Genetic analysis of field resistance to tan spot in spring wheat

Tan spot, caused by Pyrenophora tritici-repentis (Died.) Drechs., is an important constraint to wheat (Triticum aestivum L.) yield in many countries. Since the inheritance of field resistance to tan spot is poorly understood, this study was conducted to determine the genetic control of resistance in the field. Resistance was measured as disease severity caused by P. tritici-repentis race 1 in four crosses involving five wheat parents: parent 1 (P₁) = catbird; parent 2 (P₂) = Milan/Shanghai-7; parent 3 (P₃) = Alondra/Coc//Ures; parent 4 (P₄) = Bcb//Dundee/Gul/3/Gul); parent 5 (P₅) = ND/VG9144//Kal/BB/3/Yaco/4/Chil. P₁, P₂ and P₃ were resistant and P₄ was moderately resistant, whereas P₅ was susceptible to tan spot. The F₂-derived F₃ families and the parents were field evaluated at El Batán, Mexico, in 1996. When all the plants within a F₃ family expressed low levels of disease severity similar to that of the resistant parent it was classified resistant (R), otherwise the progeny was classified as susceptible (S). The progeny of the three crosses of the susceptible parent with the resistant and moderately resistant parents P₂, P₃, and P₄ segregated as 3R:13S whereas the progeny in the cross with P₁ showed a segregation ratio of 1R:15S. This suggests that each resistant parent possessed two genes conditioning resistance to tan spot severity caused by P. tritici-repentis race 1. Information on the inheritance of resistance measured as disease severity on adult plants under field conditions is of practical importance for wheat breeding programs seeking improvement in tan spot resistance.     

Genetic variation in a population of tetraploid potatoes: Response to the potato leafhopper and the potato flea beetle

Progenies from a group of tetraploid parental clones from the USDA potato breeding program were used to investigate variation in resistance to the potato leafhopper,Empoasca fabae (Harris), and the potato flea beetle,Epitrix cucumeris (Harris). The study utilized two mating designs: i) selfing and testing both the parents and the S₁ progeny; and ii) nine clones, used as males, were each crossed to three different clones, and progenies from the resulting families were tested. Statistically significant differences between families were measured in each test for both insects. Non-additive genetic variance was larger than additive genetic variance in progeny reaction to leafhopper infestation and hopperburin, but was smaller in progeny reaction to flea beetles. Environmental variation contributed heavily to the total variation of plant reaction to both insect species. Selecting individuals was indicated to be slightly more effective than selecting males on half-sib progeny performance but not as effective as selecting clones on S₁ progeny performance. Because of the large environmental variance and small additive variance for both the leafhopper and flea beetle, slow progress in increasing the level of resistance to these two species in this sample population was predicted. Resistance to leafhopper infestation was genetically quite highly correlated (positive) with resistance to hopperburn, but phenotypically the correlation was considerably smaller. Negative genotypic, phenotypic, and environmental correlations between leafhopper infestation and flea beetle infestation suggest that selecting for resistance to one of these species, in the population sampled for these tests, would tend to increase susceptibility to the other.     

Inheritance and stability of resistance to fusarium tuber rot in tetraploid potatoes

Fusarium tuber rot of potato (Solanum tuberosum L.) is one of the most economically important diseases of stored potatoes. Dry rot is caused by several species of Fusaria, particularlyFusarium sambucinum in North America. The objective of this study was to determine the inheritance of resistance to Fusarium tuber rot. A highly resistant (B0172-22) and a highly susceptible (B0178-34) potato clone were crossed as female parents with two male parents. Tubers from varying numbers of progeny (28–37) from these four crosses were inoculated withFusarium sambucinum in the laboratory. Three tubers from each progeny were inoculated at approximately monthly intervals six times in each of 2 years. The depth and diameter of the lesion were measured 40 days after inoculation. Estimates of broadsense heritability (H) for lesion depth and a 95% confidence interval about these estimates were similar in 2000 and 2001: 0.84 (0.84, 0.91) and 0.77 (0.77, 0.87), respectively, as were the estimates for lesion diameter: 0.83 (0.83, 0.90) and 0.80 (0.80, 0.89), respectively. The differences in lesion depth for the female parents were significant the second year, but not the first year. The differences in lesion diameter for the female and male parents were significant both years. The female x male interactions were not significant for either lesion depth or lesion diameter. There were significant differences between clones in each of these four families for both traits both years. In 2000 and 2001, 24% and 38%, respectively, of the clones were unstable for resistance. In only one of the four families did instability increase as susceptibility increased, suggesting that there sometimes may be a genetic component to instability per se. These results indicate that resistance to Fusarium dry rot has a large genetic component. They also indicate that lesion diameter is more informative than lesion depth for evaluating resistance.     

