Cercospora bladvlekkenziekte in suikerbiet: Epidemiologie, aspecten van de levenscyclus en ziektebeheersing

Op 15 juni 2004 promoveerde Jessica Vereijssen aan Wageningen Universiteit op haar proefschrift getiteld: 'Cercospora leaf spot in sugar beet: Epidemiology, life cycle components and disease management'. De promotor was prof. dr. MJ Jeger en co-promotoren waren dr. ir. JHM Schneider (IRS (Instituut voor Rationele Suikerproduktie)) en dr. ir. AJ Termorshuizen (Biologische Bedrijfssystemen, WU

Approaches for estimating benefits and costs of interventions in plant biosecurity across invasion phases

Nonnative plant pests cause billions of dollars in damages. It is critical to prevent or reduce these losses by intervening at various stages of the invasion process, including pathway risk management (to prevent pest arrival), surveillance and eradication (to counter establishment), and management of established pests (to limit damages). Quantifying benefits and costs of these interventions is important to justify and prioritize investments and to inform biosecurity policy. However, approaches for these estimations differ in (1) the assumed relationship between supply, demand, and prices, and (2) the ability to assess different types of direct and indirect costs at invasion stages, for a given arrival or establishment probability. Here we review economic approaches available to estimate benefits and costs of biosecurity interventions to inform the appropriate selection of approaches. In doing so, we complement previous studies and reviews on estimates of damages from invasive species by considering the influence of economic and methodological assumptions. Cost accounting is suitable for rapid decisions, specific impacts, and simple methodological assumptions but fails to account for feedbacks, such as market adjustments, and may overestimate long-term economic impacts. Partial equilibrium models consider changes in consumer and producer surplus due to pest impacts or interventions and can account for feedbacks in affected sectors but require specialized economic models, comprehensive data sets, and estimates of commodity supply and demand curves. More intensive computable general equilibrium models can account for feedbacks across entire economies, including capital and labor, and linkages among these. The two major considerations in choosing an approach are (1) the goals of the analysis (e.g., consideration of a single pest or intervention with a limited range of impacts vs. multiple interventions, pests or sectors), and (2) the resources available for analysis such as knowledge, budget and time

Cercospora leaf spot in sugar beet. Epidemiology, life cycle components and disease management

Cercospora beticola, the causal agent of Cercospora 1eaf spot (CLS) in sugar beet, causes I reductions in sugar yield and financial returns. Section I describes the epidemiology of CLS. Two disease severity indices were compared and their relation with sugar yield modelled. A linear curve fitted the Agronomica disease severity index with sugar yield in both severely and mildIy infected fields, whereas for a 1eaf disease severity assessment this relationship was exponential and linear, respectively. The Agronomica disease severity index was less laborious and improved disease assessment. Using spatial and temporal rnodels, the within-row dependencies of CLS disease severities was demonstrated. However, disease increase on a given plant was largely determined b>disease on that plant. Similarly when describing the within-plant dynamics of disease, i1 was concluded that the major determinant of disease increase on a given 1eaf layer on ? plant was from disease on that 1eaf layer. Spraying a fungicide twice changed the contribution of the lower 1eaf layers. In one mildly infected field, these 1eaf layers were more often significant in the model; in the other severely infected field less often. Ir section II studies on the life cycle of CLS are reported. It was demonstrated that C beticola is able to infect sugar beet through the root in glasshouse, climate room, and ir field experiments. The genetic variability of C. beticola was studied in an international collection of isolates. Between and within geographic regions, genetic variability is high A C. beticola specific PCR primer was developed, which enabled detection of C. beticolc in plant tissue, We showed systemic growth of C. beticola in a sugar beet seedling after root infection. Section III describes the development of a supervised control system. The use of an action threshold involving two early sprays and weather-based treatment: resulted in sugar yields and financial returns similar to calendar sprayed treatments, am saved up to two fungicide applications

Caging overrides effects of density in laboratory-based physiological estimates of reproductivity in Listronotus bonariensis (Coleoptera: Curculionidae)

