A Gene-for-Gene Relationship Between Wheat and Mycosphaerella graminicola, the Septoria Tritici Blotch Pathogen

ABSTRACT Specific resistances to isolates of the ascomycete fungus Mycosphaerella graminicola, which causes Septoria tritici blotch of wheat, have been detected in many cultivars. Cvs. Flame and Hereward, which have specific resistance to the isolate IPO323, were crossed with the susceptible cv. Longbow. The results of tests on F1 and F2 progeny indicated that a single semidominant gene controls resistance to IPO323 in each of the resistant cultivars. This was confirmed in F3 families of Flame x Longbow, which were either homozygous resistant, homozygous susceptible, or segregating in tests with IPO323 but were uniformly susceptible to another isolate, IPO94269. None of 100 F2 progeny of Flame x Hereward were susceptible to IPO323, indicating that the resistance genes in these two cultivars are the same, closely linked, or allelic. The resistance gene in cv. Flame was mapped to the short arm of chromosome 3A using microsatellite markers and was named Stb6. Fifty-nine progeny of a cross between IPO323 and IPO94269 were used in complementary genetic analysis of the pathogen to test a gene-for-gene relationship between Stb6 and the avirulence gene in IPO323. Avirulence to cvs. Flame, Hereward, Shafir, Bezostaya 1, and Vivant and the breeding line NSL92-5719 cosegregated, and the ratio of virulent to avirulent was close to 1:1, suggesting that these wheat lines may all recognize the same avirulence gene and may all have Stb6. Together, these data provide the first demonstration that isolate-specific resistance of wheat to Septoria tritici blotch follows a gene-for-gene relationship.     

Identification of a new resistance gene to a chinese blast fungus isolate in the Japanese rice cultivar aichi asahi

ABSTRACT Genetic analysis of the rice cultivar Aichi Asahi and some other Japanese cultivars for the high resistance to the blast fungus isolate CHNOS58-3-1 from China was performed. All the Japanese differential cultivars were resistant to the isolate except for 'Pi No. 4', which showed moderate resistance. Analysis of the F(2) population of a cross of the susceptible cultivar Reiho and the resistant cultivar Aichi Asahi indicated that the resistance of 'Aichi Asahi' to the isolate was conferred by one dominant gene. To identify the gene in other Japanese differential cultivars, AK lines, which were derived from a cross of 'Aichi Asahi' x 'K59' and assumed to harbor no known genes except for the new one, were used for the allelism tests. The new, completely dominant resistance gene was detected in 14 differential cultivars, but not in 'Pi No. 4', 'Yashiro-mochi', and 'K1', and was designated as Pi19(t). Pi19(t) was allelic or closely linked to Pita(2) on chromosome 12. Pi19(t) was extensively distributed among Japanese traditional local cultivars.     

Identification of avirulence genes in the rice blast fungus corresponding to three resistance genes in Japanese differentials

The pathogenicity of progeny from crosses among three Chinese isolates of Magnaporthe grisea collected from rice was tested on three Japanese differentials (Ishikarishiroke, Aichiasahi, K 59) having the blast resistance genes Pii, Pia, and Pit, respectively. Monogenic control was demonstrated for avirulence to the differentials. To identify resistance genes corresponding to the avirulence genes, the resistance and susceptibility in F₃ lines of the cultivars in response to the parents of the crosses were analyzed genetically. The three avirulence genes identified, designated Avr-Pii, Avr-Pia, and Avr-Pit, appear to correspond to resistance genes Pii, Pia, and Pit, respectively. The monogenic control of avirulence in the fungus and monogenic dominant resistance in rice cultivars supports a gene-for-gene relation in the Pii-, Pia-, or Pit-dependent resistance to the rice blast fungus in rice cultivars.     

Genetic Analysis and Identification of Amplified Fragment Length Polymorphism Markers Linked to the alm1 Avirulence Gene of Leptosphaeria maculans

ABSTRACT A gene-for-gene interaction was previously suggested by mapping of a single major locus (LEM 1) controlling cotyledon resistance to Leptosphaeria maculans isolate PHW1245 in Brassica napus cv. Major. In this study, we obtained further evidence of a gene-for-gene interaction by studying the inheritance of the corresponding avirulence gene in L. maculans isolate PHW1245. The analysis of segregating F(1) progenies and 14 test crosses suggested that a single major gene is involved in the interaction. This putative avirulence gene was designated alm1 after the resistance locus identified in B. napus. Amplified fragment length polymorphism (AFLP) markers were used to generate a rudimentary genetic linkage map of the L. maculans genome and to locate markers linked to the putative avirulence locus. Two flanking AFLP markers, AC/TCC-1 and AC/CAG-5, were linked to alm1 at 3.1 and 8.1 cM, respectively. Identification of markers linked to the avirulence gene indicated that the differential interaction is controlled by a single gene difference between parental isolates and provides further support for the gene-for-gene relationship in the Leptosphaeria-Brassica system.     