Bacterial wilt resistance in the progenies of the fusion hybrids between haploid of potato andSolanum commersonii

Bacterial wilt of potatoes, which used to be a widespread disease in tropics and subtropics, has become a threat to potato production in temperate region. The diploid species Solanum commersonii has several desirable characteristics including cold tolerance and resistance to several diseases. Selected somatic hybrids between S. tuberosum dihaploid and bacterial-wilt-resistant S. commersonii clones were cross pollinated with S. tuberosum cultivars for further selection of bacterial wilt resistance. The chromosome numbers of the fusion parents were confirmed as 24, and the three fusion hybrids crossed were all tetraploids. The chromosome number of 11 backcross 1 progenies (BC1) was 48 and that of the other six was close to the tetraploid number. Backcross 2 progenies (BC2) were obtained from only three of the 44 BC1 clones crossed. The S. commersonii parent clone, LZ3.2, was the most resistant to bacterial wilt among wild species clones tested. The first sexual progenies segregated for resistance, with one clone highly susceptible and four clones highly resistant. Three highly resistant BC1 clones, CT02-4, CT08-4, and CT10b-4, were backcrossed to cultivars. Two cross combinations produced mostly susceptible BC2 progenies; however seven clones were resistant or highly resistant for both race 1 and race 3. The highly resistant three clones, CT204-3, CT206-9 and CT206-10, were selected for the further testing as cultivars or breeding materials.     

Identification of stem soft rot (Erwinia carotovora subspeciesatroseptica) resistance in potato

Development of potato cultivars resistant to Erwinia species (Erwinia carotovora subspeciesatroseptica, E. carotovora subspeciescarotovora, andErwinia chrysanthemi) causing soft rot of stems and tubers involves identification and transfer of the resistance to breeding lines possessing desirable agronomic traits. In total, 363 accessions of six wildSolanum species, three somatic hybrids and nine sexual progeny of somatic hybrids were screened for stem soft rot resistance. Thirty-one, 50, and 67 percent of accessions ofSolanum boliviense, Solanum chacoense, and Solanumsanctarosae, respectively, were resistant or highly resistant, whereas, 100, 97, and 95 percent of accessions ofSolanum canasense, Solanum tarijense, andSolanum spegazzinii were susceptible or highly susceptible, respectively. Two tuber soft-rot resistant somatic hybrids, A937 and T355-11 produced earlier by the fusion ofSolanum brevidens (PI 218228) andSolanum tuberosum (PI203900) orS. tuberosum cv. Russet Burbank, respectively, and the sexual progeny of A937 andS. tuberosum cv. Katahdin were also highly resistant. Transfer of resistance fromS. brevidens to somatic hybrid (A937) and subsequently to the progeny suggests that the resistance is simply inherited. In addition to the identification of 65 resistant or highly resistant clones of wildSolanum species, this study has identified three tuber soft rot-resistant clones that are also resistant to stem soft rot. Potato cultivars showed a range of responses to the pathogen.     

Additive genes in wheat for resistance to stripe (yellow) rust (Puccinia striiformis Westend.)

