Dynamics of cercospora leaf spot disease determined by aerial spore dispersal in artificially inoculated sugar beet fields

Cercospora beticola is one of the most important fungal pathogens of sugar beet, causing cercospora leaf spot (CLS) disease. Due to the decreasing efficacy of various fungicides caused by resistance traits, the development of a sustainable disease management strategy has become more important. Therefore, detailed knowledge about the epidemiology of the pathogen is crucial. Until now, little was known about the spatiotemporal dispersal of C. beticola spores from the primary inoculum source. Rapid detection of C. beticola spores could facilitate a more precise and targeted disease control. Therefore, a TaqMan real-time PCR assay for detection and quantification of C. beticola spores caught with Rotorod spore traps was established. In 2016 and 2017, field trials were conducted to monitor C. beticola aerial spore dispersal and disease development within an inoculated field and in the adjacent noninoculated area. With the established detection method, C. beticola spores were successfully quantified and used as a measure for aerial spore dispersal intensity. The analysis of the spatiotemporal spread of C. beticola spores revealed a delay and decrease of aerial spore dispersal with increasing distance from the inoculated area. Consequently, disease incidence and severity were reduced in a similar manner. These results imply that spore dispersal occurs mainly on a small scale within a field, although long distances can be overcome by C. beticola spores. Moreover, secondary aerial spore dispersal from sporulating leaf spots seems to be the main driver for CLS disease development. These results provide an important basis for further improvement of CLS control strategies.     

Rain-induced dispersal in Puccinia arachidis,studied by means of a rainfall simulator

A rainfall simulator was used on groundnut (Arachis hypogaea) plots artificially infected withPuccinia arachidis in order to study urediniospore dispersal caused by various amounts of rainfall. Several trapping method were used to compare dry, splash and drip dispersal caused by rain. The spore liberation mechanisms activated by rain and the flows of spores which they generate can be studied at the infected canopy or at the sporulating lesion scales. Two systems referring to these scales were considered to discuss the results. Dry dispersal seemed preponderant. The spore content of the lesions in the canopy was strongly reduced by rain. The results indicated that light rain showers may promote disease dispersal, whereas it may be hampered by heavy showers.Samenvatting Een regensimulator, een krachtig beregeningsapparaat, werd ingezet boven een aantal aardnoot-(Arachis hypogaea) veldjes geïnoculeerd met de aardnootroest (Puccinia arachidis) om de uredosporenverspreiding te bestuderen bij verschillende hoeveelheden regenval. Diverse vangtechnieken werden toegepast om droge verspreiding, spatverspreiding en druipverspreiding ten gevolge van regen te vergelijken. De sporeninhoud van de sporenhoopjes op het gewas werd door regen sterk verminderd. De resultaten, verkregen in twee verschillende, maar elkaar aanvullende analyses, geven aan dat lichte regenbuien de ziekteverspreiding bevorderen, terwijl zware buien deze zouden afremmen.     

Relative humidity and wind velocity associated with diurnal rhythmicity of aerial dispersal ofPuccinia arachidis urediniospores

Four groundnut plots were inoculated withPuccinia arachidis during the growing season 1984 in Adiopodoumé (Ivory Coast). Rust intensity assessments and spore trappings were performed during the development of the resulting epidemics. Spore density in the air at canopy height ranged from 0 to 800 spores m^–3. A significant linear regression was found of the logit of the relative air spore content on the logit of rust intensity expressed as the number of lesions m^–2 of field. During the first sporulation wave following inoculation, the spore density in the air and the spore content of the pustules were determined at regular intervals. These data were related to weather parameters measured simultaneously. The spore content of the air ranged from 0 to 20 spores m^–3. A pronounced daily rhythmicity was found in the spore density of the air, related to a daily rhythm in the depletion and repletion of uredinia. The major explanatory variable was relative humidity, a secondary was wind velocity. The hypothesis is made that this periodicity affects the whole range of variation of aerial spore densities measured at different rust intensities.Samenvatting Vier veldjes met aardnoten in Adiopodoumé (Ivoorkust) werden in het groeiseizoen kunstmatig besmet metPuccinia arachidis. Gedurende het verloop van de daaropvolgende epidemie werd de mate van aantasting door roest bepaald en werden de gevangen sporen geteld. De sporendichtheid op gewashoogte varieerde van 0–800 sporen m^–3. Een significante lineaire regressie tussen de sporendichtheid en de roest-aantasting (uitgedrukt in het aantal lesies m^–2) kon worden vastgesteld na logit transformaties van de relatieve aantallen. Gedurende de eerste sporulatiegolf na de inoculatie werden de sporedichtheid in de lucht en de hoeveelheid sporen in de sporenhoopjes periodiek bepaald. De hoeveelheid sporen in de lucht varieerde van 0 tot 20 sporen m^–3. Er werd een duidelijke dagelijkse ritmiek van de sporendichtheid in de lucht gevonden. Deze hield verband met het dagelijks ritme in het verlies en de aanwas van sporen in de sporenhoopjes. De belangrijkste verklarende factor was de relatieve luchtvochtigheid, gevolgd door de windsnelheid. Er wordt verondersteld dat de gevonden periodiciteit geldt voor de gehele variatiebreedte aan sporendichtheid, bepaald bij verschillende niveaus van roestaantasting.     

