Orchard floor management practices that maintain vegetative or biomass groundcover stimulate soil microbial activity and alter soil microbial community composition

Groundcover management systems (GMS) are important in managing fruit-tree orchards because of their effects on soil conditions, nutrient availability, tree growth and yields. We employed a polyphasic approach, incorporating measures of soil microbial abundance, activity and community composition, to study the long-term effects of different GMS on biotic and abiotic factors in an orchard soil. Four GMS treatments – Pre-emergence residual herbicides (Pre-H), post-emergence herbicide (Post-H), mowed-sod (Grass), and hardwood bark mulch (Mulch) – were established in 2-m-wide strips within tree rows in an apple orchard in 1992, and have been maintained and monitored annually until the present. We have measured soil water and nutrient availability, tree growth, and yields annually from 1993 to 2003. Soil nematode numbers and trophic groups were evaluated in July and Oct. 2001, and Sept. 2003. Numbers of culturable bacteria and fungi, soil respiratory activity, eubacterial and fungal community composition were determined in May and Sept. 2003. The Pre-H treatment soil had the fewest culturable bacteria, while the Grass treatment had the largest population of culturable fungi. Soil nematode population size and diversity were also affected by GMS treatments; the Pre-H treatment had the lowest ratio of (bacteriovores + fungivores) to plant parasitic nematodes. Soil respiration rates were higher in the Mulch than in other treatments during a 40-day incubation period. Hierarchical cluster dendrograms of denaturing gradient gel electrophoresis (DGGE) fingerprints for eubacterial community 16S rRNA genes indicated that Post-H and Grass treatments clustered together and separately from the Pre-H and Mulch treatments, which were also grouped together. The influence of GMSs on the fungal community, as assessed by PCR-DGGE of the internal transcribed spacer (ITS) region, was not as pronounced as that observed for bacteria. Soil fungal community composition under the Mulch differed from that under other treatments. The effects of GMS on soil microbial community abundance, activity, and composition were associated with observed differences in soil organic matter inputs and turnover, nutrient availability, and apple tree growth and yields under the different GMS treatments.     

Soil physical conditions in a New York orchard after eight years under different groundcover management systems

An experiment was conducted near Ithaca, New York, to compare orchard soil and groundcover management systems (GMSs) in a silty clay loam soil (mixed, mesic, Glosaquic Hapludalf). Three replications of four GMS treatments were randomly assigned to 12 plots and maintained since 1992 in 2 m-wide strips within tree rows: (1) Sod: Red fescue (Festuca rubra) turfgrass mowed biweekly; (2) Post-HBS: Post-emergence applications of glyphosate herbicide in May and July annually; (3) Pre-HBS: Pre-emergence applications of paraquat, diuron and norflurazon herbicides each May; and (4) Mulch: A 15-cm depth layer of shredded hardwood bark mulch, renewed triennially. After eight years of continuous treatments, we compared soil physical conditions and hydraulic properties in these GMSs. Bulk density was lower and soil porosity greater under Mulch than other GMSs. Infiltration was more rapid under Mulch than other GMSs, and under Post-HBS than Sod. Saturated hydraulic conductivity ranged from 6 × 10^–4 m^.s^–1under Sod to 12 × 10^–4 m^.s^–1 under Mulch plots, and was equivalent in Sod and Pre-HBS, lower in Sod than Post-HBS or Mulch treatments, higher in Post-HBS than Pre-HBS, and higher in Mulch than all other GMSs. Volumetric soil water content at field capacity ranged from 0.47 under Mulch to 0.40 under Sod, and was not significantly different among GMSs between 0.99 and 39.22 kPa of pressure. GMS treatments and related management practices at this orchard had substantially different long-term effects on soil physical conditions. Compaction and reduced infiltration in Sod compared with other GMSs were attributed to tractor wheel traffic during mowing. Mulch treatments improved soil conditions relative to other GMSs. Greater infiltration rates and hydraulic conductivity under Post-HBS compared with Pre-HBS suggest relative advantages of post-emergence herbicides.     

