Laurent Huber

Field measurements of airborne concentration and deposition rate of maize pollen

In recent years there has been interest in the dispersal of maize (Zea mays) pollen from crops, particularly in relation to gene flow and seed quality. We report the results of experiments that measured maize pollen dispersal from a 20 m x 20 m experimental crop. The experiments were done in a commercial farm in France during the summer of 2000. Pollen production was estimated to range from 10(4) to 2 x 10(6) grains per day per plant. Pollen concentrations and deposition rates decreased rapidly with distance from the crop: concentrations decreased by about a factor of 3 between 3 and 10 m downwind of the source deposition rates at 30 m were < 10% of those at 1 m. Horizontal flux of pollen were estimated from pollen concentration and wind speed profiles using a mass balance approach, and ranged from 15 to 560 grains m-1 s-1 at 3 m from the source. Comparison of deposition rates estimated with the mass balance and direct measurement suggests that only a small proportion of the pollen released from the crop would have been still airborne at distances greater than 30 m downwind. Deposition velocity determined as the ratio of the deposition rate to the airborne concentration at 3 m from the source averaged 0.6 m s-1, which is twice as large as the settling velocity for maize pollen.

Robust cropping systems to tackle pests under climate change. A review

Agriculture in the twenty-first century faces the challenge of meeting food demands while satisfying sustainability goals. The challenge is further complicated by climate change which affects the distribution of crop pests (intended as insects, plants, and pathogenic agents injurious to crops) and the severity of their outbreaks. Increasing concerns over health and the environment as well as new legislation on pesticide use, particularly in the European Union, urge us to find sustainable alternatives to pesticide-based pest management. Here, we review the effect of climate change on crop protection and propose strategies to reduce the impact of future invasive as well as rapidly evolving resident populations. The major points are the following: (1) the main consequence of climate change and globalization is a heightened level of unpredictability of spatial and temporal interactions between weather, cropping systems, and pests; (2) the unpredictable adaptation of pests to a changing environment primarily creates uncertainty and projected changes do not automatically translate into doom and gloom scenarios; (3) faced with uncertainty, policy, research, and extension should prepare for worst-case scenarios following a “no regrets” approach that promotes resilience vis-à-vis pests which, at the biophysical level, entails uncovering what currently makes cropping systems resilient, while at the organizational level, the capacity to adapt needs to be recognized and strengthened; (4) more collective approaches involving extension and other stakeholders will help meet the challenge of developing more robust cropping systems; (5) farmers can take advantage of Web 2.0 and other new technologies to make the exchange of updated information quicker and easier; (6) cooperation between historically compartmentalized experts in plant health and crop protection could help develop anticipation strategies; and (7) the current decline in skilled crop protection specialists in Europe should be reversed, and shortcomings in local human and financial resources can be overcome by pooling resources across borders.

Influence of target characteristics on the amount of water splashed by impacting drops

Abstract Experiments were done to investigate the influence of drop diameter on the efficiency of the splash process, using a rain tower and different types of target (glass, water films, healthy and/or diseased oilseed rape leaves and tobacco leaves) and different target configurations. For horizontal glass plates, random variability in the mass of water splashed was 3–4 times greater for dry targets than for wet targets and considerably more water was splashed from wet targets. The effect of target angle was greater for large drops and the greatest amount of water was splashed from horizontal targets. More water was splashed from oilseed rape leaves than tobacco leaves. Both spore suspensions and surfactant in target liquids decreased the mean volume of water splashed and increased its variability. A power law described the relationship between drop diameter and mass of water splashed per incident drop for drops falling from a height of 11 m onto targets of different types. Using results obtained with incident drops falling from heights of 11 or 1.5 m there was no unique relationship between mass of water splashed and kinetic energy, momentum or impact force. A simulation study, which calculated splash efficiency of single drops on leaves, illustrated the importance of leaf characteristics when assessing rain-splash potential, and more generally when developing methodology to describe the influence of crop structure on the dispersal of spores by splash.