Combining permanent aerobiological networks and molecular analyses for large-scale surveillance of forest fungal pathogens: A proof-of-concept

Forest disease management relies principally on a preventive approach in which epidemiological surveillance plays a crucial role. However, efficient and cost-effective surveillance methods are not currently available for large spatial scales. Nevertheless, aerobiological networks have been set up for several decades in many countries to monitor pollen dispersal and provide real-time assessments of allergenic risk. Here, we suggest that the same approach could be used for the surveillance of forest pathogens. Using molecular methods, we analysed samples from 12 sites of the French aerobiological network, at different dates. Both metabarcoding by high-throughput sequencing (using two markers and two different bioinformatics approaches) and real-time PCR targeting eight important forest pathogens were conducted. To validate the approach, temporal and spatial trends of spore detection were compared with field disease data. The metabarcoding approach demonstrated that many fungal plant pathogens could be found in aerobiological samples. Moreover, five of the eight targeted forest pathogens were detected by real-time PCR, with temporal and spatial trends of spore capture consistent with field data. In particular, Hymenoscyphus fraxineus was detected at high frequency in aerobiological samples in the areas where ash dieback has been present for the longest period of time, and at lower frequency in areas with more recent invasion. Spore detection of seasonal pathogens showed a temporal pattern similar to that of disease reports. Overall, our study provides a proof of concept that permanent aerobiological networks combined with molecular methods may provide a useful tool for large-scale surveillance of forest pathogens.     

Evidence for sporangial dispersal leading to a single infection event and a sudden high incidence of grapevine downy mildew

In a German vineyard (PRO) between 10 and 13 June 2004, the incidence of grapevine downy mildew ( Plasmopara viticola ) increased abruptly from 0 to 99%. Infected vines bore on average between 45 and 60 lesions each, corresponding to about 220 000 lesions ha⁻¹ in a non-aggregated distribution. A second vineyard (FUT), approximately 50 m distant from PRO, had been inoculated 3 weeks before the abrupt increase in incidence of disease in PRO. Using microsatellites to ascertain the sources of inoculum and likelihood and extent of interplot spread from FUT to PRO, 555 samples were collected and 20 unique genotypes were identified, of which one caused 80% of the sampled lesions in both vineyards. Three genotypes responsible for 95% of the lesions in FUT and PRO were identified as the genotypes originally established through earlier inoculations in FUT. This is the first report of definitive and quantitative evidence of sporangial migration up to 130 m in a single infection event. The utility of molecular tools to address practical epidemiological issues in this pathosystem is illustrated. The results of this study provide an example of how P. viticola was able to rapidly colonize European vineyards after the pathogen was introduced from North America in 1878.     

Development of isolate‐specific markers for Neofusicoccum parvum and N. luteum and their use to study rainwater splash dispersal in the vineyard

Unique bands were identified in single isolates of Neofusicoccum parvum and Neofusicoccum luteum using universally primed polymerase chain reaction (UP‐PCR) analysis of isolates obtained from grapevines and non‐grapevine hosts in New Zealand, Australia, South Africa and the USA. Primers were designed to amplify a 1550 bp portion of the 1573 bp marker band from N. parvum isolate B2141 and a 510 bp portion of the 524 bp marker band from N. luteum isolate G51a2. A PCR‐RFLP assay was developed to distinguish the N. parvum isolate B2141 from other N. parvum isolates, based on a polymorphism found in the marker band using the TaqI restriction endonuclease. For N. luteum isolate G51a2, the designed primers were specific at an annealing temperature of 63°C in the PCR. The sensitivity threshold of the N. parvum and N. luteum isolate‐specific markers was 50 pg and 5 pg, respectively, when used in standard PCR with purified genomic DNA. The sensitivity of the N. parvum isolate‐specific marker was increased to 0·5 pg by nested PCR. The specificity test of both isolate‐specific markers with six other Botryosphaeriaceae spp. showed that they were specific to their respective species and isolates. Both markers were able to detect the conidia of N. parvum and N. luteum marker isolates in rainwater samples collected at different distances from an inoculation point in the vineyard. The results showed that rain splash could disperse the conidia of both of these species up to 2 m from the inoculum point in a single rainfall event.     