Rootstock genotype succession influences apple replant disease and root-zone microbial community composition in an orchard soil

Apple replant disease (ARD) is a soil-borne disease complex that affects young apple trees in replanted orchards, resulting in stunted growth and reduced yields. Newly developed rootstock genotypes with tolerance to ARD may help to control this disease. We determined the effects of rootstock genotype rotations during orchard renovation, by investigating root-zone soil microbial consortia and the relative severity of ARD on seven rootstock genotypes (M.9, M.26, G.30, G.41, G.65, G.935, and CG.6210) planted in soil where trees on four of those same rootstocks (M.9, M.26, G.30 and CG.6210) had grown for the previous 15 years. Rootstock genotyping indicated that genetic distances among rootstocks were loosely correlated with their differential responses to ARD. Root-zone fungal and bacterial community composition, assessed by DNA fingerprinting (T-RFLP), differed between M.26 and CG.6210. Soil bacterial communities were influenced most by which rootstock had grown in the soil previously, while fungal communities were influenced more by the current replanted rootstock. In a clone library of bacteria from M.26 and CG.6210 root-zone soil, β-Proteobacteria was the most abundant phylum (25% of sequences). Sequences representing the Burkholderia cepacia complex were obtained only from CG.6210 soil. Rootstock genotypes that were grown in the orchard soil previously affected subsequent ARD severity, but replanting with the same or closely related rootstocks did not necessarily exacerbate this disease problem. Our results suggest that genotype-specific interactions with soil microbial consortia are linked with apple rootstock tolerance or susceptibility to ARD.     

复合微生物菌剂对重茬苹果园土壤细菌群落的影响

【目的】了解抗重茬复合微生物菌剂对重茬苹果园土壤细菌数量及多样性的影响。【方法】对重茬苹果园土壤进行复合微生物菌剂处理后,采用自动核糖体内转录间隔区序列分析方法(Automated ribosomal intergenic spacer analysis,ARISA)对新茬苹果园、重茬苹果园土壤细菌群落多样性进行分析。【结果】经复合微生物菌剂处理2年后,重茬果园土壤OTUs总数和Shannon-Weiner指数分别为250和4.44,新茬果园OTUs总数和Shannon-Weiner指数分别为234和3.81,经One-Way ANOSIM法分析得到二者差异系数P分别为0.108 3和0.084 3。另外,重茬园和新茬园共享核心OTUs有89个,二者的Jaccard群落相似性系数为0.47,相似程度中等。【结论】复合微生物菌剂具有提高重茬果园土壤微生物多样性,促进重茬苹果园土壤微生物生态系统恢复的作用。     

Linking Soil Biotic and Abiotic Factors to Apple Replant Disease: a Greenhouse Approach

Apple replant disease (ARD) is a frequently occurring plant disease, which causes retarded growth and mortality of young apple trees in replanted orchards. The aetiology is not well understood, but soil‐borne micro‐organisms are often discussed as primary causal agents of the replant problem. A greenhouse study was conducted in Laimburg, Italy, with orchard soils from the region, with the aim of obtaining information about the influence of soil biotic and abiotic factors on the aetiology of the disease. Apple rootstocks (M9) were planted into soils cultivated with apple trees that were either fumigated with chloropicrin or not fumigated, as well as mixtures of fumigated and non‐fumigated soils. In addition, uncultivated soils (from the inter‐row, from a fallow plot and from a meadow) were taken as controls. Various parameters were measured after 62 days in a controlled pot assay. Soils fumigated with chloropicrin resulted in higher apple shoot growth and lower microbial biomass carbon than non‐fumigated soils. Uncultivated soils had generally the highest microbial biomass carbon and the highest ergosterol contents. No considerable differences between basal respiration, ergosterol content, pH, electrical conductivity, and most nutrient and metal contents were observed between fumigated and non‐fumigated soils. Denaturing gradient gel electrophoresis gels of DNA extracted from the soils revealed differences in the fungal, bacterial and actinobacterial communities of the different soils, indicating significant shifts in microbial community composition after chloropicrin treatment. This study indicates biotic factors in soil to be a causal agent of apple replant disease.     

