Useful Levels Of Resistance Research Articles

A Major Locus on Wheat Chromosome 7B Associated With Late-Maturity α-Amylase Encodes a Putative ent-Copalyl Diphosphate Synthase

Many wheat varieties have the potential to develop unacceptably high levels of α-amylase in the grains if exposed to a cool temperature shock or simply cool temperature during the early to middle stages of grain filling. This phenomenon is referred to as late maturity α-amylase (LMA). The enzyme persists in the grain until harvest and may result in wheat with a low Falling Number that does not meet receival and export specifications. Resistance to LMA is therefore a valuable target for wheat breeders and wheat industries in general. Genetic evidence implicating a locus on the long arm of chromosome 7B in variation in LMA phenotype was confirmed in this investigation. Through intensive fine-mapping an ent-copalyl diphosphate synthase (CPS), hitherto named LMA-1, was identified as the likely candidate gene associated with variation in LMA phenotype. Single Nucleotide Polymorphisms (SNPs) within the LMA-1 coding sequence of Chinese Spring, Maringa and Halberd result in either prematurely terminated or functionally altered proteins that are associated with useful levels of resistance to LMA. LMA-1 transcripts detected in de-embryonated grain tissue from around 15 days after anthesis, several days before the synthesis of α-amylase, were low in the resistant varieties Chinese Spring and Maringa compared with LMA susceptible genotype Spica. This was associated with a dramatic reduction in the concentrations of intermediates in the gibberellin biosynthesis pathway such as GA19, evidence that LMA-1 was functioning as CPS in the gibberellin biosynthesis pathway. A survey of a large collection of Australian and international wheat varieties distinguished 9 major haplotypes at the LMA-1 locus. Generally, within classes, there was notable variation for LMA phenotype and evidence for genotypes whose resistance is presumed to be due to genetic loci located elsewhere on the wheat genome. Further investigation is required to characterize the sequence of steps between LMA-1 and α-amylase synthesis as well as to gain a better understanding of the role and potential impact of other genetic loci. Diagnostic markers for sources of resistance and SNP variation reported in this study should assist breeders to deploy resistance associated with LMA-1 variants in breeding programs.

Evaluation of XL370A-Derived Maize Germplasm for Resistance to Leaf Feeding by Fall Armyworm1

The fall armyworm, Spodoptera frugiperda (J.E. Smith) (Lepidoptera: Noctuidae), is an economically important insect with larvae damaging maize (Zea mays L.) leaves and ear tissue. The pest has become resistant to several classes of insecticide and Bt-maize grown in some geographical areas. Once discovered and characterized, native sources of maize resistance to this pest could be effectively integrated with existing control tactics. The objective for this study was to test experimental lines derived from maize germplasm XL370A for resistance to leaf feeding by fall armyworm. Plants were grown in the field in 2018 and 2019, artificially infested with fall armyworm, and leaf damage scores recorded. Average 14-day scores for experimental maize lines GEMN-0095 (5.8), GEMN-0096 (5.7), and GEMN-0133 (5.6) were moderately resistant and 7- and 14-day scores for these entries were not significantly different across both years. Cuba 94 was not significantly different from the three entries with the exception of having greater 7-day damage scores in 2019. GEMN-0048 was not resistant but variability was observed in 14-day scores between 4 (resistant) and 8 (susceptible) in individual plants. The experimental lines are adapted for growth in temperate regions and might provide maize breeding programs with useful levels of resistance to fall armyworm.

Valuable New Leaf or Inflorescence Resistances Ensure Improved Management of White Rust (Albugo candida) in Mustard (Brassica juncea) Crops