A Gene-for-Gene Relationship Underlying the Species-Specific Parasitism of Avena/Triticum Isolates of Magnaporthe grisea on Wheat Cultivars

ABSTRACT To elucidate genetic mechanisms of the species-specific parasitism of Magnaporthe grisea, a Triticum isolate (pathogenic on wheat) was crossed with an Avena isolate (pathogenic on oat), and resulting F(1) progeny were subjected to segregation analyses on wheat cvs. Norin 4 and Chinese Spring. We found two fungal loci, Pwt3 and Pwt4, which are involved in the specific parasitism on wheat. Pwt3 operated on both cultivars while Pwt4 operated only on 'Norin 4'. Using the cultivar specificity of Pwt4, its corresponding resistance gene was successfully identified in 'Norin 4' and designated as Rmg1 (Rwt4). The presence of the corresponding resistance gene indicated that Pwt4 is an avirulence locus. Pwt3 was assumed to be an avirulence locus because of its temperature sensitivity. We suggest that gene-for-gene interactions underlie the species-specific parasitism of M. grisea.     

Pi34-AVRPi34: a new gene-for-gene interaction for partial resistance in rice to blast caused by Magnaporthe grisea

The japonica rice (Oryza sativa) cultivar Chubu 32 has a high level of partial resistance to blast, which is mainly controlled by a dominant resistance gene located on chromosome 11. The partial resistance to the rice blast fungus (Magnaporthe grisea) in Chubu 32 has isolate specificity; isolate IBOS8-1-1 is more aggressive on Chubu 32 than are other isolates. We hypothesized that the gene-for-gene relationship fits this case of a partial resistance gene in Chubu 32 against the avirulence gene in the pathogen. The partial resistance gene in Chubu 32 was mapped between DNA markers C1172 (and three other co-segregated markers) and E2021 and was designated Pi34. In the 32 F₃ lines from the cross between a chromosome segment substitution line (Pi34⁻) from Koshihikari/Kasalath and Chubu 32, the lines with high levels of partial resistance to the M. grisea isolate Y93-245c-2 corresponded to the presence of Pi34 estimated by graphic genotyping. This indicated that Pi34 has partial resistance to isolate Y93-245c-2 in compatible interactions. The 69 blast isolates from the F₁ progeny produced by the cross between Y93-245c-2 and IBOS8-1-1 were tested for aggressiveness on Chubu 32 and rice cultivar Koshihikari (Pi34⁻). The progeny segregated at a 1 : 1 ratio for strong to weak aggressiveness on Chubu 32. The results suggested that Y93-245c-2 has one gene encoding avirulence to Pi34 (AVRPi34), and IBOS8-1-1 is extremely aggressive on Chubu 32 because of the absence of AVRPi34. This is the first report of a gene-for-gene relationship between a fungal disease resistance gene associated with severity of disease and pathogen aggressiveness.     

Molecular mapping of the new blast resistance genes Pi47 and Pi48 in the durably resistant local rice cultivar Xiangzi 3150

The indica rice cultivar Xiangzi 3150 (XZ3150) confers a high level of resistance to 95% of the isolates of Magnaporthe oryzae (the agent of rice blast disease) collected in Hunan Province, China. To identify the resistance (R) gene(s) controlling the high level of resistance in this cultivar, we developed 286 F(9) recombinant inbred lines (RILs) from a cross between XZ3150 and the highly susceptible cultivar CO39. Inoculation of the RILs and an F(2) population from a cross between the two cultivars with the avirulent isolate 193-1-1 in the growth chamber indicated the presence of two dominant R genes in XZ3150. A linkage map with 134 polymorphic simple sequence repeat and single feature polymorphism markers was constructed with the genotype data of the 286 RILs. Composite interval mapping (CIM) using the results of 193-1-1 inoculation showed that two major R genes, designated Pi47 and Pi48, were located between RM206 and RM224 on chromosome 11, and between RM5364 and RM7102 on chromosome 12, respectively. Interestingly, the CIM analysis of the four resistant components of the RILs to the field blast population revealed that Pi47 and Pi48 were also the major genetic factors responsible for the field resistance in XZ3150. The DNA markers linked to the new R genes identified in this study should be useful for further fine mapping, gene cloning, and marker-aided breeding of blast-resistant rice cultivars.     

