Potential plant–aphid–fungal associations aiding conservation biological control of cereal aphids in Argentina

The overall aim of this study was to identify potential associations between aphids and plants as reservoirs of entomophthoralean fungi. A survey of weeds associated with wheat field borders in two different localities was performed over two years, from April 2010 to April 2012 in the Pampeana central region, Argentina. On each sampling date, five individuals of each plant species were randomly selected, and healthy and infected aphids were collected and quantified once a month. The taxonomic identities of both aphid hosts and entomophthoralean fungi were established. Aphid–fungus associations identified as favorable for inclusion in conservation biological control strategies in borders of wheat crops are the following: Sonchus oleraceus (L.)/Hyperomyzus carduellinus (Theobald), Uroleucon sonchi (L.)/Pandora neoaphidis (Remaudière & Hennebert) Humber; S. oleraceus (L.)/H. carduellinus (Theobald)/Zoophthora radicans (Brefeld) Batko; Lamium amplexicaule (L.)/Cryptomyzus korschelti Bôrner/P. neoaphidis, Z. radicans, Entomophthora planchoniana Cornu; Foeniculum vulgare (Miller)/Dysaphis apiifolia (Theobald)/Z. radicans; Morrenia brachystephana Griseb/Aphis nerii Boyer de Fonscolombe/P. neoaphidis and Brassica rapa L./Brevicoryne brassicae (L.)/P. neoaphidis.     

Endophytic fungi in the invasive weed Impatiens glandulifera: a barrier to classical biological control?

The rust fungus, Puccinia komarovii var. glanduliferae, has been introduced into the UK for biological control of the invasive weed, Impatiens glandulifera (Himalayan balsam). However, establishment of the pathogen has differed across the country, which may be partly explained by variation in plant genotype. The aim of this study was to examine whether there is a further layer of phenotypic resistance, provided by indigenous foliar endophytic fungi. Culturable endophytes were isolated from a number of different balsam populations, and the commonest species were inoculated into ‘clean’ balsam plants, to test their interactions with the rust. We found that endophyte communities within balsam are low in diversity and become more dissimilar with increasing distance between populations. Three endophytes (Colletotrichum acutatum, Alternaria alternata and Cladosporium oxysporum) were common and appeared to be antagonistic to the rust, reducing pustule number and mitigating the effect of the pathogen on plant biomass. I. glandulifera thus partially conforms to the endophyte-enemy release hypothesis, in that as an introduced species, it has an impoverished endophyte complement, acquired from the local environment. However, these endophytes represent a potential barrier to effective biological control and future weed control strategies need to find strains of rust that can overcome plant genetic resistance and the overlaying phenotypic resistance, conferred by endophytes. Future classical biological control programmes of weeds must therefore take into account the fungal bodyguards that invasive species may acquire in their introduced ranges.     

Can we forecast the effects of climate change on entomophagous biological control agents?

The worldwide climate has been changing rapidly over the past decades. Air temperatures have been increasing in most regions and will probably continue to rise for most of the present century, regardless of any mitigation policy put in place. Although increased herbivory from enhanced biomass production and changes in plant quality are generally accepted as a consequence of global warming, the eventual status of any pest species will mostly depend on the relative effects of climate change on its own versus its natural enemies' complex. Because a bottom‐up amplification effect often occurs in trophic webs subjected to any kind of disturbance, natural enemies are expected to suffer the effects of climate change to a greater extent than their phytophagous hosts/preys. A deeper understanding of the genotypic diversity of the populations of natural enemies and their target pests will allow an informed reaction to climate change. New strategies for the selection of exotic natural enemies and their release and establishment will have to be adopted. Conservation biological control will probably become the keystone for the successful management of these biological control agents. © 2013 Society of Chemical Industry     

Phytoplasmas change the source–sink relationship of field-grown sweet cherry by disturbing leaf function

Changes in the function of field-grown sweet cherry leaves infected with phytoplasma were evaluated through the analysis of photosynthesis, respiration, carbohydrates and hormones. Phytoplasmal infection caused witches' broom, small leaves, leaf yellowing and leaf rolling. The photosynthesis of infected leaves was considerably reduced, and they were unable to produce sufficient carbohydrates for their own needs. In contrast, the starch content of infected leaves was significantly increased. These results demonstrate the change in the role of infected leaves from sources to sinks. Further analysis revealed that the photosynthetic decline was related to a significant decrease in photosynthetic pigments and to marked inactivation of photosystem II (PSII). Furthermore, the loss of PSII function was due to a decrease in chlorophyll content, reduction and closure of active reaction centers, and decline in photochemical efficiency.     

