A framework for examining inter-regional aerial transport of fungal spores

Aerial transport of fungal spores has no doubt been responsible for occasionally spreading plant diseases over distances of 500 km or more. With currently available methods, however, there is little basis for estimating the likelihood of such occurrences. Models capable of estimating the relative probability of infection of a crop from known sizes of local or distant sources of pathogenic spores could help farmers with decisions about fungicide use, local sanitation and quarantine. A logical framework for estimating long-distance transport of viable spores is presented which encompasses: production of spores at the source region, escape of spores into the air above the diseased crops, transport and dilution of spores in the atmosphere, survival of airborne spores and deposition of spores onto a distant crop. Hypothetical examples of spore transport over 700 km are calculated to illustrate the relative magnitudes of the five parts of the spore transport process. Although the combined uncertainties in spore transport are estimated to be a factor of 10⁴–10⁶, this should be compared to the expected factor of 10¹³ dilution of spore concentration experienced by a cloud of spores after having been transported through the atmosphere for 700 km. The largest uncertainties lie in our ability to estimate daily spore production in a geographic region, survival of spores while airborne and deposition of spores during rainfall. Survival of spores and deposition of spores during rainfall are difficult to predict because of temporal and spatial variations in weather. To account for this, it will be necessary in the future to combine the physical model of spore transport described here with weather records and air parcel trajectory calculations to develop a “climatology” of danger of disease transmission between geographic regions.     

The effects of population density and intercropping with cowpea on the water use and growth of sorghum in semi-arid conditions in Botswana

Soil moisture was measured below sorghum crops of differing density in rainfed trials in increasingly droughty conditions from 1981/2 to 1983/4. Additional measurements were made in sorghum/cowpea intercrops and cowpea monocrops during the drought seasons of 1982/3 and 1983/4.Increasing crop density, by addition of either sorghum or cowpea, resulted in increased effective root activity, which was accompanied by small increases in actual/potential evapotranspiration (E_a/E_p) early in each season. These small increases were followed by small decreases later in the season, but these effects were rarely statistically significant. Both sorghum density and intercropping had little effect on total seasonal water use.Dry matter production increased linearly with increasing seasonal water use. Water use efficiencies of dry weight production were increased by increasing sorghum density. Intercropping in dry conditions did not have significant effects on water use efficiencies. Estimates of soil evaporation suggested that the effects of density on water use efficiency could be attributed to reductions in soil evaporation.The effects of density on reproductive yields were governed by two opposing processes: increasing crop density increased water use efficiencies of dry weight production, but decreased the proportions of dry weight allocated to grain.     

Population trends and feeding preferences of flea beetles (Phyllotreta cruciferae Goeze) in collard-wild mustard mixtures

Population densities of flea beetles (Phyllotreta cruciferae Goeze) were lower per unit collard plant in polycultures composed either of non-host plants (field bean, vetch and barley) or of additional host plants (wild mustard) than in collard monocultures. Flea beetles exhibited decreased movement patterns in the non-host polycultures, but similar movement activity in the monoculture and collard-wild mustard polyculture. Differences in flea beetle abundance between collard monoculture and collard-wild mustard polycultures disappeared when flowers were artificially removed from wild mustard. Significantly fewer flea beetles occurred in collard rows immediately adjacent to wild mustard borders, but their numbers tended to increase further away from the border. Collard plants sprayed with wild mustard extracts, and especially with allylisothiocyanate emulsions, proved to be more attractive to flea beetles than collard plants sprayed with water. Flower extracts seemed to exert greater attraction than leaf extracts. Understanding of these spatial and temporal interactions (apparently chemically mediated) could lead to development of effective trap cropping systems for flea beetle control in crucifer crops.     

Comparing surface erosion processes in four soils from the Loess Plateau under extreme rainfall events

This research aims to improve erosion control practice in the Loess Plateau, by studying the surface erosion processes, including splash, sheet/interrill and rill erosion in four contrasting soils under high rainfall intensity (120 mm h⁻¹) with three-scale indoor artificial experiments. Four contrasting soils as sandy loam, sandy clay loam, clay loam and loamy clay were collected from different parts of the Loess Plateau. The results showed that sediment load was significantly impacted by soil properties in all three sub-processes. Splash rate (4.0–21.6 g m⁻²∙min⁻¹) was highest in sandy loam from the north part of the Loess Plateau and showed a negative power relation with the mean weight diameter of aggregates after 20 min of rainfall duration. The average sediment load by sheet/interrill erosion (6.94–42.86 g m⁻²∙min⁻¹) was highest in clay loam from middle part of the Loess Plateau, and the stable sediment load after 20 min showed a positive power relation with the silt content in soil. The average sediment load increased dramatically by rill and interrill erosion (21.03–432.16 g m⁻²∙min⁻¹), which was highest in loamy clay from south part of the Loess Plateau. The average sediment load after the occurrence of rill showed a positive power relation with clay content and a negative power relation with soil organic matter content. The impacts of slope gradient on the runoff rate and sediment load also changed with soil properties. The critical factors varied for different processes, which were the aggregate size for splash erosion, the content of silt particles and slope gradient for sheet/interrill erosion, and the content of clay particles, soil organic matter and slope gradient for rill erosion. Based on the results of the experiments, specific erosion control practices were proposed by targeting certain erosion processes in areas with different soil texture and different distribution of slope gradient. The findings from this study should support the improvement of erosion prediction and cropland management in different regions of the Loess Plateau.     

Neuraminidase production by Erysipelothrix rhusiopathiae

In order to characterise neuraminidase activity by Erysipelothrix, 85 isolates of Erysipelothrix spp. from a variety of sources including human clinical, marine and terrestrial animals, and the environment were investigated for neuraminidase production. Neuraminidase activity was detected by a peanut lectin haemagglutination method. The effects of media, incubation conditions and pH on the production and activity of neuraminidase were also investigated. Enzyme activity was detected only in the supernatants of the isolates of Erysipelothrix rhusiopathiae which had been incubated in cooked meat broth and Todd Hewitt broth supplemented with horse serum after 16 and 36 h incubation at 37 degrees C. The maximum titres were reached at 40 h in cooked meat broth and 56 h in Todd Hewitt broth supplemented with horse serum. All 58 isolates and the type strain (ATCC 19414) of E. rhusiopathiae produced detectable neuraminidase activity with titres between 10 and 320. The optimal pH for the enzyme activity varied among the isolates with a pH between 6.0 and 7.0 covering the highest enzyme activity of the most. There was no statistically significant difference in the level of neuraminidase activity between isolates from different sources (p > 0.05). Neuraminidase activity was not detected in the non-pathogenic Erysipelothrix spp. such as E. tonsillarum. Neuraminidase was detected only in E. rhusiopathiae suggesting its possible role as a virulence factor. Enzyme production and activity were medium and pH dependent. The peanut lectin haemagglutination assay is a simple, rapid and sensitive method and is particularly useful for the analysis of multiple samples.