Soybean Brown Stem Rot, Phytophthora sojae, and Heterodera glycines Affected by Soil Texture and Tillage Relations

ABSTRACT Investigations were conducted to determine whether the effects of tillage practices on the prevalence of brown stem rot of soybean (caused by Phialophora gregata), Heterodera glycines, and Phytophthora sojae were confounded by soil texture in samples collected in the fall of 1995 and 1996. Soil and soybean stem samples, along with tillage information, were collected from 1,462 randomly selected fields in Illinois, Iowa, Minnesota, Missouri, and Ohio in collaboration with the National Agricultural Statistics Service. The incidence of brown stem rot was determined from 20 soybean stem pieces collected from each field in a zigzag pattern. The detection frequency of P. sojae (expressed as percent leaf disks colonized) and population densities of H. glycines were determined from soil cores also collected in a zigzag pattern. The soil samples were grouped into various textural classes, and the effect of soil texture and tillage relations on the activities of each pathogen were determined. Both tillage and soil texture affected the incidence of brown stem rot; however, there was no interaction between tillage and soil texture. Conservation tillage had a greater (P < 0.05) incidence of brown stem rot in clay loam and silty clay loam than did conventional tillage. The detection frequency of P. sojae was not affected by tillage, but a tillage x texture interaction (P = 0.013) indicated that the effect of tillage depended on soil texture. There was a greater (P < 0.05) detection frequency of P. sojae in conservation tillage than in conventional tillage in silt loam and loam soils. However, in sandy loam, the detection frequency of P. sojae was greater (P = 0.0099) in conventional tillage than in conservation tillage. Population densities of H. glycines were significantly affected by both tillage and soil texture, but overall, there was no tillage x texture interaction. There was an inverse relationship between population densities of H. glycines and percent clay (r = -0.81, P = 0.01) in no-till fields, but little or no change in nematode densities was observed with increasing clay content in tilled fields. Population densities of H. glycines were less (P < 0.05) in no-till fields than in tilled fields in silty clay loam and clay soils. There was no difference in H. glycines densities between the tillage categories in soils sandier than silty clay loam or clay. The findings emphasize the need for cautious interpretation of the effects of tillage practices on diseases and pathogens in the absence of information on soil texture.     

Spatial patterns,species richness and cover in weed communities of organic and conventional no‐tillage spring wheat systems

Heterogeneous field conditions are ubiquitous throughout agricultural systems and have given rise to the practice of site‐specific management, in an effort to increase sustainability and/or homogenise growing conditions and thereby increase crop yields. The spatial pattern of weeds in conventional systems is widely accepted to be aggregated, but there have been no scientific studies regarding the spatial pattern of weed distribution in organic systems. Using a combination of aggregation measures and quadrat variance techniques, this study compared the spatial pattern of weed distribution in conventionally managed no‐tillage spring wheat fields to those of organically managed spring wheat fields. Per cent weed cover data (by species) were collected in the summers of 2005 and 2006 from transects located in conventional no‐tillage and organic spring wheat fields. Weed cover was aggregated in both the conventional and the organic systems, but the patterns of aggregation were different for the two systems. Conventional no‐tillage systems showed a patch/gap pattern, while organic systems showed multiple scales of patchiness with few gaps. These results suggest that processes causing aggregation in the two systems may be different and that site‐specific management may be applicable to organic systems as well as conventional spring wheat systems.     

贵州省大豆孢囊线虫的发生与分布

大豆孢囊线虫病是威胁大豆生产的重大病害之一。2015-2016年,采用随机取样的方法对贵州省大豆孢囊线虫的发生和分布情况进行了调查。调查范围包括贵阳、六盘水、安顺、毕节、黔西南和遵义等6个地区下辖的14个县(区),共采集土样287份。调查结果表明,贵州省大豆孢囊线虫的总检出率为28.2%,其中毕节地区的大方县和赫章县孢囊检出率最高,分别达到46.7%和45.0%,而在遵义市、贵阳市和黔西南州孢囊检出率为0。六盘水地区的水城县孢囊发生量最大,平均孢囊数为3.4个/200g土,最大孢囊数达到17个/200g土;安顺地区的镇宁和关岭县大豆孢囊线虫发生程度最轻,每200g土样平均孢囊数分别为0.3和0.2个,最大孢囊数仅为2个/200g土。分离的孢囊主要形态特征与大豆孢囊线虫一致,并且ITS序列与甘肃大豆孢囊线虫种群序列一致性达100%。根据形态学观察和ITS序列分析比对,将贵州省采集到的孢囊线虫鉴定为大豆孢囊线虫。本研究结果可为有效防控贵州省大豆孢囊线虫病提供依据。     

