Summer Epidemics of Apple Scab: The Relationship between Measurements and their Implications for the Development of Predictive Models and Threshold Levels under Different Disease Control Regimes

A 2‐year study on epidemic progress of apple scab was conducted at Randwijk, the Netherlands, in 1998 and 1999. The summer epidemic caused by conidia was studied instead of the well‐described spring season epidemic originating from ascospores. The aim was to investigate relationships between disease measurements, i.e. disease incidence and severity measures of apple scab, and their implications for the development of predictive models and threshold levels. The study characterized good relationships between the measurements on cultivar Jonagold using regression analyses in three disease control regimes (untreated, organic and integrated). For fruit quality prediction, the relationship between fruit incidence (I_f) and leaf incidence (I_l) in the organic control regime was given by I_f = 1.966 + 0.402 × (I_l) (R² = 0.92). As a result of low level of disease in the integrated control regime, shoot incidence (I_s), with higher values than leaf incidence, was better suited for prediction. The relationship was given by I_f = ?0.162 + 0.028 × (I_s) (R² = 0.91). For the integrated control regime, disease threshold levels were constructed for timing of the final fungicide application. If an apple grower wants to keep fruit infection under 1% incidence (harvest scab threshold), the timing of the final fungicide application (action threshold) should correspond to 4% shoot scab incidence at the beginning of August. The results are compared with similar studies and their biological interpretation is discussed.     

DOTA-Functionalized Polylysine: A High Number of DOTA Chelates Positively Influences the Biodistribution of Enzymatic Conjugated Anti-Tumor Antibody chCE7agl

Site-specific enzymatic reactions with microbial transglutaminase (mTGase) lead to a homogenous species of immunoconjugates with a defined ligand/antibody ratio. In the present study, we have investigated the influence of different numbers of 1,4,7,10-tetraazacyclododecane-N-N′-N′′-N′′′-tetraacetic acid (DOTA) chelats coupled to a decalysine backbone on the in vivo behavior of the chimeric monoclonal anti-L1CAM antibody chCE7agl. The enzymatic conjugation of (DOTA)₁-decalysine, (DOTA)₃-decalysine or (DOTA)₅-decalysine to the antibody heavy chain (via Gln295/297) gave rise to immunoconjugates containing two, six or ten DOTA moieties respectively. Radiolabeling of the immunoconjugates with ¹⁷⁷Lu yielded specific activities of approximately 70 MBq/mg, 400 MBq/mg and 700 MBq/mg with increasing numbers of DOTA chelates. Biodistribution experiments in SKOV3ip human ovarian cancer cell xenografts demonstrated a high and specific accumulation of radioactivity at the tumor site for all antibody derivatives with a maximal tumor accumulation of 43.6±4.3% ID/g at 24 h for chCE7agl-[(DOTA)-decalysine]₂, 30.6±12.0% ID/g at 24 h for chCE7agl-[(DOTA)₃-decalysine]₂ and 49.9±3.1% ID/g at 48 h for chCE7agl-[(DOTA)₅-decalysine)]₂. The rapid elimination from the blood of chCE7agl-[(DOTA)-decalysine]₂ (1.0±0.1% ID/g at 24 h) is associated with a high liver accumulation (23.2±4.6% ID/g at 24 h). This behavior changed depending on the numbers of DOTA moieties coupled to the decalysine peptide with a slower blood clearance (5.1±1.0 (DOTA)₃ versus 11.7±1.4% ID/g (DOTA)₅, p<0.005 at 24 h) and lower radioactivity levels in the liver (21.4±3.4 (DOTA)₃ versus 5.8±0.7 (DOTA)₅, p<0.005 at 24 h). We conclude that the site-specific and stoichiometric uniform conjugation of the highly DOTA-substituted decalysine ((DOTA)₅-decalysine) to an anti-tumor antibody leads to the formation of immunoconjugates with high specific activity and excellent in vivo behavior and is a valuable option for radioimmunotherapy and potentially antibody-drug conjugates (ADCs).     

