Regional spore dispersal as a factor in disease risk warnings for potato late blight: A proof of concept

This study develops and tests a novel approach for including regional risk factors in operational disease risk warnings against potato late blight. The central premise is that fungicide inputs can be reduced by omitting applications on days when conditions are unsuitable for the atmospheric transport of viable sporangia. The decision support system first decides whether a specific day is ‘high risk’ (suitable for disease development in planta). Simulation studies revealed that on such high risk days, the capacity of the atmosphere to transport sporangia viably over relevant distances varies widely. An additional rule assesses this capacity, which is high when weather conditions allow a large number of spores to be released from the canopy and transported viably over long distances. When this capacity is high the original spray advice is followed, and when it is low a no-spray advice is given.The concept is implemented using the published decision support system SIMCAST complemented with models for spore release from sporangiophores, spore escape from the canopy, a newly developed model for spore dispersal and dry deposition of spores, spore survival during transportation, and weather forecast data from the mesoscale meteorological model MM5. Cultivar resistance was also incorporated into spray advice.The concept was tested in a field experiment in 2007 with three cultivars, representing a range in resistance to potato late blight from susceptible to highly resistant, and compared to a ‘stand-alone’ version of SIMCAST. In a period with normal ‘infection pressure’ (risk of disease) one third of the spray recommendations made by SIMCAST alone were modified and negated by the new system for the highly resistant cultivar. These savings came on top of a reduced, resistance-level dependent dose rate of Shirlan (a.i. Fluazinam).The results demonstrate the feasibility of including dispersal modeling and forecasted meteorology in disease warnings against Phytophthora infestans, even if the whereabouts of sources is unknown. The principles can be used in many decision contexts, but further work is needed to test and refine the method before it can be used in practice.     

Use of a spectrum model and satellite cloud data in the simulation of wheat stripe rust (Puccinia striiformis) dispersal across the Tasman Sea in 1980

Long distance dispersal of plant pathogens has been simulated using different approaches. Models to predict dispersal of airborne spores often use simplified methods to estimate solar radiation in order to estimate mortality of spores. We incorporated a spectrum model into a Lagrangian approach to simulate dispersal of wheat stripe rust from Australia to New Zealand across the Tasman Sea. To estimate cloud cover conditions, satellite data were also used in our study. The combination of a spectrum model and satellite cloud data made it easy to identify sites at which viable spores were deposited. In our study, 0.6% of deposition near New Zealand followed exposure to UV radiation less than our mortality threshold (0.9 MJ m⁻²). Model spores of wheat stripe rust arrived at sites within 40 km from Otama, New Zealand, where wheat stripe rust was first reported in November 1980. It appeared that the uredospores were transported under an overcast cloud condition, which would increase the chance of successful infection on host crops. Further studies on the relationship between solar UV radiation and mortality of pathogen uredospores would facilitate the use of the spectrum model and satellite data to predict dispersal of airborne spores.     

Dry and wet atmospheric deposition of nitrogen, phosphorus and silicon in an agricultural region

We measured atmospheric nutrient deposition as wet deposition and dry deposition to dry and wet surfaces. Our analyses offer estimates of atmospheric transport of nitrogen (N), phosphorus (P) and silicon (Si) in an agricultural region. Annual dry and wet deposition (ha^?1 year^?1) was 0.3 kg of P, 7.7 kg of N, and 6.1 kg of Si; lower than or similar to values seen in other landscapes. N:P and Si:N imply that atmospheric deposition enhances P and Si limitation. Most P and soluble reactive P (SRP) deposition occurred as dryfall and most dry-deposited P was SRP so would be more readily assimilable by plant life than rainfall P. Dry deposition of N to wet surfaces was several times greater than to dry surfaces, suggesting that ammonia (NH _x ) gas absorbtion by water associated with wet surfaces is an important N transport mechanism. Deposition of all nutrients peaked when agricultural planting and fertilization were active; ratios of NH _x :nitrate (NO _x ) hbox{reflected} the predominant use of NH _x fertilizer. Wet deposition estimates were consistent over hundreds of km, but dry deposition estimates were influenced by animal confinements and construction. Precipitation wash-out of atmospheric nutrients was substantial but larger rain events yielded higher rates of wet deposition. Methodological results showed that local dust contaminated wet deposition more than dry; insects, bird droppings and leaves may have biased past deposition estimates; and estimating dry deposition to dry plastic buckets may underestimate annual deposition of N, especially NH _x .     