The identification of a gene for race-specific resistance to Peronospora parasitica (downy mildew) in Brassica napus var. oleifera (oilseed rape)

The oilseed rape cultivar Cresor was resistant to 14 isolates of Peronospora parasitica derived from crops of Brassica napus in the UK. Segregation for resistance to one isolate among F₂ plants and F₃ progeny of crosses between Cresor and the susceptible cultivars Victor and Jet Neuf indicated that resistance was controlled by a single gene. There was evidence that genetic background and environment could influence the phenotypic expression of this resistance. Two sexual progeny isolates derived from a homothallic isolate of P. parasitica avirulent on Cresor were completely virulent on this cultivar. This suggested that the parental isolate was heterozygous at a matching locus or loci for avirulence and demonstrated the race-specific nature of the resistance.     

Rice Pi-ta gene Confers Resistance to the Major Pathotypes of the Rice Blast Fungus in the United States

ABSTRACT The Pi-ta gene in rice prevents the infection by Magnaporthe grisea strains containing the AVR-Pita avirulence gene. The presence of Pi-ta in rice cultivars was correlated completely with resistance to two major pathotypes, IB-49 and IC-17, common in the U.S. blast pathogen population. The inheritance of resistance to IC-17 was investigated further using a marker for the resistant Pi-ta allele in an F(2) population of 1,345 progeny from a cross of cv. Katy with experimental line RU9101001 possessing and lacking, respectively, the Pi-ta resistance gene. Resistance to IC-17 was conferred by a single dominant gene and Pi-ta was not detected in susceptible individuals. A second F(2) population of 377 individuals from a reciprocal cross between Katy and RU9101001 was used to verify the conclusion that resistance to IC-17 was conferred by a single dominant gene. In this cross, individuals resistant to IC-17 also were resistant to IB-49. The presence of Pi-ta and resistance to IB-49 also was correlated with additional crosses between 'Kaybonnet' and 'M-204', which also possess and lack Pi-ta, respectively. A pair of primers that specifically amplified a susceptible pi-ta allele was developed to verify the absence of Pi-ta. We suggest that Pi-ta is responsible for resistance to IB-49 and IC-17 and that both races contain AVR-Pita genes.     

Genetic and molecular analyses in crosses of race 2 and race 7 of albugo Candida

ABSTRACT The inheritance of avirulence and polymorphic molecular markers in Albugo candida, the cause of white rust of crucifers, was studied in crosses of race 2 (Ac2), using isolates MiAc2-B1 or MiAc2-B5 (metalaxyl-insensitive and virulent to Brassica juncea cv. Burgonde) with race 7 (Ac7), using isolate MsAc7-A1 (metalaxyl-sensitive and virulent to B. rapa cv. Torch). Hybrids were obtained via co-inoculation onto a common susceptible host. Putative F(1) progeny were selfed to produce F(2) progeny. The parents and F(1) progeny were examined for virulence on the differential cultivars B. juncea cv. Burgonde and B. rapa cv. Torch. Segregation of avirulence or virulence of F(2) populations was analyzed on cv. Torch. Putative F(1) hybrids were confirmed by random amplified polymorphic DNA markers specific for each parent. Avirulence or virulence of F (2) progeny to B. rapa cv. Torch suggested 3:1 in each of three populations, supporting the hypothesis of a single dominant avirulence gene. Amplified fragment length polymorphism markers also segregated in regular Mendelian fashion among F(2) progeny derived from two F(1) hybrids (Cr2-5 and Cr2-7) of Cross-2. This first putative avirulence gene in A. candida was designated AvrAc1. These results suggest that a single dominant gene controls avirulence in race Ac2 to B. rapa cv. Torch and provides further evidence for the gene-for-gene relationship in the Albugo-Brassica pathosystem.     

