Inheritance of resistance to early blight disease in a diploid potato population

Early blight disease, caused by Alternaria solani Sorauer, is a serious disease of potato foliage and tubers that occurs in most potato‐growing regions world‐wide. Developing new potato cultivars with resistance to early blight may reduce losses in the field and in storage, and lessen the need for fungicide applications. A total of 280 clones, derived from 72 maternal half‐sib families from a diploid random‐mated hybrid population of Solarium phureja×Solarium stenotomum were examined for resistance to early blight. The clones that were evaluated in a replicated field trial for 2 years in Pennsylvania, USA, had similar early blight intensity both years. Significant differences were found among families, within families and for the interaction of years × within families. Broad‐sense heritability for resistance, measured as area under the disease progress curve (AUDPC), was estimated as 0.73, with a 95% confidence interval of 0.65‐0.78, and narrow‐sense heritability was estimated as 0.61 ± 0.29 (P = 0.05). The correlation of AUDPC for early blight between years was 0.57 (P < 0.0001). These results suggest that this diploid population is worthy of use in breeding for early blight resistance.     

Half-sib progeny evaluation and selection of potatoes resistant to the US8 genotype of Phytophthora infestans from crosses between resistant and susceptible parents

The objectives of this study were to evaluate the use of potato (Solanum tuberosum L.) late blight (Phytophthora infestans (Mont.) de Bary) resistant parents in cultivar development and identify superior clones possessing moderate to high late blight resistance combined with acceptable maturity and tuber quality. Ninety-five crosses were made between eight unadapted parents with reported late blight resistance (B0718-3, Bertita, Bzura, Greta, Libertas, Stobrawa, Tollocan and Zarevo) and susceptible parents (cultivars or advanced breeding clones) adapted to North American growing conditions. A total of 408 field selected clones were assessed for late blight resistance in the greenhouse and in the field using a mixture of US8 P. infestans isolates (A2 mating type, metalaxyl resistant) that overcame all known R-genes except R8 and R9. Clones with ≤ 10% infected foliar area in the greenhouse test or ≤ 0.30 RAUDPC (relative area under the disease progress curve) value in the field in 1998 were re-tested in 1999. A total of 118 (29% of 408) putative late blight resistant clones were selected. The eight late blight resistant parents differed in both the ability to transmit late blight resistance and in the level of resistance transmitted to the progeny. The Tollocan and B0718-3 families (half-sib progeny) had the greatest degree of resistance and frequency of resistant clones. Scott-Knott cluster analysis ranked 79 clones (67% of 118) in the high and moderate late blight resistant groups. Among these 79 clones, 19 clones had vine maturity equal to or earlier than mid-season combined with acceptable tuber quality. Further selection in 2000 resulted in eight advanced selected clones (six from Tollocan and two from B0718-3 families) with the same level of resistance as the parent combined with vine maturity and tuber quality equivalent to Atlantic, a standard cultivar for chip processing in North America. The results indicate that this breeding approach can be used to select parents for late blight resistance breeding and to identify superior clones with high levels of late blight resistance and marketable vine maturity and tuber quality. This revised version was published online in July 2006 with corrections to the Cover Date.     

A simple method based on laboratory inoculum and field inoculum for evaluating potato resistance to black scurf caused by Rhizoctonia solani

A two-step method was developed to evaluate potato resistance to black scurf caused by Rhizoctonia solani. Tuber piece inoculum was first conducted in the laboratory, which was also first reported in this study. After inoculation with pathogen discs and culture for 48 h, the necrotic spots on the inoculated potato pieces were generated and measured by the crossing method. Further evaluation was conducted through field experiments using a wheat bran inoculum method. The wheat bran inoculum was placed into the pit dispersedly and surrounded seed tubers. Each cultivar or line was subjected to five treatments of 0-, 2-, 3-, 4-, and 5-g soil inoculum. The results showed that 2–4 g of wheat bran inoculum was the optimum for identifying tuber black scurf resistance. The laboratory scores positively correlated with the incidence and severity of black scurf in the field. According to the results in the laboratory, relatively resistant cultivars could be selected for further estimation of tuber black scurf resistance in field experiments. It is a practical and effective screening method for rapid identification of resistant potato germplasm, which can reduce workload in the field, shorten time required for identification.     