连作和轮作对大豆胞囊线虫群体数量及土壤线虫群落结构的影响

大豆胞囊线虫Heterodera glycinesIchinohe病害是大豆生产中的毁灭性病害,对世界大豆生产造成重大损失。采用根染色法和常规土壤线虫分类鉴定技术研究了连作和轮作两种耕作方式下大豆胞囊线虫群体数量和土壤线虫群落结构的变化。结果表明,连作使大豆田土壤中胞囊和根系上各龄期胞囊线虫数量显著增加(95.6→335.5),而轮作积累很少(10.3→31.8),感病品种中J2的侵入数量决定了大豆胞囊线虫形成胞囊的数量(22.8→95.6,129.4→335.5)。玉米-玉米-大豆的轮作方式使胞囊积累最少(10.3)。轮作大豆田土壤线虫总数高于连作,主要由于轮作大豆田土壤线虫的优势属Helicotylenchus和优势营养类群植物寄生线虫PP的相对丰度高于连作,但大豆胞囊线虫的相对丰度正相反,连作(33.65%)高于轮作(0.91%),说明Heterodera glycines可以作为土壤线虫的关键属种用以指示土壤发病的严重程度。研究中重要的优势属和关键属种cp值均为3,说明其繁殖和抗干扰能力比较强。研究证明土壤线虫优势营养类群、优势属种和关键属种可以有效地指示农业土壤健康状况和大豆田的大豆胞囊线虫病害的发生程度。     

Interactions Between the Soybean Cyst Nematode and Fusarium solani f. sp. glycines Based on Greenhouse Factorial Experiments

ABSTRACT The soybean cyst nematode, Heterodera glycines, and the fungus that causes sudden death syndrome (SDS) of soybean, Fusarium solani f. sp. glycines, frequently co-infest soybean (Glycine max) fields. The interactions between H. glycines and F. solani f. sp. glycines were investigated in factorial greenhouse experiments with different inoculum levels of both organisms on a soybean cultivar susceptible to both pathogens. Measured responses included root and shoot dry weights, H. glycines reproduction, area under the SDS disease progress curve, and fungal colonization of roots. Both H. glycines and F. solani f. sp. glycines reduced the growth of soybeans. Reproduction of H. glycines was suppressed by high inoculum levels but not by low levels of F. solani f. sp. glycines. The infection of soybean roots by H. glycines did not affect root colonization by the fungus, as determined by real-time polymerase chain reaction. Although both pathogens reduced the growth of soybeans, H. glycines did not increase SDS foliar symptoms, and statistical interactions between the two pathogens were seldom significant.     

种子处理诱导大豆抗胞囊线虫病的生防细菌筛选与鉴定

国内研究了简单芽孢杆菌Bacillussimplex包衣处理大豆种子,诱导大豆植株抗胞囊线虫的侵染。采用温室盆栽试验,对研究室通过2年田间试验筛选出的3株生防细菌进行复筛,获得最优菌株Sneb545;为了阐明其作用方式,设计了裂根试验验证菌株Sneb545诱导大豆产生抗胞囊线虫的能力,并对菌株Sneb545进行种水平鉴定。结果表明,菌株Sneb545处理种子后可以诱导苗期大豆对第一代胞囊线虫产生明显的抗性,线虫入侵总数显著降低,较对照降低72．63％,胞囊抑制率达70．63％;裂根试验结果证明,菌株Sneb545能够诱导大豆对胞囊线虫产生很强的抗性,菌株Sneb545在挑战根系中接种后,应答根系中胞囊线虫入侵量降低51．27％,土壤中胞囊减少65．82％;经形态学特征、生理生化试验测定及16SrDNA序列同源性分析,确定该菌株为简单芽孢杆菌。     

大豆胞囊线虫侵染泡桐和豌豆的研究

 形态学观测和寄生性研究结果表明,发生于我国河南省兰考县泡桐(Pauloumia sp.)和内蒙古自治区凉城县豌豆(Pisum sativum)上的胞囊线虫均属于大豆胞囊线虫(Heterodera glyeines);上述两种植物上的胞囊线虫均可侵染大豆,在根部形成有卵雌虫。而兰考和凉城两地大豆上的胞囊线虫群体可分别在泡桐成豌豆上良好繁殖。以上结果证明,泡桐和豌豆为大豆胞囊线虫的两种新寄主植物,其中,泡桐为大豆胞襄线虫侵染多年生乔木植物的首例报道。     

Effects of Herbicides on Fusarium solani f. sp. glycines and Development of Sudden Death Syndrome in Glyphosate-Tolerant Soybean