Apple Replant Disorder of Pingyitiancha Rootstock is Closely Associated with Rhizosphere Fungal Community Development

The aim of this study was to examine rhizosphere fungal community development in apple replant soil and a new planting soil in Beijing through a potted experiment with the apomictic triploid Pingyitiancha rootstock using the Illumina Miseq method. Steam sterilization significantly enhanced seedling growth in replant soil, while root segment addition did not enhance replant disease symptoms. The bulk soil of the replant site (ReSoil) and the new soil of the planting site (NewSoil) were sampled prior to nursery planting, and rhizosphere soils were r Qinping esampled after one growing season as RePlant and NewPlant, respectively. Approximately 48.3, 47.9, 59.8 and 68.8% of the operational taxonomic units (OTUs) were classified as Ascomycota and Basidiomycota in the community of ReSoil, New Soil, RePlant and NewPlant, respectively. Shannon indices suggested that planting apple nurseries increased the fungal diversity of rhizosphere soil in both soils. Apple replant disorder (ARD) was highly associated with an unbalanced microbial ecosystem with more saprophytic and pathogenic fungi, but less species known to produce antimicrobial metabolites, while Rhizopus, Pyrenochaeta and Eurotium were identified as the key factors in ARD.     

Effects of Nitrogen and Phosphorus on the Growth of Microorganisms Associated with Apple Replant Disease and on Apple Seedlings Grown in Soil Infested with these Microorganisms*

The growth of fungi causing apple replant disease (ARD) was inhibited by the addition of N and P to the growing medium. The population of bacteria antagonistic to ARD-causing fungi was significantly increased in the growing medium supplemented with N 400 P 400 mg/l or greater. The application of nitrogen alone or in combination with phosphorus to soil infested with fungi or bacteria that cause apple replant disease significantly increased seedling height. The addition of P alone to these infested soils did not have any effect on seedling height. Significant increases in seedling height occurred with N applications when seedlings were grown in soil to which bacteria that are antagonistic to fungi causing ARD had been added. These results suggest that the application of N, with or without P, to apple replant soils may suppress the growth of ARD-causing fungi or bacteria and promote the growth of antagonistic bacteria.     

环渤海湾地区连作苹果园土壤中酚酸类物质变化

分析了山东昌邑、栖霞、蓬莱,辽宁大连、抚宁、绥远,河北昌黎、青县等地苹果园连作土壤中酚酸物质的组成和含量,结果表明:连作障碍发生的苹果园土壤中酚酸类物质的组成和含量在不同地区、不同土层厚度间存在显著差异。苹果园土壤中酚酸类物质的含量从春季到秋季随时间的延长逐渐减少,但过程缓慢。且非连作园土壤中酚酸类物质含量显著少于连作土。连作土壤中酚酸物质的组成和含量在不同季节间差异显著,尤其春季土壤中酚酸物质的种类与夏季和秋季显著不同。这可能是因为植物在不同季节分泌的酚酸类物质种类和含量有差别.不同土层中酚酸物质的分布因季节不同而有显著差异。夏季土壤中酚酸物质主要分布在浅层土壤中,而秋季则主要分布在深层土壤中。这可能是由于浇水等果园管理措施和自然降水对土壤中的酚酸物质产生的淋溶作用,使得大量的酚酸物质向深层土壤运动,最终造成了秋季果园连作土中随土层加深而酚酸物质含量增加的现象。不同地区苹果园连作土中,对羟基苯甲酸、(+)-儿茶素、咖啡酸、阿魏酸含量与非连作土无显著差异,而焦性没食子酸、绿原酸和根皮苷显著高于非连作土。焦性没食子酸、绿原酸和根皮苷可能是引起山东、辽宁、河北地区苹果园连作障碍的关键酚酸物质。     

环渤海湾苹果产区老果园与连作果园土壤线虫群落特征

应用类群多样性、类群丰富度、个体密度和功能类群指数等群落参数,于2009年5-10月分3次对环渤海湾苹果栽植区老果园不同位置和连作果园取样研究了土壤线虫群落特征。结果表明:苹果连作对果园土壤生物环境具有明显的恶化作用,连作果园土壤环境条件恶劣;对功能类群指数的统计表明,苹果连作明显改变了果园土壤中植物寄生性线虫r-选择和k-选择的比例,与自由生活线虫比较,连作对植物寄生线虫影响更明显;连作提高了果园的土壤线虫PPI/MI,说明连作对果园土壤健康的扰动最大;土壤不同食性线虫数量统计结果显示,环渤海湾苹果栽植区植食性线虫数量未达线虫伤害阈值,线虫不是引起环渤海湾地区苹果连作障碍的主要原因。     

Does apple replant disease affect the soil patch selection behaviour and population growth of Collembolans?