Precipitation as the most affecting factor on soil–plant environment conditions affects the mycorrhizal spore numbers in three different ecological zones in Turkey

Mycorrhizal symbiosis is the one of the most important relationship between microbiota and plants to sustain plant nutrition in relatively unfavourable conditions. Somehow this relation is threatened by time, therefore, definition of the factors effecting mycorrhizal symbiosis has become essential. The aim of this study was to determine the differences in specific mycorrhizal parameters such as sporulation and soil–plant environment conditions in three different regions of Turkey. During 1996?2002, 53 soil series were selected from natural and agricultural plant communities in three different agro-ecological zones of Turkey: Central Anatolia (CA), the Southeastern Anatolian (SA) project area and the Coast of Mediterranean (CM). The arbuscular mycorrhizal fungus (AMF), spore numbers and mycorrhizal root colonization were related to the annual average precipitation, soil characteristics and host plant identity.

In the CM zone (average annual precipitation of 650?mm), soils found under natural vegetation contained a maximum value of 108?spores?g^?1, with bare soils containing a minimum number of 0.1?spores?g^?1. In the CA zone (330?mm annual average precipitation), the maximum number of spores in the soil samples was 46.5?spores?g^?1 with a minimum of 6.8?spores?g^?1 and in the SA soil samples (380?mm annual average precipitation), a maximum of 48.4?spores?g^?1 and a minimum of 14.2?spores?g^?1 were recorded. The overall mean number of mycorrhizal spores g^?1 soil was 15.5?±?14.4, 22.2?±?8.6 and 27.9?±?25.4 for the CA, SA and CM zones, respectively. Mean spore numbers differed in only two of the three zones, with the third zone being intermediate. Precipitation was the most affecting factor on the sporulation of AMF. Also host plant species and certain soil parameters, such as positive correlations with CaCO₃ and N-min and a negative correlation with organic matter, have an influence on sporulation.

The key finding is that the cropping system has a large impact on spore numbers/abundance. Seventeen standing crops as well as bare soil, fallow and natural areas were compared. There are a large number of factors which can affect mycorrhizal development; in the present work, it seems that soil and crop management, and environmental factors (such as precipitation) affect sporulation and root colonization. Covering land surface with mycorrhiza-dependent cover crop, irrigation and less soil till may increase indigenous mycorrhizal spores.     

Relative contributions of sedimentation and impaction to deposition of particles in a crop canopy

Lycopodium spores were released steadily into the air during 20–30 min from a line source positioned within a wheat crop. The spores were trapped on sticky strips held at angles, π, of 0, 30, 60 and 90° with respect to the horizontal and oriented to face the mean wind direction and on sticky, vertical glass rods. The aerial spore concentration, C, was measured by small suction traps. Deposits of the spores on wheat leaves were obtained from sections of leaves whose posture in the canopy was nearly horizontal, nearly vertical, or at angles between 30 and 60°. Number of spores per m² for all trapping surfaces were obtained by counting under a microscope. Experiments were conducted on seven different days, encompassing friction velocities, u^*, of 0.27–0.50 m s⁻¹. The rate of deposition on angled surfaces, D(π), was given approximately by D(θ) = D(0) cos (θ) + D(90) sin (θ), where D(0) and D(90) were the observed rate of deposit on horizontal and on vertical surfaces, respectively. Below mid-canopy height, inertial impaction of spores was negligible, so that D(90) = 0 for all the trap surfaces. There, D(0) was mainly due to sedimentation and was very nearly equal to v_s·C, where v_s is the settling speed of the spore in still air. Near the top of the canopy, deposition on sticky surfaces was enhanced by inertial impaction and turbulent deposition, so that D(0) was about twice that expected from sedimentation and D(90) was about five times larger than expected from inertial impaction at the mean wind speed. Nevertheless, considering the vertical distribution of leaf area and the angles of leaves in a wheat canopy, the rate of deposition of spores for the entire depth of a wheat canopy can be calculated with a probable underestimation of only 20% by simply assuming sedimentation on horizontally projected area and impaction on vertically projected area.     

