Useful Levels Of Resistance Research Articles

Genetics and race variability of the lucerne-Colletotrichum trifolii pathosystem in Australia

The Australian-bred lucerne cultivars, Trifecta and Sequel, were found to possess useful levels of resistance to both Colletotrichum trifolii races 1 and 2. Race 2 has only been previously observed in the United States and surveys did not reveal its presence in Australia. Multilocus finger printing using random amplified polymorphic DNA (RAPDs) analysis revealed low diversity (<10% dissimilarity) within Australian C. trifolii collections, and between the Australian race 1 isolates and a US race 2 isolate. Studies on the inheritance of resistance to C. trifolii race 1 in individual clones from Trifecta and Sequel revealed the presence of 2 different genetic mechanisms. One inheritance was for resistance as a recessive trait, and the other indicated that resistance was dominant. The recessive system has never been previously reported, whereas in the US, 2 completely dominant and independent tetrasomic genes An1 and An2 have been reported to condition C. trifolii resistance. It was not possible to fit the observed segregations from our studies to a single-gene model. In contrast to US studies, clones of cv. Sequel exhibiting the recessive resistance reacted differently to spray and stem injection with C. trifolii inoculum, being resistant to the former and susceptible to the latter, providing additional evidence for the presence of a different genetic mechanism conditioning resistance to those previously reported in the US. As C. trifolii is one of the most serious diseases of lucerne worldwide, the future development of molecular markers closely linked to the dominant and recessive resistances identified in these studies, and the relationships between these resistances and An1 and An2 as determined by genetic mapping, appear to be useful areas of future study.

Root‐Knot Nematode Resistance of F2 Cotton Hybrids from Crosses of Resistant Germplasm and Commercial Cultivars

The southern root‐knot nematode (RKN), Meloidogyne incognita (Kofoid and White) Chitwood, is a major pest of cotton (Gossypium hirsutum L.). A high level of resistance has been identified in eight germplasm lines; however, these lines are not as productive in yield as current cultivars. This study was conducted to determine the possibility of using F2 populations from crosses of these RKN resistant germplasm lines with five current cultivars as a means of combining RKN resistance with high yield potential. Two F2 populations were evaluated for their level of nematode resistance. Resistance in these F2 populations was greater than the cuitivar parents but less than the RKN resistant germplasm parents. The resistance level of the F2 population, however, was nearer the germplasm parenthan the cultivar parent, indicating that the F2 hybrids possess a useful level of resistance. Yield and fiber properties among the F2 hybrids indicated that by careful selection of parents, F2 populations that were equal in yield and fiber quality to the best of the cultivar parents could be produced. Results of this study indicate that F2 populations may provide a rapid way to combine high levels of RKN resistance with the higher yield and superior fiber properties of current cultivars.

The legume pod borer Maruca testulalis, and its principal host plant, Vigna unguiculata (L.) Walp. — use of selective insecticide sprays as an aid in the identification of useful levels of resistance

Cowpea varieties were evaluated for resistance to Maruca testulalis using differential insecticide sprays that provided a range of pest densities and crop damage. The latter was expressed as a pod evaluation index, Ipe, which uses the degree of pod production or pod load (PL) and pod damage (PD). The different spray treatments included a monocrotophos spray which does not control the pod borer, one or two sprays of a protective mixture of cypermethrin and dimethoate at different stages in the crop phenology, and full protection with the same mixture. Unsprayed cowpea produces very few pods so it was not included as a treatment. None of the tested varieties was resistant to flower damage, and apart from TVnu 72, the resistant control, none were highly resistant to the borer. Some varieties e.g. MRx nos. 2-85F, 10-85S, 17-85S, 48-85S, 58-85S, 66-85S, 67-85S and Zonkwa Local had relatively high Ipe values, after receiving only one protective spray, and were considered to be moderately resistant. These varieties can either be grown under minimum insecticide protection (i.e. one insecticide spray) to exploit their potential, in the appropriate cropping or agronomic systems which reduce pest densities, or in locations where borer infestation is low. The Ipe statistic is related to larval counts in flowers ( R e = 0.74–0.79), thus if larval counts in flowers were not taken, it would suffice as an indirect estimate of the M. testulalis load. This can be done in conjunction with the rapid visual estimate (RVE) of flower infestation and/or damage which is also more time efficient, even if slightly less accurate than direct larval counts.