AbstractField resistances/susceptibilities against Albugo candida race 2V were determined for 29 Indian Brassica juncea varieties and compared with resistant varieties from China (6) and Australia (7). ‘Basanti’ (AUDPC incidence 46.7; AUDPC severity 29.2) represents the first high‐level resistance to race 2V in Indian varieties. Several others showed lower but still useful levels of resistance, including Narendra Ageti Rai‐4 (AUDPC incidence 150.6; AUDPC severity 66.8) and JM1 (AUDPC incidence 167.1; AUDPC severity 83.7). Highly susceptible Indian varieties had AUDPC incidence values >200 and severity >100. ‘Basanti’ had least stagheads/plant (0.32), while Narendra Ageti Rai‐4 had lowest % plants with stagheads (2.48). In contrast, almost half of Indian varieties had stagheads/plant >1 and % plants with stagheads >4, and >26 for ‘Kranti’. The resistance in ‘Basanti’ paves the way forward towards significantly improved white rust management in mustard in India. JM06011, JM06021, JR049 from Australia and CJB‐003 from China had zero leaf incidence. There were significant (P < 0.001) relationships between disease incidence with severity (R2 0.92), stagheads/plant (R2 0.69) and also % plants with stagheads (R2 0.60); between disease severity with stagheads/plant (R2 0.68) and also % plants with stagheads (R2 0.69); and between stagheads/plant with % plants with stagheads (R2 0.59).

Identification and mapping of quantitative trait loci for leaf rust resistance derived from a tetraploid wheat Triticum dicoccum accession

The Triticum turgidum ssp. dicoccum (2n = 4x = 28) accession MG5323 showed a useful level of resistance to leaf rust disease. A segregating population of 110 recombinant inbred lines (RILs), derived from a cross between cv Latino (T. turgidum spp. durum), susceptible to leaf rust, and MG5323 was evaluated for reactions of seedlings to two different Puccinia triticina isolates. Genotyping of the RILs was performed with different molecular markers (SSR, EST-SSR and SNP), leading to the construction of a linkage map containing 10,840 loci covering 14 chromosomes, with an average marker density of 0.22 cM/marker. Linkage analysis allowed the identification of three different regions significantly associated with leaf rust resistance, with MG5323 contributing the resistant alleles. A major resistance gene was detected on the short arm of chromosome 1B, explaining a total phenotypic variation ranging from 41.37 to 49.51 %. Two additional minor resistance genes located on chromosome 7B explained a phenotypic variation ranging between 17.77 and 25.81 %. No obvious positional relationships were observed when the map position of the genes was compared with those of other previously identified wheat leaf rust resistance genes, suggesting that new resistance sources to leaf rust were identified in the tetraploid background. A significant positive epistatic effect was detected between quantitative trait loci (QTLs) for each trait, indicating that different QTLs contribute different degrees of resistance. Analysis of the leaf rust responses of the RILs demonstrated that only lines bearing resistant alleles at both loci showed effective leaf rust resistance, indicating that the genes identified behave as complementary genes.

Estrategias para seleccionar frijol común con mayor resistencia a mustia hilachosa.

Higher levels of web blightresistance is needed to increase common bean yield and seedquality in the hot and humid regions of Central America andthe Caribbean. Only moderate levels of resistance have beenidentified in common bean. Moreover, virulence patterns ofthe web- blight pathogen, Thanatephorus cucumeris (Frank)Donk, vary among geographic locations. Fields screening forweb-blight resistance is limited to seasons that favor thedevelopment of the disease. In the field, it is also difficult toseparate disease avoidance due to plant architecture andphysiological resistance. Greenhouse and laboratorytechniques have been developed that facilitate the screeningof bean lines for physiological resistance. A recurrentselection scheme is proposed to select beans with higherlevels of web-blight resistance. Inter-specific crosses may beused to broaden the genetic base of resistance to web-blight.P. coccineus lines with useful levels of resistance have beenidentified. Transgenic bean plants with herbicide resistancewould be useful for minimum tillage. A cover of deadvegetation on the soil surface could help to reduce the spreadof web-blight.

Resistance to Lettuce Aphid (Nasonovia ribisnigri) Biotype 0 in Wild Lettuce Accessions PI 491093 and PI 274378