Is the onset of septoria tritici blotch epidemics related to the local pool of ascospores?

To elucidate the early epidemic stages of septoria tritici blotch, especially the relationship between the onset of epidemics, the local availability of primary inoculum, and the presence of wheat debris, the early disease dynamics and airborne concentration in Zymoseptoria tritici ascospores were concomitantly assessed at a small spatiotemporal scale and over two years, using spore traps coupled with a qPCR assay. One plot, with the crop debris left, provided a local source of primary inoculum, while the other plot, without debris, lacked any. According to the assay's limits of detection, daily spore trap samples were classified as not detectable or not quantifiable, detectable, and quantifiable. The proportions of samples assigned to the different classes and numbers of spores in samples classified as quantifiable were significantly different between years, time periods (from November to March) and spore trap location (field with or without debris). The effect of year on the airborne ascospore concentration was high: 22 daily peaks with more than 230 ascospores m^?3 day^?1 were identified in the autumn of 2012/13, but none in the autumn of 2011/12. The local presence of wheat debris had no obvious effect on the amount of airborne ascospores or on the earliness of the two epidemics, especially in the year with high inoculum pressure (2012/13). These results suggest that the amount of primary airborne inoculum available in a wheat crop is not a limiting factor for the onset of an epidemic.     

How widespread is Parthenium hysterophorus and its biological control agent Zygogramma bicolorata in South Asia?

Parthenium hysterophorus is a weed of global significance causing severe economic, environmental, human and animal health problems in Asia, Africa, Australia and the Pacific. In South Asia, P. hysterophorus occurs in India, Pakistan, Sri Lanka, Bangladesh and Nepal. A host-specific leaf-feeding beetle Zygogramma bicolorata from México was introduced into India in 1984, as a biological control agent for P. hysterophorus . In this study, a GIS-based distribution map of P. hysterophorus and its biological control agent Z. bicolorata in South Asia based on meta-analysis is presented. The map highlights the limited published information on P. hysterophorus incidence in many of the states and territories in India, as well as in neighbouring Bangladesh, Bhutan, Nepal and Pakistan. Incidence of Z. bicolorata was recorded as three distinct clusters, covering many states in India. In Pakistan, Z. bicolorata was recorded in the Punjab region bordering India. A CLIMEX model based on the current distribution of Z. bicolorata in India suggests that the geographic range of this agent in India and Pakistan can extend to other P. hysterophorus -infested areas in the region. The CLIMEX model also suggests that all of Bangladesh and Sri Lanka, and parts of Nepal are climatically suitable for Z. bicolorata.     

Is the emergence of Dothistroma needle blight of pine in France caused by the cryptic species Dothistroma pini?

Dothistroma needle blight (DNB) emerged in France in the past 15 years. This disease is induced by two closely related species: Dothistroma septosporum and D. pini. Although both species are currently present in France, only D. septosporum was reported in the past. We investigated whether a recent arrival of D. pini in France could be a cause of the DNB emergence. We analyzed herbarium specimens of pine needles with DNB symptoms using polymerase chain reaction techniques to study the past frequency of D. pini in France. We also determined the present distribution within the country of D. septosporum and D. pini and compared it with the spatial pattern of DNB reported in the Département de la Santé des Forêts (DSF; French forest health monitoring agency) database. Although D. pini was detected on herbarium specimens from 1907 and 1965, it was not frequent in France in the past. Today, it is frequent, although not present throughout the country, being absent from the north and the east. There is no relationship between the D. pini distribution in France and the spatial pattern of DNB reported in the DSF database. Thus, the emergence of DNB in France cannot be explained by a recent arrival of D. pini.     