Evaluation of some equations for estimating evapotranspiration in the south of Iran

The Penman–Monteith (PM) equation is the most common method of estimating reference crop evapotranspiration (ET _o) for different climates of the world. This equation needs full weather data, however, few stations with complete weather data exist in Fars Province, in the south of Iran. Therefore, other equations based on more readily available weather data, such as temperature and rainfall, can be used instead of the PM equation in Fars Province. Four calibrated equations have been proposed in previous studies for Fars Province using weather data up to 2000. These equations were the Hargreaves equation (H), a new equation based on monthly temperature and rainfall (R), the Thornthwaite equation (T) and the Blaney–Criddle equation (B). Using weather data for 2001 to 2006 from 14 stations in Fars Province and outside the province, this study determined the best equations for estimating ET _o in each month and each station, rather than using the PM equation. The results revealed that equations H, R, T and B showed a good correlation to the PM equation, and can be used to estimate monthly ET _o in the study area. Also, the best equation for each location in Fars Province in each month of the year can be determined by using prepared distribution maps. Furthermore, the results showed that there was no specific relationship between the climate at the station and the best equation for estimating ET _o.     

Rainfall erosivity estimation over the Tibetan plateau based on high spatial-temporal resolution rainfall records

The Tibetan Plateau (TP) in China has been experiencing severe water erosion because of climate warming. The rapid development of weather station network provides an opportunity to improve our understanding of rainfall erosivity in the TP. In this study, 1-min precipitation data obtained from 1226 weather stations during 2018–2019 were used to estimate rainfall erosivity, and subsequently the spatial-temporal patterns of rainfall erosivity in the TP were identified. The mean annual erosive rainfall was 295 mm, which accounted for 53% of the annual rainfall. An average of 14 erosive events occurred yearly per weather station, with the erosive events in the wet season being more likely to extend beyond midnight. In these cases, the precipitation amounts of the erosive events were found to be higher than those of the daily precipitations, which may result in implicit bias as the daily precipitation data were used for estimating the rainfall erosivity. The mean annual rainfall erosivity in the TP was 528 MJ mm·ha^?1·h^?1, with a broader range of 0–3402 MJ mm·ha^?1·h^?1, indicating a significant spatial variability. Regions with the highest mean annual rainfall erosivity were located in the forest zones, followed by steppe and desert zones. Finally, the precipitation phase records obtained from 140 weather stations showed that snowfall events slightly impacted the accuracy of rainfall erosivity calculation, but attention should be paid to the erosion process of snowmelt in the inner part of the TP. These results can be used as the reference data for soil erosion prediction in normal precipitation years.     

Effect of crop type and crop growth on atmospheric nitrogen deposition

Varying atmospheric nitrogen (N) depositions for different crops were observed at Bad Lauchstädt (Saxony‐Anhalt, Germany) when using the ITNI system (ITNI = Integrated Total Nitrogen Input), which is based on the ¹⁵N isotope dilution method. These differences were only partly explained by climatic influences. The effects of crops on the atmospheric N deposition measured by the ITNI system are discussed. For this purpose, data of six different plant species recorded in 1998 were re‐analyzed. It was found that the airborne N input is closely correlated with the morphology and metabolism of crops. Daily atmospheric N depositions of 129.0–360.8 g per hectare were measured for the plant species used. The nutritional supply of plants, especially with N, is another factor of influence on the N input from the atmosphere which should be considered. To investigate this aspect, a pot experiment was conducted with the grass Lolium perenne at three different N levels. An increase in the airborne N uptake (corresponding to N fertilization) was observed as biomass production rose.     