Forty-nine lines of wheat (F₆ generation) containing minor additive genes conditioning resistance to stripe (yellow) rust (Puccinia striiformis Westend.) were evaluated to several virulence types (physiologic races) of the pathogen obtained from the USA and Europe. All wheat lines showed some level of resistance to all virulence types of P. striiformis but there were differences in level of resistance between crosses and between lines within a particular cross. Generally, all lines expressed the most resistance at the seedling stage when grown at higher temperatures (15°C during the dark period; 25°C during the light period) than at 2°C (dark period) and 20°C (light period). Lines with more resistance genes showed the most resistance, and this resistance was least sensitive to temperature change. Mature plants in the field had levels of resistance equal to, or greater than, those of their seedling counterparts. One parent of each cross evaluated was either a commercial cultivar or an advanced wheat selection, and was rated as susceptible. In many cases, these susceptible parents contributed to resistance in the progeny and thus the progeny showed transgressive segregation for resistance. Some parent cultivars, which were completely susceptible as seedlings and moderately resistant as mature plants, also contributed to seedling resistance in the progeny when combined with parents known to contain minor additive genes for resistance to P. striiformis. The advantages of using minor additive genes are discussed in relation to the control of stripe rust of wheat and to crop protection generally.     

Resistance toFusarium sambucinum in wild and cultivatedSolanum species

Several species ofFusarium cause both dry rot in storage and seedpiece decay in the field, which results in severe losses in all major potato-growing regions. In temperate climates twoFusarium species are the principle causes of tuber dry rot-F.sambucinum andF. coeruleum. Resistance toF. sambucinum was identified in cultivated and wildSolanum species. Relatively high levels of resistance exist in the following 2EBN species:S. boliviense (PI#’s 265860, 310975, 310974, 545853, 545889, 545964),S. gourlayi ssp.gourlayi (PI#’s 473344, 545865),S. microdontum (PI#’s 310979, 473171),S. sancta-rosae (PI# 498392),S. kurtzianum (PI# 472930),S. fendleri ssp.fendleri (PI# 262895),S. gandarillasii (PI#’s 265866, 283076),S. oplocense (PI# 498281) andS. vidaurrei (PI# 498332). The resistance is highly heritable but appears, at least in some of the species, to be associated with recessive alleles.     

Evaluation of potato clones for resistance to tuber and stem rot induced byErwinia chrysanthemi

Tubers of 149 clones ofSolarium tuberosum subsp.andigena (andigena) from four self-pollinated (S₁) families (Jak 072, 702440, 700718 and Och 5331) obtained from true seed were rated for resistance toErwinia chrysanthemi. The clones were classified into four arbitrary categories based on disease reaction: resistant, intermediate, susceptible, and very susceptible. In family Jak 072, 23% of the clones were resistant and 61% intermediate. In family Och 5331, 22% of the clones were susceptible and 67% very susceptible; and in families 702440 and 700718, 38% and 29% of the clones were intermediate and 45% and 40% were susceptible, respectively. Tubers of 11 clones chosen to represent each one of the four categories were inoculated withE. chrysanthemi (Ech), E. carotovora subsp.carotovora (Ecc) andE. carotovora subsp.atroseptica (Eca). Clones responded similarly toEch andEca. However,Ecc induced significantly more rot thanEch orEca, presumably favored by the temperature of incubation (25°C). Clones Jak 072-17 and Jak 072-18 were resistant to the three bacterial pathogens. Above ground stems of 6 of the 11 selected clones inoculated withEch were susceptible to stem rot. No correlation was found between the response of tubers and above ground stems to tuber and stem rot induced byEch.     