Clonality and long-distance migration of Puccinia striiformis f.sp. tritici in north-west Europe

The biotrophic fungus Puccinia striiformis f.sp. tritici , a basidiomycete that causes yellow rust on wheat, is spread by wind-dispersed spores. Analysis of amplified fragment length polymorphism (AFLP) variation showed that the fungus frequently migrates between the UK, Germany, France and Denmark. There is no biological evidence for sexual or parasexual reproduction under natural conditions, and this was supported by the lack of recombination, as revealed by AFLP, over the time and area represented by the samples in this study. A phylogeographic analysis revealed that there was effectively a single, clonal population in the four countries, up to 1700 km apart, consistent with a 'continent-island' model in which Denmark is the recipient of migrants from other countries. In five cases, specific pathogen clones were dispersed between the UK and Denmark, and on at least two recent occasions clones were also spread from the UK to Germany and France, causing outbreaks of yellow rust on wheat cultivars that were previously resistant to the disease in these countries. The agronomic consequences of migration were enhanced because of the limited genetic diversity for yellow rust resistance in wheat cultivars in the area. These results demonstrate that long-distance migration of pathogen clones, coupled with low diversity in the host species, may cause previously useful resistance genes to become ineffective for disease control on a continental scale.     

Genetic structure and spatial distribution of the mycoparasite Sphaerellopsis filum on Melampsora larici-epitea in a short-rotation coppice willow planting

AFLP analysis was used to examine the genetic structure of the mycoparasite Sphaerellopsis filum on willow rust Melampsora larici-epitea using two sets of samples of a willow clone in a short-rotation coppice planting. The first set was collected from a plot (short strip) in 1998, 1999 and 2000 and the second from another plot (long strip) in 2000. In all, 228 S. filum isolates were typed, 139 AFLP loci scored and 54 AFLP genotypes identified. In the short strip, genotypes sampled in 1998 were no longer detected in subsequent years and genotype diversity fluctuated (0·25–0·68) over the years, indicating that migration had a major impact on the genotype structure. Two distinct groups (average Nei and Li's similarity coefficient between the groups = 0·047) were identified. The group B genotypes were sampled only in 2000. Within the groups, the average similarities were > 0·96. Both the index of association test and the parsimony tree length permutation test suggest that there was a significant clonality in group A while recombination cannot be ruled out in group B. Localized clusters of AFLP genotypes were detected using the software SaTScan that is based on the spatial scan statistic. Possible mechanisms involved in the spread of S. filum in willow coppice plantations are discussed.     

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.     

Secondary dispersal, spatial dynamics and effects of herbicides on reproductive capacity of a recently introduced population of Bromus sterilis in an arable field

An arable field was subdivided and subjected to either deep inversion ploughing or non‐inversion cultivation after viable seeds of Bromus sterilis had been sown into oilseed rape stubble. After sowing in isolated plots distributed within the field, sequences of cropping treatments for the establishment of two successive winter wheat crops were applied. Each subfield was split into an uphill and a downhill direction for soil cultivation. The field had a 10° slope. In the season following seed introduction, 2.6% of the introduced seeds had successfully germinated and established in the non‐inversion cultivation regime, when no effective graminicide was applied. Ploughing eradicated B. sterilis. Using differential global positioning system (DGPS) mapping of the whole field population, emerged plants were observed up to 8.7 m (uphill treatment) and 21.3 m (downhill treatment) of their initial source. The median distance seeds were transported was 2.3 m uphill and 4.8 m downhill. Post‐emergence application of the herbicide propoxycarbazone slightly reduced weed density and seed weight, and almost halved weed seed production. Application of fenoxaprop‐P‐ethyl was followed by higher density of plants, tillers and seeds of B. sterilis. Seed viability was unaffected by herbicide use. Thus, in the second wheat crop following seed rain, the weed population was dispersed more widely in the field, such that 20–30% of seeds were dispersed more than 5 m distance from the first year's foci of infestation. The relevance of soil cultivation to secondary dispersal of B. sterilis is discussed.     