Estimates for the long-range transport of air pollution

Different atmospheric, source, and surface conditions can result in substantially different ranges of atmospheric transport of air pollutants; for example, even for anthropogenic S and N, the ranges can vary from about 10¹ to 10⁵ km. In this report, the emphasis is on indicating some of the reasons for the great variability of these ranges. Thus, some of the complexities of dry deposition, atmospheric chemistry, and precipitation scavenging are described, and it is demonstrated how synoptic-scale meteorologic conditions can control both dry and wet deposition. On the other hand, it is suggested that the mean, tropospheric-residence time of particulate S and N from fossil-fuel combustion in temperate latitudes is probably about a week, but the amount of this material remaining airborne after a week can be large, since the amount is expected to have a log-normal distribution over an ensemble of realizations. Applications of the results to the U.S./Canadian acid-rain issue, to episodic-deposition events, and to global-scale atmospheric pollution are indicated.     

Soil texture and irrigation influence the transport and the development of Pasteuria penetrans, a bacterial parasite of root-knot nematodes

The transport of the spores of Pasteuria penetrans was studied in three contrasted textured soils (a sandy, a sandy-clay and a clay soils), cultivated with tomato, inoculated with juveniles of Meloidogyne javanica and watered with 25 or 150 mm day⁻¹. One month after inoculation of the nematodes, 53% of the spores inoculated were leached by water flow in the sandy soil but only 14% in the sandy-clay soil and 0.1% in the clay soil. No nematodes survived in the clay soil, while the population was multiplied both in the sandy and in the sandy-clay soils. But juveniles of M. javanica were more infected by P. penetrans in the sandy-clay soil than in the sandy soil. Comparing different combinations of bare soils containing 1.1-57% of clay showed that the best spore percolation and retention balance occurred in soils amended with 10-30% clay. However, the spore recoveries decreased when the soil was enriched with more than 30% clay. The role of clay particles on the extractability of spores and on their availability to attach to the nematode cuticle in the soil is discussed.     

影响丛枝菌根真菌孢子萌发的几种因素研究

对丛枝菌根真菌孢子萌发的几种影响因素进行了研究。结果表明,土壤是丛枝菌根真菌孢子萌发最适宜的培养基;寄主植物根的分泌物对孢子萌发有显著的促进作用。重金属Cd和Pb含量过高时(50mg kg)抑制真菌孢子的萌发。培养基中有效P含量较低时(KH2PO4添加量为0～80mg L),对孢子萌发影响较小,高浓度的有效P(KH2PO4添加量大于100mg L)对孢子萌发有一定的抑制作用。培养基的pH值过高(pH8.0以上)或过低(pH5.5以下)抑制孢子萌发。生长激素对孢子萌发率没有显著性影响。对于有休眠现象的丛枝菌根真菌,4℃低温处理4～6周,可打破休眠孢子的休眠,显著提高休眠孢子的萌发率。     

基于图像处理的小麦条锈病菌夏孢子模拟捕捉的自动计数

利用孢子捕捉器进行气传植物病原真菌孢子捕捉,实现田间病原真菌孢子数量的监测,对于气传植物真菌病害的预测预报和防治决策具有重要意义。目前对捕捉到的孢子多采用传统显微镜孢子计数方法,由于孢子个体小、数量大,利用这种计数方法费时费力,易造成较大计数误差。为了获得一种孢子捕捉器捕捉孢子的自动计数方法,提高计数的准确性和工作效率,本研究利用透明胶带、凡士林玻片和Eppendorf离心管3种方法模拟捕捉小麦条锈病菌夏孢子,利用显微镜照相技术获得孢子图像,在MATLAB软件环境下,对图像进行基于最近邻插值法的缩放处理、基于K-means聚类算法的分割处理、形态学操作修饰和分水岭分割等一系列的处理,实现夏孢子的自动计数和标记。结果表明,3种模拟方法获得的孢子图像经过处理后,均可获得较好的孢子计数结果。透明胶带、凡士林玻片、Eppendorf离心管模拟捕捉条锈病菌夏孢子的平均计数准确率最低分别为98.5%、98.7%、99.9%,Eppendorf离心管模拟捕捉条锈病菌夏孢子和小麦白粉病菌分生孢子的平均计数准确率为99.8%。本研究为实现田间利用孢子捕捉器捕捉孢子的自动计数提供了一种简便、快捷、准确、高效的方法。     

