Diseases of tropical crops: Problems and controls

Summary

Tropical crops are diverse as to botanical origins and as to function; they have numerous diseases, some of them very damaging. A few characteristic crops and their diseases and causal agents are listed for purposes of illustration. There are several approaches to control of which the cultural, legal and chemical have some uses; by far the most important is by breeding ‘horizontal resistance’ (HR) which is cheapest and ‘environmentally friendly’. Sugarcane is specifically instanced as the best example in the world of outstandingly successful HR breeding programmes; vast areas of a clonal crop are grown in good health without the use of chemical controls.     

Evaluation of Hydrangea macrophylla for Resistance to Leaf‐Spot Diseases

Garden hydrangea (Hydrangea macrophylla) is a popular ornamental plant that can be devastated by leaf‐spot diseases. Information is needed to determine susceptibility of commercial cultivars to leaf‐spot diseases. To address this need, 88 cultivars of H. macrophylla were evaluated for their resistance to leaf‐spot diseases in full‐shade (2007–2008), full‐sun (2007–2008) and partial‐shade (2009–2010) environments in McMinnville, TN, USA. Ten cultivars [‘Ami Pasquier’, ‘Ayesha’, ‘Blue Bird’, ‘Forever Pink’, ‘Fuji Waterfall’ (‘Fujinotaki’), ‘Miyama‐yae‐Murasaki’, ‘Seafoam’, ‘Taube’, ‘Tricolor’ and ‘Veitchii’] were rated resistant (R) or moderately resistant to leaf spot under each of the three environments. In 2007–2008, approximately 51% of the cultivars were rated R in full shade, but only 5% were R in full sun. In 2009–2010, only 1% of the cultivars were rated R in partial shade. Although environmental parameters including temperature and rainfall influence disease severity and host reaction, a shaded environment was least favourable for leaf‐spot disease development, which demonstrates that establishing hydrangea in shaded environment can be an effective tool along with cultivar selection for managing leaf‐spot diseases on hydrangea. Six pathogens, Corynespora cassiicola, Cercospora spp., Myrothecium roridum, Glomerella cingulata (Anamorph: Colletotrichum gloeosporioides), Phoma exigua and Botrytis cinerea, were associated with leaf‐spot diseases of garden hydrangea. Of the leaf‐spot pathogens, C. cassiicola was most frequently isolated (55% of all isolates), followed by Cercospora spp. (20%) and other pathogens (25%). Because symptoms attributed to each leaf‐spot pathogen were similar, cultivars were selected for resistance to multiple leaf‐spot pathogens.     

