A host-pathogen simulation model: powdery mildew of grapevine

An epidemiological model simulating the growth of a single grapevine stock coupled to the dispersal and disease dynamics of the airborne conidia of the powdery mildew pathogen Erysiphe necator was developed. The model input variables were either climatic (temperature, wind speed and direction) or related to the pathogen (location and onset of primary infection). The environmental input variables dictated plant growth and pathogen spread (latent period, infection, lesion growth, conidial spore production and release). Input parameters characterized the crop production system, the growth conditions and the epidemiological characteristics of the pathogen. Output described, at each time step, number, age and pattern of healthy and infected organs, infected and infectious leaf area and aerial density of spores released. Validation of the model was achieved by comparing model output with experimental data for epidemics initiated at different times of host growth. Epidemic behaviour for two contrasting years of crop development and 7 phenological stages at the time of primary infection (PI) was examined. For PI occurring at day 115 a vine with late budbreak (1998) showed 58% of primary leaves diseased at flowering compared with only 19% for a vine with early budbreak (2003). Depending on the phenological stage at PI (1–4 leaves), the proportion of diseased primary leaves decreased from 42% to 6% at flowering. Simulations suggested that differences resulted from the interplay between the timing of the first sporulation event, the phenological stage at the time of initial infection, and the age structure and spatial distribution of the leaf population.     

A qualitative host-pathogen interaction in the Theobroma cacao-Moniliophthora perniciosa pathosystem

The aim of this study was to test whether resistance of clones of Theobroma cacao (cocoa) varied between isolates of Moniliophthora (formerly Crinipellis ) perniciosa , the cause of witches' broom disease. Developing buds of vegetatively propagated T. cacao grown in greenhouses in the UK were inoculated with 16 000 spores of M. perniciosa per meristem in water, under conditions where water condensed on the inoculated shoot for at least 12 h after inoculation. The proportion of successful inoculations varied between clones and was inversely correlated with time to symptom production or broom formation. A specific interaction was demonstrated among three single-spore isolates of M. perniciosa and the clone Scavina 6 (SCA 6) and a variety of susceptible clones. Isolates Castenhal-I and APC3 were equally likely to infect SCA 6 and the other clones, but isolate Gran Couva A9 never infected SCA 6, although it was as virulent on the other clones. The interaction was maintained when the wetness period was extended to 70 h. Offspring of SCA 6 × Amelonado matings were all susceptible to both Castenhal-I and GC-A5, with no evidence of greater variability in susceptibility to GC-A5 than Castanhal-I. This suggests recessive inheritance of a single homozygous factor conferring resistance to GC-A5, from SCA 6. The progenies were slightly more susceptible to Castanhal-I than GC-A5. The implications for managing the disease are discussed.     

A gene-for-gene relationship in the Triticum-ustilago system,and some remarks on host-pathogen combinations in general

Summary In theTriticum-Ustilago system a gene-for-gene relationship presumably exists. Two sets of resistance factors and two sets of incompatibility factors in the host, corresponding to four sets of complementary factors for virulence in the pathogen, can explain the interactions of 8 variety groups of the host with 6 physiologic races of the pathogen.Some comments are made on the applicability of the gene-for-gene concept in systems involving either a biotrophic or a perthotrophic pathogen as one of the partners.Samenvatting Bij de combinatie tarwe-stuifbrand bestaat vermoedelijk een gen-om-gen verband. Twee paar resistentie-en twee paar incompatibiliteitsfactoren in de gastheer, corresponderende met vier paar complementaire virulentiefactoren in het pathogeen, kunnen de interacties verklaren van 8 rassengroepen van de gastheer met 6 fysio's van het pathogeen.Naar aanleiding van de publikaties vanPerson (1959) enPerson et al. (1962) worden enkele opmerkingen gemaakt betreffende de toepasbaarheid van het gen-om-gen begrip in combinaties waarbij hetzij een biotroof hetzij een perthotroof pathogeen een van de partners is.     