我国小麦秋苗条锈病发生规律及其区间菌源传播关系

全国冬小麦秋苗均可遭受来自西北和西南越夏区条锈菌的侵染与危害, 条锈菌在寄主上进一步繁殖和发展, 引起本地或者外地小麦条锈病流行。不同麦区秋苗条锈病发生时间、发生程度及其菌源传播规律各不相同, 年度间亦存在差异。根据小麦条锈病发生流行频率、病菌越冬和越夏情况、秋季菌源和春季菌源的有无与多少、提供时间及其影响范围与作用, 结合地理生态条件、气候特点、小麦种植区划与栽培模式等, 将中国小麦条锈病发生流行区域划分为8个明显不同的生态区系, 即关中、华北春季流行区; 成都平原、江汉流域冬季繁殖区; 西北、川西北越夏易变区; 云贵高原越夏冬繁区; 新疆冬春麦常发区; 西藏高原青稞、小麦常发区; 南方晚播冬麦偶发区; 内蒙古、东北春麦偶发区。通过病害实地调查、病菌群体遗传多样性和高空气流轨迹分析, 进一步揭示了区间菌源传播关系。陇南、陇中、陇东、宁南、海东、陕西宝鸡以及川西北和云贵高原等地区离越夏区较近, 冬小麦播种较早, 秋苗条锈病发生早、发病重, 秋季随西北气流传播到平原冬麦区和海拔较低的冬麦区侵染危害秋播麦苗, 其菌源数量对全国小麦条锈病发生流行程度起着至关重要的作用, 是中国小麦条锈病的秋季菌源基地, 面积约67万hm2; 成都平原、江汉流域、陕南、豫南、云贵坝区等麦区, 离条锈菌越夏区相对较远, 小麦播种期也较晚, 秋苗发病较晚较轻, 但冬季气候温和, 雨露条件充沛, 条锈菌在冬季可以不断侵染和繁殖, 在早春可积累大量菌源, 然后向北部和西部广大麦区扩散传播, 引起小麦条锈菌春季侵染, 是中国小麦条锈病的春季菌源基地, 面积约200万hm2。云贵高原越夏冬繁区可为我国广大麦区特别是长江中下游麦区提供部分菌源, 荆州等鄂东南地区是云贵菌源向长江下游麦区传播的中转站。云南与甘肃之间存在大量的基因流, 推测云南可能是中国小麦条锈菌重要的起源中心。西南和西北秋季菌源对长江流域麦区的相对重要性有待进一步研究明确。     

大螟幼虫田间扩散及成虫飞行能力研究

大螟的抗性演化将成为制约Bt水稻产业化发展的关键因素,为实施预防性的害虫抗性治理策略,明确大螟成、幼虫的运动习性具有非常重要的意义,本文通过田间调查和飞行磨测试的方法研究了大螟幼虫在田间的迁移扩散行为和成虫的飞行能力。结果表明,大螟幼虫在不同水稻穴之间及同一水稻穴内的不同分蘖间均存在频繁的迁移扩散,且其在分蘖期水稻上的迁移扩散能力显著高于孕穗期水稻。在整个幼虫发育过程中,大螟在分蘖期和孕穗期水稻上的平均迁移扩散距离分别为62.29cm和51.02cm,最大扩散距离为120cm。吊飞结果表明,大螟的有效飞行时间为4～5d,能多次飞行,约有75.5%的个体累计飞行距离≤5km。因此,对于幼虫扩散能力相对较强、成虫飞行能力相对较弱、求偶前期相对较短的大螟来说,建议其抗性治理策略应设置为分区种植的庇护所,且庇护所与Bt稻田的距离最好不超过5.0km。     

Establishment of, and preliminary impact studies on, the rust, Maravalia cryptostegiae, of the invasive alien weed, Cryptostegia grandiflora in Queensland, Australia

The rust fungus Maravalia cryptostegiae , from south-west Madagascar, was introduced into Australia in 1995 as a classical biological control agent against the highly invasive rubber-vine weed Cryptostegia grandiflora , a woody climber endemic to Madagascar. The rust was released at 69 sites between 1995 and 1997 and is now established throughout the plant's exotic range in Queensland, estimated at over 40 000 km². Dispersal was low in the first 3–4 months but was virtually linear thereafter, and the rust spread over 100 km within the first year; after 3 years it was recorded 550 km away from the nearest release site. Spraying both dry and aqueous inoculum of uredinioid teliospores from the ground using mist-blowers, as well as from the air by atomizing spore suspensions, resulted in rust-induced defoliation, producing an overall reduction in fecundity and biomass of the weed. In sites with low water tables, weed growth decreased markedly, with a reduction in plant volume from 9 m⁻³ to 1 m⁻³ over a 4-year period. Both rust- and drought-induced stress combined to cause up to 75% plant mortality at some sites, and at all monitored sites, seedling recruitment was virtually nil. Improved growth of indigenous grasses amongst rubber-vine thickets has increased fuel loads and created opportunities to use fire as a component of an integrated approach to the management of this economically and ecologically damaging weed.     