PATHOLOGY OF CYST-NEMATODES

1. The cyst-nematodes, which belong to the genus Heterodera Schmidt, are obligate plant parasites. Several species cause crop diseases of economic importance. These nematodes differ greatly in structure from the other, vermiform, plant-parasitic and soil-inhabiting nematodes, for although the migrant larval phase of cyst-nematodes is vermiform, this phase is only transient. The principal phases are the sedentary female, which feeds upon the plant root, and the cyst, which derives from the female and consists of the eggs surrounded by mucilage within the dead female cuticle. The term ‘egg’ includes the contents within the shell, which at maturity is usually a coiled, second stage, larva. 2. The migrant larva may be parasitized by many species of predacious and endozoic fungi which have been much studied, but these do not parasitize the females or cysts. Until 1974 only one parasite of the females was known, and that imperfectly. On the other hand, many fungi have been cited as parasites of eggs inside cysts, notably the species now known as Phialophora malorum and Cylindrocarpon destructans. Insect-pathogenic fungi have also been cited as egg parasites but evidence is now against their pathogenicity to Heterodera. No other classes of organism are known to parasitize the females or cysts, although Collembola have been reported as predators. 3. Studies published since 1973, including the present, show a total of seven (or eight) pathogenic fungi. There are two (or three) pathogens of the females, two major and three minor pathogens of the eggs. The former comprise Catenaria auxiliaris and two Entomophthora-like fungi, possibly the same species but both imperfectly known. The major egg pathogens are Verticillium chlamydosporium and an incompletely known species termed the ‘contortion fungus’ from the symptoms on infected larvae newly hatched from eggs. The minor egg pathogens are a ‘black yeast’ taxonomically fairly similar to but not identical with Phialophora malorum; a sterile ‘crystal-forming fungus’ and Cylindrocarpon destructans. The main features of these pathogens are described and illustrated. 4. Mycorrhizal fungi belonging to Glomus Tulasne & Tulasne are sometimes associated with cysts. They ordinarily sporulate inside empty cysts, but in one remarkable example from Heterodera schachtii growing on tomato host in glasshouse culture a Glomus apparently behaved as a pathogen. Pythium species may invade cysts maintained under unfavourable conditions. Miscellaneous other fungi are also found occasionally in eggs. 5. In addition to undergoing disease caused by fungi, eggs sometimes undergo oily degeneration. This disorder is not of numerical significance, but a high proportion of cysts are diseased through apparently non-specific causes whose symptoms can be summarized as ‘lysed, shrivelled, coagulated or decayed’. Although the majority of diseased females are killed by fungal pathogens, some 25–40% of diseased cysts are destroyed through non-specific causes. Overall disease in ten populations of Heterodera schachtii examined from roots was 9% of females and 26% of young cysts; in 76 populations sieved from soils a mean of 14% of older cysts were diseased. Where the cyst-nematode host had been grown in monoculture for 11 years higher proportions of cysts were diseased although few pathogens of females were present. Heterodera avenue cysts were parasitized to a similar extent. 6. Of the pathogens of females, Catenaria auxiliaris was widespread in H. schachtii and uncommon in H. avenue, whereas Entomophthora-like fungi were widespread in H. avenue and rare in H. schachtii. Of egg pathogens, Verticillium chlamydosporium and the ‘contortion fungus’ were common in H. schachtii and H. avenae, and the ‘contortion fungus’ was also found in H. glycines. The minor egg pathogens were much less common.     

Antibody-Based Resistance to Plant Pathogens

Plant diseases are a major threat to the world food supply, as up to 15% of production is lost to pathogens. In the past, disease control and the generation of resistant plant lines protected against viral, bacterial or fungal pathogens, was achieved using conventional breeding based on crossings, mutant screenings and backcrossing. Many approaches in this field have failed or the resistance obtained has been rapidly broken by the pathogens. Recent advances in molecular biotechnology have made it possible to obtain and to modify genes that are useful for generating disease resistant crops. Several strategies, including expression of pathogen-derived sequences or anti-pathogenic agents, have been developed to engineer improved pathogen resistance in transgenic plants. Antibody-based resistance is a novel strategy for generating transgenic plants resistant to pathogens. Decades ago it was shown that polyclonal and monoclonal antibodies can neutralize viruses, bacteria and selected fungi. This approach has been improved recently by the development of recombinant antibodies (rAbs). Crop resistance can be engineered by the expression of pathogen-specific antibodies, antibody fragments or antibody fusion proteins. The advantages of this approach are that rAbs can be engineered against almost any target molecule, and it has been demonstrated that expression of functional pathogen-specific rAbs in plants confers effective pathogen protection. The efficacy of antibody-based resistance was first shown for plant viruses and its application to other plant pathogens is becoming more established. However, successful use of antibodies to generate plant pathogen resistance relies on appropriate target selection, careful antibody design, efficient antibody expression, stability and targeting to appropriate cellular compartments.     

The influence of host and non‐host companion plants on the behaviour of pest insects in field crops

Companion plants grown as ‘trap crops’ or ‘intercrops’ can be used to reduce insect infestations in field crops. The ways in which such reductions are achieved are being described currently using either a chemical approach, based on the ‘push‐pull strategy’, or a biological approach, based on the ‘appropriate/inappropriate landing theory’. The chemical approach suggests that insect numbers are reduced by chemicals from the intercrop ‘repelling’ insects from the main crop, and by chemicals from the trap‐crop ‘attracting’ insects away from the main crop. This approach is based on the assumptions that (1) plants release detectable amounts of volatile chemicals, and (2) insects ‘respond’ while still some distance away from the emitting plant. We discuss whether the above assumptions can be justified using the ‘appropriate/inappropriate landing theory’. Our tenet is that specialist insects respond only to the volatile chemicals released by their host plants and that these are released in such small quantities that, even with a heightened response to such chemicals, specialist insects can only detect them when a few metres from the emitting plant. We can find no robust evidence in the literature that plant chemicals ‘attract’ insects from more than 5 m and believe that ‘trap crops’ function simply as ‘interception barriers’. We can also find no evidence that insects are ‘repelled’ from landing on non‐host plants. Instead, we believe that ‘intercrops’ disrupt host‐plant finding by providing insects with a choice of host (appropriate) and non‐host (inappropriate) plant leaves on which to land, as our research has shown that, for intercropping to be effective, insects must land on the non‐host plants. Work is needed to determine whether non‐host plants are repellent (chemical approach) or ‘non‐stimulating’ (biological approach) to insects.     