An excised-leaf inoculation technique for evaluating host-pathogen interactions and quantitative resistance of bermudagrass genotypes to dematiaceous hyphomycetes

ABSTRACT Excised leaves of bermudagrass were inoculated with mycelium of isolates of Bipolaris, Exserohilum, Curvularia, and Drechslera spp. in water agar plates to evaluate differences in susceptibility of leaf tissue, virulence of pathogens, and quantitative resistance of bermudagrass genotypes. Isolates of nine species of pathogens induced similar symptoms of light- to dark-brown necrosis and bordering chlorosis in excised leaves that were not distinct for individual species or genera. Severity of symptoms induced by most isolates increased progressively from younger to older leaves. Within and across leaf positions, numerous significant differences in virulence of isolates within fungal species and between species were observed. Among 40 randomly selected bermudagrass genotypes, a continuous quantitative gradient was observed for mean scores of disease severity in excised leaves inoculated with E. rostratum. Numerous significant differences were observed within this gradient, and severity of symptoms in the most susceptible genotypes was approximately double that in the most resistant. When intact foliage of genotypes from the resistant and susceptible extremes of the gradient was inoculated with spores of E. rostratum, corresponding differences in severity of symptoms and significant (P = 0.05) correlations between results with excised leaves and intact foliage were observed. However, the range of differences in disease severity between genotypes was more narrow in intact foliage than in excised leaves. Results indicate that the excised leaf inoculation technique can be used to evaluate the relative resistance of bermudagrass genotypes to E. rostratum for use in programs to breed for quantitative host resistance.     

Systems Modelling as an Approach to Pest Management

Emphasis is placed upon the value of a systems approach as a tool for interrelating the complex interdisciplinary components of the increasingly desirable pest management programs. A brief outline of the parameters incorporated and construction of a mathematical model are given. Selected examples are given of models being implemented at present, and the benefits arising from them, such as in the cotton, alfalfa and apple ecosystems in the United States. Future effectuation, in Europe, of complete pest management schemes for plant diseases and nematodes, insects and mites, birds, rodents and weeds is needed, but will require concerted co-operation between governments and organizations to initiate programs comparable to those implemented in the United States. Provision should be made for some form of extension/education system, such that the methods and benefits will be realized.     

Pheromone-baited traps as an aid in studying the phenology of the potato tuber moth,Phthorimaea operculella (Zell.), in Israel

Potato tuber moths were found to be on the wing in Israel throughout the year, as revealed by captures in traps based on pheromone-baited caps which remained attractive to males, without replacement of the pheromone sources, for over a year.     

Recent developments in our understanding of the plant cuticle as a barrier to the foliar uptake of pesticides†

The plant cuticle is a highly complex membrane which forms the outer surface of the aerial portion of plants. The nature of the plant cuticle is reviewed with particular regard to its action as a potential barrier to the penetration of pesticide molecules; the role of the cuticular waxes is highlighted. The physicochemical properties of the cuticle influence the behaviour of spray droplets and, in turn, may affect the rate and efficiency of cuticle penetration. The permeation of active ingredients is influenced by their solubility characteristics as indicated by octanol/water (log K_ow) and cuticle/water (K_cw) partition coefficients. Penetration of hydrophilic compounds (low log K_ow) may be enhanced by hydration of the cuticle, while transcuticular transport of non-polar solutes (high log K_ow) is increased by factors which reduce wax viscosity. The use of in-vitro models involving isolated cuticle membranes, isolated cuticle waxes, or isolated leaves has helped to focus on the activities of the cuticle in the absence of other physiological factors. Using these systems, the role of the waxes as a transport-limiting barrier has been identified and the factors influencing sorption, permeance and desorption examined. The action of surfactants, in vitro and in vivo, has been briefly addressed in regard to their role in facilitating cuticle penetration; other steps involving surfactant/solute/cuticle are complex, and synergy appears to depend on a number of factors including test species, concentration of active ingredient, surfactant type and concentration. Adjuvants may greatly influence the surface properties of the droplet, predispose the cuticle to solute transport, and enhance pesticide activity. The nature of these complex inter-relationships is discussed. © 1999 Society of Chemical Industry     

Shifting focus from the population to the individual as a way forward in understanding,predicting and managing the complexities of evolution of resistance to pesticides