This contribution examines the potential for error when the results from laboratory-based experiments are extrapolated to inform field ecological analyses. The effects on the reproductivity of different population densities of Argentine stem weevil Listronotus bonariensis (Kuschel) were studied using caged populations of immature (pre-reproductive) and mature (reproductive) adults. Three densities of weevils equivalent to 125/m² (low), 375/m² (medium) and 1125/m² (high) from field-collected pre- and reproductive populations were confined in plastic containers for either 11 or 21 days with ad libitum food and oviposition-site resources. The impact of the medium and high densities on weevil reproductive physiology was assessed by dissection at 11 and 21 days. Data were collected on sexual maturity, wing muscle development, and the presence of eggs and β-carotene crystals. Only very weak treatment effect responses were observed. A significantly higher percentage of immature females showed oöcyte resorption at high densities at 11 days, but by 21 days this had disappeared. Males had significantly less developed testes at high densities at 11 days, but again by 21 days this effect was not significant. Mature females had significantly larger wing muscles at high density after 11 days of caging; however, by 21 days this effect had gone. For both immature and mature females, there was no effect of density on the relationship between eggs per female and wing muscle index. The low density treatment was not included in this analysis due to low weevil numbers surviving at the end of the experiment. It is concluded that caging itself had a far greater effect on the physiology of the weevils than any imposed population density treatments, which is contrary to the findings of field population-dynamics analyses. Care should be taken in extrapolating laboratory trials involving this genus, and indeed it may occur more widely, to explain field situations.AGMARDT Postdoctoral Fellowship. Grant Number: 99

Root infection of sugar beet by Cercospora beticola in a climate chamber and in the field

Sugar beet root infection by Cercospora beticola, the causal agent of Cercospora leaf spot (CLS), was studied in a climate chamber and in the field. In the climate chamber, root incubation of susceptible seedlings with a conidial suspension resulted in disease incidences that were significantly different for two sugar beet cultivars (Auris: 0.8 ± 0.14 and A00170: 0.5 ± 0.18; P 95) in our climate chamber or after canopy closure in the field. Quantification of root infection and long term survival in soil is necessary to assess its contribution to the epidemiology and life cycle of Cercospora beticola. Cultural methods such as a wider crop rotation, management of crop debris and ploughing systems may provide control strategies alternative to or reducing fungicide input

Comparison of two disease assessment methods for assessing Cercospora leaf spot in sugar beet

Two disease severity scales for Cercospora leaf spot (CLS) assessment were compared. CLS was assessed in two experimental fields in the Netherlands in 1999 using two scales: a single leaf severity assessment, as used in the IPS-system (Integriertes Pflanzenschutzsystem) in Germany, referred to as DSIPS, and a whole plant assessment, Agronomica diagram from Italy, referred to as DSAGR. To obtain a range of disease severities, fungicides were applied at defined action thresholds based on disease incidence and severity. There was an exponential relationship between DSIPS and DSAGR (R2 = 86%) for pooled data with little change in the whole plant assessment above DSIPS = 5%. An exponential curve best fitted DSIPS and root and sugar weight in fields severely infected with CLS, whereas a linear curve was found for mildly infected fields. A linear curve fitted DSAGR best with root and sugar weight in both severely and mildly infected fields. No relationship was found between both DSIPS and DSAGR and sugar content. The use of DSAGR was less time consuming in monitoring and was done with greater accuracy, efficiency and level of reproducibility than DSIPS. These results demonstrate that CLS assessment can be less time-consuming and more practical in application

Possible root infection of Cercospora betiicola in sugar beet

A potential primary infection site of the foliar pathogen Cercospora beticola in sugar beet is described. Sugar beet seedlings of the susceptible cv. Auris were grown in a standard soil for 14 days. A monoconidial culture of a C. beticola isolate was grown to produce conidia. In experiment 1, roots were immersed in a conidial suspension of isolate code IRS 00-4, or in tap water (control), for 2 days. After incubation seedlings were potted in a peat – fine river sand mixture and placed at low relative humidity (RH)

Laboratory bioassay and greenhouse evaluation of Trichogramma cordubensis strains from Portugal

A simple, inexpensive chamber was developed and tested as an evaluative tool to monitor Trichogramma cordubensis dispersal in the laboratory. The chamber consisted of a continuous, winding channel which was cut into an aluminum block. Wasps were released at one end of the channel and allowed to walk in the channel for 21 h and to parasitize Mamestra brassicae eggs placed 3.4 m from the point of wasp introduction. Comparisons between two T. cordubensis populations demonstrated that one population (TCM) dispersed more in the chamber and located host eggs more successfully than the other population (TCD). Subsequent greenhouse releases confirmed that the TCM population dispersed more readily and had significantly higher parasitism rates on sentinel Ephestia kuehniella eggs on tomato plants. The potential utilization of this chamber as a tool to evaluate quality of Trichogramma populations, mainly dispersal activity, is discussed