Importance of different sources of inoculum and dispersal methods of conidia of Colletotrichum musae, the causal agent of banana anthracnose, for fruit contamination

Different populations of Colletotrichum were characterized and quantified on floral parts of banana plants from flowering until harvest. Isolates of Colletotrichum found to be pathogenic and attributed to the species C. musae (77% of isolates) were differentiated from other species by abundant sporulation, a short mycelium, and rapid growth. Colletotrichum musae was isolated from floral parts mainly during the month following bunch emergence. The respective involvement of different sources of inoculum (leaves, bunch bracts, floral parts) in the levels of fruit contamination was evaluated. When the floral parts and bunch bracts were removed at flowering, the severity of anthracnose disease was considerably reduced. The severity of the disease is strongly correlated with cumulative rainfall during the first 35 days after bunch emergence, and was considerably reduced when rainwater runoff over the bunches was limited by placing plastic sleeves over them. The disease was not observed on banana fruit grown under shelters, protected from rain. The results obtained from this study show clearly that contamination of fruit by conidia takes place largely due to the trickling of rainfall over the floral parts, which are the main source of inoculum. The application of these results for integrated control is discussed.     

宁夏不同农业生态区土壤养分时空变化特征

根据宁夏耕地土壤监测结果,分析了不同农业生态区土壤氮磷钾养分含量的时空变化特征。结果表明,在1970～2000年的30a里,土壤全氮、全磷和全钾含量没有发生显著变化,而土壤碱解氮和有效磷含量呈较显著或显著增长态势,土壤速效钾含量在山区没有明显增减,而在灌区近15a呈明显下降态势。从两个地区比较来看,土壤氮磷钾养分含量总体上都是灌区高于山区,其中地区间差异最大的是土壤有效磷含量,具有极显著差异的是全氮、碱解氮和有效磷含量,全钾和速效钾含量在地区间存在较显著或显著差异,全磷量在地区间没有显著差异。从地区内变异系数来看,除土壤全钾外,其他都是山区显著大于灌区,山区变异系数的大小依次为全氮>有效磷>速效钾>碱解氮>全磷>全钾,灌区变异系数的大小依次为碱解氮>有效磷>速效钾>全氮>全磷>全钾。不论山区还是灌区,土壤全钾量的变异系数都是最小。     

Temporal and spatial variations of net primary productivity and its response to groundwater of a typical oasis in the Tarim Basin,China

Net primary productivity (NPP) of the vegetation in an oasis can reflect the productivity capacity of a plant community under natural environmental conditions. Owing to the extreme arid climate conditions and scarce precipitation in the arid oasis regions, groundwater plays a key role in restricting the development of the vegetation. The Qira Oasis is located on the southern margin of the Taklimakan Desert (Tarim Basin, China) that is one of the most vulnerable regions regarding vegetation growth and water scarcity in the world. Based on remote sensing images of the Qira Oasis and daily meteorological data measured by the ground stations during the period 2006-2019, this study analyzed the temporal and spatial patterns of NPP in the oasis as well as its relation with the variation of groundwater depth using a modified Carnegie Ames Stanford Approach (CASA) model. At the spatial scale, NPP of the vegetation decreased from the interior of the Qira Oasis to the margin; at the temporal scale, NPP of the vegetation in the oasis fluctuated significantly (ranging from 29.80 to 50.07 g C/(m²•month)) but generally showed an increasing trend, with the average increase rate of 0.07 g C/(m²•month). The regions with decreasing NPP occupied 64% of the total area of the oasis. During the study period, NPP of both farmland and grassland showed an increasing trend, while that of forest showed a decreasing trend. The depth of groundwater was deep in the south of the oasis and shallow in the north, showing a gradual increasing trend from south to north. Groundwater, as one of the key factors in the surface change and evolution of the arid oasis, determines the succession direction of the vegetation in the Qira Oasis. With the increase of groundwater depth, grassland coverage and vegetation NPP decreased. During the period 2008-2015, with the recovery of groundwater level, NPP values of all types of vegetation with different coverages increased. This study will provide a scientific basis for the rational utilization and sustainable management of groundwater resources in the oasis.