Is there need for leaf-galling grape phylloxera control? Presence and distribution of Dactulosphaira vitifoliae in Swiss vineyards

Grape phylloxera (Dactulosphaira vitifoliae, FITCH) was a major pest in Vitis vinifera grape production in the late 19th and early 20th century in Europe. Roots of V. vinifera varieties are susceptible to D. vitifoliae attack whereas the leaves are not affected. The pest has been controlled by using American grapevine varieties as rootstocks. However, in recent years, grape producers have reported higher incidences of leaf galls induced by D. vitifoliae on V. vinifera leaves. Research on the occurrence of D. vitifoliae on grapevine leaves and roots has shown that D. vitifoliae is present in all wine-growing regions of Switzerland. In more than 80% of vineyards claiming to be infested, D. vitifoliae was found on leaves of varieties with interspecific crosses in their parentage. Three locations with leaf-galling individuals on V. vinifera were detected. D. vitifoliae individuals were found emerging from soil potentially associated with the commonly used rootstock varieties, which are assumed to be phylloxera tolerant. As climate change may necessitate the relocation of some viticultural areas to new wine-growing regions, there will be a greater need to control D. vitifoliae on leaves of interspecific crosses and the inspected planting material to limit further spread of the pest.     

Transgenic crops and beyond: how can biotechnology contribute to the sustainable control of plant diseases?

Disease resistance is without argument the best technological approach to control diseases in plants since no management input is required by the grower once the resistant variety has been planted. The biggest problems in using disease resistance lie in the facts that effective sources of resistance are not available for many important diseases, especially those caused by necrotrophic pathogens; and that pathogen populations adapt to the utilisation of novel sources of resistance, most notably for air-borne biotrophic pathogens. Several biotechnological approaches have been developed to produce disease resistant plants, the most recent known as NBT – New Breeding Technologies. This review focuses on recent advances in those technologies which adapt the knowledge obtained using molecular genetic approaches for the study of plant-microbe interactions to combat plant diseases.     

Multiple facets of response to fungicides – the influence of azole treatment on expression of key mycotoxin biosynthetic genes and candidate resistance factors in the control of resistant <Emphasis Type="Italic">Fusarium</Emphasis> strains

The spread of fungicide resistant and/or tolerant phytopathogenic fungi is an important factor affecting crop protection. However, the mechanisms of fungal response to fungicide application are not entirely characterised. In particular, the contribution of previously known resistance factors and the final influence of fungicide treatments on metabolism of surviving mycelia (e.g. mycotoxin increased release and biosynthesis potentially causing contamination of the crops) merit investigation, in order to improve future molecular diagnostics of fungicide resistant strains. The performed experiments have shown distinct expression changes for homologs of a known fungicide resistance factor Flr1 (yeast; DHA1 family of major facilitator superfamily transporters) after azole application in cultured fusaria. Two distantly related homologs of that gene were selected, based on the unsupervised clustering and phylogenetic analysis of transporter sequences. One of these (FGSG_02865), was found to occur across the Fusarium sambucinum complex (F. graminearum, F. culmorum, F. cerealis) and was upregulated starting 24 h after fungicide treatments. This delayed response may point to possible involvement of DHA1 antiporters in a generalised response to stress resulting from fungicide treatment. Additional expression profiling was conducted for the mycotoxin biosynthetic genes (trichothecene and zearalenone gene clusters) in strains of Fusarium sambucinum complex cereal pathogens. The expression changes, when subjected to treatment with the fungicides (flusilazole, carbendazim), show that even an effective treatment (in this study, the benzimidazole fungicide carbendazim) applied to the grown mycelium, can result in enhanced activation of mycotoxin biosynthetic genes in fungal cells which survive the treatment. Our results suggest that increased mycotoxin contamination can be strongly influenced not only by the amount or the type of antifungal compound, but also the timing of fungicide exposition (stage of infection).     

Are herbicides a once in a century method of weed control?