Nitrogen availability and susceptibility of tomato leaves to Botrytis cinerea

The aim of this study was to investigate the effect of nitrogen availability on susceptibility of tomato leaves to the fungal pathogen Botrytis cinerea. Plants with varying nitrogen availability were grown by adding N daily in exponentially increasing amounts to a nutrient solution at different rates. Leaves of plants grown at low nitrogen availability had a high leaf C/N ratio (21 g g^-1) and were about 2.5 times more susceptible to primary lesion formation by B. cinerea compared to plant grown at high nitrogen availability, which had a low leaf C/N ratio (11 g g^-1). Leaf C/N ratio accounted for 95% of variation in susceptibility. This relationship between C/N ratio and susceptibility persisted when plants were grown with exponential P addition and optimal N supply, and was thus independent of plant growth rate or related factors. We could not explain the effect of nitrogen availability by variation in the most obvious N-based resistance compound α-tomatine because more susceptible leaves with a high C/N ratio contained more α-tomatine. These leaves also contained more soluble carbohydrates. The level of soluble carbohydrates correlated positively with susceptibility, independent of the growth method. We therefore suggest that the effect of N availability on susceptibility must be explained by variation in levels of soluble carbohydrates and speculate about the role of these carbohydrates in the infection process. This revised version was published online in June 2006 with corrections to the Cover Date.     

Disease spread in small-size directed networks: Epidemic threshold, correlation between links to and from nodes, and clustering

Network epidemiology has mainly focused on large-scale complex networks. It is unclear whether findings of these investigations also apply to networks of small size. This knowledge gap is of relevance for many biological applications, including meta-communities, plant–pollinator interactions and the spread of the oomycete pathogen Phytophthora ramorum in networks of plant nurseries. Moreover, many small-size biological networks are inherently asymmetrical and thus cannot be realistically modelled with undirected networks. We modelled disease spread and establishment in directed networks of 100 and 500 nodes at four levels of connectance in six network structures (local, small-world, random, one-way, uncorrelated, and two-way scale-free networks). The model was based on the probability of infection persistence in a node and of infection transmission between connected nodes. Regardless of the size of the network, the epidemic threshold did not depend on the starting node of infection but was negatively related to the correlation coefficient between in- and out-degree for all structures, unless networks were sparsely connected. In this case clustering played a significant role. For small-size scale-free directed networks to have a lower epidemic threshold than other network structures, there needs to be a positive correlation between number of links to and from nodes. When this correlation is negative (one-way scale-free networks), the epidemic threshold for small-size networks can be higher than in non-scale-free networks. Clustering does not necessarily have an influence on the epidemic threshold if connectance is kept constant. Analyses of the influence of the clustering on the epidemic threshold in directed networks can also be spurious if they do not consider simultaneously the effect of the correlation coefficient between in- and out-degree.     

Phylogenesis of spermatogenesis in Annulipalpia caddisflies: An ultrastructural analysis on philopotamidae spermiogenesis

Characters currently used in phylogenetic analyses are insufficient for determining the status of several families of the order Trichoptera (caddisflies). Comparative spermatology can contribute to solving this problem. In the suborder Integripalpia, the sperm axoneme displays the pattern of 9 + 2 pairs of parallel microtubules found in many animal species. In the suborder Annulipalpia, however, axonemes bear remarkable aberrations. Consistently, in Chimarra florida (Philopotamidae, Annulipalpia) we found that the number of central microtubules varies from zero to four in axonemes of the spermatids and that spermatozoa lack axonemes. We propose that, similarly, the axoneme pattern became unstable in the Annulipalpia ancestor, from which branched two phylogenetic lines having different kinds of axoneme aberrations: (a) families in which the number of central microtubules of the axoneme exceeds two (e.g., Philopotamidae, Polycentropodidae) and (b) families in which the number of central microtubules of the axoneme does not exceed two (e.g., Hydropsychidae).     

A generic theoretical model for biological control of foliar plant diseases

We have developed a generic modelling framework to understand the dynamics of foliar pathogen and biocontrol agent (BCA) populations in order to predict the likelihood of successful biocontrol in relation to the mechanisms involved. The model considers biocontrol systems for foliar pathogens only and, although it is most applicable to fungal BCA systems, does not address a specific biocontrol system. Four biocontrol mechanisms (competition, antibiosis, mycoparasitism and induced resistance) were included within the model rubric. Because of the wide range of mechanisms involved we use Trichoderma/Botrytis as an exemplar system. Qualitative analysis of the model showed that the rates of a BCA colonising diseased and/or healthy plant tissues and the time that the BCA remains active are two of the more important factors in determining the final outcome of a biocontrol system. Further evaluation of the model indicated that the dynamic path to the steady-state population levels also depends critically on other parameters such as the host-pathogen infection rate. In principle, the model can be extended to include other potential mechanisms, including spatio-temporal heterogeneity, fungicide effects, non-fungal BCA and strategies for BCA application, although with a cost in model tractability and ease of interpretation.     

