Isolation and characterization of endophytic streptomycete antagonists of Fusarium wilt pathogen from surface-sterilized banana roots

A total of 131 endophytic actinomycete strains were successfully isolated from surface-sterilized banana roots. These isolates belonged to Streptomyces (n=99), Streptoverticillium (n=28), and Streptosporangium (n=2) spp. The remaining 2 isolates were not identified. About 18.3% of the isolates inhibited the growth of pathogenic Fusarium oxysporum f. sp. cubense on banana tissue extract medium. The most frequently isolated Streptomyces sp. strain S96 was similar to Streptomyces griseorubiginosus. About 37.5% of the S. griseorubiginosus strains were antagonistic to F. oxysporum f. sp. cubense. The antagonism of strain S96 was lost when FeCl(3) was introduced into the inhibition zone. In vivo biocontrol assays showed that the disease severity index (DSI) was significantly (P=0.05) reduced and mean fresh weight increased (P=0.001) in plantlets treated with strain S96 compared to those grown in the absence of the biocontrol strain. These findings indicate the potential of developing siderophore-producing Streptomyces endophytes for the biological control of fusarium wilt disease of banana.     

蔓割病不同抗性甘薯品种的茎部细胞结构观察

蔓割病是我国南方薯区甘薯主要病害之一,本研究采用直接观察、显微和超微结构观测等方法,对高抗、中感和高感蔓割病的3个甘薯品种(高抗品种:金山57,中感品种:热薯1号,高感品种:新种花)接种蔓割病菌28 d植株充分发病后其茎基部细胞的侵染结构进行了观察。结果表明,在用清水作对照处理时,3个品种茎部组织的细胞形态和结构正常且完整,细胞代谢强。高抗品种金山57无论是接种组还是对照组,均未发现病原菌丝的存在,其茎下部、中部和上部细胞结构相对完整。中感品种热薯1号,蔓割病菌从其茎基部侵入后导致茎基部细胞破损坏死,而中部寄主-病原互作较为活跃和典型,造成养分运输受阻,茎基部接种后病原菌丝会沿着寄主茎部的维管束和其他组织一直向寄主的茎部末端蔓延,遭受侵染后的部位其细胞反应与高感品种新种花类似。高感品种新种花遭受蔓割病原菌侵染后,病原菌菌丝从茎基部新鲜剪口侵入后进入表皮细胞、皮层、维管束并在甘薯茎基部蔓延至中部、上部,直至寄主整株枯死,甘薯茎的木质部导管出现侵填体,质壁分离;与此同时,细胞壁的沉积物及乳突在病原菌的入侵处形成,各种无定型物质或纤丝构成的织网迅速包围入侵菌丝。     

Identification of I‐7 expands the repertoire of genes for resistance to Fusarium wilt in tomato to three resistance gene classes

The tomato I‐3 and I‐7 genes confer resistance to Fusarium oxysporum f. sp. lycopersici (Fol) race 3 and were introgressed into the cultivated tomato, Solanum lycopersicum, from the wild relative Solanum pennellii. I‐3 has been identified previously on chromosome 7 and encodes an S‐receptor‐like kinase, but little is known about I‐7. Molecular markers have been developed for the marker‐assisted breeding of I‐3, but none are available for I‐7. We used an RNA‐seq and single nucleotide polymorphism (SNP) analysis approach to map I‐7 to a small introgression of S. pennellii DNA (c. 210 kb) on chromosome 8, and identified I‐7 as a gene encoding a leucine‐rich repeat receptor‐like protein (LRR‐RLP), thereby expanding the repertoire of resistance protein classes conferring resistance to Fol. Using an eds1 mutant of tomato, we showed that I‐7, like many other LRR‐RLPs conferring pathogen resistance in tomato, is EDS1 (Enhanced Disease Susceptibility 1) dependent. Using transgenic tomato plants carrying only the I‐7 gene for Fol resistance, we found that I‐7 also confers resistance to Fol races 1 and 2. Given that Fol race 1 carries Avr1, resistance to Fol race 1 indicates that I‐7‐mediated resistance, unlike I‐2‐ or I‐3‐mediated resistance, is not suppressed by Avr1. This suggests that Avr1 is not a general suppressor of Fol resistance in tomato, leading us to hypothesize that Avr1 may be acting against an EDS1‐independent pathway for resistance activation. The identification of I‐7 has allowed us to develop molecular markers for marker‐assisted breeding of both genes currently known to confer Fol race 3 resistance (I‐3 and I‐7). Given that I‐7‐mediated resistance is not suppressed by Avr1, I‐7 may be a useful addition to I‐3 in the tomato breeder's toolbox.     

