Friederike de Mol

History of Oilseed Rape Cropping and Geographic Origin Affect the Genetic Structure of Plasmodiophora brassicae Populations

The soilborne pathogen Plasmodiophora brassicae causes clubroot on Brassica crops, a common disease in many oilseed rape growing regions. Here, we investigate genetic diversity and geographic differentiation of P. brassicae populations from different regions in Germany. We compared three regions that differ in oilseed rape cropping history, oilseed rape acreage, and incidence of clubroot. These regions were either spatially separated or separated by the former inner German border. Plasmodiophora isolates were collected from 59 fields (29, 17, and 13 fields per region, respectively) and 174 amplified fragment length polymorphism (AFLP) markers were analyzed. Every field isolate showed a unique genotype pattern; that is, no genotype was shared among the regions and different fields. The mean gene diversity was 0.27, suggesting that P. brassicae is a genetically diverse species. The comparison of indexes (gene diversity, genotypic diversity, and linkage disequilibrium) between the regions does not support our hypotheses that cropping history, oilseed rape acreage, and incidence of clubroot affect these estimates. Principal component analysis (PCA), fixation index (FST), and generalized linear model (GLM) were suitable to specify regional differences. PCA revealed two clusters of isolates based on the geographic origin of the isolates and FST showed that these clusters were highly differentiated. Hypotheses about association of genotypes with different spatial scales were tested with GLM: the region, reflecting the cropping history, and the individual field had a significant effect on the AFLP pattern. We propose that individual field isolates represent a discrete population and that geographic differentiation results from low levels of gene flow due to the limited dispersal of this soilborne pathogen and from localized selection pressure as unifying force on the genotypes.

Approaches to early detection of herbicide resistance in Apera spica-venti regarding intra- and inter-field situations

Herbicides are still the most effective way of weed control. Evolved resistance to herbicides may become a serious and escalating problem in crop production systems worldwide. The challenge in avoiding the dissemination of resistant populations is an early exploration of resistance. Our study aimed at answering the questions: (1) Can the intra-field distribution of an Apera spica-venti population be used to indicate the first steps in the evolution of resistance? (2) Is the selection of field populations based on casual observations of farmers an approach to get a reasonable overview on the resistance status of field populations in monitoring? To answer these questions an intra-field study of one population in Germany and the results of a two years monitoring in north-eastern Germany regarding the actual resistance status of A. spica-venti were used. Dose-response bioassays with isoproturon showed considerable differences in the susceptibility of samples collected within the same field. Herbicide efficacy was not plant density-dependent, but it was related to spatial plant distribution depending on the processing direction in the field. The inter-field situation survey confirmed resistance to at least one herbicide in 15 out of 38 populations. The occurrence of resistance was associated with a mixed tillage system (ploughing and not-ploughing in alternation) and normal winter wheat sowing dates. Fourteen farmers were correct in their assumptions regarding absence of herbicide resistance in their fields. However, resistance was only confirmed in 56% of the fields in which farmers suspected resistance indicating that poor herbicide performance can be caused by other reasons as for example poor environmental conditions.

Antipredator Behavioral Traits of some Agriotes Wireworms (Coleoptera: Elateridae) and their Potential Implications for Species Identification

Many insects show a typical antipredator behavior. The two simplest tactics are immobilization and flight. The antipredator behavior of wireworms, the soil-dwelling polyphagous larvae of click beetles, has not yet been investigated. Birds are known to attack wireworms. Our experimental method (picking up and dropping wireworms) mimics the first attack of a predator, such as a bird. In Europe, some Agriotes species are of particular importance as crop pests. As these species differ considerably, in ecology, damage threshold, and pathogen resistance, knowing more about the species specific biology and behavior will support the establishment of successful control measures. In an extensive microcosm study, we observed the antipredator behavior of wireworms belonging to four different Agriotes species (A. lineatus, A. obscurus, A. sordidus, and A. ustulatus) before and during digging into the soil after a startle-stimulus. We recorded wireworm immobility frequency and duration, and locomotor activity at three temperatures. We also analyzed genetic and morphologic attributes of A. lineatus larvae from two different origins. Following behavior types were found reflecting different escape tactics: (a) distinct tonic immobility (A. lineatus), (b) brief/inconsistent tonic immobility (A. obscurus, A. sordidus), and (c) immediate fleeing/burying (A. ustulatus). Additionally, we found small but significant differences in morphology, behavior, and genetics (PCR results) between A. lineatus larvae originating from the Netherlands and those originating from Germany. The biological information we gathered about each species will further increase the understanding of these insects and improve interpretation of future experimental data. In addition, the described behavioral differences between Agriotes obscurus and A. lineatus wireworms may represent a useful additional criterion in morphological species identification.

Risk Potential of Clubroot Disease on Winter Oilseed Rape

Clubroot disease caused by Plasmodiophora brassicae is an important disease of cruciferous plants. Although the pathogen is widespread and has been reported to cause high yield losses, the impact on winter oilseed rape (OSR) has not been experimentally verified. To quantify the risk potential of P. brassicae, we conducted two closely linked experiments. A semicontrolled experiment used artificial soil infestation at inoculum densities between 106 and 108 spores liter-1 of soil to detect the impact on seed yield and yield components of a susceptible and resistant OSR cultivar. A greenhouse experiment was implemented using the soil of the semicontrolled experiment after cropping the two cultivars to quantify the influence of cultivar resistance on soil inoculum. According to cumulative link mixed models, disease rating was positively correlated with the amount of inoculum. Linear regression analyses revealed a negative correlation between seed yield and inoculum density. Yield losses of 60% already appeared at the lowest inoculum density. Plant losses and reduced seed per pod were accountable for yield losses. Although the resistant cultivar showed clubroot symptoms, seed yield was not affected by the pathogen. The greenhouse experiment revealed that clubroot severity in subsequent OSR was reduced after cropping the resistant cultivar. This study showed significant yield damage of P. brassicae already at low infestation levels.