Leaf wetness duration in a field bean canopy

Abstract The experimental testing of a multi-layer model to simulate surface wetness duration (SWD) in a canopy is presented. A previous model was modified by including the heat conduction from dry areas of leaves to water deposits. Field tests after sprinkling during humid and cloudy weather are reported for 10 days in the summers of 1985 and 1986. The discussion is focused on the importance of heat conduction and the influence of water distribution on leaves in determining SWD.

Influence of simulated rain on dispersal of rust spores from infected wheat seedlings

Spores of both Puccinia recondita f. sp. tritici and P. striiformis (brown rust and yellow rust of wheat) are thought to be primarily dispersed by wind. The results of experiments, using a rain simulator with uniform drop sizes (2.5, 3.4, 4.2 or 4.9 mm), on the effect of rain on dispersal of brown (leaf) rust and yellow (stripe) rust spores are reported. Experiments on both pathogens were done in still air; additional experiments were done on brown rust with simulated wind and rain. Spore dispersal was estimated by trapping spores on wheat plants and assessing the disease symptoms which subsequently developed under optimum conditions. Simulated rainfall of each the four drop sizes tested dispersed spores of both pathogens. In still air spore dispersal patterns were similar to those usually found for pathogens which are characteristically splash-dispersed. Rain exhausted the source of spores in about 20 min for the four drop sizes. When the plants were kept under optimal conditions for sporulation, the source of brown rust spores available for dispersal was restored to its initial numbers in about 2 h after depletion. For yellow rust, spore numbers in the source had not been restored to their original value after 6 h, even under optimal conditions. In the wind tunnel experiments, simulated rain did not inhibit the dispersal of brown rust spores by wind. Large incident drops dispersed more spores of both pathogens than small drops. A simulation study based on the experimental relationships obtained was done. Although these experiments clearly show that rainfall has the potential to spread both brown rust and yellow rust of wheat, the understanding of the exact role of rain dispersal in the epidemiology of both diseases requires further investigation.

Rain-splash and spore dispersal: a physical perspective

Recently, there has been considerable work on spore dispersal by rain-splash and three review articles have been published (Fitt and McCartney, 1986; Fitt et al., 1989; Madden, 1992) that discuss spore dispersal from a biological point of view. Topics covered included the characteristics of splash-dispersed fungi, mechanisms of splash dispersal and methods for studying them, the dispersal of specific pathogen spores by different types of rain under controlled conditions or in field situations, and recent spatial distribution and modelling studies.

Removal of urediniospores of brown (Puccinia recondita f.sp. tritici) and yellow (P. striiformis) rusts of wheat from infected leaves submitted to a mechanical stress

The passive spore removal from colonies due to mechanical stress was compared in the brown (Puccinia recondita f.sp. tritici) and yellow (P. striiformis) rusts of wheat. Mechanical stress was applied using either a miniaturized wind tunnel or a centrifuge. In wind-tunnel experiments, a wind of minimum velocity of 1.3 and 1.8 m s-1 for P. recondita f.sp. tritici and P. striiformis, respectively, applied for at least 10 seconds, was necessary to remove spores. The interaction between wind velocity and cumulated duration was significant for both rusts. At low wind velocity, a longer duration was required to remove the spores than at high wind velocity, and vice versa. In centrifugation experiments, the maximum spore removal occurred for angular velocities of 103 and 2 103 rotations min-1, for P. recondita f.sp. tritici and P. striiformis, respectively, applied for 5 min. Calculation of the aerodynamic and centrifugal forces showed that the forces necessary to remove spores are greater for P. striiformis than for P. recondita f.sp. tritici. This difference can be related to the size of the dispersal unit, which is larger in P. striiformis than in P. recondita f.sp. tritici due to spore clustering. These observations are consistent with the differences in the mean spore dispersal distance, which is usually smaller in P. striiformis than in P. recondita f.sp. tritici.