Sheath-blight resistance QTLS in japonica rice germplasm

Sheath blight (SB), caused by Rhizoctonia solani, is a serious disease of cultivated rice (Oryza sativa L.) for which genetic resistance is in demand by breeders. With the goal of resistance (SBR)-QTL discovery in U. S. japonica breeding material, 197 doubled-haploid lines from a cross between MCR10277 (resistant) and Cocodrie (susceptible) were evaluated in field and greenhouse assays with U. S. and Colombian pathogen isolates and genotyped at 111 microsatellite marker loci. Four SBR QTLs from MCR10277 were identified, together accounting for 47% of field genetic variation. In all trials the strongest effect was provided by a chromosome-9 QTL, qsbr_9.1, but some QTLs differed for U. S. and Colombian R. solani isolates. SBR QTLs coincided with only two of several height or heading-time QTLs, suggesting that the relationship between these developmental traits and SBR is not simple. For the U. S. isolates, a microchamber greenhouse assay revealed the same QTLs as did field inoculation.     

Late blight resistance in a diploid full-sib potato family

Late blight, caused by Phytophthora infestans (Mont.) de Bary, is the most destructive disease of potato worldwide. As this pathogen can rapidly overcome major race‐specific resistance genes, identifying the basis for enhanced quantitative resistance has become a crucial element for implementing advanced breeding strategies. A population of 230 full‐sib progeny derived from a cross between two diploid hybrid Solanum phureja × S. stenotomum clones was evaluated for foliage resistance against late blight in replicated trials at multiple locations in Pennsylvania between 1999 and 2002. In field experiments, plants were evaluated visually for per cent defoliation, and area under the disease progress curve (AUDPC) was determined. The two parents and three control cultivars (‘Atlantic’, ‘Kennebec’ and ‘Katahdin’) were included in all trials. In all three experiments, the presence of a significant number of clones exhibiting transgressive segregation were observed. There were significant differences among environments as well as among clones, and the clone × environment interaction was also significant. Stability analysis revealed that 37 clones made a significant contribution to the overall environment × clone interaction. Broad‐sense heritability for resistance, measured as AUDPC, was estimated as 0.67. The overall results indicate the presence in this potato family of a high level of field resistance against late blight. This segregating diploid family appears to be a good candidate for quantitative trait loci mapping to identify and characterize the genetic components of partial late blight resistance.     

Components of quantitative resistance to yellow rust in ten spring bread wheat cultivars and their relations with field assessments

Latency period, infection frequency, lesion length, lesion growth, disease severity and percentage of infected leaf parts were assessed on 10-day-old seedling leaves and flag leaves of ten bread wheat (Triticum aestivum L.) cultivars after inoculation with urediospores of Puccinia striiformis Westend. f. sp. tritici. For all components significant genotypic differences were detected. Components of resistance tended to be associated. A long latency period was associated with a low infection frequency, small lesions, a low disease severity and a low percentage of infected leaf parts. The latency period, measured as time period until first pustule appearance (LP1), was highly correlated with the latency period measured as time period until 50% of the pustules appeared (LP50). Assessment of latency period of large numbers of cultivars could therefore be reliably done by measuring LP1 which is less time consuming than measuring LP50. Latency period, infection frequency and disease severity were highly correlated with disease development data from field experiments. These results suggest that selection in the greenhouse for one of these components should result in cultivars with high levels of quantitative resistance. Disease severity after uniform inoculation in the greenhouse can be used for monocyclic evaluations because it is the easiest to assess. This revised version was published online in July 2006 with corrections to the Cover Date.     