Factors that Influence the Transport of Bacillus cereus Spores through Sand

The goal of this study is to clarify the surface-chemical and microphysical variables that influence bacterial spore transport through soil, thereby defining the factors that may affect spore transport velocity. Bacillus cereus spores were continuously monitored in a soil column under saturated conditions with experimental variations in soil grain size (0.359 and 0.718 mm), pH (7.2 and 8.5), and water flow rate (1.3 and 3.0 mL/min). Increasing soil grain size, flow rate, and pH resulted in enhanced spore movement. Spore transport increased 82% when soil grain size was doubled. An increase in effluent flow rate from 1.3 to 3.0 mL/min increased spore movement by 71%. An increase in pH increased spore transport by 53%. The increase in hydrodynamic forces resulting from the larger grain size soil and higher flow rate functioned to overcome the hydrophobic nature of the spore’s coat, and the interparticle bonding forces between the spore and soil particles.     

Nonlinear responses of soil erosion to climate change: a modelling study on the UK South Downs

A modelling approach is used to estimate some effects of changed climate upon rates of soil erosion on agricultural land on the UK South Downs.Previous studies have concentrated only on estimating shifts in long-term mean erosion rate: these were found to be approximately linear. However such simple shifts mask changes in the underlying distributions of annual erosion. A first series of simulations indicated that, under a wetter climate, erosion rates in wet years will generally increase more than rates in dry years. Under a “best guess” rainfall scenario with a 10% increase in winter rainfall, annual erosion increased by up to 150%. Erosion rates for individual years were shown to change in more complex nonlinear ways however, with decreases as well as increases occurring. These could be explained by the interaction of timing of rainfall with changes in the rate of crop growth.Most earlier work also assumed an equilibrium climate for the simulations, with climatic parameters such as mean monthly rainfall having stabilised at some new value, usually for a 2 × CO₂ atmosphere. This however leads to an “initial conditions” problem: how will soil characteristics have changed by the time of CO₂ doubling? A decrease in erodibility of about 20% by the time of CO₂ doubling was indicated, resulting from changed soil profile properties. However, a second series of runs employed “transient” weather sequences (i.e. with a trend imposed). For these, present-day soil profiles could legitimately be used.     

北京市降雨侵蚀力及其空间分布

 通过对北京地区20个气象站雨量资料的回归分析,发现可用公式R=5.2562F1.3057F来估算北京的降雨侵蚀力,其中FF是由逐月雨量计算而来的一种指标。用此公式计算了北京113个站点的R值,绘制了降雨侵蚀力等值线图,发现北京的R值变化于2144.0～6682.7MJ·mm·hm-22·h-1·a-1。由北部和西部山地所组成的弧形山脉R值最高,并呈现向西北、东南方向递减的趋势。研究结果可为北京的水土保持规划和评价提供依据。     

Background pollution in the Arctic air mass and its relevance to North American acid rain studies

The Arctic air mass is a unique meteorological feature of the northern hemispheric atmosphere. Possessing well-defined meteorological characteristics, it occupies not only the polar region but also a large fraction of the Canadian and Eurasian land masses during the period November to April. Poor pollutant removal by precipitation and dry deposition within the air mass and a strong transport pathway between Eurasian mid-latitudinal sources and the north, result in elevated levels of acidic anthropogenic aerosols and gases in the air mass during winter. In summer, weak north/south transport and strong pollutant removal between the Arctic and mid-latitudes and within the Arctic, results in lower airborne concentrations of acidic pollutants. Due to the presence of the relatively polluted Arctic air mass, ‘background’ air concentrations of SO₄ ⁼, SO₂ and total NO₃ ^? are elevated in western Canada during winter. Typical mean monthly concentrations from December to March are 0.8 to 2.1, 1.0 to 2.4 and 0.1 ? 0.6 μg m^?3, respectively. In the absence of the neutralizing influence of alkaline soil dust, the acidity of snow forming in western Canada during winter is expected to range from 5 to 20 μeq l ^?1.     