First record of Fusarium vascular wilt on grapevine in Egypt

During the summer season of 2003 and 2004, wilt syndromes of grapevine leaves (Cv. crimson) and vascular discolouration of roots have been observed in 2-year-old grapevine plants in the field at two sides in Gharbeia Governorate, Egypt. First, symptoms of wilt began on bottom leaves borderline as chlorosis and then these turned to necrotic spots and the leaves died. Wilt symptoms were spread to apical associated with vascular discolouration of roots and stem basal. Routine isolations of discoloured root tissue from diseased plant yielded eight isolates of Fusarium oxysporum Schlechtend only where no other fungi were developed. Microscopic examination revealed the presence of three shapes of microconidia, first is avoid shape non-septate measuring 2.5–3.0 μm × 6–10 μm, second is cylindrical with one septa measuring 2.6 μm × 17.0 μm and third shape also cylindrical with two septate measuring 3.0 μm × 20.0 μm. Macroconidia was rarely with three septate measuring 3.5– 4.0 μm × 35.0–38.0 μm, and chlamydospores were found singly or in pairs or chains. F. oxysporum isolates attacked grapevine plants (Cv. crimson) causing vascular wilt (66.7%) and root-rot syndrome (33.3%). In vitro isolates of F. oxysporum causing wilt of grapevine (Cv. crimson) varied for producing lytic enzymes, i.e. polygalacturonase (PG) and cellulase. The reactions of several grapevines (Cvs.) with a virulent isolate of F. oxysporum indicated the presence of two different symptoms, i.e. vascular wilt only on grapevine plants (Cv. crimson) and root-rot on the other grapevine (Cvs.), i.e. superior, Thompson, King robi and flame seedless. All F. oxysporum isolates caused vascular wilt of grapevine Cv. crimson, successfully reisolated from symptomatic vascular infected tissue and complete identification on the basis of colony, conidia morphology and host range at formae speciales level as F. oxysporum f. sp. herbemontis (Tochetto) Gordan. This is the first report of Fusarium wilt on grapevine in Egypt.     

Acremonium as an endophytic bioagent against date palm Fusarium wilt

A total of 250 endophytic fungal isolates, representing 30 morphotaxa, were isolated and characterised, they were collected from the different living symptomless parts of date palm trees of orchards of six Egyptian governorates. Colonisation was greater in samples from the midrib than in those from laminar tissue and slightly greater at the tip of the lamina compared with the base of the leaf. Acremonium spp. were frequently isolated as date palm root endophytes. Acremonium isolates were screened in Petri dishes to select the highest antagonistic one against an Algerian isolate of Fusarium oxysporum f.sp. albedinis. Two-week-old axenically reared date palm seedlings grown in Petri dishes were directly injected with spore suspension (1.5?×?10⁷ spores/ml) of a pure culture of the virulent antagonistic isolate of Acremonium sp. One week after endophytic colonisation, date palm seedlings were then challenged with the pathogen, Fusarium albedinis. The challenged seedlings exhibited a significant reduction in wilt symptom percentage (by 87.0%), while the seedlings exposed to Fusarial toxin without pathogen exhibited the wilt disease symptoms. This indicates that the endophyte ably depresses any toxic action of F. albedinis. The endophytic fungus was recovered from sites distant from the point of inoculation after six?months from the application, indicating that the Acremonium sp. has the potential to move throughout the tissue plant, even the end time of trial. The Acremonium mode of action, as a biocontrol agent, was discussed.     