马铃薯种质资源室内接种晚疫病的抗性鉴定

为了获得具有抗马铃薯晚疫病的品种,本研究以100份马铃薯种质资源为试材,采用室内离体叶片接种法和室内成株接种法进行晚疫病抗性评价,用高感品种Favorita接种后的症状为对照。室内离体接种鉴定结果为:‘CIP09-2’、‘CIP09-13’、‘DR-6’、‘DR-9’、‘花云’、‘R1R3’、‘云薯501’、‘R2’、‘BE13-3’等9份材料表现为高抗,占供试材料的9%;其它的15份表现中抗、44份表现抗病、23份表现感病和9份表现高感。欧氏距离类平均法聚类分析表明,聚为抗性品种的种质最多,其来源与马铃薯晚疫病的抗性没有太大关系。本研究对其中4份材料进行室内成株接种,结果表明:‘青薯10号’的抗病性高于离体叶片接种的抗性,为高抗品种;‘青薯9号’、‘2017ch-6’和‘费乌瑞它’抗性表现与离体叶片接种抗性鉴定结果一致。本研究发现所有供试马铃薯种质资源的抗性材料较丰富,可为抗病育种提供优质的亲本材料。     

Inheritance of early blight resistance in diploid potatoes

Early blight (Alternatia solani) is a fungal disease in hot and humid environments, which causes leaf, stem and tuber lesions. Early blight resistance should be incorporated into potato cultivars because the fungicide spraying is an expensive solution for developing countries. The diploid cultivated species Solanum tuberosum group Phureja and group Stenotomum are sources of resistance alleles. The elucidation of the inheritance for early blight resistance must help to decide what could be the best breeding procedure to improve this diploid germplasm and transfer the resistance to the tetraploid level. Three experiments were carried out under controlled and field conditions to determine the heritability of this trait using nested and diallel mating designs with haploid, species and haploid-species hybrids. The narrow-sense heritability estimates were relatively high (0.64–0.78). This means that additivity was the most important type of gene action for determining resistance to early blight at the diploid level. The results suggested that diploid parents showing highest levels of resistance, throughout the cycle of disease development, can be used in 4x×2x crosses to obtain resistant tetraploid progenies to this fungal disease.     

Characterization of the resistance to <Emphasis Type="Italic">Phytophthora infestans</Emphasis> in local potato cultivars in Bolivia

This experiment was carried out to investigate whether and how much field resistance to late blight, caused by Phytophthora infestans, is present in the local cultivated potato germplasm. In total 36 entries were compared in a field experiment in an area highly conducive to late blight development. Of the 36 cultivars 32 were local cultivars belonging to five Solanum species, S. tuberosum (1 accession), S. andigena (18), S. juzepczukii (2), S. stenotomum (9) and S. ajanhuiri (2). The other four cultivars were derived from breeding programmes, one being the Dutch cultivar Alpha used as a highly susceptible control. The 36 cultivars were planted according to a simple 6 × 6 lattice design with three replicates. Each replicate was divided in six incomplete blocks each with six cultivars. The disease severity was assessed weekly during 9 weeks starting 48 days after planting. The area under the disease progress curve (AUDPC) was used as a measure of the field resistance. Nine isolates from surrounding potato fields were tested for their virulence to the resistance genes R1–R11 using 22 differential cultivars. The components of the field resistance of 19 of these cultivars were compared in the greenhouse using a local isolate with virulence to all known R-genes, except to R9. The nine isolates represented seven races with a race complexity varying from 7 to 10 virulence factors. All isolates carried virulence against R1, R2, R3, R7, R10 and R11, while virulence against R9 was absent. The AUDPC among the 32 local cultivars ranged from very large, significantly larger than that of ‘Alpha’ to very small. The AUDPC from S. stenotomum accessions ranged from very large to intermediate, those from S. andigena accessions from large to very small. Especially among the S. andigena accessions interesting levels of field resistance were found. Four components of field resistance were assessed, latency period (LP), lesion size (LS), lesion growth rate (LGR) and relative sporulation area (RSA). All four showed a considerable variation among the cultivars. The LP ranged from 3½ to 6 days. The LS ranged from 225 mm² to 20 mm². The LGR varied about six-fold, the RSA more than 10-fold. The components tended to vary in association with one another. LP and LGR were well associated with each other and had a significant correlation with the AUDPC.     

