Evidence for single gene resistance in apple to brownheaded leafroller, Ctenopseustis obliquana, and implications for resistance to other New Zealand leafrollers

The survival and development rate of Ctenopseustis obliquana (Walker) (Lepidoptera: Tortricidae) larvae, and weight of pupae were measured on detached mature leaves of two apple (Malus domestica Borkh.) progenies derived from the resistant (R) parent ‘Prima’ crossed with the susceptible (S) cultivars ‘Liberty’ (n = 44) and ‘Red Delicious’ (n = 35). The R:S ratio in both these modified backcross families did not differ significantly from the 1:1 expected in the case of monogenic resistance, carried in a heterozygous condition in the resistant parent. The survival to pupation on the individual seedlings was either zero/very low (R) or high (S). With all resistant seedlings being heterozygous, this indicates that the resistance allele shows complete dominance over the susceptible allele. We have named this putative gene Cob1. The expression of the resistance was found to be influenced by both the colony of C. obliquana used and the time of the season when resistance was assessed. In a separate experiment with another tortricid, there was no survival of Planotortrix octoDugdale larvae on the apple cultivar ‘Prima’ and high survival on the cultivars ‘Liberty’ and ‘Red Delicious’. The similarity of the responses of the two leafroller species to these cultivars, and other published evidence concerning Planotortrix excessana (Walker) and Ctenopseustis herana(Felder and Rogerhofer), suggest that the resistance discovered to C. obliquana may be effective against all four endemic tortricid species. The implications of these findings for apple breeding and leafroller control in New Zealand are discussed.     

Oligogenic inheritance for resistance to Zucchini yellow mosaic virus in Cucurbita pepo

‘True French’ is an open‐pollinated cultivar of the Zucchini (Courgette) Group of Cucurbita pepo and is susceptible to Zucchini yellow mosaic virus (ZYMV). Using C. moschata‘Menina’ as the source of ZYMV resistance and following six generations of backcrossing, a true‐breeding line nearly isogenic to ‘True French’, designated 381e, was recovered that carried ZYMV resistance, albeit not at as high a level as in ‘Menina’. ‘True French’ and accession 381e were crossed, and their reciprocal F₁, F², and backcross progenies were grown in a chamber and inoculated with a highly virulent, non‐aphid‐transmissible strain of ZYMV. Nearly all F¹ plants and all plants of the backcross to 381e were classified as resistant. Segregation to resistant and susceptible individuals occurred in the backcross to the susceptible parent, in accordance with a 3:5 three‐gene ratio of resistant: susceptible. The F₂ segregated in accordance with a ratio of 45 resistant : 19 susceptible, which would be obtained if there was one major gene for resistance, Zym‐1 (Zym), and two other genes, herein designated Zym‐2 and Zym‐3, both of which for complementary to Zym‐1. The presence of Zym‐1 and either Zym‐2 or Zym‐3 is necessary for resistance to be expressed in young plants, but the presence of all three might be necessary for resistance to continue to be expressed during subsequent development of the plants. Evidently, Zym‐2 and Zym‐3 are ubiquitous in C. moschata but their susceptible alleles are much more common in C. pepo. As the level of resistance of 381e to ZYMV is not as high as that of C. moschata‘Menina’, additional, as yet unidentified, genes must be involved in conferring high resistance to this virus.     

Stacking three late blight resistance genes from wild species directly into African highland potato varieties confers complete field resistance to local blight races