Induced resistance in rice against insects

Vaccinations are the mainstay of western preventive medicine, and they have been used to protect some crops against disease and insect pests. We consider rice as a model for protection using induced resistance since it is one of the most important staple crops and there have been significant new developments in: cross-resistance among rice insects, chemical pathways involved in induced resistance, sequencing the rice genome and expression of genes conferring resistance against rice insect pests. Insect attack has been found to cause lesions that kill planthopper eggs and early stages of gall midges. Damaged plants released volatiles that made them less likely to be chosen by planthoppers and more attractive to parasitoids. Chemical elicitors have been developed for dicotyledonous plants and these can induce resistance in rice, although rice does not fit models developed to explain signalling in dicots. For example, salicylic acid did not increase in rice after infection by pathogens and did not appear to be the mobile signal for induced resistance against pathogens although it was involved in induced responses to phloem-feeding insects. Jasmonic acid acted as a signal in some induced responses to pathogens as well as chewing insects. Many of the genes associated with induced resistance in rice have recently been mapped, and techniques are being developed to incorporate them into the genome of cultivated varieties. Attempts to control insect pests of rice will affect interactions with pathogens, predators and parasites, and other organisms in this agroecosystem.     

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.     

Toward an understanding of the molecular basis of quantitative disease resistance in rice

Rice crops are severely damaged by diseases caused by bacterial, fungal, and viral pathogens. Application of host resistance to these pathogens is the most economical and environmentally friendly approach to solve this problem. Quantitative resistance conferred by quantitative trait loci (QTL) is a valuable resource for the improvement of rice disease resistance. Although numerous resistance QTL against rice diseases have been identified, these resources have not been used effectively in rice improvement because the genetic control of quantitative resistance is complex and the genes underlying most of the resistance QTL remain unknown. This review focuses on the latest molecular progress in quantitative disease resistance in rice. This knowledge will be helpful for characterizing more resistance QTL and turning the quantitative resistance into actual resources for rice protection.     

The genetic structure of the plant pathogenic fungus Melampsora larici-populina on its wild host is extensively impacted by host domestication

Wild and cultivated plants represent very different habitats for pathogens, especially when cultivated plants bear qualitative resistance genes. Here, we investigated to what extent the population genetic structure of a plant pathogenic fungus collected on its wild host can be impacted by the deployment of resistant cultivars. We studied one of the main poplar diseases, poplar rust, caused by the fungus Melampsora larici‐populina. A thousand and fifty individuals sampled from several locations in France were phenotyped for their virulence profile (ability to infect or not the most deployed resistant cultivar ‘Beaupré’), and a subset of these was genotyped using 25 microsatellite markers. Bayesian assignment tests on genetic data clustered the 476 genotyped individuals into three genetic groups. Group 1 gathered most virulent individuals and displayed evidence for selection and drastic demographic changes resulting from breakdown of the poplar cultivar ‘Beaupré’. Group 2 comprised individuals corresponding to ancestral populations of M. larici‐populina naturally occurring in the native range. Group 3 displayed the hallmarks of strict asexual reproduction, which has never previously been demonstrated in this species. We discuss how poplar cultivation has influenced the spatial and genetic structure of this plant pathogenic fungus, and has led to the spread of virulence alleles (gene swamping) in M. larici‐populina populations evolving on the wild host.     