Combined application of native Trichoderma isolates possessing multiple functions for the control of Fusarium wilt disease in banana cv. Grand Naine

Fusarium wilt caused by Fusarium oxysporum f. sp. cubense (Foc) is considered as a lethal disease of bananas worldwide. To manage the disease effectively, 20 rhizospheric and 43 endophytic Trichoderma isolates obtained from 12 different Foc resistant banana accessions were evaluated against Foc in vitro and in vivo. In vitro screening among Trichoderma isolates for their multiple functions (mycelial and spore germination inhibition, hydrogen cyanide, chitinolytic enzymes, non-volatile and volatile metabolites production) in suppressing Foc and promoting plant growth (IAA production and phosphate solubilisation) indicated that the multiple biocontrol actions were significantly higher in 6 isolates of rhizospheric Trichoderma and 10 isolates of endophytic Trichoderma compared to other isolates. The greenhouse evaluation of individual application of these rhizospheric and endophytic Trichoderma isolates against Fusarium wilt pathogen in cv. Grand Naine (AAA) indicated significant suppression of Fusarium wilt disease and increased plant growth characters as compared to Foc pathogen inoculated plants. However, none of these individual Trichoderma isolates recorded complete suppression of Fusarium wilt disease. Therefore, the greenhouse evaluation involving combination of rhizospheric Trichoderma sp. NRCB3 + endophytic Trichoderma asperellum Prr2 recorded 100% reduction of Fusarium wilt disease and increased plant growth parameters up to 250% when compared to individual isolates application and Foc alone-inoculated plants. Further, the field evaluation of this combination of Trichoderma isolates applied for three times: (1) at 15 days before planting, (2) second month after planting and (3) fourth month after planting resulting in significant reduction of Fusarium wilt disease and also increase in bunch weight as compared to untreated control plants. Therefore, these Trichoderma isolates may be used in combination for the effective suppression of Fusarium wilt disease in banana.     

尖孢镰刀菌古巴专化型MAPK FoSlt2敲除突变体的转录组分析

尖孢镰刀菌古巴专化型Fusarium oxysporum f. sp. cubense（FOC）是威胁香蕉生产的重要土传病原真菌。丝裂原活化蛋白激酶（mitogen-activated protein kinase,MAPK）FoSlt2信号通路在调控尖孢镰刀菌古巴专化型的生长发育、细胞壁完整性和致病性方面发挥着重要作用。为了揭示FoSlt2信号通路的致病机理和寻找农药靶标,本研究利用高通量RNA-seq技术对该病菌野生型菌株和FoSlt2敲除突变体菌株的转录组进行了比较分析,结果表明差异表达基因共有2 164个,其中上调表达基因有1 184个,下调表达基因有980个。Gene Ontology（GO）功能分析结果表明,差异表达基因主要参与在结合、催化分子功能组和代谢过程、细胞过程生物学通路中。KEGG 功能富集分析结果表明,差异基因主要参与戊糖和葡糖醛酸盐转换、氨基糖和核苷酸糖、氨基葡聚糖降解、磷酸肌醇和碳类物质代谢通路,说明这些通路与尖孢镰刀菌古巴专化型的生长发育和致病性相关。该研究为尖孢镰刀菌古巴专化型致病机制的阐明奠定了理论基础。     

绿豆尖镰孢枯萎病抗性鉴定方法

绿豆是我国的主要食用豆类之一。由尖镰孢引起的绿豆枯萎病是一种严重的土传病害,病原菌从根部侵入,引起植株矮化,叶片黄化、枯萎,根茎部维管束变褐,严重时导致植株死亡。防治枯萎病最经济、有效的方法是培育利用抗病品种。本研究在控制条件下以具有不同抗性表型绿豆品种为材料,分别对接种方法、植株生育期、接种体浓度、接种体处理时间及接种后植株培养温度等影响绿豆抗性表型的因素进行比较研究,以期建立一个快速、准确和高效的绿豆枯萎病抗性鉴定方法,为抗病资源的筛选和抗病育种提供技术支持。结果表明,绿豆枯萎病苗期抗性鉴定最适宜的接种方法为剪根浸根法,最适宜接种体浓度为105~106孢子/m L,接种最佳植株生育期为2叶期,最短有效接种体浸根时间为2 min,最适宜发病温度为25℃,接种后14 d调查病情。     

Control of chickpea wilt (Fusarium oxysporum f.sp. ciceris) using Trichoderma spp. in Ethiopia

Thirty-two Trichoderma isolates were collected from soils grown with chickpea in central highlands of Ethiopia. The eight isolates were identified by CAB-International as Trichoderma harzianum, T. koningii and T. pseudokoningii. In in vitro tests, all Trichoderma isolates showed significant (P < 0.05) differences in their colony growth and in inhibiting the colony growth of Fusarium oxysporum f.sp. ciceris, race 3. In potted experiment, four Trichoderma isolates were tested as seed treatment on three chickpea cultivars (JG-62 susceptible, Shasho moderately susceptible and JG-74 resistant) against F. oxysporum f.sp. ciceris, race 3. The result showed that T. harzianum and unidentified Trichoderma isolate T23 significantly reduced wilt severity and delayed disease onset. The degree of wilt severity and delay of disease onset varied with chickpea cultivars. Our study revealed that biological control agents such as Trichoderma can be a useful component of integrated chickpea Fusarium wilt management.     