Reaction of sugar beet S1 lines and cultivars to different isolates of Macrophomina phaseolina and Rhizoctonia solani AG-2-2IIIB

Interactions of 17 sugar beet lines and cultivars with four isolates of Macrophomina phaseolina (the causal agent of charcoal rot) and one isolate of Rhizoctonia solani (the causal agent of crown and root rot) were studied in separate experiments under greenhouse conditions. The isolates of Macrophomina were taken from their host plants, sugar beet (two isolates), soybean and sesame. In the first experiment, the colonized toothpick was used as inoculum. In the second experiment, six-month-old sugar beet plants were inoculated with barley seeds colonized with M. phaseolina. For the inoculation of sugar beet lines with R. solani, the colonized corn seeds were used. Root symptoms were recorded four weeks after inoculation, by estimating the proportion of the root surface infected by the pathogens, using a 1–9 standard scale. Our results showed a significant difference among lines and cultivars in their resistance to these two pathogens. Line B8618 was found to be considerably resistant to the isolates of the both pathogens. The inoculation methods of Macrophomina isolates had no significant effect on the results. The interaction between isolate and cultivar was not also significant in Macrophomina-resistant lines. Therefore, it appears that the response of sugar beet lines to the tested fungal isolates was not differential. These resistant lines showed a high resistance to all the tested M. phaseolina isolates. Our results revealed that the Macrophomina-resistant lines also showed resistance to R. solani. Furthermore, the sugar beet drought tolerant lines (M293, M362 and M345) were susceptible to the tested M. phaseolina and R. solani isolates.     

Search for sources of early blight (Alternaria solani) field resistance not associated with vegetative late maturity in tetraploid potato germplasm

Summary A total of 934 potato cultivars and clones from different breeding programmes across the world were tested for their reaction to early blight (Alternaria solani) in Brasilia-DF, Brazil. This field trial was set up in order to identify tetraploid potato genotypes with useful levels of resistance to early blight (EB) that were not correlated with late maturity or lateness. Artificial inoculation (spread ofA. solani infected leaves over plant canopy) was employed even though EB occurs naturally at epidemic levels in the test area. Four disease assessments were made every 10 days, beginning 45 days after planting. Based on the evaluation made 65 days after planting, only 27 clones were identified that had less lesioned leaf area than the resistant standard (cv. Aracy). Area under disease progress curve (AUDPC) was also used as a criterion of resistance. Using AUDPC as a parameter, 22 genotypes were identified presenting values lower than or identical to cv. Aracy. Differences in the commercial yield were recorded in genotypes with identical response to EB, suggesting that distinct levels of tolerance to EB also exist. In spite of the strong correlation between EB resistance and late maturity, some genotypes derived from the parental materials NDD 277-2, Kufri Jyoti, CIP 377888-7, Maine-28, and the clones CFS 69.1 and I-853 were identified as having good levels of EB field resistance not linked with the negative phenotypic trait of lateness. These materials presented medium to medium-early maturity with a vegetative cycle of around 90–95 days. Some selected genotypes presented more acceptable agronomic characteristics including resistance toPhytophthora infestans, potato leaf roll luteovirus (PLRV) and potato virus Y (PVY). The EB resistance sources identified in this work can be recommended as parental materials for widening the genetic base for EB disease resistance in breeding programmes for highland tropical and subtropical areas.     