The efficacy of any pesticide is an exhaustible resource that can be depleted over time. For decades, the dominant paradigm – that weed mobility is low relative to insect pests and pathogens, that there is an ample stream of new weed control technologies in the commercial pipeline, and that technology suppliers have sufficient economic incentives and market power to delay resistance – supported a laissez faire approach to herbicide resistance management. Earlier market data bolstered the belief that private incentives and voluntary actions were sufficient to manage resistance. Yet, there has been a steady growth in resistant weeds, while no new commercial herbicide modes of action (MOAs) have been discovered in 30 years. Industry has introduced new herbicide tolerant crops to increase the applicability of older MOAs. Yet, many weed species are already resistant to these compounds. Recent trends suggest a paradigm shift whereby herbicide resistance may impose greater costs to farmers, the environment, and taxpayers than earlier believed. In developed countries, herbicides have been the dominant method of weed control for half a century. Over the next half‐century, will widespread resistance to multiple MOAs render herbicides obsolete for many major cropping systems? We suggest it would be prudent to consider the implications of such a low‐probability, but high‐cost development. © 2017 Society of Chemical Industry     

Do we have the tools to manage resistance in the future?

Pesticide resistance is a major factor affecting world food and fibre production, but that has been contained so far by the availability of diverse modes of action. Overcoming resistance by switching to a new mode of action is a concept easily grasped by growers but threatened by losses through resistance and new registration requirements. Opportunities for innovation and development of a diversity of novel modes of action exist through harnessing recent advances, fundamental to all eukaryotes and largely funded for medical rather than agricultural objectives, in understanding cell biology and development. The cystoskeleton, cell wall synthesis, signal transduction and RNAi are discussed as examples where new targets are now exposed. However, new modes of action will be delivered not only by sprayer or seed treatment but also through transgenic crops, although these still need to be transferred from experiment to practice. Improvements in modelling protein structures and target-site changes, supplemented by rapid diagnostics to detect resistance early, will improve resistance risk management and integrate chemical, biopesticide, transgenic and conventional breeding around the concept of diversity in modes of action. However, before agronomy can translate this into practical antiresistance strategies, there is a need to direct more resources to the biochemistry and cell biology of pests, diseases and weeds to translate these new discoveries into key tools needed to manage resistance in the future.     

Interactions in the Brassica napus–Pyrenopeziza brassicae pathosystem and sources of resistance to P. brassicae (light leaf spot)

Pyrenopeziza brassicae, cause of light leaf spot (LLS), is an important pathogen of oilseed rape and vegetable brassicas and has a wide geographic distribution. Exploitation of host resistance remains the most sustainable and economically viable solution for disease management. This study evaluated 18 oilseed rape cultivars or breeding lines for host resistance against P. brassicae in glasshouse experiments. Selected cultivars/lines were inoculated with eight single-spore isolates of the pathogen obtained from three different regions in England. Analysis of P. brassicae infection-related changes on host plants identified leaf deformation as a characteristic feature associated with P. brassicae infection, this showed poor correlation to LLS severity measured as the amount of pathogen sporulation on infected plants. Resistant host phenotypes were identified by limitation of P. brassicae sporulation, with or without the presence of a necrotic response (black flecking phenotype). Investigation of this pathosystem revealed significant differences between cultivars/lines, between isolates, and significant cultivar/line-by-isolate interactions. In total, 37 resistant and 16 moderately resistant interactions were identified from 144 cultivar/line-by-isolate interactions using statistical methods. Most of the resistant/moderately resistant interactions identified in this study appeared to be nonspecific towards the isolates tested. Our results suggested the presence of isolate-specific resistant interactions for some cultivars. Several sources of resistance have been identified that are valuable for oilseed rape breeding programmes.     

Is the current state of the art of weed monitoring suitable for site‐specific weed management in arable crops?

Weed monitoring is the first step in any site‐specific weed management programme. A relatively large variety of platforms, cameras, sensors and image analysis procedures are available to detect and map weed presence/abundance at various times and spatial scales. Remote sensing from satellites or aircraft can provide accurate weed maps when the images are obtained at late weed phenological stages. Cameras located on unmanned aerial vehicles (UAVs) have been shown to be adequate for early‐season weed detection in a variety of wide‐row crops, providing images with relatively high spatial resolutions. Alternatively, weed detection/mapping systems from ground‐based platforms can achieve even higher resolutions using a variety of non‐imaging and imaging technologies. These ground systems are suited, in some cases, for real‐time site‐specific weed management. Despite this rich arsenal of technologies, their commercial adoption is, apparently, low. In this study, we describe the state of the art of remotely sensed and ground‐based weed monitoring in arable crops and the current level of adoption of these technologies, exploring major constraints for adoption and trying to identify research gaps and bottlenecks.