Apple replant disease (ARD) is common to all major apple-growing regions in the world. It occurs when new apple trees are replanted on sites where previously the same or closely related crop species were grown. Biotic (fungi, bacteria and nematodes) and abiotic soil factors (poor soil structure, nutrition) contribute to the development and severity of ARD. However, the aetiology of ARD and effects on higher trophic levels are still unknown. In that sense, Collembola might play an important role, since they are one of the dominant mesofauna groups in many soils. They act as decomposer, fungivores and predators, representing different trophic levels in soil food webs. Therefore, any effect of ARD on the occurrence of Collembola could have ecological impacts on the soil quality and health. Here, we examined the colonization behaviour of two Collembolan species, Folsomia candida and Sinella curviseta, in choice tests and population growth tests using Apple Replant Diseased soil (ARD) and non-ARD soil samples from different field sites and standardized laboratory bioassays. Additionally, Collembola behaviour was quantified by continuous video observations to investigate short-term behavioural changes. Results showed that both Collembolan species significantly preferred colonization of the non-ARD soils compared with ARD soils, independent of the origin of the soil samples or specific disinfection treatments. Moreover, the detailed video analysis of the foraging behaviour indicates rapid colonization of soil samples and low dispersal rates. Most likely, volatile compounds and to a lesser extent feeding stimulants play a vital role for the colonization process for both Collembolan species. Finally, results showed negative effects of ARD on population growth of both Collembolan species already after an 8-week period, implying strong nutritional deficiencies in ARD affected soils. The hypothesis that ARD causing microorganisms directly affected orientation, colonization and population development of Collembola is discussed.     

Global RNA association with the transcriptionally active chromosome of chloroplasts

Key message

Gene expression studies in roots of apple replant disease affected plants suggested defense reactions towards biotic stress to occur which did not lead to adequate responses to the biotic stressors.

Abstract

Apple replant disease (ARD) leads to growth inhibition and fruit yield reduction in replanted populations and results in economic losses for tree nurseries and fruit producers. The etiology is not well understood on a molecular level and causal agents show a great diversity indicating that no definitive cause, which applies to the majority of cases, has been found out yet. Hence, it is pivotal to gain a better understanding of the molecular and physiological reactions of the plant when affected by ARD and later to overcome the disease, for example by developing tolerant rootstocks. For the first time, gene expression was investigated in roots of ARD affected plants employing massive analysis of cDNA ends (MACE) and RT-qPCR. In reaction to ARD, genes in secondary metabolite production as well as plant defense, regulatory and signaling genes were upregulated whereas for several genes involved in primary metabolism lower expression was detected. For internal verification of MACE data, candidate genes were tested via RT-qPCR and a strong positive correlation between both datasets was observed. Comparison of apple ‘M26’ roots cultivated in ARD soil or γ-irradiated ARD soil suggests that typical defense reactions towards biotic stress take place in ARD affected plants but they did not allow responding to the biotic stressors attack adequately, leading to the observed growth depressions in ARD variants.

     

甲壳素对连作条件下平邑甜茶幼苗生长及土壤环境的影响

研究在苹果连作土壤中添加甲壳素对苹果幼苗生长、土壤酶及土壤真菌群落结构的影响,探讨甲壳素缓解苹果连作障碍的可能性,为防控苹果连作障碍提供依据。盆栽条件下,以平邑甜茶幼苗为试材,在苹果连作土壤中分别添加0,0.5,1.0和2.5g/kg的甲壳素,测定了连作土壤中添加不同量的甲壳素后,幼苗生物量、根系保护酶活性、土壤主要酶(蔗糖酶、脲酶、磷酸酶等)活性以及土壤中真菌群落结构的变化。9月份结果表明,与对照相比,1.0 g/kg的甲壳素处理连作土,可显著提高平邑甜茶幼苗株高和干鲜重,分别比对照增加了36.8%、82.1%和100.8%;甲壳素处理能增加幼苗根系保护酶活性,其中1.0 g/kg甲壳素处理SOD、POD和CAT活性最高,其次为0.5 g/kg,而2.5 g/kg甲壳素处理显著抑制了幼苗根系保护酶活性。1.0 g/kg甲壳素处理可提高土壤中细菌/真菌值,并且提高了土壤中蔗糖酶、脲酶、蛋白酶、磷酸酶、过氧化氢酶和多酚氧化酶活性,分别比对照提高了8.6%、40.5%、81.1%、15.3%、18.7%和49.8%,2.5 g/kg甲壳素处理则降低土壤酶活性或者使土壤酶活性与对照相当。根据T-RFLP的图谱中OUT的数量、种类及丰度,分别计算了不同处理土壤的真菌多样性,发现1.0 g/kg甲壳素处理的连作土具有最高的多样性、均匀度和丰富度指数,分别比对照增加了52.2%、8.0%和87.1%。主成分分析(PCA)结果显示,不同剂量甲壳素处理的连作土壤中真菌被PC2分成了两部分,其中0.5 g/kg和1.0 g/kg的甲壳素添加量分布在PC2的负方向上,而CK和2.5g/kg的甲壳素处理分布在PC2的正方向上,这说明添加不同量的甲壳素对连作土壤真菌群落多样性有显著影响,添加量太多或者太少均会造成土壤真菌多样性下降,只有适量的甲壳素可提高真菌群落结构多样性。实验结果表明1.0 g/kg的甲壳素可提高连作平邑甜茶幼苗生物量,改善连作土壤环境,有效缓解平邑甜茶的连作障碍。     