The lettuce aphid, Nasonovia ribisnigri Mosley (Hemiptera: Aphididae), is a major insect pest of lettuce, Lactuca sativa L, in many commercial lettuce production areas around the world. Resistance to lettuce aphid biotype 0 (Nr:0) was first reported in Lactuca virosa L. accession IVT 280 and characterized as complete, i.e., virtually no aphids survived, and genetically dominant to partial resistance in L. virosa accession IVT 273. Complete and partial resistances to Nr:0 were conditioned by two alleles, Nr (complete resistance) and nr (partial resistance), but the genetic relationship to susceptibility was not reported. We previously reported two new potential sources of unique genes for resistance to Nr:0 in Lactuca serriola L. accession PI 491093 and L. virosa PI 274378. We report on the genetic and phenotypic nature of resistance to Nr:0 in these two wild lettuce accessions. Resistance to Nr:0 in PI 274378 is complete and allelic to complete resistance in IVT 280. Resistance to Nr:0 in PI 491093 was partial, recessive to complete resistance in ‘Barcelona’ that was derived from IVT 280, but dominant to susceptibility in ‘Salinas’. We propose the revised gene symbols for resistance to Nr:0: Nr:0C for complete resistance and Nr:0P for partial resistance, which was originally designated as nr but may now be regarded as the symbol for susceptibility to all strains of lettuce aphid. The dominance relationships among these three alleles are Nr:0C (in IVT 280, ‘Barcelona’) > Nr:0P (in PI 491093) > nr (in susceptible genotypes). Expression of partial resistance in PI 491093 was variable in controlled infestation tests, but in a naturally infested field test provided a potentially useful level of resistance to Nr:0. Partial resistance, where complete resistance has not been widely deployed, may either alone or as a component of integrated pest management delay or prevent emergence of genotypes that overcome complete resistance controlled by Nr:0C.

Screening of pepper accessions for resistance against two thrips species (Frankliniella occidentalis and Thrips parvispinus)

Thrips are damaging pests in pepper worldwide. They can cause damage directly by feeding on leaves, fruits or flowers, and also indirectly by transferring viruses, especially tomato spotted wilt virus (TSWV). Although thrips are among the most damaging pests in pepper, until now there is no commercial variety with a useful level of resistance to thrips. This is at least partly due to the lack of knowledge on resistance levels in pepper germplasm of QTLs and/or genes for resistance, and of information about resistance mechanisms to thrips in pepper. This paper describes our research aimed at developing practical and reliable screening methods for thrips resistance in pepper and at identifying pepper accessions showing a strong resistance to thrips. Thirty-two pepper accessions from four species of pepper (Capsicum annuum, C. baccatum, C. chinense and C. frutescens) and two species of thrips (Frankliniella occidentalis and Thrips parvispinus) were used in this study. Our results indicate that the laboratory based leaf disc test and the detached leaf test can be used as reliable screening methods for thrips resistance in pepper. We observed a large variation for resistance to thrips in Capsicum that can be exploited in breeding programs.

Morphological and molecular characterization of melon accessions resistant to Fusarium wilts

Fusarium wilt incited by Fusarium oxysporum f. sp. melonis (F.o.m) is one of the most widespread and devastating melon diseases. While resistance to physiological races 0, 1, and 2 is relatively frequent in different botanical varieties, sources of resistance to race 1,2 are restricted to a few Far-Eastern accessions. In this work, the results of a screening for resistance to F.o.m. race 1,2 among 32 accessions are presented. Three Japanese accessions (‘Kogane Nashi Makuwa’, ‘C-211’, and ‘C-40’) showed the highest resistance levels, but useful levels of resistance were also detected in one Russian ‘C-160’ and two Spanish (‘C-300’ and ‘Mollerusa-7’) accessions. These resistant materials, together with other accessions previously described as resistant to F.o.m. races 0, 1, and/or 2 have been morphologically and molecularly characterized. Based on cluster analysis, these accessions have been grouped according to the botanical subspecies they belong to. Assessment of genetic diversity indicated that the resistant accessions to races 0, 1 and 2, are scattered along the established clusters. On the other hand, high levels of resistance to the race 1,2 could be found only among accessions belonging to Cucumis melo subsp. agrestis, nevertheless, a certain degree of resistance to this race could also be found within some accessions belonging to subsp. melo. As far as we know, this is the first report of resistance to F.o.m race 1,2 found out from the Far-Eastern melon material. Based on fruits characteristics, it appears that several inodurus and cantalupensis accessions could be exploited in breeding programs as resistance sources to F.o.m races 0, 1 and/or 2 for the improvement of these melon types. The accessions with the highest levels of resistance to the race 1,2 appeared to be very distant both molecularly and morphologically from the commercial types. Nevertheless ‘C-160’, ‘C-300’, and ‘Mollerusa-7’ classified as var. inodorus are morphologically very similar to the Spanish commercial types and might be used as resistant sources in breeding these melon types.