Considered responsible for one million deaths in Ireland and widespread famine in the European continent during the 1840s, late blight, caused by Phytophthora infestans, remains the most devastating disease of potato (Solanum tuberosum L.) with about 15%–30% annual yield loss in sub‐Saharan Africa, affecting mainly smallholder farmers. We show here that the transfer of three resistance (R) genes from wild relatives [RB, Rpi‐blb2 from Solanum bulbocastanum and Rpi‐vnt1.1 from S. venturii] into potato provided complete resistance in the field over several seasons. We observed that the stacking of the three R genes produced a high frequency of transgenic events with resistance to late blight. In the field, 13 resistant transgenic events with the 3R‐gene stack from the potato varieties ‘Desiree’ and ‘Victoria’ grew normally without showing pathogen damage and without any fungicide spray, whereas their non‐transgenic equivalent varieties were rapidly killed. Characteristics of the local pathogen population suggest that the resistance to late blight may be long‐lasting because it has low diversity, and essentially consists of the single lineage, 2_A1, which expresses the cognate avirulence effector genes. Yields of two transgenic events from ‘Desiree’ and ‘Victoria’ grown without fungicide to reflect small‐scale farm holders were estimated to be 29 and 45 t/ha respectively. This represents a three to four‐fold increase over the national average. Thus, these late blight resistant potato varieties, which are the farmers’ preferred varieties, could be rapidly adopted and bring significant income to smallholder farmers in sub‐Saharan Africa.     

Morphology and discrimination features of pollen from Italian olive cultivars (Olea europaea L.)

Pollen morphology of 14 cultivars of Olea europaea subsp. europaea var. europaea was analysed in order to discriminate main pollen types. The cultivars were selected from the most spread and early flowering crops grown in Italy. Morphometric parameters were observed on acetolysed pollen by means of light microscopy and scanning electron microscopy. Polar axis (P), equatorial diameter (E), P/E ratio, maximum distance between colpi in mesocolpium, distance between the apices of two colpi, exine thickness, maximum length of lumina in mesocolpium and in apocolpium, and exine reticulum thickness in mesocolpium have been measured. According to P and E, the 14 olive cultivars of this study can be divided into the three groups of small (P: 21.75 µm, E: 22.55 µm; ‘Manna’ and ‘Tonda di Cagliari’), large (P: 25.1 µm, E: 26.1 µm; ‘Pescarese’ and ‘Rotondella di Sanza’) and medium size (P: 23.49 µm, E: 24.54 µm, ‘Carolea’, ‘Grossa di Cassano’, ‘Giarraffa’, ‘Nocellara messinese’, ‘Nocellara del Belice’, ‘Santagatese’, ‘Intosso’, ‘Maiatica di Ferrandina’, ‘Nostrale di Fiano Romano’, ‘Santa Caterina’). Maximum length of lumina and exine thickness are useful parameters for further distinction of olive pollen groups, since these parameters are able to provide a specific pollen profile for each cultivar.     

Inheritance of resistance to lettuce root aphid in the lettuce cultivars 'Avoncrisp' and 'Lakeland"

The inheritance of resistance in two lettuce cultivars to lettuce root aphid, Pemphigus bursarius, was studied in a series of laboratory and field experiments at Wellesbourne between 1989 and 1992. A source of total resistance in the cv. ‘Avoncrisp’ which is linked to the downy mildew resistance gene Dm6, was shown to be governed by a single dominant gene. There were no maternal effects evident in the inheritance of this resistance. The basis of the high level of resistance which exists in the cv. ‘Lakeland’ (formerly known as ‘Jubilee’) was also shown to be controlled by the same dominant gene. The linkage between Dm6 and root aphid resistance was broken in ‘Lakeland’ as this cultivar does not possess the Dm6 gene. The linkage was presumably broken when the original cross between the parents of cv. ‘Lakeland’, ‘Calmar’ and ‘Avoncrisp’ was made. Under laboratory conditions small numbers of aphids commence development on cv. ‘Lakeland’ but colonies fail to develop and under field conditions the resistance provides adequate Protectión against the pest. The resistance in both ‘Avoncrisp’ and ‘Lakeland’ was effective against a population of lettuce root aphid collected from an endive crop in southern France as well as being effective against the Wellesbourne population of this aphid.     