连作苹果土壤酚酸对平邑甜茶幼苗的影响

为探讨连作(重茬)苹果土壤中酚酸类物质的积累与苹果连作障碍的关系,在砂培条件下,取连作果园土壤中实际浓度的酚酸类物质处理平邑甜茶幼苗,探讨了连作2a的果园土壤中实测浓度的根皮苷、间苯三酚、根皮素、对羟基苯甲酸和肉桂酸对平邑甜茶幼苗根系线粒体指标、抗氧化酶活性、膜过氧化程度及活性氧(ROS)含量的影响。结果表明:连作土壤中实际浓度的5种酚酸类物质均使平邑甜茶幼苗生长受到抑制,根系受影响程度高于地上部分,表现为根冠比降低;线粒体膜通透性转换孔(MPTP)开放程度增大,线粒体膜电位降低,细胞色素Cyt c/a比值下降;降低了幼苗根系中超氧化物歧化酶(SOD)、过氧化物酶(POD)和过氧化氢酶(CAT)活性,增加了过氧化氢(H2O2)、超氧阴离子自由基(O·-2)以及丙二醛(MDA)的含量。土壤浓度的5种酚酸类物质中,以根皮苷处理抑制效果最显著,间苯三酚处理抑制力最小。因此,根皮苷是引起苹果连作障碍的主要酚酸,实践中应重点考虑对根皮苷的降解以缓解苹果连作障碍。     

连作苹果园土壤真菌的T-RFLP分析

为探讨连作苹果园不同土壤空间真菌群落结构,应用T-RFLP(Terminal Restriction Fragment Length Polymorphism)技术,比较了3个连作园不同取样位置(行间、原穴、株间)和不同土层(0—30 cm、30—60 cm)的土壤真菌多样性,并结合不同样品TRFLP图谱的差异,采用多样性指数分析、聚类分析和主成分分析(PCA),分析了3个连作园土壤真菌群落结构特征。结果表明,磁窑、道朗和金城连作园的土壤真菌多样性存在差异,各采样地区的Shannon多样性指数在0.43—2.47之间,Pielou均匀度指数在0.17—0.85之间,Simpson优势度指数在0.12—0.81之间,Margalef丰富度指数最高的是金城树穴0—30 cm土层(R=4.55),最低的是磁窑行间30—60 cm土层(R=0.77)。在调查的不同取样位置、不同土层中,原树穴具有最高的多样性指数、均匀度指数、丰富度指数和最低的优势度指数;0—30 cm土层的土壤真菌多样性指数、均匀度指数、丰富度指数均高于30—60cm土层,而优势度指数的趋势正好相反;PCA和聚类分析结果显示磁窑、道朗和金城连作园的土壤真菌群落结构均有明显差异,3个连作园的土壤真菌各自构成一个独立的群落结构,这些群落能够适应各自的土壤环境并成为环境的优势群落。     