杂交小麦杂种一代白粉病抗性表现规律的研究

为探讨杂交小麦杂种一代白粉病的抗性表现规律,以陕西不同地区小麦白粉病流行菌种做菌源,通过田间和苗期接种,对9份抗、感白粉病小麦亲本材料和9份大田感病品种及其杂交F1代进行抗病性鉴定,并结合SCAR和SSR分子标记进行了检测.结果表明:(1)以N95175、N9209为亲本的杂交F1含有Pm21基因,苗期和成株期抗病性均很好,与抗病亲本的抗病性表现基本一致,符合显性遗传,其抗病性主要由显性单基因控制.(2)以N9134、N9227A为亲本的杂交F1含有PmAS846基因,成株期抗病性较好,苗期大部分表现抗病;以N9227A为亲本的杂交F1中有40%的杂交组合表现中感,该抗性基因不完全符合单基因显性遗传,其表达在一定程度上受遗传背景及环境的影响.(3)中国春为苗期感病、成株期抗病材料,其与抗病亲本杂交F1代的抗病性接近或略高于抗病亲本;与感病品种杂交F1代的抗病性介于双亲之间.(4)白粉病遗传属于细胞核遗传,正反交F1代苗期抗病性无明显差异,杂交种的抗病性与双亲相关.因此, 在组配小麦强优势杂种组合时应尽量选用抗病的品种作为亲本之一,并考虑慢病性品种的应用.     

Analysis of host plant resistance to multiple Fusarium species associated with stalk rot disease in sorghum [Sorghum bicolor (L.) Moench]

Fusarium spp. is among the largest and most important pathogen groups that attack major grain crops including sorghum. The objectives of this study were to compare the virulence of Fusarium spp. associated with sorghum and examine the mode of resistance of genotypes to the disease. Eight Fusarium species – F. verticillioides, F. thapsinum, F. andiyazi, F. proliferatum, F. nyagamai, F. pseudoanthophilum, F. brevicatenulatum, and F. pseudonygamai – were studied using three sorghum genotypes under greenhouse condition. Three of these pathogens (F. verticillioides, F. thapsinum, and F. andiyazi) were selected for genetic analysis of resistance under field conditions. Sorghum genotypes with contrasting stalk rot reactions were selected for use in both the greenhouse and field experiments. Two weeks after flowering, plants were inoculated with liquid inoculum culture (5 × 10⁴ conidia ml⁻¹) of the different pathogens. Plants were harvested 4 weeks after inoculation and rated for disease severity on the basis of lesion length and number of nodes crossed by the lesion. Among the pathogens, F. thapsinum resulted in consistently higher disease scores in all genotypes under all environments. Likewise, genotype SC599 showed the greatest and most stable resistance as inbred as well as in hybrid combinations as shown by consistently high GCA for resistance to all pathogen species. We recommend that future screening exercises for Fusarium stalk rot resistance utilize F. thapsinum as the causal organism and include the resistant genotype SC599 as a control.     

Resistance of potato clones to the green peach aphid and potato leafroll virus

During 1980 and 1981 potato cultivars and breeding selections, including cultivated species and their hybrid derivatives, were evaluated for resistance to the green peach aphid (GPA),Myzus persicae (Sulzer), and to potato leafroll virus (PLRV). Criteria used were the number of aphids which colonized the clones in free choice field experiments and the number of plants derived from these experiments which showed symptoms of PLRV infection. Generally, greater resistance to GPA was found inSolarium tuberosum gp.andigena selections and hybrids than in gp.tuberosum cultivars. There were approximately fourfold differences in season-mean GPA levels among the clones tested each year. Forty-two families, representing a cross-section of the USDA breeding populations at the University of Idaho Research and Extension Center, Aberdeen, showed a similar range in colonization levels. Resistance to GPA colonization appeared to be more prevalent in gp.andigena, gp.phureja, and gp.stenotonum derivatives. There was a weak positive correlation (r² = .34, P = .01) between foliar total glycoalkaloids and season-mean GPA colonization levels for six clones representing the range of observed resistance to GPA. Resistance to GPA colonization was apparently not directly related to resistance to PLRV infection. Katahdin, for example, was relatively susceptible to GPA colonization but very resistant to PLRV infection whereas selection A69657-4 (gp.andigena) was among the most resistant to GPA colonization but among the more susceptible to PLRV infection. Breeding for resistance to GPA colonization therefore may not be as promising for PLRV control as developing PLRV resistant cultivars.     

