The effect of non-inversion tillage and light availability on dispersal and spatial growth of Cynodon dactylon

The invasive ability of Cynodon dactylon is dependent on self dispersal and on cultivation practices. Tillage can seriously change patch biomass and spatial structure, spreading vegetative propagules of the weed. The objectives of this study were: (i) to quantify the effect on non‐inversion tillage on dispersal, establishment and colonization of C. dactylon and (ii) to propose a simple model considering soil cultivation effects and light availability on spatial growth of weed patches. Two experiments were carried out, exploring different soils and environmental conditions. Spatial distribution of vegetative units differed when tillage was conducted with different non‐inversion implements and could be described by simple functions. A minimum patch biomass seems necessary before vegetative structures are vulnerable to movement by cultivation. Only a small proportion of the biomass dispersed from original patches was able to establish. However, simulation showed that the area colonized by C. dactylon mostly increased by means of tillage dispersal, both with and without crop competition, in one growing cycle. It appears sensible to consider changing cultivation practices to reduce weed dispersal and to use crop competition for light to create unsuitable habitats limiting weed colonization.     

Awn of darnel (Lolium temulentum L.) as an anthropogenic dispersal organ: A case study in Malo, south-western Ethiopia

Darnel ( Lolium temulentum ), a weed of wheat and barley, is either awned or awnless. The role of the darnel awn in dispersal was investigated in relation to human's agricultural practices in cereal cultivation in a Malo village, Gaytsa, south-western Ethiopia. In 24 free-threshing wheat fields and 11 emmer wheat fields, 129 and 88 spikes (one spike per plant), respectively, of darnel were randomly sampled. At the market in Gaytsa, 11 free-threshing wheat grain samples and eight emmer wheat grain samples of 150–200 g were collected. For each grain sample, the number of wheat grains and awned and awnless grains of darnel were recorded. The relative percentage of the awned form in free-threshing wheat fields was 8.53% and that in emmer wheat fields was 70.45%. The relative percentage of awned grains in free-threshing wheat grains from the market collection was 3.55% and that in emmer wheat grain lots was 75.20%. The morphology of awned darnel grains resembles emmer wheat grains and awnless darnel grains resemble free-threshing wheat grains. Thus, grain mimicry controls the close association of the two forms of darnel with the wheat species. The lack of an awn itself functions for darnel grain as a dispersal mechanism in free-threshing wheat cultivation systems compared with other wild grass species.     

Periodicity and gradients in dispersal ofAlternaria linicola in linseed crops

Conidia ofAlternaria linicola produced on infected linseed crops were mainly dispersed by wind. The numbers of conidia in the air above linseed crops collected by a Burkard spore sampler were greatest between 1200 h and 1300 h, when the relative humidity was lowest. Although numbers of conidia collected decreased with increasing height within and above the crop canopy, air-borneA. linicola conidia were present up to 80 cm above the crop canopy. Conidia ofA. linicola were transported by wind up to at least 40 m downwind from an artificial line inoculum source, but their numbers decreased with increasing distance from the source. In 1991, 1992, and 1993, the dispersal ofA. linicola conidia above linseed crops followed a seasonal periodicity which was influenced by weather conditions and cultural practices. The greatest numbers of conidia were collected during July, August and early September and coincided with periods favourable for sporulation and with an increase in the incidence of the disease in the senescent crop. Air-borneA. linicola conidia produced on point or line inoculum sources (naturally infected linseed stem debris) were responsible for the spread of the disease in linseed crops. In 1992 and 1993, the disease was first detected downwind from the sources, but by the end of the growing seasons, it had spread in all directions and up to 20 m and 60 m from the sources, respectively. Disease gradients were initially steep near the inoculum sources but they became flatter with time due to the secondary spread of the disease.     