Characterization of the Rust Fungus, Puccinia emaculata, and Evaluation of Genetic Variability for Rust Resistance in Switchgrass Populations

Several fungal pathogens have been identified on ornamental and native stands of switchgrass (Panicum virgatum L.). Diseases of switchgrass, particularly rust, have been largely neglected and are likely to become the major limiting factor to biomass yield and quality, especially when monocultured over a large acreage. Based on teliospore morphology and internal transcribed spacer-based diagnostic primers, the rust pathogen collected from switchgrass research fields in Oklahoma was identified as Puccinia emaculata. Furthermore, to identify genetically diverse source(s) of rust resistance, several switchgrass genotypes from both upland (cv. ‘Summer’ and ‘Cave-in-Rock’) and lowland (cv. ‘Alamo’ and ‘Kanlow’) ecotypes were evaluated in Ardmore, Oklahoma during 2008 and 2009 and in growth chamber assays. Field and growth chamber evaluations revealed a high degree of genetic variation within and among switchgrass cultivars. In general, Alamo and Kanlow showed moderate resistance to P. emaculata, while Summer was highly susceptible. Distinct ecotypic variations for reactions to rust were also prevalent with the lowlands maintaining a high level of resistance. These results suggest the potential for improvement of rust resistance via the selection of resistant individuals from currently available cultivars. Further, the selection pressure on the pathogen would also be reduced by employing several rust resistant cultivars in production-scale situations.     

Disease resistance in tomato: practical situation

Tomato crops are parasitised by a large and ever increasing number of pathogens. For tomato, the genetic control of pathogens is very used in practice. The sources of resistance occur monogenic and dominant. Their sources are the wild species closely related to the cultivated form. Many open-pollinated varieties presently cultivated posses genetic resistance to three or four pathogens. With the increasing use of F1 hybrids it is possible to use varieties cumulating four to six resistances. During the last years several new resistances were identified and breeders are introducing these in modern varieties cumulating various resistances and particular qualities now required by specific markets, notably fruit long life and good taste. Some interesting resistances are difficult to select. There are the cases with the oligogenic partial resistances also when the resistance tests are difficult to carry out. In some cases using molecular markers can permit the selection. The wild species of Lycopersicon continue to be the hope to control diseases and pests. Nevertheless, modern varieties derived from interspecific crosses can be sources of new resistances sometimes linked to the expected resistances. New hopes appear with the transgenosis, notably to control various viruses. The present practical situation of the genetic resistance is presented for various cultural conditions.     

Annual Medicago: from a model crop challenged by a spectrum of necrotrophic pathogens to a model plant to explore the nature of disease resistance

BACKGROUND: Annual Medicago spp., including M. truncatula, play an important agronomic role in dryland farming regions of the world where they are often an integral component of cropping systems, particularly in regions with a Mediterranean or Mediterranean-type climate where they grow as winter annuals that provide both nitrogen and disease breaks for rotational crops. Necrotrophic foliar and soil-borne pathogens dominate these regions and challenge the productivity of annual Medicago and crop legume species. SCOPE: This review outlines some of the major and/or widespread diseases these necrotrophic pathogens cause on Medicago spp. It then explores the potential for using the spectrum of necrotrophic pathogen-host interactions, with annual Medicago as the host plant, to better understand and model pathosystems within the diseases caused by nectrotrophic pathogens across forage and grain legume crops. CONCLUSIONS: Host resistance clearly offers the best strategy for cost-effective, long-term control of necrotrophic foliar and soil-borne pathogens, particularly as useful resistance to a number of these diseases has been identified. Recently and initially, the annual M. truncatula has emerged as a more appropriate and agronomically relevant substitute to Arabidopsis thaliana as a model plant for legumes, and is proving an excellent model to understand the mechanisms of resistance both to individual pathogens and more generally to most forage and grain legume necrotrophic pathogens.     

Insights to plant–microbe interactions provide opportunities to improve resistance breeding against root diseases in grain legumes

Root and foot diseases severely impede grain legume cultivation worldwide. Breeding lines with resistance against individual pathogens exist, but these resistances are often overcome by the interaction of multiple pathogens in field situations. Novel tools allow to decipher plant–microbiome interactions in unprecedented detail and provide insights into resistance mechanisms that consider both simultaneous attacks of various pathogens and the interplay with beneficial microbes. Although it has become clear that plant‐associated microbes play a key role in plant health, a systematic picture of how and to what extent plants can shape their own detrimental or beneficial microbiome remains to be drawn. There is increasing evidence for the existence of genetic variation in the regulation of plant–microbe interactions that can be exploited by plant breeders. We propose to consider the entire plant holobiont in resistance breeding strategies in order to unravel hidden parts of complex defence mechanisms. This review summarizes (a) the current knowledge of resistance against soil‐borne pathogens in grain legumes, (b) evidence for genetic variation for rhizosphere‐related traits, (c) the role of root exudation in microbe‐mediated disease resistance and elaborates (d) how these traits can be incorporated in resistance breeding programmes.     