Inheritance of mefenoxam resistance in selfed populations of the homothallic oomycetePhytophthora erythroseptica (Pethybr.), cause of pink rot of potato

The inheritance of mefenoxam resistance in the oomycetePhytophthora erythroseptica, causal organism of pink rot of potato, was investigated in two successive selfed generations of single oospore isolates. Fourteen parental isolates from three mefenoxam-sensitivity phenotypes (sensitive, intermediately resistant, and resistant) were used to produce S1 and S2 progeny isolates by selfing each isolate and germinating single oospore cultures from each isolate. The two generations of isolates were tested for mefenoxam sensitivity using the radial growth inhibition assay and calculating EC₅₀ values. Twenty-four-hundred isolates from S1 and S2 generations were tested for mefenoxam sensitivity. Results demonstrated the lack of segregation for mefenoxam sensitivity among S1 and S2 progeny isolates from the resistant and sensitive parents. The majority of the S1 progeny isolates from the parents with intermediate resistance also possessed intermediate resistance, but demonstrated substantial quantitative shifts in response to mefenoxam more often toward increased insensitivity. A few isolates from the S1 progeny were sensitive to mefenoxam, and one isolate was mefenoxam-resistant. Similarly, the majority of the S2 progeny isolates from intermediately resistant parents also possessed intermediate resistance with some quantitative shifts in sensitivity to mefenoxam similar to those observed in S1 progeny, as well as a few isolates that were sensitive to mefenoxam. These results do not support the hypothesis that resistance to mefenoxam inP. erythroseptica is controlled by a single gene exhibiting incomplete dominance. Alternative hypotheses are proposed such as the genetics of mefenoxam resistance inP. erythroseptica is probably under the control of more than one major gene and perhaps some minor genes of additive effect.     

Characterization of Fusarium spp. Causing Potato Dry Rot in China and Susceptibility Evaluation of Chinese Potato Germplasm to the Pathogen

Dry rot is a serious potato disease and causes significant losses in China. Research efforts on potato dry rot have been very limited as well as the attempts to characterize the pathogen in the major potato production regions of China. A total of 260 Fusarium isolates were identified in 698 potato tubers collected in six important potato production regions of northern China, out of which five different Fusarium species, Fusarium sambucinum, F. avenaceum, F. oxysporum, F. equiseti, and F. acuminatum, were isolated. The identification of each species was confirmed by sequencing analysis of ∼700 bp DNA fragment derived from the translation elongation factor-1 alpha gene. F. sambucinum was found to be the predominant species accounting for 56% of the isolates. Different pathogenicity was found to be associated with five most common Fusarium species. Sixty-seven clones used in China were identified as susceptible to F. sambucinum, indicating little scope for developing resistant cultivars using the currently available potato germplasm.     

Inheritance of early blight resistance from open-pollinated 4×-2× potato hybrids

Early blight, caused byAlternaria solani Sorauer, is a serious disease of potatoes that occurs in most potatogrowing regions in the world. There is little resistance to early blight among commercial potato cultivars. However, resistance to early blight in diploid (Solanum phureja-S. stenotomum) potatoes has been identified, and was found to be highly heritable and readily transferred to the tetraploid level via4×-2× crosses. The purposes of this study were to identify good levels of early blight resistance in open-pollinated4×-2× (Solanum tuberosum×S. phureja-S. stenotomum) hybrids selected for horticultural characteristics, to estimate broad-sense heritability for early blight resistance in these hybrids, and to investigate the general and specific combining ability for resistance to early blight from some of these early blight resistant tetraploid selections. Four early blight resistant clones were crossed as female parents with four different male parents in a design II mating scheme to generate 16 families. Approximately 20 randomly chosen offspring per family were visually evaluated for early blight resistance in 1995 and 1996 in a randomized complete block design in Pennsylvania. Broad-sense heritability for early blight resistance among the open-pollinated4×-2× hybrids which were originally selected for horticultural characteristics was 0.91 with a 95% confidence interval of 0.80, 0.94. There were significant differences among female and male parents for area under the disease progress curve, indicating the importance of general combining ability for early blight resistance in this germplasm. The female x male source of variation was not significant, indicating that specific combining ability was not important. The greatest number of resistant progeny were observed in families where both parents were derived from the early blight resistant population; however, at least one highly resistant progeny was produced in all families. These results suggest that the early blight resistance in these clones can be readily incorporated into the commercial tetraploid breeding population.     