Complete resistance to leaf and staghead disease in AustralianBrassica junceagermplasm exposed to infection byAlbugo candida(white rust)

White rust (Albugo candida) can be a highly destructive disease of Brassica juncea. Identification ofnewsources of host resistance to A. candida is a key priority in Australia for the B. juncea breeding program.Afew genotypes from China and Australia have been recently identified with useful levels of resistance to A. candida. In this study, a line of B. juncea (JM06011) from Australia showed complete resistance to white rust disease during the whole growing season and overall performed better than three Chinese genotypes (CBJ-001, CBJ-003, CBJ-004) and one other Australian genotype (JR049) that were previously reported to be promising and that were used as resistant controls. JM06011 showed no symptoms of the disease on leaves and a total absence of stagheads, and this is the first report of complete resistance to white rust among Australian B. juncea germplasm. More Australian genotypes, JM06010, JM06021, JM06004 and JM06013, showed a lower but still useful level of resistance to A. candida based on both incidence and severity of disease on leaves when screened under the natural environmental conditions existing in Western Australia. Overall, genotypes from China were relatively more susceptible than those from Australia with the exception of the three resistant controls. There was no relationship between foliar disease and staghead incidence across the genotypes tested.

A Novel Source of Resistance in Wheat to Pyrenophora tritici-repentis Race 1.

Tan spot, caused by the fungus Pyrenophora tritici-repentis, causes serious yield losses in wheat (Triticum aestivum) and many other grasses. Race 1 of the fungus, which produces the necrosis toxin Ptr ToxA and the chlorosis toxin Ptr ToxC, is the most prevalent race in the Great Plains of the United States. Wheat genotypes with useful levels of resistance to race 1 have been deployed, but this resistance reduces damage by only 50 to 75%. Therefore, new sources of resistance to P. tritici-repentis are needed. Recombinant inbred lines developed from a cross between the Indian spring wheat cvs. WH542 (resistant) and HD29 (moderately susceptible) were evaluated for reaction to race 1 of the fungus. Composite interval mapping revealed quantitative trait loci (QTL) on the short arm of chromosome 3A explaining 23% of the phenotypic variation, and the long arm of chromosome 5B explaining 27% of the variation. Both resistance alleles were contributed by the WH542 parent. The QTL on 5BL is probably tsn1, which was described previously. The 3AS QTL (QTs.ksu-3AS) on 3AS is a novel QTL for resistance to P. tritici-repentis race 1. The QTL region is located in the most distal bin of chromosome 3AS in a 2.2-centimorgan marker interval. Flanking markers Xbarc45 and Xbarc86 are suitable for marker-assisted selection for tan spot resistance.

Resistance to Hessian fly (Diptera: Cecidomyiidae) in a Canadian spring wheat cultivar

AbstractThe Canadian spring wheat (Triticum aestivum L.; Poaceae) cultivar ‘Superb’ was less susceptible to damage by Hessian fly, Mayetiola destructor (Say), than the spring wheat cultivars ‘AC Barrie’, ‘AC Foremost’, ‘McKenzie’, ‘AC Domain’, and ‘Glenlea’ in Manitoba. The partial resistance of ‘Superb’ was similar, at the seedling stage, to that of ‘Guard’, which possesses the resistance gene H18. Females laid eggs readily on all cultivars, providing no evidence for antixenosis, but few larvae developed on seedlings of ‘Superb’ and ‘Guard’, showing that antibiosis against larvae is the mechanism of resistance in these seedlings. In the field, where infestation of spring wheat takes place about 4 weeks after the seedling stage, ‘Guard’ continued to show high levels of resistance, but ‘Superb’ was less resistant, although still more resistant than highly susceptible cultivars. Infested stems of ‘Superb’ and ‘Nordic’ were less likely to break than infested stems of other cultivars, showing that these two cultivars are partially tolerant to infestation. Infested stems of ‘Guard’ and other cultivars showed high levels of stem breakage and are intolerant. Yield losses due to infestation by Hessian fly were mostly caused by the breakage and falling over of infested stems, which prevented the seeds on these stems from being harvested. Infested stems of all susceptible cultivars that remained standing at harvest had lower seed masses and fewer seeds per spike than uninfested stems, which contributed to yield loss. ‘Grandin’, a parent of ‘Superb’, is the probable source of resistance in ‘Superb’, but the pedigree of ‘Grandin’ provides no clue as to the gene(s) involved. The partial antibiosis and tolerance expressed by ‘Superb’ is sufficient to reduce losses to Hessian fly by 65% in comparison with a susceptible cultivar such as ‘AC Barrie’. ‘Superb’ is the first Canadian spring wheat cultivar identified to have an agronomically useful level of resistance to Hessian fly.