Management of strawberry grey mould using mixtures of biocontrol agents with different mechanisms of action

Experiments were conducted using combinations of commercially available biological control agents (BCAs) to control infection of detached strawberry leaves by Botrytis cinerea Pers.;Fr. Combinations were tested either as mixtures, or as sequential treatments before and after disease loading. Five BCAs were initially tested, giving a range of mechanisms of antagonism. Only three (Sentinel?, Serenade? and Trianum?) were effective in single agent experiments and were taken forward for combinatorial tests. Biocontrol efficacies, relative to the control, varied considerably among replicate trials, but all treatments involving Sentinel (BCA: Trichoderma atroviride P. Karsten LC52) and Trianum (BCA: Trichoderma harzianum Rifai T22) were very effective, either in sequential combination with other BCA or alone. Serenade (BCA: Bacillus subtilis (Ehrenberg) Cohn) was least effective, and sometimes did not result in any significant reduction in disease. Combinations of BCAs as mixtures resulted in less control (i.e., higher disease) than when the most effective BCA within the combination was applied alone, indicating possible antagonism between the BCAs. However, when two BCAs were applied sequentially about 48 h apart, there was no strong evidence for antagonism between the BCAs; this was also confirmed in an experiment using whole plants in controlled environment cabinets. We conclude that there was usually no benefit in applying two BCAs either together or sequentially, so in general we would not advise combinations as the method of choice. Thus, research is necessary to investigate possible interference among specific BCAs, since several BCAs may be applied simultaneously under field conditions to control several diseases.     

The number of links to and from the starting node as a predictor of epidemic size in small-size directed networks

Much recent modelling is focusing on epidemics in large-scale complex networks. Whether or not findings of these investigations also apply to networks of small size is still an open question. This is an important gap for many biological applications, including the spread of the oomycete pathogen Phytophthora ramorum in networks of plant nurseries. We use numerical simulations of disease spread and establishment in directed networks of 100 individual nodes at four levels of connectivity. Factors governing epidemic spread are network structure (local, small-world, random, scale-free) and the probabilities of infection persistence in a node and of infection transmission between connected nodes. Epidemic final size at equilibrium varies widely depending on the starting node of infection, although the latter does not affect the threshold condition for spread. The number of links from (out-degree) but not the number of links to (in-degree) the starting node of the epidemic explains a substantial amount of variation in final epidemic size at equilibrium irrespective of the structure of the network. The proportion of variance in epidemic size explained by the out-degree of the starting node increases with the level of connectivity. Targeting highly connected nodes is thus likely to make disease control more effective also in case of small-size populations, a result of relevance not just for the horticultural trade, but for epidemiology in general.     

Populations of bacteria and actinomycetes associated with sclerotia ofPhymatotrichum omnivorum buried in Houston black clay

Sclerotia ofPhymatotrichum omnivorum (Shear) Duggar (the causal agent of root rot of cotton) were produced in the laboratory and then buried at a depth of 45 cm at three sites in Texas situated on Houston black clay soils with various cropping histories. The sites included a native grassland prairie, a field in continuous cotton production, and a field in which cotton, corn, and sorghum were grown in rotation. Samples of sclerotia were retrieved monthly over a 12 month period. Populations of bacteria and actinomycetes were enumerated using dilution-plate techniques and isolates were screened (in vitro) for their ability to produce substances inhibitory toP. omnivorum. The sclerotia supported large numbers of bacteria (including fluorescent pseudomonads) and actinomycetes. Numbers associated with sclerotia ranged from 10⁶–10⁹ cells per gram of sclerotia plus adherent soil and were 2–3 orders of magnitude greater than numbers from soil at the same depth but free of sclerotia. Bacteria and actinomycetes antagonistic toP. omnivorum were isolated from sclerotia buried at each of the three sites. Up to 26% of the isolates inhibited growth ofP. omnivorum.