Non-specific resistance in brown planthopper resistant indica rice varieties against Plodia interpunctella (Lepidoptera: Phycitidae)

Mechanisms of host plant resistance against insect pests can be manifold. Resistance screenings generally use single target insect pests, but the resistance thus screened may not always be specific to the target insect species. We conducted a test for non‐specific resistance in indica rice varieties with resistance genes against brown planthopper (BPH), by using the Indian meal moth, Plodia interpunctella. The test system was very simple, and only required the non‐pest moth to be reared on rice flour. We compared the survival rate, developmental period and adult weight of the moth on three rice varieties: ‘Nipponbare’, a BPH‐susceptible japonica variety, and ‘Thai Collection 11’ and ‘Pokkali’, two resistant indica varieties. Our results were straightforward and demonstrate that resistance in the two resistant rice varieties is not BPH specific, because development of the moth was retarded and adult body weight was reduced.     

Role of enterotoxigenic Escherichia coli prophage in spreading antibiotic resistance in a porcine-derived environment

Enterotoxigenic Escherichia coli (ETEC) cause acute secretory diarrhoea in pigs, posing a great economic loss to the swine industry. This study analysed the prevalence and genetic characteristics of prophages from 132 ETEC isolates from symptomatic pigs to determine their potential for spreading antibiotic resistance. A total of 1105 potential prophages were identified, and the distribution of the genome size showed three ‘overlapping’ trends. Similarity matrix comparison showed that prophages correlated with the ETEC lineage distribution, and further identification of these prophages corroborated the lineage specificity. In total, 1206 antibiotic resistance genes (ARGs) of 52 different categories were identified in 132 ETEC strains; among these, 2.65% (32/1206) of ARGs were found to be carried by prophages. Analysis of flanking sequences showed that almost all the ARGs could be grouped into two types: ‘bla_TEM-1B’ and ‘classic class 1 integron (IntI1)’. They co-occurred with a strictly conserved recombinase and transposon Tn3 family but with a difference: the ‘bla_TEM-1B type’ prophages exhibited a classic Tn2 transposon structure with 100% sequence identity, whereas the ‘IntI1 type’ co-occurred with the TnAs2 transposon with only 84% sequence identity. These results imply that ARGs might be pervasive in natural bacterial populations through transmission by transposable bacteriophages.     

Inheritance of partial resistance to the leaf aphids Macrosiphum euphorbiae and Uroleucon sonchi in lettuce

The genetics of partial resistance (PR) of lettuce to the leaf aphid species Macrosiphum euphorbiae and Uroleucon sonchi was studied in F₁ and F₂ generations of six crosses between four cultivars. Partial resistance inherited mainly additively. With M. euphorbiae the estimates of broad-sense F₂ heritability were low and non-significant in crosses between parents of the same resistance level (S x S and PR x PR), and high (0.69-0.76) in crosses between parents of different resistance level (PR x S). The level of resistance in lettuce cultivars ‘Charan’ and ‘Marbello’ led to a marked reduction in percentage of plants with colonies of M. euphorbiae in a field experiment. With U. sonchi the highest estimates of F₂ heritability were found in ‘Charan x ‘Avoncrisp’ (0.51) and ‘Charan’ x ‘Chou de Naples' (0.48). In the cross ‘Charan’ x ‘Marbello’ (PR x PR) no significant segregation for resistance to M. euphorbiae was found, indicating that these cultivars may have the same genetic constitution for resistance to M. euphorbiae.‘Charan’ has additional genes for resistance to U. sonchi, that are not, or only weakly linked to the gene(s) for resistance to M. euphorbiae. The heritability estimates for the partial resistance to M. euphorbiae and U. sonchi indicate that it is possible to effectively select for these traits in plant breeding programmes.     