EVALUATION OF TOMATO GERMPLASM AND SELECTION OF BREEDING LINES FOR RESISTANCE TO BEGOMOVIRUSES IN GUATEMALA

EVALUATION OF TOMATO GERMPLASM AND SELECTION OF BREEDING LINES FOR RESISTANCE TO BEGOMOVIRUSES IN GUATEMALA

Trichomes of Lycopersicon species and their hybrids: effects on pests and natural enemies

Abstract1 The cultivated tomato, Lycopersicon esculentum, is an economically important worldwide crop. Current pest management techniques rely heavily on pesticides but trichome‐based host‐plant resistance may reduce pesticide use.2 A review of the literature is provided on trichomes of wild Lycopersicon species and the effects of trichome‐based host‐plant resistance on arthropods. Solvents have been used to remove glandular trichome exudates and the resulting dimminution of their effects quantified. Correlational approaches to assess the relationship between the different trichome types and effects on pests have also been used.3 Most studies have focused on Lepidoptera and Hemiptera, although some work has included Coleoptera, Diptera and Acarina, and both antibiotic and antixenotic effects have been demonstrated.4 Natural enemies are a cornerstone of international pest management and this review discusses how the compatibility of this approach with trichome‐based host‐plant resistance is uncertain because of the reported negative effects of trichomes on one dipteran, one hemipteran and several Hymenoptera.5 For trichome‐based host‐plant resistance to be utilized as a pest management tool, trichomes of wild species need to be introgressed into the cultivated tomato. Hybrids between the cultivated tomato and the wild species Lycopersicon hirsutum f. glabratum, Lycopersicon pennellii and Lycopersicon cheesmanii f. minor have been produced and useful levels of resistance to Acarina, Diptera and Hemiptera pests have been exhibited, although these effects may be tempered by effects on natural enemies.6 This review proposes that studies on genetic links between fruit quality and resistance, field studies to determine the compatibility of natural enemies and trichome‐based host‐plant resistance, and a strong focus on L. cheesmanii f. minor, are all priorities for further research that will help realize the potential of this natural defence mechanism in pest management.

New generation maize inbred lines resistant to diplodia ear rot, caused by Stenocarpella maydis (Berk) Sutton

In an attempt to obtain superior levels of resistance to Stenocarpella maydis ear rot, three elite maize inbred lines (DO620Y, E739 and B37) from different genetic backgrounds that had previously been shown to have useful levels of resistance were crossed in all combinations. Additionally, two susceptible experimental maize populations were crossed to a resistant experimental inbred line as donor parent. Progeny in each of the five populations was selfed and selected for resistance over five successive generations, following artificial inoculation of plants at the localities Potchefstroom, Cedara and Hazyview. The five populations progressed from the S1 during 1997/98 to the S5 during 2000/01. A progressive improvement in resistance occurred over seasons in all progenies. After three cycles of selection, approximately 50% of the selections showed less than 10% infected ears, with a total of 84 selections showing no infection. Selections displaying the lowest disease incidence at any one locality were not necessarily the most resistant at another locality, possibly owing to significant pathogen × environment interactions. It is therefore possible that resistance in new generation inbred lines may be more stable than in the donor parents. Using I137TN as tester, a number of resistant selections were identified during 2001/02 that had combining ability for yield that compared favourably with commercial standards.

