. trifolii isolates, when inoculated onto the Australian differential clones, gave responses that were in agreement with their North American reactions. A RAPD analysis was conducted on 9 Australian C. trifolii isolates including races 1, 2, and 4; two C. destructivum and one C. gloeosporioides isolate were included as known outliers. For the C. trifolii isolates, 94.6% similarity was found regardless of host origin or race, compared with 2.2% similarity between this group and the C. gloeosporioides and C. destructivum isolates, confirming that the new races belong to C. trifolii. Currently, it is hypothesised that only plants carrying genes An1 and An2 are resistant to the 3 races. Of 22 cultivars screened against the 3 races, only UQL-1, Hallmark, and Pioneer 54Q53 had >30% of plants resistant to the 3 races in separate screenings. The research highlights the need to find new sources of resistance to C. trifolii in lucerne.

A model comparing the value of broad next-gen sequencing (NGS)-based testing to single gene testing (SGT) in patients with nonsquamous non-small cell lung cancer (NSCLC) in the United States.

2020 ◽  
Vol 38 (15_suppl) ◽  
pp. 9529-9529
Author(s):  
Nathan A. Pennell ◽  
Jie Zhou ◽  
Brian Hobbs
Keyword(s):  
Single Gene ◽  
Cost Savings ◽  
The United States ◽  
Survival Duration ◽  
Statistical Estimates ◽  
Gene Testing ◽  
Current State ◽  
Life Years ◽  
The Us ◽  
Highly Effective

9529 Background: Patients (pts) with nonsquamous (ns) NSCLC should be tested for actionable driver oncogenes (ADOs), and highly effective treatments (tx) may be available for these pts. Although EGFR and ALK single gene testing (SGT) is relatively common (>80%) in the US, testing for less common ADOs is rare. Unidentified pts with ADOs have survival comparable to pts without alterations. We interrogate plausible testing configurations and discuss their implications for the US population. Methods: Simulation was used to evaluate various levels of testing with SGT or NGS on the basis of life years gained (LYG) as well as cost per LYG. Expected prevalence of ADOs among nsNSCLC pts as well as the survival distribution of pts in the presence versus absence of an ADO tx strategy were calibrated based on current literature. Survival duration for each simulated pt was generated from Weibull distributions fit to statistical estimates of median and 5-year survival. With appropriate match between ADO and targeted tx, the Weibull distribution offered a median additional 2 years of life. ADOs included in NGS: EGFR, ALK, ROS1, BRAF, RET, MET, NTRK. SGT: EGFR and ALK. Results: Each incremental 10% increase in NGS instead of SGT produces 2630 additional LYG and a cost savings per LYG between -$49 to -$109. At the current 80% testing rate, replacing SGT with NGS would result in an additional 21,019 LYG with reduced cost per LYG of -$599. Increasing testing from 80% to 100% of eligible pts would increase LYG by 15,017 over the current state. If 100% of eligible pts were tested with NGS and every identified pt received tx, the cost per LYG of this strategy would be $16,641.57. Conclusions: In a hypothetical model where highly effective tx is available to all identified pts with ADOs, broad NGS testing compared to SGT for EGFR/ALK leads to large gains in life years at reduced cost per LYG compared to SGT, supporting universal NGS testing of all advanced nsNSCLC pts. Conversely, lower levels of testing or only testing for common ADOs (as is the current state) result in large numbers of pts being unidentified and not experiencing these benefits. [Table: see text]

scholarly journals Characteristics and Frequency of Aphanomyces euteiches Races 1 and 2 Associated with Alfalfa in the Midwestern United States

Plant Disease ◽  
2001 ◽  
Vol 85 (7) ◽  
pp. 740-744 ◽  
Author(s):  
D. K. Malvick ◽  
C. R. Grau
Keyword(s):  
Root Rot ◽  
High Frequency ◽  
Rapd Analysis ◽  
Random Amplified Polymorphic Dna ◽  
Aphanomyces Euteiches ◽  
History Of ◽  
Race 1 ◽  
Race 2 ◽  
Genotypic Characteristics ◽  
Distribution Frequency

Aphanomyces root rot of alfalfa, caused by Aphanomyces euteiches, kills seedlings and causes decline of established plants in slowly drained soils. Two races of A. euteiches that are pathogenic to alfalfa have been identified. Despite the contribution of race 1 resistance to establishment and yield of alfalfa, race 1-resistant alfalfa cultivars perform poorly in some fields infested with A. euteiches. Many isolates of A. euteiches obtained from the soils of problematic fields are of a race 2 phenotype. The purpose of this study was to determine distribution, frequency, and pathogenic and genotypic characteristics of race 1 (R1) and race 2 (R2) isolates from 21 fields: 13 in Wisconsin, 7 in Minnesota, and 1 in Kentucky. A. euteiches was successfully isolated from the soil of 16 of the 21 fields; 405 isolates were obtained from Wisconsin, 4 from Minnesota, and 48 from Kentucky. Pathogenicity and race phenotype of isolates were characterized on Saranac (susceptible to R1 and R2 isolates) and WAPH-1 (resistant to R1 and susceptible to R2 isolates) alfalfa populations. One Wisconsin field with no recent history of alfalfa production had a high frequency (51%) of R2 isolates, and 43% of all isolates were R2 from fields with a history of alfalfa production. In a location that was planted continuously to pea for 30 years, 27% of the isolates were R2. Random amplified polymorphic DNA (RAPD) analysis of three R1 and three R2 isolates with eight primers generated 43 total polymorphic bands; however, none of the bands were uniquely associated with race phenotype. Cluster analysis based on RAPD bands revealed no consistent genotypic distinctions between R1 and R2 isolates of A. euteiches. Evaluation of eight commercial alfalfa cultivars for resistance to two R1 and two R2 isolates demonstrated that most are susceptible to R2 isolates; however, those selected for R2 resistance express resistance to R2 isolates. The results suggest that R2 isolates represent a widespread risk to alfalfa cultivars having resistance only to R1 isolates in fields with varied cropping histories.

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