Number of genes controlling slow rusting resistance to leaf rust in five spring wheat cultivars

The number of genes controlling slow rusting resistance to leaf rust (Puccinia triticina) was estimated in five spring wheat (Triticum aestivum) cultivars using quantitative formulae. Parents and F₆ families were evaluated in replicated field trials under epidemics initiated by artificial inoculation. The F₆ families resulted from a diallel cross involving the fast-rusting cultivar Yecora 70 and five slow-rusting wheat cultivars: Sonoita 81, Tanager ‘S’, Galvez 87, Ures 81, and Moncho ‘S’. The area under the disease progress curve (AUDPC) was used to measure leaf rust severity over time. Results indicate that cultivar Sonoita 81 has three or four genes, Tanager ‘S’ has two or three genes, Galvez 87 has three genes, and both Ures 81 and Moncho ‘S’ have two genes for slow rusting resistance to leaf rust. Based on this result and previously reported moderate to high narrow-sense heritability estimates for slow rusting resistance in these materials, early-generation selection for slow leaf rusting would be effective.     

The influence of genetic background on resistance to the cabbage aphid (Brevicoryne brassicae) in Kale (Brassica oleracea var. acephala)

Resistance to Brevicoryne brassicae has been identified in the progeny of two selected kale (B. oleracea var. acephala) plants, one from the F1 hybrid cultivar ‘Arsis RS’ and one from the landrace ‘Butzo’. These plants were crossed with susceptible B. oleracea morphotypes that have different periods to flowering. The type of susceptible plant line used had an effect on the resistance phenotypc of the progeny. Tested F2 populations derived from these crosses show that resistance is not under simple genetic control. This, in addition to variation in aphid numbers within accessions, suggests that separation of genetic components of control from environmental ‘noise’ for any accession may only be possible by the production of double haploid plant lines.     

The mildew resistance of Hordeum bulbosum and its transference into H. vulgare genotypes

Mildew from susceptible genotypes (SI and S2) of Hordeum bulbosum was found to be avirulent on all H. vulgare genotypes tested, including such cultivars as Proctor with no known genes for resistance to mildew. The H. bulbosum genotype SI (2n =14) proved resistant to all isolates of mildew from H. vulgare. The mildew isolates used for these tests possessed most of the common virulence factors which attack the current ‘vulgare’ cultivars in Western Europe. Some H. bulbosum genotypes were resistant to the ‘bulbosum’ mildew. Attempts at combining resistance from both species are presented and discussed.     

Interspecific transfer of resistance to Plum pox virus from almond to peach by grafting

Grafting almond variety ‘Garrigues’ onto ‘GF305’ peach seedlings heavily infected by Plum pox virus (PPV) progressively produces the disappearance of viral symptoms and drastically reduces virus accumulation in ‘GF305’ rootstock, in most cases to undetectable levels. This response appears to be specific between almond and peach, as it was not consistently observed by grafting ‘Garrigues’ onto other Prunus species such as plum (‘Adesoto’) or apricot (‘Real Fino’). The ability to induce resistance to PPV in ‘GF305’ was transmitted to the sexual descendants of Garrigues. Furthermore, grafting ‘Garrigues’ onto ‘GF305’ before PPV inoculation completely prevented virus infection, showing that the resistance is constitutive and not induced by the virus. This fact suggests that resistance may be due to the transfer of a defence factor from ‘Garrigues’ almond through the graft union and its interaction with specific factors of ‘GF305’ peach to produce the antiviral response. These results open new avenues to potential protection against PPV in peach, the most economically important species among stone fruits.     

Effects of simulated drought stress on carotenoid contents and expression of related genes in carrot taproots