Assessment of levels of resistance of cotton cultivars to isolates of Xanthomonas malvacearum (E.F. Smith) Dowson in Nigeria

Nine cotton cultivars were assessed for resistance to bacterial blight by inoculation of the mid-veins of leaves of seedlings at the four- to six-leaf stage with six isolates of Xanthomonas malvacearum (E.F. Sm.) Dowson, obtained from the three cotton-growing zones of northern Nigeria. The length of lesions 15 days after inoculation formed the basis of assessment. REBA B-50 and RASA (76)101 were highly resistant, while RSA(68)27 and BJA-592 were classified as resistant. The moderately resistant group consisted of Arkansas 52-21, Samaru 77 and Samaru 72. Samaru 71 was less susceptible than Samaru 26J, the susceptible control. There were also differences in virulence amongst the six isolates of X. malvacearum used in the test, suggesting the possible presence of races of the pathogen.     

Assessing progenies of potato for resistance to early blight

Summary Progenies from crosses between cultivars varying widely in resistance to early blight (Alternaria solani Sor.), were assessed for resistance as true seedlings in a glasshouse in Scotland. The resistance of a representative sample of surviving genotypes from each progeny was compared with samples of the same progenies not previously exposed to the fungus, both in the glasshouse in Scotland and in the field in Israel. The exposed population was more resistant. Resistance was identified more effectively in adult plants from tubers in the glasshouse than in true seedlings and agreement between glasshouse and field assessment was better when progenies were compared rather than individual genotypes. The mid parent and progeny mean scores of the unexposed population were correlated at both sites, thus confirming that the resistance is heritable. Selecting resistant individuals at the seedling stage is suggested as a useful tool for resistance breeding, having first chosen the best parents for crossing.     

Agronomic performance of potato germplasm selected for high resistance to verticillium wilt

A three-year study was conducted from 1991 through 1993 to evaluate the field performance of potato clones that had been selected for resistance to wilt symptom development and stem colonization byVerticillium dahliae. The total yield, size distribution, and specific-gravity of these highly resistant clones were compared with standard cultivars and two parent clones with high Verticillium resistance and high yield, A66107-51 and A68113-4. Two groups of Verticillium resistant germplasm were selected in 1991. One group was the highly resistant progeny from a cross between A66107-51 and A68113-4. The second group consisted of hybrids between wild species accessions with high Verticillium resistance and cultivated diploid and tetraploid germplasm. Twenty-one out of 125 progeny clones from the A66107-51 × A68113-4 cross were highly resistant to Verticillium infection and were tested in yield trials for two years. Another eight selections had high Verticillium resistance but such low yield of seed tubers that they were tested only one year for yield. Eleven of the original 29 highly resistant selections were significantly lower in total yield than Russet Burbank. Only one clone was significantly higher in yield than RB, and none outyielded either of the resistant parents. Reddale and Century Russet had moderate to high Verticillium resistance, respectively. Both outyielded Russet Burbank and were comparable with the high yielding resistant parents. Of 15 interspecific hybrids tested in 1991, only four had sufficient yield to produce seed for yield trials and good wilt resistance after three successive seasons of evaluation. We concluded from this study that selecting at early stages of variety development strictly for Verticillium resistance is likely to be inefficient. Rather, selection should be for yield and other agronomic criteria in Verticillium infested fields.     