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

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

Resistance of hybrid canola ( Brassica napus L.) to flea beetle ( Phyllotreta spp.) damage during early growth

The hybrid canolas, Brassica napus hyb. K4-87 and B. napus hyb. K36-87, were tested for two modes of resistance, antixenosis and tolerance, against flea beetles, Phyllotreta spp. In feeding tests in which choices were offered between the hybrids and a standard inbred canola cultivar, B. napus cv. Westar, the cotyledons of the hybrid K4-87 and the cotyledons and first true leaves of the hybrid K36-87 showed a higher level of antixenosis. As neither parent of K36-87 had a higher level of antixenosis, this trait in the hybrid may be due to heterosis. In no-choice feeding tests, however, cotyledon damage and seedling biomass 1 week after feeding damage did not differ among K4-87, K36-87, and Westar. The hybrids showed no tolerance to feeding damage either in the laboratory or in the field. On the basis of plant biomass and yield data from Manitoba and Saskatchewan, the hybrids do not exhibit sufficient resistance to provide protection in the field. These results do not support the hypothesis that hybrid canola will contribute an agronomically useful level of resistance against flea beetle damage under commercial growing conditions.

Field Resistance to Russian Wheat Aphid in Barley: I. Sympton Expression

Russian wheat aphid (RWA), Diuraphis noxia (Mordvilko), is a serious pest of wheat (Triticum aestim L.) and barley (Hordeum vulgare L.). This research was conducted to identify barley genotypes resistant to RWA under field conditions. Spring barley genotypes were evaluated during summer 1989 (109 genotypes) and winter 1990 (111 genotypes) at El Batán, Mexico. Hill‐plots of 8 to 10 seeds were infested with 90 to 110 greenhouse‐reared RWA per hill. Hills were visually rated on a 1 to 6 scale (1 no symptoms) for RWA damage at tillering (Zadoks 10 to 29), jointing (Zadoks 30 to 35), and post‐heading (Zadoks 59 to 65). Genotypes differed significantly (P < 0.01) in reaction to RWA at individual scoring dates, and averaged over all scoring dates. Fifteen genotypes were classified as resistant (score < 2.51) based on overall mean score; seven of these were resistant at all scoring dates. The latter included selections from the crosses SMA1/SB401, API/CM67//ACER,ASE/2CM//B.7.6.B.B, GLORIA/COME, CI1255/ARAMIR//GLORIA/3/CI2325/CI12225//BOY*2/3*SURB,ECT/6/M/N//V/4/USB//13914/C/3/SMA/5/SMA, and SD729/POR/3/APM/ATHS//GVA/4/ORE.Significant (P < 0.01) genotype ✕ crop cycle and genotype ✕ growth stage interactions suggest the possibility of improving detection of material with useful levels of resistance by screening in the environment where the material will ultimately be grown, and observing plants at various stages during crop growth. Despite the complication of interaction effects, field screening using artificially infested hill‐plots was effective in identifying barley genotypes that expressed RWA resistance across a broad range of growing conditions and crop development.