Induction of systemic resistance in chickpea against Fusarium wilt by Bacillus strains

Plant growth promoting rhizobacteria (PGPR) strains Rb29 (B. amyloliquefaciens MF352007), Bs1 (B. subtilis MF352017) and Bt1 (B. tequilensis MF352019) were tested for growth promotion and for their ability to induce systemic resistance against Fusarium wilt, a vascular disease of chickpea, using two methods that include whole plant and a split-root system. Bacillus strains and Fusarium oxysporum f. sp. ciceris (FOC) were inoculated on separate halves of roots of chickpea seedlings at the same time and then planted in separate pots either in superposition or one side of the other. All Bacillus strains systemically induced resistance against FOC, and significantly (p < 0.05) reduced the wilt disease by 98–100%. Application of Bacillus strains effectively enhanced plant growth, leading to increased plant height, root length, a fresh and dry weight of shoots and roots. These results help to explain the role of strains of Bacillus in growth promotion and biological control of Fusarium wilt in chickpea. This is the first report of systemic-induced resistance against Fusarium wilt in chickpea obtained by application of Bacillus strains to a root system spatially separated from the FOC-inoculated root.     

Antifungal activity of streptavidin C1 and C2 against pathogens causing Fusarium wilt

Fusarium wilt is caused by the soil-inhabiting fungus Fusarium oxysporum ff. spp. and is one of the most devastating plant diseases, resulting in losses and decreasing the quality and safety of agricultural crops. We recently reported the structures and biochemical properties of two biotin-binding proteins, streptavidin C1 and C2 (isolated from Streptomyces cinnamonensis strain KPP02129). In the present study, the potential of the biotin-binding proteins as antifungal agent for Fusarium wilt pathogens was investigated using recombinant streptavidin C1 and C2. The minimum inhibitory concentration of streptavidin C2 was found to be 16 µg ml^–1 for inhibiting the mycelial growth of F. oxysporum f.sp. cucumerinum and F. oxysporum f.sp. lycopersici, while that of streptavidin C1 was found to be 64 µg ml^–1. Compared with the nontreated control soil, the population density of F. oxysporum f.sp. lycopersici in the soil was reduced to 49·5% and 39·6% on treatment with streptavidin C1 (500 µg ml^–1) and C2 (500 µg ml^–1), respectively. A greenhouse experiment revealed that Fusarium wilt of tomato plants was completely inhibited on soil drenching using a 50-ml culture filtrate of the streptavidin-producing strain KPP02129.     

Characterization of emerging populations of Fusarium oxysporum f. sp. conglutinans causing cabbage wilt in China

Fusarium oxysporum f. sp. conglutinans (FOC) causes Fusarium wilt, a disease of cabbage that has brought about significant economic loss throughout northern China since it was first detected in 2001. To characterize the Chinese FOC isolates, we compared the cultural characteristics, pathogenicity and races between the Chinese isolates and the type strains (race 1: 52,557 and race 2: 58,385). The Chinese FGL‐03‐6 isolate had cultural characteristics similar to those of strain 52,557, including colony growth rate, colony and spore characteristics and responses to temperature changes, while the strain 58,385 grew faster, produced more pigment and spores and was more adaptable to temperature fluctuations. The lethal temperature for all strains was 60°C, and the optimal temperatures for pathogen growth on potato dextrose agar and pathogenicity on plants were 25°C and 25 to 30°C, respectively. Tests for race and pathogenicity indicated that different cabbage cultivars had similar resistance reactions to FGL‐03‐6 and 52,557. However, the pathogenicity of FGL‐03‐6 was similar to that of 58,385, which infected quickly and caused more severe disease symptoms. This study further provides information regarding characterizing different strains of F. oxysporum f. sp. conglutinans.     