Spatial and temporal patterns of Lolium rigidum–Avena sterilis mixed populations in a cereal field

Through a detailed case study of a two‐species (Lolium rigidum and Avena sterilis) weed community at contrasting scales, this paper examined factors that affect weed distribution across space and time in a commercial wheat field in north‐east Spain. A. sterilis showed relatively stable spatial distribution and spatial structure of its population over time at large scale, with well‐defined patches, although weed density rose quickly. L. rigidum showed poorly defined patches that were not stable across time. Interaction between species could explain to some degree the spatial distribution at large scale: a negative relationship was detected between the spatial structures of both weed populations. At fine scale, both species showed a clear interaction effect from primary dispersal (more important in A. sterilis) and secondary dispersal from combine harvesting (more important in L. rigidum).     

Modelling the escape of Chondrostereum purpureum spores from a larch forest with biological control of Prunus serotina

Chondrostereum purpureum is being developed as a microbial herbicide to controlPrunus serotina in coniferous forests. After biological control, the fungus produces basidiospores that may reach and infect non-target plants. As part of a risk analysis, spore escape from a forest was quantified. For this purpose, a stratified model was developed for a forest of 250 m with 250 m. A turbulent diffusion equation was used to describe spore fluxes for each layer.In a simple calculation procedure, the spore fluxes for the different layers were calculated simultaneously. The downwind escape fraction was found to vary from 0.19 to 0.45, and the upward escapte fraction from 0.07 to 0.23. These escape fractions increased with increasing wind speed.Number of spores released from basidiocarps, and escape fractions could be used as inputs for a Gaussian plume model to calculate spore immission into orchards.     

Major emerging alien plants in Austrian crop fields

Globalisation and increasing trade have led to the introduction of alien plants that are highly competitive and difficult to control in agriculture in central Europe. Our study set out to analyse the invasion process and agricultural impact of the six emerging alien plants Abutilon theophrasti, Ambrosia artemisiifolia, Cyperus esculentus, Datura stramonium, Panicum schinzii and Sorghum halepense in Austria, based on a large distribution data set (694 occurrence records in crop fields). We found that all study species have increased in abundance and range, especially after the year 2000. The rate of spread was highest for A. artemisiifolia and lowest for C. esculentus. At present, records of the study species were largely associated with areas of high summer crop concentration in eastern, southern and north‐western Austria. Based on the CORINE land‐cover data set, we found that the agricultural area at risk of being invaded increased over time. At present, it ranges between 45 500 and 168 000 ha (approximately 2.4% and 8.7% of the total agricultural area) depending on the species. The invasion success of the study species is probably associated with frequent human‐mediated dispersal, specific crop‐dominated rotations and herbicide use. Our results suggest that the study species will successfully spread further. We conclude that these emerging alien species will cause substantial impacts on crops in Austria and probably in other countries of central Europe.     

Role of rainfall in the development of coffee berry disease in Coffea arabica caused by Colletotrichum kahawae,in Cameroon

The development of coffee berry disease (CBD) epidemics (caused by Colletotrichum kahawae) in Cameroon was monitored over two successive years (2004 and 2005) on coffee trees protected from rainfall by transparent plastic sheets and on unprotected control trees. This work was done to assess how rain affected disease development when it did not fall directly onto the coffee trees and to determine the influence of primary inoculum on the severity of CBD. Weekly observations over the 2 years showed that there were 1·1% diseased berries on coffee trees completely protected from rainfall, compared with 45% diseased berries on unprotected coffee trees. Disease severity on unprotected trees during the 2 years of the experiment was estimated at 53% diseased berries, compared with 27% on trees only protected in the first year. These results confirmed rainfall as one of the key physical factors in the development of Arabica CBD. They also provided evidence of a subsequent effect of protecting coffee trees from rainfall in 2004 on the severity of CBD in 2005. This suggested some practices that might lead to very effective cultural control of CBD in regions where severe epidemics of the disease occur.     

Weed populations and pathogens

Plant:pathogen systems are complex, highly spatial and fluctuate considerably. The dynamics of their individual populations depends on factors such as their patchiness, shapes and scales of dispersal distributions, genetic polymorphism and their responses to their biotic and abiotic environments. We review knowledge of these population properties in the context of weed management and biological control and discuss their likely influence on the population dynamics of pathogens.