Reaction of 578 Spring Spelt Wheat Accessions to 35 Races of Wheat Stem Rust

A survey of spelt wheat accessions (Triticum aestivum ssp. spelta L.) was made to characterize their reactions to stem rust (Puccinia graminis Rob. ex Desm. f. sp. tritici). Spelt wheat genotypes cross readily with bread wheat (Triticum aestivum L.) and could be used as donor parents in bread wheat breeding programs. Because of the paucity of information on their reaction to stem rust in the literature, the objectives of this study were to: (i) screen spelt accessions for possible new sources of resistance to wheat stem rust, and (ii) postulate and confirm the presence of known genes for resistance to stem rust. Infection‐type data from seedlings artificially infected with stem rust were used to postulate Sr genes present in 578 spring spelt accessions from the USDA National Small Grain Collection. Conventional genetic studies were used to confirm the hypothetical resistance genotypes derived from infection‐type data. Genetic studies confirmed the presence of postulated genes for resistance Sr9e, Sr9f, Sr18, SrLC, and SrMcN; Sr9g was absent. Other genes for resistance postulated to be present included Sr9a, Sr9d, Srl0, Sr15, Sr17, Srl9, and Sr20. Infection‐type data proved the absence of Sr5, Sr6, Sr8a, Sr9b, Sr11, Sr13, Sr14, Sr16, Sr24, Sr25, Sr26, Sr29, Sr36, and SrTmp in these spelt accessions. Several unidentified genes that were postulated to be present may provide useful levels of resistance when transferred in combinations to bread wheat.

A glasshouse progeny test for resistance to gangrene (Phoma foveata)

Tubers were obtained from 80 glasshouse-grown seedlings of each of twelve progenies, inoculated withPhoma foveata Foister, and the mean percentage of infected tubers recorded. There was a good correlation (r=0.798) between this assessment and that made previously on field-grown tubers of eleven of the progenies. A gangrene test on glasshouse-grown tubers is suggested as a more rapid means of identifying progenies with useful levels of resistance.

Improved expression of streptomycin resistance in plants due to a deletion in the streptomycin phosphotransferase coding sequence.

Previous studies have shown that a chimeric streptomycin phosphotransferase (SPT) gene can function as a dominant marker for plant cell transformation. The SPT marker previously described by Jones and co-workers has a limited value since it conferred a useful level of resistance only to a fraction (10%) of Nicotiana plumbaginifolia transgenic lines. Expression of resistance was species specific: no such resistant transformants were found in N. tabacum. In this paper we describe an improved SPT construct that utilizes a mutant Tn5 SPT gene. The mutant gene, SPT*, encodes a protein with a two amino acid deletion close to its COOH-terminus. In N. tabacum cell culture the efficiency of transformation with the improved streptomycin resistance marker was comparable to kanamycin resistance. When the chimeric SPT* gene was introduced linked to a kanamycin resistance gene, streptomycin resistance was expressed in most of the transgenic N. tabacum lines.

Susceptibility of Australian wheats to Pyrenophora tritici-repentis

The response of Australian wheats to yellow spot caused by Pyrenophora tritici-repentis has been examined under controlled conditions with a juvenile-plant screening system. About three-quarters of contemporary Australian wheats are highly susceptible to the disease and very few cultivars possess useful levels of resistance. Also, the majority of Australian wheats now undergoing final evaluation before releasing are highly susceptible to yellow spot. Modern wheats grown in Queensland and northern New South Wales are, as a group, more susceptible than those they replaced, and this may have contributed to the upsurge in yellow spot in the region. In two comparisons between ratings on juvenile plants under controlled conditions with field disease assessments and yield measures, the juvenile plant ratings and field disease assessments explained similar proportions of the variation in yield measures.

Evaluation of Chrysanthemum Cultivars and Insecticides for Control of Damage by a Leafminer, Liriomyza trifolii (Diptera: Agromyzidae)1

Chrysanthemum cultivars grown for pot flower production in greenhouses exhibited various and potentially useful levels of resistance to Liriomyza trifolii (Burgess). Leafminer control with insecticides was significantly ( P <0.05) enhanced when applied to resistant varieties. Evaluations of insecticides showed an initial tolerance of the South Carolina population to permethrin and moderate control (71%) by foliar treatment with encapsulated methyl parathion. Permethrin regained efficacy after a 6-month period during which pyrethroid sprays were withheld. Methamidophos, cypermethrin, and permethrin treatments resulted in 90 +% control in subsequent tests, but methamidophos was phytotoxic to foliage of most cultivars at standard application rates. Tank mixes of pyrethroids and encapsulated methyl parathion gave greater efficacy than either chemical used alone. Fenvalerate treatments provided moderate control. Oxydemeton methyl showed no efficacy against the leafminer.