The evolution of resistance to pesticides is often conceptualised and modelled at a population level, but population‐based approaches ignore important aspects of variability between individuals within populations that may be essential drivers of resistance. Here it is argued that individual‐based modelling has the potential to generate new insights and perspectives, thus deepening our understanding of the complexities of the evolutionary dynamics of resistance to pesticides. Copyright © 2012 Society of Chemical Industry     

针茅狭跗线螨生物学及其寄主专一性研究

针对针茅芒刺对草地畜牧业的危害,采取生物防治措施控制其发生与危害.通过调查明确了针茅芒刺的天敌类群,并对针茅芒刺优势天敌--针茅狭跗线螨生物学及寄主专一性进行了研究,阐明了针茅狭跗线螨对针茅芒刺的抑制机理,明确了其寄主范围,以确保在针茅草原利用该天敌防治针茅芒刺的安全性.     

Mulched cover crops as an alternative to conventional weed management systems in vineyards

Conventional methods of weed management in vineyards rely primarily on herbicides and tillage. The desire to adopt alternatives to these methods is driven by environmental and economic reasons. Weed suppression and grape yield under mulched cover crop systems at two rainfed northern California vineyards were similar to, and at times exceeded, those under conventional tillage or herbicide management. Cover crop productivity was positively correlated with weed suppression and mulch decomposition rates and seemed to be determined primarily by location and then by cover crop type. The mulch from mowed cover crops averaged 603(± 94) gm⁻² at the two sites. Weed suppression was linked to light interception by the mulch cover for most weed species. Subterranean clover planted directly in the vine row significantly reduced weed cover where it established. The increased dominance of the perennial Convolvulus arvensis and reduction of certain annual species was indicative of species compositional changes in all treatments. Profits under the cover cropping systems exceeded those under conventional tillage and herbicide systems by €  794 ha⁻¹ averaged over the duration of the experiment at both locations.     

Sodium Alginate as an Adjuvant of an Antagonistic Bacterium, Pseudomonas fluorescens Strain A11RN, to Enhance Biocontrol of Turfgrass Snow Mold Caused by Typhula ishikariensis

Pretreatment with sodium alginate (20 g/m²) prior to application of a biocontrol agent, Pseudomonas fluorescens strain A11RN, enhanced the control turfgrass snow mold caused by Typhula ishikariensis biotype B. Disease severities in two fields relative to untreated control plots were 53% and 61% in plots treated with both the strain and sodium alginate, which were significantly lower than 69% and 77% in plots treated only with the strain. The antagonist population remained above the threshold for effective control (ca. 1×10⁷ cfu/cm³) at least for the first month under snow in pretreatment with sodium alginate. The population of the biocontrol agent after snowmelt on turfgrass was five times higher in sodium alginate-amended plots than in unamended plots. Sodium alginate alone did not reduce disease severity. Sodium alginate might have supported the survival of the applied strain. Received 7 June 1999/ Accepted in revised form 20 December 1999     

Lesion expansion as an epidemic component

ABSTRACT A simulator for the enlargement of cohorts of circular lesions on cohorts of host tissue was used to examine five epidemiological parameters: radial rate (mm day(-1)) of lesion expansion, k (exp); maximum basic infection rate, R (m); proportion of lesion area as infectious, f; initial lesion size (mm(2)), z; and proportion of susceptible host sites, s. Based on the proportion of disease severity at day 50 and the proportion of the total disease that originated solely from lesion expansion, k(exp) was the most sensitive of the five parameters. A radial rate of only 0.1 mm day(-1) resulted in a proportion of >0.7 of the diseased area that came from lesion expansion. In an extensive survey of phytopathological literature, many plant pathogens had radial rates greater than 0.1 mm day(-1), which would result in a proportion of >0.95 of the total disease that comes from lesion expansion. Susceptible host sites, s, was a sensitive parameter, as this determined the host area into which lesions could expand. Naturally, R(m) was a sensitive parameter for the proportion of disease on day 50, as it controlled the overall speed of the epidemic. Initial lesion size was a relatively insensitive parameter, although z interacted significantly with s. The greatest proportion of disease that originated from lesion expansion occurred with fast k(exp), small z, and low values of s, R(m), and f. The model was validated with lesion numbers and severities obtained in natural epidemics of Cercospora medicaginis on alfalfa and Exserohilum turcicum on maize. We recommend that the 'epidemic quintuplet' used to describe polycyclic epidemics be expanded to the 'epidemic sextuplet' with the inclusion of k(exp), since lesion expansion is a major component of many polycyclic epidemics.     