Evidence of genetic transmission of antibiosis and antixenosis resistance of sorghum to the spotted stemborer, Chilo partellus (Lepidoptera: Pyralidae)

Spotted stemborer, Chilo partellus (Swinhoe) (Lepidoptera: Pyralidae), is the most important pest of sorghum in Asia and south and eastern Africa. Host plant resistance is an important control tactic for controlling this pest. Two breeding lines 27B × PB 15881-3 and 463B × PB 15881-3 with their parents, resistant and susceptible genotypes were evaluated in the field, glasshouse and laboratory for different resistance parameters. Breeding lines and genotypes varied significantly in foliar damage ratings, percentage of stem length tunneled, percentage of plants with deadhearts, larval survival, larval and pupal weights, larval and pupal duration, and percentage pupation and adult emergence in diets amended with leaf powder of different sorghum genotypes. The breeding lines 27B × PB 15881-3 and 463B × PB 15881-3 showed antixenosis and antibiosis to C. partellus in terms of reduced eggs per plant, larval survival and development. The levels of antixenosis and antibiosis of both breeding lines were similar to their resistant parents. Results indicate that transmission of characteristics responsible for resistance to the progeny from the resistant parent occurred.     

Horizontal resistance insolanum tuberosum to colorado potato beetle (leptinotarsa decemlineata say)

Horizontal resistance (HR) was assessed as a strategy to reduce potato defoliation by the Colorado potato beetle (CPB). Hybrids amongSolanum tuberosum varieties grown in Iowa were first evaluated with a detached leaflet feeding bioassay. Beginning in 1998, resistance was increased annually over three years by intermating and selecting the most resistant l%–2% of each generation of 300–400 plants using the bioassay. When C (= mg leaf tissue consumed_* neonate larva^-1 over 48 h) was measured for the 1% most-resistant genotypes of all generations grown side by side in 2000, it ranged from 6.1 mg_* larva^-1 in the parents to 2.3 for progeny of the F₃ generation compared to 2.7 in ND2858-1, a resistant check. Field trials in Wisconsin showed less defoliation in the F₁ and F₂ selected genotypes than in the parents. A second breeding program using moreS. tuberosum parents with greater genetic diversity, and much larger breeding populations, resulted in significantly faster accumulation of resistance, further suggesting that recurrent mass selection is effective. Foliar glycoalkaloid analyses revealed low levels of solanine and chaconine in the F₃ selections; no leptines or leptinines were detected. Because the parents wereS. tuberosum varieties in which vertical resistance to CPB is unknown, and because the resistance to feeding from parents to the F₃ resistant plants was linear and gradual while ranging from initially very low to considerably higher in the third year, horizontal resistance is strongly implicated. The development of a significant degree of resistance after three rounds of breeding using relatively small breeding populations suggests that HR can be developed from varieties ofSolanum tuberosum lacking initial high levels of resistance to CPB.     

Segregation of leptine glycoalkaloids and resistance to Colorado potato beetle (Leptinotarsa decemlineata (Say)) in F2Solanum tuberosum (4x) ×S. chacoense (4x) potato progenies

Solanum chacoense Bitter is resistant to the Colorado potato beetle (CPB),Leptinotarsa decemlineata (Say). Resistance has been associated with the presence of a rare class of glycoalkaloids, the leptines. In this study, seven tetraploid, F2S. tuberosum xS. chacoense families were evaluated for foliar production of leptines I and II, leptinines I and II, and α-solanine and α-chaconine; and screened for resistance to CPB in the laboratory and field. Resistance was correlated with the concentrations of glycoalkaloids on a family and an individual basis. Leptine concentrations ranged from undetectable to a high of 18.0 mg/g dry weight. All of the progeny produced solanine and chaconine. Family 9623 had the highest mean leptine concentration and the lowest mean leaf disk feeding and CPB defoliation levels. Family 9616 had the lowest mean glycoalkaloid concentration and ranked as one of the most susceptible families. Regression analyses of solanine + chaconine, leptine I and II, and leptinine I and II foliar concentrations versus leaf disk consumption and field defoliation revealed that only increased foliar levels of leptines resulted in decreased CPB feeding. The regression models for leptines versus leaf disk consumption and field defoliation were highly significant, accounting for 17% and 26% of the variation in consumption and defoliation, respectively. To the best of our knowledge, this is the first work reporting the impact of leptine and leptinine concentrations on CPB feeding in tetraploid,S. tuberosum xS. chacoense potato hybrids. Results are discussed within the context of breeding for resistance to CPB.