EVOLUTIONARY EFFECTS OF SELECTION ON AGE AT REPRODUCTION IN LARVAL AND ADULT: DROSOPHILA MELANOGASTER

Two sets of three replicate lines of Drosophila melanogaster were artificially selected by reproduction at either a ‘young’ or an ‘old’ age. The pure lines, the hybrids between the lines within a selection regimen and the base stock from which the lines were derived were compared for longevity, early and late fertility, development time, larval viability and adult thorax length. Comparison of hybrid with pure lines showed some evidence for inbreeding depression in the lines from both selection regimes. Comparison of hybrid lines with the base stock did not provide evidence for any trade-off in either males or females between early fertility on the one hand and late life fertility and longevity on the other. Nor was there any clear evidence of a trade-off between pre-adult and adult fitness components. There was evidence of inadvertent selection for rapid development in both selection regimens, especially in the females of the ‘young’ lines, and this complicated the interpretation of the responses and correlated responses to selection. An improvement in adult performance in the ‘old’ line males relative to the base stock appeared to be attributable to reversal of mutation accumulation. Comparison of the hybrid ‘young’ and ‘old’ lines with the base stock did not support the idea that the superior longevity and late life fertility of the ‘old’ lines relative to the ‘young’ lines could be accounted for by the effects of mutation accumulation in the ‘young’ lines. The results point to the need to compare selected lines with their base stock when deducing responses and correlated responses to selection and to avoid unintentional selection. In this type of experiment, larval density should be standardized during selection, and adults should not be under pressure for rapid maturation.     

INTROGRESSION AND INCORPORATION. STRATEGIES FOR THE USE OF CROP GENETIC RESOURCES

• 1 In recent decades there has developed a very general appreciation of the need to build, maintain and use collections of crop plants with the primary object of sustaining genetic advance by plant breeding on into the indefinite future. The fact of genetic erosion is universally acknowledged and substantial advances in some practical aspects of genetic resource conservation work have been made. Many good collections have been assembled, though some crops have been poorly served, even ignored, and maintenance, study and utilization of those that have been collected have often left much to be desired.
• 2 This review is devoted to the most neglected aspect of all, namely utilization. Traditionally, collections have been regarded as sources of ‘genes’, usually disease resistances, to be exploited by backcrossing (Introgression) into adapted stocks. Thousands of such backcrosses have been made, with very varied success; there have been successes but also many failures, due usually to the weakness of ‘vertical resistances’ as protection against adaptable pathogens. By contrast, the need to broaden the genetic bases of many crops, far beyond the confines possible for Introgression programmes, has often been recognized but rarely explored. Thus a distinction is drawn between Introgression and Incorporation as the two fundamental methods of using crop collections (see Fig. 1).
• 3 Incorporation implies the systematic exploitation of a large array of genetic variability in such a way as to generate a mass of newly adapted stocks usable as parents in breeding programmes. Genetic principles are simple and obvious, always based on recurrent cycles of recombination and mass selection. Progress is likely to be slow (indeed usually must be slow), a fact which has often been a discouragement to such programmes in the past.
• 4 Examples of long-term, systematic Incorporation programmes are few, and the more important ones are summarized. They relate topotatoes, sugar cane and maize, which are well advanced; and cocoa, oil palm and (prospectively) rubber, which are well begun. Time scales depend upon the biology of the crop and methods; typically, a few decades are necessary to make serious progress.
• 5 In discussion, the following points are emphasizedthe essential nature of Incorporation (= Base broadening) in all well-bred crops in the longer term and into the indefinite future; the weakness of phenotypic assessment of genetic potential (usually referred to as ‘evaluation’); the fact that Incorporation is esentially simple, even if slow; the nomenclatural confusion that surrounds the terms Genetic Enhancement, Prebreeding, Genetic Vulnerability; the fact that the common stereotype of a crop collection as a collection of seed packets is often wrong because many crops are clonal and/or have short-lived, unstorable seeds; and the socio-politico-bureaucratic complications that arise because of the current collapse of publicly supported research, the associated short-term ideology, and the fact that serious genetic resource work, including Incorporation programmes, demands long-term commitment to the interests of our successors, not just to ourselves.