Modeling insect resistance to insecticides using velvetbean caterpillar (Anticarsia gemmatalis) as an example

Evolution of insect resistance to insecticides, using the velvetbean caterpillar (Anticarsia gemmatalis) as an example, was evaluated with the help of a computer simulation model. The effects of the following factors were studied: mortality rate, dominance and initial frequency of the resistant gene, migration rate, reproductive disadvantage and action level. According to results of the model, mortality rate, genetic dominance and frequency, and insect migration were the most critical factors involved with the rate of evolution of insect resistance to insecticides. Mortality rate can be directly managed, and migration indirectly through preservation of refugia, as part of a program of resistance management, which should include insecticides with different modes of action on the insect, and at minimum efficient rates. © 1998 SCI     

Formation of satellite uredinia as an important trait related to grapevine colonization by Phakopsora meliosmae-myrianthae

Phakopsora meliosmae-myrianthae, the causal agent of Asian grapevine leaf rust, significantly reduces the photosynthetic efficiency of grapevine leaves in green symptomless tissues surrounding lesions. This study took a close look at grapevine leaf colonization kinetics by P. meliosmae-myrianthae and compared it to P. pachyrhizi–soybean and Uromyces appendiculatus–bean colonization. It is already known from the literature that soybean rust, similar to grapevine rust, has a negative effect on leaf photosynthesis greater than would be expected based on visual lesions. However, in contrast to soybean and grapevine rusts, the effect of bean rust on leaf photosynthesis is proportional to the diseased leaf area. Colonization progress was monitored by fungal biomass assessed via histological staining and quantitative polymerase chain reaction (qPCR). Individual lesions of P. meliosmae-myrianthae on grapevine, P. pachyrhizi on soybean and U. appendiculatus on common bean leaves were evaluated every 3–4 days, and the number of uredinia was counted. Staining showed that mycelial colonization did not extend beyond the lesion border. The number of P. pachyrhizi and P. meliosmae-myrianthae uredinia within the lesions increased over time (on average 14-fold), whereas the number of U. appendiculatus uredinia remained the same. These findings were corroborated by qPCR, which revealed a greater increase in fungal biomass for Phakopsora spp. than for U. appendiculatus until 12 days post-inoculation. The high number of satellite uredinia within lesions might be directly related to the impact of this pathogen in photosynthetic efficiency on symptomless areas of diseased grapevine leaves. This study identified accelerated formation of satellite uredinia as an important feature of grapevine colonization by P. meliosmae-myrianthae.     

Susceptibility of Clematis Varieties and Species to Stem Infection by Phoma Clematidina as an Indicator for Resistance to Wilt

The susceptibility to stem rot and wilt caused by Phoma clematidina was tested for a range of clematis varieties and species widely grown in Europe, and compared with their susceptibility to wilt in practice as judged by amateur and commercial growers in two postal surveys. Most small flowering clematis species did not die back when unwounded stems were inoculated with spores of P. clematidina, and were also reported in the surveys as being very resistant to wilt. In contrast, large flowering hybrids were found to be highly susceptible both to stem infection by P. clematidina and to wilt in practice according to growers. A significant positive linear relationship existed between the susceptibility scores for disease in the stem inoculation test and those for wilt from the surveys. Fungal isolations were made from the stems of naturally wilted clematis plants from British nurseries and private gardens. Unlike the many other fungal species isolated, P. clematidina occurred very frequently, especially in stems of large flowering varieties. Together with the results from the grower surveys and stem inoculation trial, this indicates that this fungus is a regular cause of clematis wilt. The stem inoculation test with P. clematidina described in this study would be useful for European breeders and growers to determine the susceptibility of new clematis varieties to wilt before they are marketed.