Early effects of tillage and crop rotation on arbuscular mycorrhizal fungal propagules in an Ultisol

The aim of this work was to study the early influence of conventional tillage (CT) and no-tillage (NT) on arbuscular mycorrhizal fungal (AMF) propagules. A short 2-year-course crop rotation, i.e. trial consisting of a succession of wheat and oat, was studied in a typic Chilean Ultisol from the second to fourth year after the beginning of the experiment. Measurements included mycorrhizal characteristics and some soil properties in order to explain their influence on AMF propagules. Soil samples were taken yearly in autumn (fallow period) and in early spring (flowering). Significant differences in AMF hyphal length were observed between NT and CT in the first year, but such differences disappeared thereafter. No differences in metabolically active hyphae were obtained with wheat or oat under the two tillage systems. Mycorrhizal root colonization was always higher under NT than under CT. The number of AMF spores was also higher under NT than under CT, ranging from 158 to 641 spores per 100 cm³. Twenty-two AMF species including eight Glomus spp., six Acaulospora spp., four Scutellospora spp., one Archaeospora sp., one Diversispora sp., one Entrophospora sp. and one Pacispora sp. were observed in both agro-ecosystems. Higher spore number of Acaulospora spp. was found under wheat than under oat and under CT than under NT, whilst more spores of Scutellospora spp. were observed under NT than under CT. From all mycorrhizal characteristics, spore number could be visualized as an early and useful indicator of the effect of tillage systems on mycorrhizal propagules in short-term experiments.     

Inoculation of seeds and soil with basidiospores of mycorrhizal fungi

It was demonstrated that basidiospores of the fungus Rhizopoyon luteolus, mycorrhizal for Pinus radiata, could be used successfully as seed inoculum after freeze-drying and storage for 3 months at 22°C, provided the inoculum level was increased 100-fold. Spore inoculum applied to seed could be held dry for at least 2 days before planting provided inoculum was increased 10-fold. On sowing freshly inoculated seed to sterile soil, 3 × 10³ basidiospores/seed were adequate for infection but maximum mycorrhizal infection occurred with 3 × 10⁴ spores/seed.A dose-response curve was obtained for mycorrhizal infection when basidiospores were applied to soil. As few as 100 spores/290 cm³ pot were sufficient for mycorrhizal infection although infection increased with greater spore dose to a maximum of 10⁵ spores/pot. Plant growth response was related to intensity of infection. It is suggested that the percentage germination of basidiospores in the rhizosphere may be considerably greater than those reported in studies with synthetic medium. A rhizosphere effect on germination of basidiospores was demonstrated and a method developed to facilitate studies of spore germination in the rhizosphere.     

Site variability effect on field measurement of symbiotic nitrogen fixation using the 15N isotope dilution method

The sensitivity of the isotope dilution method involving the application of ¹⁵N-enriched fertilizers to estimate symbiotic N₂ fixation in legumes under field conditions is analysed with respect to the variability of atom% ¹⁵N excess in the fixing and non-fixing (reference) crops. Field data collected along a 96 m transect with 63 plots split into two strips, one sown with ryegrass and the other with alfalfa, showed that the sensitivity of the methodology increased with the level of fixation. The results further showed that the sensitivity of the ¹⁵N isotope dilution method for estimating N₂ fixed can be greatly improved by selecting fairly homogeneous sites, although use of a heterogenous site is quite practicable if the non-fixing reference crop is sited reasonably close to the test plots. Errors associated with mismatch between reference and fixing crops or heterogeneity of site become smaller as the amount of fixation increases. The data also revealed that relative comparisons of N₂ fixing abilities of legume treatments based on their ¹⁵N enrichments may not be valid for soils with highly variable N content. Under such circumstances, the use of a reference crop sited close to each legume treatment is necessary.     

A simple biomonitor for measuring ammonia deposition in rural areas

Summary Knowledge of the contribution of NH₃ to the total deposition of N in rural areas is sparse, because the determination of NH₃ deposition is costly and labour-intensive. A simple biomonitor consisting of barley plants grown in pots with an inert growth medium is therefore proposed for estimating total N deposition, including NH₃. The rise in total N of the plant-soil system reflects the deposition. The biomonitor was tested near a dairy farm. Different N contents in the green biomass reflected differences in deposition, and the deposition correlated very well with NH₃ levels in the area and in two background stations. The biomonitors were placed above the crop, the measurements thus representing total N at the edge of a plant community. In 1 month the deposition in the NH₃ plume was 8 kg N ha^–1.