Recent Progress in Breeding for Fireblight Resistance in Apples and Pears in North America1

AbstractResistance in apple is evaluated by needle–inoculation of succulent shoot tips with 106–107 cells of a virulent isolate of Erwinia amylovora (Burr.) Winslow et al. (the incitant of fireblight) and measurement of the resulting cortical lesion when extension is complete. Data are now available on practically all commercial cultivars, some of which have a useful level of resistance. Some newer cultivars, particularly those with resistance to scab (Venluria inaequalis [Cooke] Wint.) derived from Malus floribunda, have good resistance to E. amylovora. A very high level of resistance is present in Asiatic Malus species, including M. x robusta, M. x sublo–bata, M. x atrosanguinea, and M. prunifolia, and in the North American species M. fusca. This type of resistance appears to be inherited polygenically, and because of its detectability in young seedlings can be used conveniently in breeding.Objectives of pear breeding programs are aimed at developing superior fruit quality combined with resistance to fireblight, psylla, and Fabraea leaf spot. Many high quality cultivars of Pyrus communis are extremely susceptible to E. amylovora and sensitivity appears to be controlled by a dominant gene Se. A high level of resistance is present in P. ussuriensis but varies considerably between clonal selections of other Pyrus species. Pear seedlings from controlled pollinations are artificially inoculated in the glasshouse with a similar bacterial suspension as used for apples, and only the most resistant ones are selected and planted in the field for future evaluation. In Beltsville, heritability studies of crosses between non–sensitive parents have indicated that selection for resistance within progenies results in a high degree of genetic gain. Interspecific hybridization has an advantage over P. communis crosses mainly when insect or fungus resistance is required.

Leafminer Resistance in Muskmelon1

Abstract Potentially useful levels of resistance to the leafminer, Liriomyza sativae Blanchard (= munda Frick), in Cucumis melo L. were identified in greenhouse and field studies in different years and widely separated locations. Resistant lines were identified as those having the least number of mines per leaf in a given test. Two apparently distinct sources of resistance were found in Plant Introduction (PI) 282448 from Africa and PI 313970 from India. In a single test of F1 hybrids the resistance of PI 282448 appeared to be controlled by recessive genes, while that of PI 313970 appeared to be controlled by partially dominant genes. Plant introductions from Central Asia and East Asia were, in general, the most susceptible.

Resistance in Red Raspberry to Amphorophora agathonica Hottes and Aphis rubicola Oestlund1

Abstract Twenty-two cultivars and three selections of red raspberry were evaluated for resistance to the aphids Amphorophora agathonica and Aphis rubicola in New York. ‘Canby’, ‘Lloyd George’ and ‘NY 632’ were immune to A. agathonica; ‘Washington’ and ‘Mailing Exploit’ demonstrated potentially useful levels of resistance. No immunity to Aphis rubicola was observed, but several cultivars manifest considerable resistance. Resistance to A. agathonica was not correlated with resistance to A. rubicola.

Resistance to a Spotted Stem Borer in Sorghum and Maize123

A useful level of resistance to Chilo zonellus (Swinhoe) exists in both maize (corn) and sorghum breeding material used in East Africa. Ratings for leaf feeding proved useful in separating resistant and susceptible maize inbreds but not maize hybrids or sorghum varieties. For these, larval counts and exit holes per plant might be used, but yield also would be an important criterion. Thus, a technique of applying recurrent selection to composite populations by infesting every plant with 1–3 egg masses in the leaf whorl and of basing the final selection on yield is likely to prove an effective method of developing types of maize and sorghum possessing enhanced resistance to C. zonellus .

Resistance in Sweetpotatoes to Damage by Soil Insects2

Sweetpotato varieties and breeding lines varied widely in susceptibility to root damage by soil insects in the field. Sources of resistance to most major pests of sweetpotato roots were found. Some commercial lines proved to have useful levels of resistance to some insects, but lines selected specifically for resistance were generally most resistant. Resistance to some of the insects appeared to be controlled by common factors.