Cultivar and crop management influences on fusarium head blight and mycotoxins in spring wheat (Triticum aestivum) in New Zealand

Cultivar and crop management influences on fusarium head blight (FHB) of wheat (Triticum aestivum L.) were investigated in cultivar field trials and commercial wheat crops in the North Island of New Zealand over two growing seasons. There were consistent differences between cultivars in their susceptibility to FHB. The Chinese wheat ‘Nanjing’ had the lowest level of FHB, mycotoxins, and Fusarium infection in grain. Although no New Zealand cultivars approached an equivalent level of resistance, FHB in some cultivars was low in most situations, and these cultivars had a useful level of resistance. FHB and mycotoxin levels varied widely between crops surveyed. Two Fusarium mycotoxins, deoxynivalenol (DON) and nivalenol (NIV), were detected in grain samples from crops and trials. Overall, DON levels were higher than NIV in crops in both years. FHB incidence and levels of Fusarium infection and mycotoxins in grain were closely related in samples from a particular crop, but the relationships were much less apparent between crops. F. graminearum predominated in grain samples, although F. avenaceum, F. culmorum, and F. poae were also common. Highest levels of F. graminearum were recorded in grain samples from crops that followed maize, whereas F. avenaceum and F. poae were more common in samples from crops that did not follow maize.

Fusarium Head Blight Reaction of Langdon Durum‐Triticum dicoccoides Chromosome Substitution Lines

Fusarium head blight (FHB), caused by Fusarium graminearum Schwabe, is a serious disease problem on durum wheat (Triticum turgidum L. var. durum) in the USA. To date, the resistance to FHB available in hexaploid wheat (T. aestivum L.) sources has not been transferred successfully to tetraploid durum. In the 1980s, USDA geneticist L.R. Joppa produced a set of disomic lines [LDN(DIC)] derived from wild emmer (T. turgidum L. var. dicoccoides). Each line had a different pair of chromosomes from T. dicoccoides substituted for the corresponding ‘Langdon’ durum chromosomes. The purpose of this study was to determine if any of the LDN(DIC) lines showed useful levels of resistance to FHB. We tested these lines for FHB response by inoculation with F. graminearum in the greenhouse. Inoculation was accomplished via the single spikelet method, in which a droplet of conidiospore suspension is placed into one spikelet per spike followed by mist–high humidity for 3 d to establish infection. After 21 d, spikes were scored for extent of FHB symptoms and the average disease severity of the lines compared. A low disease severity score in this test indicated the presence of resistance to FHB in that line. Each test was replicated and the tests were done five times. In each test, some lines differed significantly. One line, LDN(DIC‐3A), was consistently less susceptible and another line, LDN(DIC‐2A), was consistently more susceptible than the parent Langdon durum, which itself showed an intermediate FHB reaction. Several other LDN(DIC) lines showed a trend either for increased or reduced susceptibility to FHB. Since each line differs by a single chromosome pair, the results suggest that genes affecting FHB resistance are present on several different T. dicoccoides or Langdon durum chromosomes.

Mapping of four major rice blast resistance genes from ’Lemont’ and ’Teqing’ and evaluation of their combinatorial effect for field resistance

A framework linkage map was developed using 284 F10 recombinant inbred lines (RILs) from a ’Lemont’×’Teqing’ rice cultivar cross. Evaluation of a subset of 245 of these RILs with five races of the rice blast pathogen permitted RFLP mapping of three major resistance genes from Teqing and one major gene from Lemont. All mapped genes were found to confer resistance to at least two blast races, but none conferred resistance to all five races evaluated. RFLP mapping showed that the three resistance genes from Teqing, designated Pi-tq5, Pi-tq1 and Pi-tq6, were present on chromosomes 2, 6 and 12, respectively. The resistance gene from Lemont, Pi-lm2, was located on chromosome 11. Pi-tq1 is considered a new gene, based on its reaction to these five races and its unique map location, while the other three genes may be allelic with previously reported genes. Lines with different gene combinations were evaluated for disease reaction in field plots. Some gene combinations showed both direct effects and non-linear interaction. The fact that some of the lines without any of the four tagged genes exhibited useful levels of resistance in the field plots suggests the presence of additional genes or QTLs affecting the blast reaction segregating in this population.

Influence of pollen selection for Alternaria helianthi resistance on the progeny performance against leaf blight in sunflower (Helianthus annuus L.)