Heritability of resistance to foliar late blight in a diploid hybrid potato population of Solanum phureja×Solanum stenotomum

The emergence of new races of Phytophthora infestans has necessitated the search for additional sources of potato germplasm with resistance to late blight. This study examined 281 clones, derived from 72 families of a diploid random-mated hybrid population of Solanum phureja×Solanum stenotomum. The clones were evaluated in a replicated field trial for 2 years with the control cultivar ‘Atlantic’ in Pennsylvania, USA. The P. infestans US-8 A2 mating type culture was used to inoculate spreader rows of susceptible S. tuberosum cv.‘Russet Burbank’. Percent defoliation caused by the late blight fungus was estimated visually in each plot three times near the end of the growing season. Area under the disease progress curve (AUDPC) was estimated and showed that late blight was more severe in 1997 than in 1996. Genetic differences among clones and significant, but small, clone–environment interaction were detected for AUDPC. Broad-sense and narrow-sense heritability estimates, over years, were 0.79 ± 0.05 (P = 0.05) and 0.78 ± 0.29, respectively. Seventy-five percent of the diploid clones had a significantly lower mean AUDPC than Atlantic. These results support the idea that this diploid population is worthy of use in breeding for late blight resistance in tetraploid potato cultivars.     

Venturia inaequalis resistance in local Spanish cider apple germplasm under controlled and field conditions

Host resistance is a key method for the integrated management of apple scab caused by Venturia inaequalis, which is one of the most important diseases of apple. Artificial inoculation of 92 cider apple cultivars with a mixed inoculum of V. inaequalis identified 19 weakly resistant and 19 resistant cultivars. Twelve of these resistant cultivars were previously classified as having low susceptibility to fire blight, and four of them showed complete or weak resistance to races (1), (1, 6) and (6, 7, 13) of V. inaequalis. The analysis of a selection of 72 cultivars for 6 years under field conditions identified 14 cultivars that were classified as resistant to apple scab under high disease pressure involving one to six Mills periods of severe risk of infection each year. Eight out of these 14 cultivars previously showed high levels of resistance to fire blight, which would allow the incorporation of genetic resistance in the integrated production of cider apples in Spain through their use in breeding programs.     

Fire blight resistance and genetic trueness-to-type of four somaclonal variants from the apple cultivar Greensleeves

Four somaclonal variants regenerated from adventitious buds of the apple variety Greensleeves were preselected on the basis of their reduced fire blight susceptibility. The present study aimed at assessing precisely their level of fire blight resistance through various inoculation techniques (on in vitro leaves and microcuttings, on greenhouse plants and in field conditions). Overall results of these tests indicated that one clone (R 46/3) was clearly less susceptible than the control. This clone was also characterized as a ‘spur’ variant, with a reduced growth which can explain its limited susceptibility to fire blight. A second clone (R 20/63) was slightly less susceptible than the control in greenhouse and field tests, but this low level of resistance was overcome by high concentrations of inoculum. The absence of variation in chromosome number and isozyme patterns confirmed the genetic trueness-to-type of these four somaclones. This revised version was published online in August 2006 with corrections to the Cover Date.     

Potential of a Fusarium eumartii culture filtrate on the screening for wilting resistance in potato

Summary Fusarium solani f.sp. eumartii Carp. Snyder and Hansen (Fusarium eumartii) is a soil inhabitant that induces the so-called Potato Wilt and Stem End Rot disease. Prior to wilting, the pathogen induces peculiar small bronze spots on the leaflets. Failure to isolate F. eumartii from infected leaflets suggests the involvement of a toxin in the disease. The fungus was grown in liquid Richard's medium and thereafter a filtrate was obtained dialyzing (MW cutoff 12,000–14,000) and sterilizing the culture by filtration (0.22 m). Potato leaves treated with both the pathogen or the filtrate showed symptoms of bronze spots and significantly higher electrolyte leakage when compared to controls. Tomato leaves showed neither bronze spots nor electrolyte leakage after plant inoculation with the pathogen or with the filtrate treatment. Both, the absence of visible symptoms and the lack of electrolyte leakage in tomato could be associated to a certain degree of host specificity of the F. eumartii filtrate towards potato. The filtrate also induced symptoms similar to infections by F. eumartii in adult plants and in vitro plantlets of cultivars Huinkul MAG and Kennebec. Callus responses to the filtrate were related to responses of the cultivars to the pathogen in greenhouse. These results show the potential of the culture filtrate of F. eumartii for use in screening for wilting resistance.     