Carotenoids are liposoluble pigments found in plant chromoplasts that are responsible for the yellow, orange, and red colors of carrot taproots. Drought is one of the main stress factors affecting carrot growth. Carotenoids play important roles in drought resistance in higher plants. In the present work, the carotenoid contents in three different-colored carrot cultivars, ‘Kurodagosun’ (orange), ‘Benhongjinshi’ (red), and ‘Qitouhuang’ (yellow), were determined by ultra-high-performance liquid chromatography (UPLC) after 15% polyethylene glycol (PEG) 6000 treatment. Real-time fluorescence quantitative PCR (RT-qPCR) was then used to determine the expression levels of carotenoid synthesis- and degradation-related genes. Increases in β-carotene content in ‘Qitouhuang’ taproots under drought stress were found to be related to the expression levels of DcPSY2 and DcLCYB. Increases in lutein and decreases in α-carotene content in ‘Qitouhuang’ and ‘Kurodagosun’ under PEG treatment may be related to the expression levels of DcCYP97A3, DcCHXE, and DcCHXB1. The expression levels of DcNCED1 and DcNCED2 in the three cultivars significantly increased, thus suggesting that NCED genes could respond to drought stress. Analysis of the growth status and carotenoid contents of carrots under PEG treatment indicated that the orange cultivar ‘Kurodagosun’ has better adaptability to drought stress than the other cultivars and that β-carotene and lutein may be involved in the stress resistance process of carrot.

     

Life cycle of Chattonella marina (Raphidophyceae) inferred from analysis of microsatellite marker genotypes

Red tides of Chattonella spp. have caused continuous damage to Japanese aquaculture, however, the life cycle of this organism remains incompletely understood. To further investigate this matter, we assessed genotypes at 14 microsatellite markers in three varieties of Chattonella marina, viz., C. marina var. antiqua, C. marina var. marina, and C. marina var. ovata, to establish whether Chattonella undergoes asexual diploidization or sexual reproduction. After genotyping 287 strains of C. marina, all but one of these strains was shown to be heterozygous for at least some loci, and thus, in the diploid state, suggesting that Chattonella strains undergo sexual reproduction. In addition, we performed single‐cell amplification on ‘small cells’ that are derived from vegetative cells under dark and low‐nutrient conditions. The results indicated the existence of two types of small cells. The ‘Small cell Type 1’ was found to be heterozygous, genotypically equivalent to the vegetative cells, and is therefore diploid. These small cells may change to resting cells (cysts) directly. The ‘Small cell Type 2’ was homozygous at all analyzed loci, suggesting that these small cells are haploid and may be derived by meiosis. As fusion between small cells has previously been observed, the ‘Small cell Type 2’ may be the gamete of Chattonella. We present a construct of the full life cycle of Chattonella marina based on our own and previous results.     

Susceptibility of Cut Rose Flower Cultivars to Infections by Different Isolates of Botrytis cinerea

A method was developed for the inoculation of flowers of cut roses with conidia of Botrytis cinerea. Flower buds were inoculated by spraying of conidial supensions that were ultrasonicated for 10 s. The differences in susceptibility between 8 rose cvs to infections of isolate Bc-33 and the differences in pathogenicity between 14 5. cinerea isolates to cv. ‘Sonia’ were evaluated. Isolates obtained from rose flowers caused higher infection rates than those obtamed from various other hosts. The CVS ‘Madelon’, ‘Melody’ and ‘Sonia’ were found to be highly susceptible, whereas the cvs ‘Caramboie’, ‘Gabriella’, ‘Pasadena’ and ‘Rubinette’ were only slightly susceptible. The lower disease severity in the less susceptible cvs was based on a retardation of the growth of mfection hyphae in the petals, which may have been due to partial resistance. The formation of symptoms is effectuated already by the initial stages in the infection process. Thus, the apparent existing partial resistance to infections of B. cinerea cannot prevent the loss of ornamental value.     

Evolutionary trends on extant cat skull morphology (Carnivora: Felidae): a three‐dimensional geometrical approach