Relationship between Avirulence Genes of the Same Family in Rice Blast Fungus Magnaporthe grisea

A genetic cross between rice-field isolates of Magnaporthe grisea produced progeny segregating for avirulence/ virulence on six rice cultivars among nine race differentials, while on three other cultivars, Shin 2 (Pik-s), Aichi Asahi (Pia) and Ishikari Shiroke (Pii), parental and progeny isolates were all virulent. Based on segregation ratios in 115 progeny isolates, avirulence on Kanto 51 (Pik), Yashiro-mochi (Pita), Fukunishiki (Piz) and Toride 1 (Piz-t) is under monogenic control. On Tsuyuake (Pik-m) and Pi No. 4 (Pita-2), however, a disproportionate ratio in the segregation was observed, suggesting that avirulence on these two cultivars is controlled by two or more genes. Assuming that the avirulence gene AvrPik-m consists of at least two genes, AvrPik-m1 and AvrPik-m2, each of which functions in the whole gene AvrPik-m, and that one of AvrPik-m1 and AvrPik-m2 is AvrPik, we could account for the disproportion in the avirulence/virulence segregation of the progeny. This hypothesis would also be consistently applied for avirulence gene AvrPita-2. There seem to be two types of the avirulence genes : AvrPik-m, that is comprised of the tightly linked genes, AvrPik-ml (=AvrPik) and AvrPik-m2, and AvrPita-2, that is comprised of the loosely linked genes AvrPita-2A (=AvrPita) and AvrPita-2B. As one possible explanation of the rice resistant reaction to blast, multiple specificity was suggested for the first time for the blast fungus. On the contrary, the avirulence genes AvrPiz and AvrPiz-t were inherited independently, despite the corresponding genes for resistance (Piz and Piz-t) being located at the same locus. The cross of rice blast isolates (races 447 and 337) produced only 25 kinds of races in the progeny, although theoretically about 64 kinds of races should be produced if six avirulence genes segregated independently. Because no progeny are with AvrPik (or AvrPita) and without AvrPik-m (or AvrPita-2), the number of races theoretically should be 36 at most. A number of strains, such as races 377 and 737, with a single avirulence gene were obtained from this cross. These strains may be valuable for analysis of resistance genes in rice plant. Received 19 August 2002/ Accepted in revised form 11 November 2002     

Natural Variation at the Pi-ta Rice Blast Resistance Locus

ABSTRACT The resistance gene Pi-ta protects rice crops against the fungal pathogen Magnaporthe grisea expressing the avirulence gene AVR-Pita in a gene-for-gene manner. Pi-ta, originally introgressed into japonica rice from indica origin, was previously isolated by positional cloning. In this study, we report the nucleotide sequence of a 5,113-base pair region containing a japonica susceptibility pi-ta allele, which has overall 99.6% nucleotide identity to the indica Pi-ta allele conferring resistance. The intron region shows the levels of sequence diversity that typically differentiate genes from indica and japonica rices, but the other gene regions show less diversity. Sequences of the Pi-ta allele from resistant cultivars Katy and Drew from the southern United States are identical to the resistance Pi-ta sequence. Sequences from susceptible cultivars El Paso 144 and Cica 9 from Latin America define a third susceptibility haplotype. This brings the total number of Pi-ta haplotypes identified to four, including the resistance allele and three susceptibility alleles. The Pi-ta locus shows low levels of DNA polymorphism compared with other analyzed R genes. Understanding the natural diversity at the Pi-ta locus is important for designing specific markers for incorporation of this R gene into rice-breeding programs.     

小麦抗白粉病基因Pm21 的抑制基因

 小麦-簇毛麦6VS. 6AL 易位染色体含有抗白粉病基因Pm21,在我国的小麦育种中被广泛应用。近年来,一些含有Pm21 基因的小麦品种(系)开始感染白粉病。为探索含Pm21 的品种(系)感染白粉病的原因,本研究在6VS. 6AL 易位系与小麦品系(种)R14 和川农12 的杂交后代中利用分子标记CINAU17-1086 和CINAU18-723 辅助选择的遗传背景相对简单的F7 和F8 近等基因系为材料,研究了小麦抗白粉病基因Pm21 的抗病性表达。结果发现,在3 个含有6VS. 6AL 易位染色体的感病F6 植株繁殖的F7 近等基因系中发生了白粉病抗性的分离,分离比率符合13 感病︰ 3 抗病的理论值。在随机选取的F7 感病小麦单株所繁殖的F8 近等基因系中,有7 / 13 的株系一致地重感白粉病,有6 / 13 的株系发生了抗白粉病的分离,其中2 / 13 的株系分离比符合3 感病︰ 1 抗病、4 / 13 的株系分离比符合13 感病︰ 3 抗病的分离模式。这一结果指出,小麦株系中的抗白粉病基因Pm21 的抗性表达受小麦基因组中的一对显性抑制基因所控制,该基因来源于小麦品种(系)川农12或R14,建议命名为SuPm21。本研究指出,在把外源基因引入小麦的研究中,有利的外源基因与不含抑制基因的受体遗传资源同等重要。     