Rooting of bean leaves and use in germplasm evaluation for common bacterial blight resistance

Summary A simple protocol for leaf rooting in beans (Phaseolus vulgaris L.) was developed and used to investigate the reaction of Xanthomonas campestris pv. phaseoli (Xcp), causal pathogen of common bacterial blight disease, in detached versus attached bean leaves. Trifoliate leaves of different sizes (one-third, two-thirds, and fully expanded), either with or without the pulvinus attached to the petioles, were excised from 20 day-old plants of six bean cultivars/lines. Leaf cuttings were cultured in potting medium and then incubated for 5 to 10 days under transparent polyethylene plastic cover in the greenhouse. Roots were readily initiated along the petioles of the leaf cuttings, whether the pulvinus was present or absent. All leaves which were two-thirds expanded and fully expanded developed roots 5 to 7 days after culture. Eighty to 90 percent of the leaves which were one-third expanded formed roots 8 to 10 days after incubation. Laminae of the rooted leaf cuttings were viable and green during the 2 to 3 months period in culture after removing the plastic cover. The common bacterial blight reactions were similar for inoculated attached leaves, detached rooted leaves (inoculated either after or prior to rooting), and moistened detached leaves incubated without rooting. The latter were only usable for evaluation of the Xcp reaction in growth chamber experiments but not under greenhouse conditions. The rooted leaves would be useful for screening bean lines for multiple disease resistance, especially if the pathogens require different environments for disease expression.Abbreviations CBB Common Bacterial Blight - Xcp Xanthomonas campestris pv. phaseoli (Smith) Dye     

Evaluation of Verticillium wilt resistance in commercial cultivars and advanced breeding lines of cotton

Verticillium wilt (VW), caused by Verticillium dahliae Kleb, is one of the most destructive diseases in cotton (Gossypium spp.). The most efficient and cost-effective method of controlling the disease is the use of resistant cotton cultivars. Most commercial cultivars and elite breeding lines are developed under non-VW conditions and their responses to the disease are currently unknown. This study was conducted to evaluate current commercial cotton cultivars and advanced breeding lines for VW resistance. In 2011–2013, a total of 84 cultivars from major US seed companies, 52 advanced breeding lines from the US public breeding programs, and 87 introgression lines from a cross between Acala 1517-99 × Pima PHY 76 from the New Mexico Cotton Breeding Program, were evaluated for VW resistance in the greenhouse. Cotton cultivars and breeding lines were evaluated in ten separate replicated tests by inoculation with a defoliating-type isolate of V. dahliae. While leaf severity rating and percentages of infected plants, infected leaves and defoliated leaves were found to be significantly and positively correlated with one another, leaf severity rating and percentage of infected leaves were best choices because of their relatively low coefficients of variation and higher resolutions to differentiate resistant genotypes from susceptible ones. The heritabilities for the VW resistance traits ranged from 0.58 to 0.80 with an average of 0.67, indicating that variation in VW resistance is predominantly due to genetic factors. Of the 223 commercial cultivars and advanced lines, six Upland cultivars (FM 9160B2F, FM 9170 B2F, NG 4010 B2RF, Nitro 44 B2RF, DP 1219 B2RF, and ST 4288 B2F), five advanced lines (Ark 0403-3, MD 10-5, MD 25ne, NC11AZ01, and PD 0504), two introgression lines from Upland × Pima (NM11Q1157 and 08N1618), and four Pima cultivars (COBALT, DP 357, PHY 800, and PHY 830) had higher levels of resistance to VW. The resistance shown by most of these cultivars in the greenhouse was consistent with their performance in previous field tests. Based on the initial VW resistance, 19 highly or moderately resistant genotypes were chosen for re-evaluation and 30 genotypes were also assessed more than once for VW resistance in different tests, most of which had concordant performance. These cultivars and advanced lines should be useful resources to improve VW resistance in cotton breeding.     