     

抗真菌植物基因工程的策略和进展

所有高等植物都受多种真菌的侵害,水稻的240多种病害中真菌性痫害占90％。,可见真菌病害是世界范围内危害作物产蘑的主要因素之一,是长期以来作物育种学家一直在努力攻克的难题。目前国     

Transgenic tomato plants expressing the <Emphasis Type="Italic">Arabidopsis NPR1</Emphasis> gene display enhanced resistance to a spectrum of fungal and bacterial diseases

Development of effective disease-resistance to a broad-range of pathogens in crops usually requires tremendous resources and effort when traditional breeding approaches are taken. Genetic engineering of disease-resistance in crops has become popular and valuable in terms of cost and efficacy. Due to long-lasting and broad-spectrum of effectiveness against pathogens, employment of systemic acquired resistance (SAR) for the genetic engineering of crop disease-resistance is of particular interest. In this report, we explored the potential of using SAR-related genes for the genetic engineering of enhanced resistance to multiple diseases in tomato. The Arabidopsis NPR1 (nonexpresser of PR genes) gene was introduced into a tomato cultivar, which possesses heat-tolerance and resistance to tomato mosaic virus (ToMV). The transgenic lines expressing NPR1 were normal as regards overall morphology and horticultural traits for at least four generations. Disease screens against eight important tropical diseases revealed that, in addition to the innate ToMV-resistance, the tested transgenic lines conferred significant level of enhanced resistance to bacterial wilt (BW) and Fusarium wilt (FW), and moderate degree of enhanced resistance to gray leaf spot (GLS) and bacterial spot (BS). Transgenic lines that accumulated higher levels of NPR1 proteins exhibited higher levels and a broader spectrum of enhanced resistance to the diseases, and enhanced disease-resistance was stably inherited. The spectrum and degree of these NPR1-transgenic lines are more significant compared to that of transgenic tomatoes reported to date. These transgenic lines may be further explored as future tomato stocks, aiming at building up resistance to a broader spectrum of diseases.     

Genetic mapping of pear sawfly (<Emphasis Type="Italic">Caliroa cerasi</Emphasis>) and pear blister mite (<Emphasis Type="Italic">Eriophyes pyri</Emphasis>) resistance in an interspecific pear family

Whilst minor pests of pear, both sawfly larvae (pear slug) and pear blister mite can at times cause sufficient damage in commercial and particularly in organic pear production for treatment to be required. In the course of breeding new pear cultivars, resistance to both pests was identified in an interspecific pear family raised from a cross between ‘PremP003’ and ‘Moonglow’. The replicated seedling family was subjected to uninhibited insect development for both pests in an insect-proof cage, providing ample infestations for resistance segregation. Using an existing genetic map for the family, one major quantitative trait locus (QTL) for resistance to pear blister mite was located to linkage group 13 (LG13) of ‘PremP003’. For pear slug, we mapped three QTLs for oviposition antixenosis, one each on LG7 and LG9 of ‘Moonglow’ and another on LG10 of ‘PremP003’, and one resistance QTL for leaf damage to LG9 of ‘Moonglow’ at a distance of 8.1 cM below the oviposition QTL. Incorporating these resistances into future cultivars could contribute to a reduction in pesticide use in pear production, especially in combination with the resistances for pear psylla (Cacopsylla pyri) and fire blight (Erwinia amylovora) recently mapped in the same population using marker-assisted selection.     

A curious case of resistance to a new encounter pathogen: myrtle rust in Australia

Resistance genes (R genes) in plants mediate a highly specific response to microbial pathogens, often culminating in localized cell death. Such resistance is generally pathogen race specific and believed to be the result of evolutionary selection pressure. Where a host and pathogen do not share an evolutionary history, specific resistance is expected to be absent or rare. Puccinia psidii, the causal agent of myrtle rust, was recently introduced to Australia, a continent rich in myrtaceous taxa. Responses within species to this new pathogen range from full susceptibility to resistance. Using the myrtle rust case study, we examine models to account for the presence of resistance to new encounter pathogens, such as the retention of ancient R genes through prolonged ‘trench warfare’, pairing of resistance gene products and the guarding of host integrity.