Gene Action and Linkage of Avirulence Genes to DNA Markers in the Rust Fungus Puccinia graminis

ABSTRACT Two strains of the wheat stem rust fungus, Puccinia graminis f. sp. tritici, were crossed on barberry, and a single F(1) progeny strain was selfed. The parents, F(1), and 81 F(2) progeny were examined for virulence phenotypes on wheat differential cultivars carrying stem rust resistance (Sr) genes. For eight Sr differentials, phenotypic ratios are suggestive of single dominant avirulence genes AvrT6, AvrT8a, AvrT9a, AvrT10, AvrT21, AvrT28, AvrT30, and AvrTU. Avirulence on the Sr; (Sr 'fleck') differential showed phenotypic ratios of approximately 15:1, indicating epistatic interaction of two genes dominant for avirulence. Avirulence on Sr9d favored a 3:13 over a 1:3 ratio, possibly indicating two segregating genes-one dominant for avirulence and one dominant for avirulence inhibition. Linkage analysis of eight single dominant avirulence genes and 970 DNA markers identified DNA markers linked to each of these avirulence genes. The closest linkages between AvrT genes and DNA markers were between AvrT6 and the random amplified polymorphic DNA marker crl34-155 (6 centimorgans [cM]) AvrT8a and the amplified fragment length polymorphism marker eAC/mCT-197 (6 cM) and between AvrT9a and the amplified fragment length polymorphism marker eAC/mCT-184 (6 cM). AvrT10 and AvrTU are linked at distance of 9 cM.     

Significance of PWT4–Rwt4 interaction in the species specificity of Avena isolates of Magnaporthe oryzae on wheat

We examined whether PWT4, an avirulence gene of Avena isolates of Magnaporthe oryzae toward wheat, corresponded to Rwt4, a resistance gene identified in wheat cultivar Norin 4, in a one-to-one manner. Twelve wheat cultivars were inoculated with 65X1, an F₁ culture with PWT4 derived from a cross between an Avena isolate (Br58) and a Triticum isolate (Br48). Three wheat cultivars (Norin 26, Shin-chunaga, Cheyenne) were resistant and therefore selected as possible carriers of Rwt4. The three cultivars were then inoculated with a population derived from a backcross of 61M2 carrying PWT4 with Br48 carrying pwt4. Segregation analyses revealed that PWT4 operates against the three cultivars. If PWT4 corresponds to Rwt4 in a one-to-one manner, all three cultivars should carry Rwt4. To test if this is the case, the three cultivars were crossed with Chinese Spring (a noncarrier of Rwt4) and Norin 4. When F₂ seedlings from Chinese Spring × Norin 26, Chinese Spring × Shin-chunaga, and Chinese Spring × Cheyenne were inoculated with 61M2, resistant and susceptible seedlings segregated in a 3 : 1 ratio. On the other hand, crosses between the three cultivars and Norin 4 yielded no susceptible F₂ seedlings. These results indicate that all three cultivars carry Rwt4. Considering all results, we concluded that PWT4 corresponds to Rwt4 in a one-to-one manner. An inoculation test with Chinese Spring–Cheyenne chromosome substitution lines indicated that Rwt4 is located on chromosome 1D.     

Genetic and molecular analyses of the incompatibility between Lolium isolates of Pyricularia oryzae and wheat

Pyricularia oryzae isolates from Lolium spp. (annual ryegrass and perennial ryegrass) show evidence of recent events in evolution of this fungus. A wheat blast isolate found in Kentucky in 2011 was assumed to originate from annual ryegrass isolates. Genetic analyses revealed that the incompatibility between a Lolium isolate and common wheat cultivars is controlled by two gene pairs, Rmg6–A1 with a strong effect and R2-A2 with a weak effect, implying that this incompatibility is conditioned by simple gene-for-gene interactions. Disruption of the A1 avirulence gene led the Lolium isolate to gain virulence on common wheat. These results suggest a mechanism for host jumping by the blast fungus.     