The skulls of 33 extant cat species were characterized through three‐dimensional geometric morphometrics using 20 landmarks. A principal component analysis (PCA) was performed with Procrustes fitted coordinates, and the PC‐scores were phylogenetically corrected by independent contrasts method. Three PCs allowed for the definition of five cat skull patterns. PC1: ‘snouted/massive‐headed cats’ (genus Panthera) opposing the ‘round‐headed small cats’ (genus Oncifelis, Prionailurus rubiginosus, Prionailurus bengalensis, among other small cats); PC2: ‘tapering‐headed cats’ (Neofelis nebulosa, Herpailurus yagouaroundi, Prionailurus planiceps) opposing the ‘stout‐headed cats’ (Acinonyx jubatus, Uncia uncia, Otocolobus manul, Felis margarita, and Felis nigripes); and PC3: ‘low profiled‐headed cats’ (mostly, Pr. planiceps). A sixth pattern, the ‘generalized skull’, observed in the Caracal lineage, genus Lynx, Leopardus pardalis, and Catopuma temminckii, indicates a morphological convergence among midsized‐cats. The morphological trends ‘snouted/massive’ and ‘round’ clearly denote a co‐evolution between size and shape. The other skull patterns evolved unrelatedly to the size (i.e. their allometric variations are not a size function). Nevertheless, each species comprises an amalgam of these patterns, so the influence of the size permeates, in some extent, the skull morphology along all cat lineages. The felid ecomorphological fit to hypercarnivory is obvious; however, different skull shapes in same‐sized species with similar habits, indicate that the variation in the skull morphology may result from phenotypic fluctuations, whose adaptive value (if indeed there is any) is still obscure. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2011, 103 , 176–190.     

Small shelly fossils and trace fossils near the Precambrian-Cambrian boundary in the Yukon Territory, Canada

In the past an ‘explosion’ in diversity and abundance of small shelly fossils and of trace fossils has served to mark the base of the Cambrian. However, no evidence has been presented to prove that the ‘explosions’ of the two groups were synchronous. We describe small shelly fossils and trace fossils from the same phosphatic limestone beds that indicate that the two events were separate in time. The small shelly fossils are Anabarites trisulcatus, Hyolithellus cf. H. isiticus, Microcornus? sp., Protohertzina anabarica, P. unguliformis, P. sp. A, Pseudorthotheca sp. A, Rushtonia? sp. A, four types of tuberculate plates and one type of reticulate plate. These fossils represent a restricted, ‘pre-explosion’ fauna and are assigned to the Anabarites-Circotheca-Protohertzina Assemblage Zone, an uppermost Precambrian zone in the Meishucun Stage, Yunnan Province, China. A point at the top of this zone has received strong international endorsement for future designation as the base of the Cambrian. Associated with the small shelly fossils are the trace fossils Cruziana sp. A, Cruziana? sp. B, Rusophycus sp. A, Palaeophycus rubdark and arthropod scratch marks. If found in isolation, this trace fossil assemblage would be considered as post-Precambrian because it includes large, highly organized arthropod traces that are traditionally accepted as occurring above the trace fossil ‘explosion’. We therefore conclude that the trace fossil ‘explosion’ predates the small shelly fossil ‘explosion’. If the proposed location of the base of the Cambrian in Yunnan is accepted, the small shelly fossil ‘explosion’ concept and its relationship to the boundary would not be greatly modified. The trace fossil ‘explosion’, however, would no longer indicate the base of the Cambrian and the ranges of some trace fossils would be extended into the Precambrian.     

Determining the host‐plant resistance mechanisms for Mamestra brassicae (Lepidoptera: Noctuidae) pest in cabbage

Six cabbage (Brassica oleracea var. capitata) varieties with different levels of resistance to Mamestra brassicae (Lepidoptera: Noctuidae) were investigated in order to assess whether antibiosis and antixenosis mechanisms are involved in the resistance to this pest or not. Four experiments were conducted to determine the effect of variety and plant ontogeny on larval behaviour, adult oviposition and leaf damages in non‐choice and choice tests. Larval survival, time to development and larval weights differed depending on the varieties and plant stages that we tested. At the pre‐head stage, larval mortality was higher, larvae died faster, time to pupation was shorter, pupae were lighter and the percentage of viable pupae and growth index (GI) values were lower than larvae reared from plants at the head stage. The commercial hybrid ‘Corazón de buey’ and the local variety named ‘BRS0535’ exhibited antibiosis to M. brassicae as they reduced its survival and growth and delayed its development time. In addition, these varieties were the most resistant after artificial infestation in terms of head foliage consumption and number of larvae per plant. Oviposition tests demonstrated that resistance found in ‘Corazón de buey’ and BRS0535 could be also based on antixenosis mechanisms as they resulted in fewer egg batches on plants, whereas BRS0402 could be classified as resistant because M. brassicae larvae showed less preference for it. Thus, resistance to M. brassicae found in cabbage crops may be due to the joint action of several factors involving antibiosis and antixenosis. We found significant differences in the resistance of BRS0535 depending on the plant ontogeny as it loses its resistance while developing. Further studies are required to identify the mechanism of antibiotic resistance which is present in this variety at the pre‐head stage and the changes that occur in plant defence as it grows.     