Seven genotypes of sunflower, including populations and hybrids, showing differential susceptibility to Alternaria leaf and stem blight were crossed to CMS 234A with pollen selection and without pollen selection. The pathogen culture filtrate was used as selective pressure at stylar tissue by applying it 1 h before pollination. Distilled water applied to stigmas and styles served as control. Two sets of seven hybrids, one set from selective and the other from non-selective fertilization, were evaluated for reaction to Alternaria leaf and stem blight during the rainy season under natural epiphytotic conditions. Selection for resistant pollen on the stigmatic surface resulted in a corresponding increase in progeny resistance. The study demonstrates the transmission of the selected trait from the pollen generation to progeny. Further, it was observed that the effect of pollen selection was high in the progenies of moderately resistant parents compared to progenies of highly susceptible parents. The effect of successive pollen selection was studied by backcrossing the progeny derived through selective fertilization to the fertile parent using selective fertilization. Successive pollen selection further improved disease resistance of progeny. However, the improvement was not very great. Hence, repeated cycles of selection are required to achieve a useful level of resistance in the case of sunflower, since resistance is polygenetically controlled.

Association of Fusarium Head Blight Resistance with Gliadin Loci in a Winter Wheat Cross

Fusarium head blight (FHB) is one of the major diseases of wheat (Triticum aestivum L.). Genetic control of resistance to Fusarium spp. has been studied in resistant Chinese spring wheat cultivars, but little is known about inheritance of lower but useful levels of resistance found in winter wheat. This study was undertaken to characterize the genetic control of FHB resistance and to identify possible associations of resistance with several marker loci in winter wheat. Recombinant inbred lines were derived from the cross of a susceptible winter wheat, F1054W, with a moderately resistant parent, Sincron, not related to the Chinese germplasm. The parents had different alleles at five marker loci, RhtB1, Rht8, W2, GliB1, and GliD1. Recombinant inbred lines were classified for alleles at the RhtB1 and Rht8 loci by plant height and seedling response to exogenous gibberellic acid. Alleles at the GliB1 and GliD1 loci were determined from the gliadin spectrum detected by starch gel electrophoresis and alleles at the W2 locus by phenotypes in the field. The area under disease progress curve (AUDPC) and relative weight of inoculated spikes (RWIS), following injection of Fusarium graminearum Schwabe inoculum into central florets of flowering spikes, were determined for 3 yr in the field. A significant increase in resistance was associated with the allele GliR1, suggesting location of a Fusarium head blight resistance quantitative trait locus (QTL) on chromosome T1BL.1RS. A smaller but significant increase in resistance was associated with allele GliD1b, indicating the presence of another QTL on chromosome 1D. No association between Fusarium resistance and RhtB1, Rht8, or W2 was found. The effects of favorable alleles on chromosomes 1B and 1D were cumulative. Selection for genotypes possessing GliR1 and GliD1b in this population would increase the probability of obtaining lines with higher resistance to Fusarium head blight.

Sprout Damage and Preharvest Sprout Resistance in Hard White Winter Wheat

Increased production of hard white winter (HWW) wheat, Triticum aestivum L., in the Great Plains introduces the possibility of preharvest sprouting (PHS) that is often not expressed among widely grown hard red winter (HRW) cultivars. Field experiments were conducted to learn the severity of sprouting likely to occur in contemporary HWW wheat. Specific objectives were to (i) determine the level of sprout damage in Great Plains environments for HWW and HRW genotypes, (ii) assess the level of PHS resistance among the two classes, and (iii) determine the relative merits of germination tests and spike‐wetting treatments. Twenty‐four HWW experimental lines, three HWW cultivars, and five HRW cultivars were evaluated at Stillwater and Lahoma, OK, in 1995 and 1996. Differences in sprout damage measured at two harvest dates were clearly evident between HRW and HWW entries. Pre‐harvest sprout resistance measured at physiological maturity varied widely among HWW lines. No line was as resistant as Plainsman V, a HRW cultivar previously not known to possess high PHS resistance. Two lines were equivalent to the most resistant HWW cultivar, Rio Blanco. Major genes for red kernel color did confer some protection from pre‐harvest sprouting, but their absence did not preclude useful levels of resistance. Chaff tissue played a minor role in influencing genetic differences in resistance. Germination percent at physiological maturity was strongly correlated with field measurements of sprout damage and showed high repeatability. Future improvement of PHS resistance is needed in HWW wheat and should emphasize this component.