Genetic distribution of the avirulence gene AVRPiz-t in Thai rice blast isolates and their pathogenicity to the broad-spectrum resistant rice variety Toride 1

Rice blast disease, caused by the filamentous fungus Pyricularia oryzae, is one of the most destructive diseases in rice worldwide. Breeding of resistant rice cultivars remains a cost-effective and environment-friendly means for controlling blast disease, but the resistance tends to break down over time because of the pathogen's rapid adaptation. In this study, AVRPiz-t gene sequences of 46 rice blast isolates were evaluated using a Southern blot analysis. The AVRPiz-t gene was present in 24 of 46 (52.2%) rice blast isolates. The pathogenicity assay showed that all blast isolates were avirulent against Japanese rice cv. Toride 1, which carries several rice blast resistance genes including Piz-t, Pii, Pi37, and Pi-ta. Screening for the Piz-t gene in Thai rice germplasm revealed that less than 20% of rice varieties harbour the Piz-t gene. Therefore, the Toride 1 rice variety could serve as an effective donor of rice blast resistance to be used in rice breeding programmes in Thailand. This study provides evidence for co-evolution between the rice blast resistance gene Piz-t and the rice blast fungal avirulence gene AVRPiz-t. Understanding this relationship will facilitate the sustainable development of breeding for rice blast resistance in the future.     

Identification and mapping of a rice blast resistance gene Pi-g(t) in the cultivar Guangchangzhan

A single dominant blast resistance gene was identified in Chinese indica rice ( Oryza sativa ) cv. Guangchangzhan (GCZ), which shows complete resistance to Japanese isolate Ken53-33 of Magnaporthe grisea . Genetic analysis of the backcross (BC₁) and second-generation (F₂) populations from a cross between susceptible cv. Lijiangxintuanheigu (LTH) and GCZ indicated that the resistance was conferred by one dominant gene. This gene was mapped on the long arm of chromosome 2 and flanked by RM166 and RM208 at distances of 4·00 ± 4·90 and 6·30 ± 4·89 cM (centiMorgans), respectively. It was designated tentatively as Pi-g(t) .     

Genetic analysis of host–pathogen incompatibility between Lolium isolates of Pyricularia oryzae and wheat

Lolium isolate TP2 of Pyricularia oryzae, causal agent of gray leaf spot of perennial ryegrass (Lolium perenne), is virulent on perennial ryegrass, but avirulent on wheat cultivars. Genetic analysis of wheat F₂ populations revealed that the resistance of wheat cultivars Chinese Spring, Shin-chunaga, and Norin 4 to TP2 was conditioned by two genes, R1 and R2. R1 was highly effective, while R2 was less effective. The strong resistance gene R1, designated Rmg6, was mapped on chromosome 1D using microsatellite markers. For revealing genetic mechanisms of avirulence, TP2 was crossed with Triticum isolate Br48. Segregation analysis of their F₁ progenies revealed that the avirulence of TP2 on the three wheat cultivars was conditioned by two unlinked genes, one (A1) highly effective and the other (A2) less effective. These results suggest that the incompatibility between TP2 and the common wheat cultivars is conditioned by two gene pairs; the Rmg6–A1 interaction results in strong resistance, and the R2–A2 interaction results in moderate resistance.     

Genetic Analysis of Avirulence/Virulence of an Isolate of Magnaporthe grisea from a Rice Field in Texas

ABSTRACT An isolate of Magnaporthe grisea, Tm4, from a rice field in Texas was crossed with a fertile laboratory strain, 70-6. The progenies showed segregation of avirulence/virulence on rice cvs. Newbonnet, Lemont, Lebonnet, Leah, and Katy. The avirulent/virulent segregation ratios were 29:6 on Newbonnet, Lemont, and Lebonnet; 28:7 on Leah; and 33:2 on Katy. There was cosegregation on the first three cultivars. Several avirulent progenies were backcrossed to virulent parent 70-6. Three generations of backcrossing avirulent progenies to 70-6 led to segregation ratios that suggested certain strains had only one avirulence gene. Strains avirulent only on cv. Katy or only on cvs. Newbonnet, Lemont, and Lebonnet were test crossed with virulent siblings. Strains that gave progeny ratios approximating 1 avirulent:1 virulent when crossed with virulent siblings were selected for further test crossing. Intercrosses between strains with possible single avirulence genes were made to determine whether these strains had the same or different avirulence genes. Many lines still segregated two genes for avirulence after three generations of backcrossing. This is based on the recovery of virulent progenies from crossing two avirulent siblings.