Skeletal muscle is protected from disuse in hibernating dauria ground squirrels

The purpose of this study is to test the hypothesis that muscle fibers are protected from undue atrophy in hibernating dauria ground squirrels (Spermophilus dauricus, Brandt). Muscle mass, fiber cross sectional area (CSA, video analysis) and fiber type distribution (m-ATPase staining) were determined in extensor digitorum longus (EDL) muscle from non-hibernating control animals (Pre-H), from animals who hibernated for one (H1) or two (H2) months, and from animals 2-4 days after arousal (Post-H; N = 8 each). Muscle wet weight decreased less than body weight in hibernating animals, resulting in a steady increase in muscle-to-body mass ratio (+ 37% in Post-H compared to Pre-H, p < 0.001). In the Pre-H group, Type I (6.3 ± 2.0%) and II (93.7 ± 2.0%) fiber CSAs were 1719 ± 201 and 2261 ± 287 μm², respectively. There was a tendency (n.s.) of larger CSA of type I in hibernators compared to pre-H. In the Post-H group, fiber CSA and type distribution were not different from Pre-H. We are the first to report data on EDL fiber type distribution and confirm a protective effect that prevents muscle atrophy in spite of prolonged disuse during hibernation in dauria ground squirrels.     

残次苹果发酵产物对连作土壤环境及‘平邑甜茶’幼苗生长的影响

在盆栽条件下,研究残次苹果发酵产物对连作土壤环境及平邑甜茶幼苗生长的影响,为减轻苹果连作障碍提供理论依据。试验以连作土壤为对照(CK),设置溴甲烷灭菌连作土壤(T₁)、连作土壤施加苹果发酵产物(T₂)、连作土壤施加灭菌苹果发酵产物(T₃)处理,测定了土壤和平邑甜茶幼苗的相关指标。结果表明: T₁、T₂、T₃能显著促进平邑甜茶幼苗的生长,其中以T₁和T₂的效果较好,其根系呼吸速率、株高、地径、鲜质量、干质量分别比CK提高了107.3%、50.6%、42.4%、171.7%、225.3%和104.4%、50.6%、42.3%、171.8%、225.5%。T₂和T₃提高了连作土壤中养分转化相关酶的活性,过氧化氢酶、脲酶、中性磷酸酶和蔗糖酶活性分别比CK提高了44.5%、169.5%、23.4%、169.3%和23.7%、72.6%、1.5%、121.5%;T₁处理的过氧化氢酶和蔗糖酶活性与CK无显著差异,脲酶和中性磷酸酶活性分别降低了40.8%和41.6%。T₂土壤铵态氮、硝态氮、速效磷和速效钾含量分别较CK提高了18.6%、50.6%、14.0%和36.7%;T₃只提高了速效氮的含量,铵态氮和硝态氮含量分别提高了7.0%和23.6%;T₁与CK相比速效养分含量有所降低。T₁和T₂显著降低了土壤放线菌和真菌数量,增加了细菌数量;T₃处理的细菌、放线菌和真菌数量均较CK显著降低。实时荧光定量PCR分析发现,T₁、T₂和T₃处理的层出镰孢菌、串珠镰孢菌、腐皮镰孢菌和尖孢镰孢菌基因拷贝数均较CK有不同程度的降低。表明苹果发酵产物处理能抑制连作土壤病原菌,改善土壤环境,促进平邑甜茶幼苗生长,可作为一种减轻苹果连作障碍的方法。     

Apple replant disease: the influence of soil phosphorus and other factors on the growth responses of apple seedlings to soil fumigation with chloropicrin

The effects of fumigating field soils before replanting apple are frequently predicted from bioassays where apple seedlings are grown in pots, some containing untreated soil and the others containing chloropicrin-fumigated soil. The results from 418 such bioassays made during eight years are discussed with reference to the effects of certain physical and chemical soil features. Soil phosphorus (P) content and pH had large effects on growth in fumigated (but not untreated) soils; they therefore had large effects on the growth response to soil fumigation. There were no indications of important effects due to soil content of potassium or magnesium or to soil texture. In fumigated soils, the height of seedlings was generally greater, the greater the soil P content, but in untreated soils it was mostly independent of P (range 1 – 156 mg P litre^-1soil). The differing effects of native P on growth in untreated or fumigated soils were attributed respectively to the presence and absence of vesicular-arbuscular mycorrhizas: their absence in fumigated soils was considered (for apple) to be an artefact in the seedling bioassay. The results for untreated soils indicated that the apple mycorrhizal system was very efficient in retrieving P when there was little available. Growth responses to soil fumigation were usually greater in acidic than in alkaline soils for any given level of P. When soils were P-amended before testing, the effects of native P and pH virtually disappeared and the proportion of orchard soils with economically significant growth responses increased from 39% to 67%.     