Identification of resistance against Fusarium oxysporum f.sp. lini in linseed germplasm

Screening of germplasm/varieties was made to find out the sources of resistance against F. oxysporum f. sp. lini. Screening was conducted on 78 available germplasm/varieties during 2003–2004 and 2004–2005 in rabi season of linseed under natural conditions. Out of total 78 entries, 27 cultures were found to be resistant to disease as the disease incidence in these cultivars were between 0 and 10%. Twenty-three cultivars fell in moderately resistant category with 10.1–25% wilt incidence. Nine genotypes were found moderately susceptible sho'wing 25.1–50% disease incidence, 14 genotypes were found susceptible showing 50.1–75% and 6 genotypes were found highly susceptible to disease (above 75%).     

Microsatellite marker-based detection of Fusarium wilt pathogens of Psidium guajava L.

Wilt of Psidium guajava L., incited by Fusarium oxysporum f. sp. psidii and Fusarium solani is a serious soil-borne disease of guava in India. Forty-two isolates each of F. oxysporum f. sp. psidii (Fop) and F. solani (Fs) collected from different agro climatic zones of India showing pathogenicity were subjected to estimate the genetic and molecular characterisation in terms of analysis of microsatellite marker studies. Out of eight microsatellite markers, only four microsatellite markers, viz. MB 13, MB 17, RE 102 and AY212027 were amplified with single band pattern showing the character of identical marker for molecular characterisation and genetic identification. Microsatellite marker MB 13 was amplified in F. oxysporum f. sp. psidii and F. solani isolates. Product size of 296 bps and 1018 bps were exactly amplified with a single banding pattern in all the isolates of F. oxysporum f. sp. psidii and F. solani, respectively. Microsatellite markers, viz. MB 17, RE 102 and AY212027 were also exactly amplified with a single banding pattern. MB 17 was amplified in F. oxysporum f. sp. psidii isolates with a product size of 300 bp. RE 102 and AY212027 were amplified in F. solani isolates with the product size of 153 bp and 300 bp, respectively. Therefore, amplified microsatellite marker may be used as identifying DNA marker.     

Investigation of the diversity of effector genes in the banana pathogen,Fusarium oxysporum f. sp. cubense,reveals evidence of horizontal gene transfer

It is hypothesized that the virulence of phytopathogenic fungi is mediated through the secretion of small effector proteins that interfere with the defence responses of the host plant. In Fusarium oxysporum, one family of effectors, the Secreted In Xylem (SIX) genes, has been identified. We sought to characterize the diversity and evolution of the SIX genes in the banana‐infecting lineages of F. oxysporum f. sp. cubense (Foc). Whole‐genome sequencing data were generated for the 23 genetic lineages of Foc, which were subsequently queried for the 14 known SIX genes (SIX1–SIX14). The sequences of the identified SIX genes were confirmed in a larger collection of Foc isolates. Genealogies were generated for each of the SIX genes identified in Foc to further investigate the evolution of the SIX genes in Foc. Within Foc, variation of the SIX gene profile, including the presence of specific SIX homologues, correlated with the pathogenic race structure of Foc. Furthermore, the topologies of the SIX gene trees were discordant with the topology of an infraspecies phylogeny inferred from EF‐1α/RPB1/RPB2 (translation elongation factor‐1α/RNA polymerase II subunit I/RNA polymerase II subunit II). A series of topological constraint models provided strong evidence for the horizontal transmission of SIX genes in Foc. The horizontal inheritance of pathogenicity genes in Foc counters previous assumptions that convergent evolution has driven the polyphyletic phylogeny of Foc. This work has significant implications for the management of Foc, including the improvement of diagnostics and breeding programmes.     

香蕉枯萎菌基因组DNA提取方法的研究

以香蕉枯萎菌菌株为试验材料,在SDS～CTAB法和高盐沉淀法等基础上加以改进,对两种提纯香蕉枯萎菌基因组DNA的方法进行了比较研究。结果表明：高盐沉淀法是适合于香蕉枯萎菌基因组DNA提取的方法。该方法提取的DNA OD260／OD280的比值为1．841,DNA产量为0．81mgDNA／g菌丝体。基因组DNA经琼脂糖凝胶电泳得到一条带型较宽且清晰的DNA谱带,基本无DNA碎带;将提取的DNA直接用于PCR扩增,得到带多而且清晰、整齐、基本无拖尾的RAPD图谱。     