Eastern filbert blight disease resistance from <Emphasis Type="Italic">Corylus americana</Emphasis> ‘Rush’ and selection ‘Yoder #5’ maps to linkage group 7

Eastern filbert blight (EFB), caused by the pyrenomycete Anisogramma anomala, is a serious threat to the hazelnut industry in the Pacific Northwest. EFB is endemic in eastern North America where it occasionally produces small cankers on the wild American hazelnut (Corylus americana). In contrast, most cultivars of European hazelnut (Corylus avellana) are susceptible. Genetic resistance is the most promising disease control method and is an objective of the Oregon State University hazelnut breeding program. ‘Gasaway’ resistance, which is governed by a dominant allele at a single locus, has been extensively used in the program. However, ‘Gasaway’ and some of its offspring have been infected by EFB isolates from New Jersey, Minnesota, and Michigan. Efforts to create new cultivars with durable EFB resistance include identifying and studying new resistance sources. In this study, resistant accessions C. americana ‘Rush’ and interspecific hybrid selection ‘Yoder #5’ were crossed with susceptible C. avellana selections and the resulting segregating seedling populations were inoculated by either exposure of potted trees under a structure topped with diseased branches or field exposure supplemented by tying diseased branches to each tree. Disease response was scored when cankers were visible 20 months after inoculation. Resistance from both sources segregated in a 1:1 ratio, indicating control by a single locus and a dominant allele for resistance. DNA extracted from the seedlings was amplified with previously mapped microsatellite markers. Resistance from both C. americana ‘Rush’ and ‘Yoder #5’ was placed on linkage group 7 in the same position as resistance from C. avellana ‘Ratoli.’ Linked microsatellite markers B753, GB372, and B509 will be useful for marker-assisted selection and the pyramiding of genes for durable EFB resistance. Assessing response to EFB is challenging, whether the plants are inoculated under a structure topped with diseased wood or in a humidity chamber in the greenhouse, or by exposure in the field. The pathogen has a 2-year life cycle, and there is a 15-month wait between inoculation and symptom expression. A small number of escapes is commonly encountered, and resistant plants occasionally develop small cankers. Our approach of studying segregation ratios and then mapping with microsatellite markers should be a useful approach for disease resistance studies in many tree crops.     

Quantitative Differences in Powdery Mildew Resistance Among Spring Barley Cultivars

Quantitative powdery mildew resistance in compatible host-pathogen-combinations was measured by the number of pastules/cm² leaf area. Spring barley cultivar ‘Proctor’ was significantly less infected than ‘Golden Promise”. Using these two cultivars (having no effective major resistance gene) as controls, MO- and AR-resistant cultivars were inoculated with virulent mildew isolates. ‘Mona”, ‘Grit’ and ‘Nudinka’ had a higher or, at least, the same level of quantitative resistance as ‘Proctor”. None of the remaining cultivars showed the high susceptibility expressed by ‘Golden Promise”. Ranking of host genotypes was nearly constant while that of mildew isolates varied considerably. Only a small portion of the observed variance was due to interaction between host cultivars and pathogen isolates. ‘Triesdorfer Diva’ gave a resistant infection type after inoculation with different AR-virulent isolates, indicating that this cultivar has major resistance other than that conditioned by gene Ml-a12.