Growth rate of fine root systems influences rootstock tolerance to replant disease

Aims

Plant tolerance to herbivory has often been linked to plant growth rate, with faster growing plants that present high tissue turnover rates expected to be more tolerant than slower-growing plants. We tested whether this relationship also holds for rootstock growth rate and tolerance to apple replant disease (ARD).

Methods

An ARD susceptible rootstock, M.26 and ARD tolerant rootstock, CG.6210 were grown in soil from an apple replant site (FS) and in pasteurized soil (PS) from the same site. Total below ground biomass production was determined by harvesting a subset of plants per soil treatment and rootstock at 11, 17, and 23 weeks after planting. Root samples were collected prior to each harvesting date to determine root respiration and total carbon (C) and nitrogen (N) content. Root dynamics were tracked during the growing season by digitally photographing root observation windows.

Results

Total root biomass, first and second order roots, and second-to-first order root ratio were higher in CG.6210 than in M.26 in both soil treatments. Roots of CG.6210 were thinner and had lower N concentration than those of M.26. Roots of M.26 had longer lifespans than those of CG.6210, and the mortality risk of M.26 roots was 56 % that of CG.6210 roots.

Conclusion

Our study indicates that rootstocks with faster growing root systems can tolerate ARD infection by investing fewer resources in individual root construction that can be shed more readily.     

Residual herbicide treatments reduce Andropogon gayanus (Gamba Grass) recruitment for mine site restoration in northern Australia

Weed invasion is a major threat to Australian tropical savannas, and controlling weeds is essential for successful re‐establishment of native species on disturbed sites. Gamba Grass (Andropogon gayanus) is an African grass which has invaded large areas of tropical savanna across northern Australia. Current management strategies in northern Australia focus on fire and glyphosate to effectively control mature plants; however, re‐establishment of infestations from the soil seed bank remains a major challenge to eradication efforts. This study focused on the effects of soil seed bank treatments on Gamba Grass recruitment on a mine site in northern Australia. Adult Gamba Grass plants within test plots were killed with glyphosate to exclude resource competition. Chemical, physical and biological treatments were then applied, and the treatment effects on subsequent Gamba Grass seedling emergence and survival quantified. Seedling emergence was significantly reduced by three of the four residual herbicide treatments tested. The most effective herbicide treatments, dalapon and sulfometuron, reduced emergence by 90% compared to the standard glyphosate treatment alone. This equated to a reduction in Gamba Grass seedling emergence from 1 seedling/m² to 1 seedling 10 m^?2, a major improvement for Gamba Grass management. These residual herbicide treatments significantly reduced the population density of Gamba Grass for at least 5 months after emergence. The physical and biological treatments did not have a significant effect on seedling emergence. This significant reduction in Gamba Grass seedling emergence and survival can substantially improve Gamba Grass management. Reducing re‐colonisation from the soil seed bank using residual herbicides provides a valuable management tool for land managers, integrating readily with established strategies for controlling the mature plants.     

Rootstock genotype and orchard replant position rather than soil fumigation or compost amendment determine tree growth and rhizosphere bacterial community composition in an apple replant soil

Apple replant disease (ARD) is a complex soilborne disease syndrome that often causes problems when renovating old orchard sites. Soil fumigants sometimes control ARD, but biological and cultural alternatives are needed. In this study the growth of two widely used clonal apple (Malus domestica) rootstocks (M7 and M26) were compared to three new rootstocks from the Cornell-Geneva series (CG16, CG30 and CG210 (a.k.a. CG6210)) in an orchard site with a history of ARD, in Ithaca, NY. Trees were planted in two distinguishable positions – the previous tree rows versus the old inter-row grass lanes. Additionally, we compared the effects of compost amendment and fumigation with dichloropropene plus chloropicrin on tree growth on each replant rootstock. Rhizosphere bacteria and actinobacteria were assessed using PCR-DGGE for the rootstocks M7, M26, CG30 and CG210. Tree growth on the rootstocks M7, M26 and CG16 was suppressed in the old tree rows relative to grass lanes, while trees on CG30 and CG210 rootstocks grew equally well in both positions. The species composition of rhizosphere bacteria and actinobacteria differed significantly between the planting positions and between the rootstocks M7 and M26 compared to CG30 and CG210. In contrast, the preplant compost or fumigation soil treatments had no effect on tree growth and little impact on rhizosphere bacterial community composition. Abbreviations: ARD – apple replant disease; DGGE – denaturing gradient gel electrophoresis; PCR – polymerase chain reaction.