Using virtual 3-D plant architecture to assess fungal pathogen splash dispersal in heterogeneous canopies: a case study with cultivar mixtures and a non-specialized disease causal agent

BACKGROUND AND AIMS Recent developments in plant disease management have led to a growing interest in alternative strategies, such as increasing host diversity and decreasing the use of pesticides. Use of cultivar mixtures is one option, allowing the spread of plant epidemics to be slowed down. As dispersal of fungal foliar pathogens over short distances by rain-splash droplets is a major contibutor to the spread of disease, this study focused on modelling the physical mechanisms involved in dispersal of a non-specialized pathogen within heterogeneous canopies of cultivar mixtures, with the aim of optimizing host diversification at the intra-field level. METHODS Virtual 3-D wheat-like plants (Triticum aestivum) were used to consider interactions between plant architecture and disease progression in heterogeneous canopies. A combined mechanistic and stochastic model, taking into account splash droplet dispersal and host quantitative resistance within a 3-D heterogeneous canopy, was developed. It consists of four sub-models that describe the spatial patterns of two cultivars within a complex canopy, the pathway of rain-splash droplets within this canopy, the proportion of leaf surface area impacted by dispersal via the droplets and the progression of disease severity after each dispersal event. KEY RESULTS Different spatial organization, proportions and resistance levels of the cultivars of two-component mixtures were investigated. For the eight spatial patterns tested, the protective effect against disease was found to vary by almost 2-fold, with the greatest effect being obtained with the smallest genotype unit area, i.e. the ground area occupied by an independent unit of the host population that is genetically homogeneous. Increasing both the difference between resistance levels and the proportion of the most resistant cultivar often resulted in a greater protective effect; however, this was not observed for situations in which the most resistant of the two cultivars in the mixture had a relatively low level of resistance. CONCLUSIONS The results show agreement with previous data obtained using experimental approaches. They demonstrate that in order to maximize the potential mixture efficiency against a splash-dispersed pathogen, optimal susceptible/resistant cultivar proportions (ranging from 1/9 to 5/5) have to be established based on host resistance levels. The results also show that taking into account dispersal processes in explicit 3-D plant canopies can be a key tool for investigating disease progression in heterogeneous canopies such as cultivar mixtures.

Laboratory study of fungal spore movement using Laser Doppler Velocimetry

The dispersal of fungal spores (diameter between 5 and 50 μm) is a process which can be divided into three stages: (1) release from the surface, (2) transport through the air and (3) deposition on the surface. The analysis of these processes requires accurate measurements of spore velocity. The objective of this work is to demonstrate that the Laser Doppler Velocimetry (LDV) is a useful tool in determining spore velocities for the study of spore dispersal. Aiming at the experimental characterisation of both release and sedimentation of biological particles, this paper reports LDV measurements for fungal spores of two wheat rusts (yellow rust caused by Puccinia striifomis and brown rust caused by Puccinia recondita) and club moss (Lycopodium clavatum). The release of yellow rust spores was observed at a minimum wind speed (threshold velocity), the release occurring in intermittent events. Similar behaviour was observed with the brown rust spores. LDV was used to measure the settling speed of single spores and spore clusters in still air. This technique was then shown to be a promising tool in the measurement of settling speeds of particles in a polydisperse spore population.

Research and Development Priorities in the Face of Climate Change and Rapidly Evolving Pests

Agriculture faces the challenge of meeting increasing food demands whilst simultaneously satisfying ever stringent sustainability goals. Taken together with the ever increasing rate of integrated globalisation and other anthropogenic impacts, this challenge is further complicated by climate change. Climate change is indeed increasingly recognised as a considerable risk to agriculture in the European Union, particularly with respect to direct impacts on crop production and yield stability. A major impact threat is the further risk from new and emerging invasive alien species, and potential novel pathogenically aggressive adaptations in existing indigenous pests and pathogens, which, hitherto, have been managed with conventional practices and approaches.