Impact of compost application on Fusarium wilt disease incidence and microelements contents of basil plants

Fusarium oxysporum was isolated from stem of basil plants showing symptoms of wilt, stem blight and collar root rot. Pathogenicity tests indicated that F. oxysporum f. sp. basilici is the causal agent of this disease. This is the first report of this pathogen in Egypt. The suppressive effects of six types of composts on Fusarium wilt disease incidence in basil were evaluated under greenhouse conditions. The effectiveness of these composts and their relation to the microelements content in treated plants was also assessed. Soil treatments with Khaya and Eucalyptus composts significantly reduced the infection percentage and disease severity of basil wilt. Otherwise, the applications of Araucaria, Datura, Ficus and Azadirachta composts showed no effect on both infection percentage and disease severity. Moreover, the Khaya and Eucalyptus compost treatments increased the levels of Iron (Fe), Zinc (Zn) and Manganese (Mn) in treated basil plants than application of Araucaria, Datura, Ficus and Azadirachta composts. In the case of Copper (Cu) content, it was significantly higher only in Eucalyptus-compost-treated plants than in other compost applications. These composts not only reduced the disease incidence but also increased both fresh and dry weight (FW and DW) and microelements contented in basil treated plants. In general, although soil amendment with either Khaya or Eucalyptus compost can reduce the disease incidence of Fusarium wilt on basil plants, microelements contented; FW and DW of these effects can be variable depending on their levels added in soil. According to the results of this study, it can be concluded that the use of compost in the soil as an organic fertiliser increased exchangeable form of microelements in the soil and also the availability of these elements by basil plants.     

香蕉枯萎病菌果胶甲基酯酶基因的克隆与生物信息学分析

旨在了解香蕉枯萎病菌(Fusarium oxysporumf.sp.cubense)4号生理小种(FOC4)PME基因序列特征,根据同源物种PME相关序列设计引物,利用PCR和RT-PCR技术,克隆FOC4序列基因和开放阅读框,命名为Foc4Pme。结果表明,所获得的PME基因均含有2个内含子和3个外显子,990 bp的片段,编码329个氨基酸。预测编码蛋白有信号肽,具有1个功能位点,其分子质量和等电点分为34.894 8 kD和9.17,该蛋白为稳定存在的蛋白。该蛋白疏水性最大值为2.022,最小值为-2.156,大部分区域为亲水区。该基因编码的蛋白具有一定保守性,进化上与镰刀菌亲缘关系最近。     

Analysis of Vegetative Compatibility Groups of Fusarium oxysporum from Eruca vesicaria and Diplotaxis tenuifolia

From 2002 to 2004, wilted plants of different species of rocket (Eruca vesicaria and Diplotaxis spp.) were found for the first time in Europe, in greenhouse cultivations in Piedmont and Lombardy, northern Italy. The causal agent of the disease was found to be Fusarium oxysporum. Vegetative compatibility analysis was carried out on 46 isolates of the fungus, 41 of them obtained from wilted rocket (E. vesicaria and D. tenuifolia) and five reference strains, in order to increase the knowledge on the causal agent of recent epidemics of Fusarium wilt on rocket in Italy. The analysis showed the presence of two vegetative compatibility groups (VCGs) (VCG 0101 and VCG 0220) pathogenic on both kinds of rocket. The two VCG populations, which were classified as formae specialesconglutinans and raphani, respectively, are spread in the area of epidemics but are not related to the host species from which they were isolated (D. tenuifolia or E. vesicaria). This finding shows the heterogeneity of the causal agent of Fusarium wilt on rocket in Italy.     

Genetics of resistance to Fusarium oxysporum f.sp. cucumerinum races 1 and 2 in cucumber line Wisconsin-2757

The mode of inheritance of resistance to Fusarium oxysporum f.sp. cucumerinum races 1 and 2 in Wisconsin-2757 (WI-2757), a gynoecious cucumber (Cucumis sativus L.), was determined by analysing segregation of F₁, F₂ and BC₁ populations of crosses with susceptible cultivar Straight-8. Resistance to either race 1 or race 2 in WI-2757 was conferred by a single dominant gene. In allelism tests, resistance to either race in WI-2757 was determined by the gene Fcu-1, which also confers resistance in line SMR-18.