Early generation selection for potato tuber quality in progenies of late blight resistant parents

Developing disease resistant cultivars is one of the major objectives for a potato (Solanum tuberosum L.) breeding program, but many resistant clones have not achieved commercial acceptance because of late maturity and non-marketable tuber characteristics. Selection for tuber quality should have greater emphasis inbreeding disease resistant cultivars. The objectives of this study were 1) to evaluate the ability of late blight (Phytophthora infestans (Mont.) de Bary) resistant parents to transmit chip-processing (tuber appearance, specificgravity, and chip-color) or tablestock (tuber appearance) quality to the offspring and 2) to compare selecting for tuber quality in single-hill versus eight-hill generations. We made crosses among eight unadapted potato cultivars (B0718-3,Bertita, Bzura, Greta, Libertas, Stobrawa, Tollocan, and Zarevo) with reported late blight resistance with adapted susceptible cultivars/breeding clones to generate 95populations (4,750 seedlings). Approximately 10% of the progeny from each cross were selected from single-hill plots based on tuber appearance, number, shape, and internal defects. These selected clones (408) were evaluated for tuber appearance, specific gravity, and chip-color. The same evaluations in the following year were made on tuber samples from eight-hill plots. Libertas and Tollocan were the best parents for transmitting chip-color; B0718-3, Zarevo, and Tollocan for transmitting tuber appearance; and Bzura, Libertas, and Zarevo for transmitting high specific gravity to the highest percentage of the offspring. Overall, 50% and 56% of the clones based on single- and eight-hill generation, respectively, were considered to possess chip-processing quality; over 90% of the clones had acceptable tablestock quality. A total of 71% of the clones possessing acceptable chip-processing and 95% of the clones possessing acceptable tablestock quality selected in both generations were identified in single-hill plots. The evaluation of tuber quality characteristics in single-hill generation not only permitted the identification of clones with acceptable chip-processing and tablestock, but also increased the amount of clonal information for the following generation of selection. In crosses between late blight resistant and susceptible clones, selection for tuber quality traits can be initiated in single-hill generation using a moderate selection intensity and precede late blight testing. This revised version was published online in August 2006 with corrections to the Cover Date.     

Vertical resistance to late blight in wild potato species from Argentina

The possible presence of vertical resistance to late blight conferred by R genes in Argentine wild Solanum species, which presumably have not evolved under the pressure of the fungus, was investigated. Solanum microdontum, S. commersonii and S. chacoense clones were tested in the greenhouse and with detached leaves in the laboratory after inoculation with a complex race and a non-virulent race of Phytophthora infestans. Two progeny tests were carried out to assess the performance of contrasting parents in regards to their resistance. A varying frequency of incompatible clones was detected among the species, with S. microdontum having the higher proportion of clones yielding incompatible reactions both in the laboratory and in the greenhouse. S. chacoense and S. commersonii showed a small but still considerable frequency of incompatible clones. The progeny tests confirmed the presence of R genes in a S. commersonii cross and their absence in a S. chacoense cross. The unexpected presence of R genes, its evolutionary significance and the consequences on the use of these species in breeding is discussed. This revised version was published online in August 2006 with corrections to the Cover Date.     

不同品种及有关外因对水稻纹枯病抗性的影响

选择9个具不同纹枯病抗性的水稻品种，采用3个纹枯病菌株和2种接种方法，在2个生长季进行水稻品种纹枯病抗性的综合评价。结果表明，生长季、接种方法和菌株极显著地影响水稻品种对纹枯病的抗性反应，其中，以生长季因子的效应最大。不同接种方法对水稻纹枯病发病程度的影响因生长季的不同有所变化。不同品种在同一生长季、接