ABSTRACT Sudden death syndrome of soybean, caused by Fusarium solani f. sp. glycines, is a disease of increasing economic importance in the United States. Although the ecology of sudden death syndrome has been extensively studied in relation to crop management practices such as tillage, irrigation, and cultivar selection, there is no information on the effects of herbicides on this disease. Three herbicides (lactofen, glyphosate, and imazethapyr) commonly used in soybean were evaluated for their effects on the phenology of F. solani f. sp. glycines and the development of sudden death syndrome in four soybean cultivars varying in resistance to the disease and in tolerance to glyphosate. Conidial germination, mycelial growth, and sporulation in vitro were reduced by glyphosate and lactofen. In growth-chamber and greenhouse experiments, there was a significant increase in disease severity and frequency of isolation of F. solani f. sp. glycines from roots of all cultivars after application of imazethapyr or glyphosate compared with the control treatment (no herbicide applied). Conversely, disease severity and isolation frequency of F. solani f. sp. glycines decreased after application of lactofen. Across all herbicide treatments, severity of sudden death syndrome and isolation frequency were lower in disease-resistant than in susceptible cultivars. Results suggest that glyphosate-tolerant and -nontolerant cultivars respond similarly to infection by F. solani f. sp. glycines after herbicide application.     

Regional Assessment of Soybean Brown Stem Rot, Phytophthora sojae, and Heterodera glycines Using Area-Frame Sampling: Prevalence and Effects of Tillage

ABSTRACT The prevalence of brown stem rot (caused by Phialophora gregata), Heterodera glycines, and Phytophthora sojae in the north central United States was investigated during the fall of 1995 and 1996. Soybean fields were randomly selected using an area-frame sampling design in collaboration with the National Agricultural Statistics Service. Soil and soybean stem samples, along with tillage information, were collected from 1,462 fields in Illinois, Iowa, Minnesota, Missouri, and Ohio. An additional 275 soil samples collected from Indiana were assessed for H. glycines. For each field, the incidence and prevalence of brown stem rot was assessed in 20 soybean stem pieces. The prevalence and recovery (expressed as the percentage of leaf disks colonized) of P. sojae and the prevalence and population densities of H. glycines were determined from the soil samples. The prevalence of brown stem rot ranged from 28% in Missouri to 73% in Illinois; 68 and 72% of the fields in Minnesota and Iowa, respectively, showed symptomatic samples. The incidence of brown stem rot was greater in conservation-till than in conventional-till fields in all states except Minnesota, which had few no-till fields. P. sojae was detected in two-thirds of the soybean fields in Ohio and Minnesota, whereas 63, 55, and 41% of the fields in Iowa, Missouri, and Illinois, respectively, were infested with the pathogen. The recovery rates of P. sojae were significantly greater (P 相似文献   

葡萄跳叶甲和捕食性天敌草间小黑蛛的空间格局及其联系

利用地学统计学原理和方法分析了葡萄跳叶甲种群和草间小黑蛛种群之间的空间结构和空间相关性。结果表明:不同时期葡萄跳叶甲种群的空间格局均呈聚集分布,其半变异函数皆为球型曲线,变程在1.164～8.508之间;不同时期草间小黑蛛种群的空间格局也均呈聚集分布,其半变异函数也皆为球型曲线,变程在2.977～7.799之间,两者变程在大多数时间变动趋势基本一致,充分说明了草间小黑蛛种群对葡萄跳叶甲在空间位置上具有一定的追随关系;用聚集强度指标分析,两者空间格局一致。草间小黑蛛聚集行为是环境因子所致,葡萄跳叶甲密度小于2头/株时,聚集原因是环境因子,密度大于2头/株时,本身密度是聚集的原因之一。     

山东省寄生烟草的孢囊线虫种类鉴定及种内群体间rDNA-ITS-RFLP分析

 在山东省5个植烟县/市采集烟草线虫土壤样本24份,其中10份土样中分离到孢囊线虫的孢囊,对之进行系统的形态学观测。孢囊阴门锥突出,双半膜孔,下桥和泡囊发达,阴门裂长（44.1±3.5） μm,阴门窗长（50.4±4.8） μm、宽（36.6±4.1） μm,下桥长（83.8±4.7） μm、宽（11.0±0.9） μm。利用大豆孢囊线虫特异性引物对分离到的孢囊线虫群体进行了分子鉴定。形态学和分子鉴定结果表明,山东省寄生烟草的孢囊线虫为大豆孢囊线虫Heterodera glycines Ichinohe,1952。对这10个孢囊线虫群体rDNA-ITS区进行了PCR扩增产物用7种限制性内切酶进行酶切和测序比对。发现,所有孢囊线虫群体均能扩增出一条大约1 000 bp大小的片段;用Alu Ⅰ、Bsh 1236 Ⅰ、Hae Ⅲ、Hha Ⅰ 、Mva Ⅰ 、Rsa Ⅰ酶切后,产生的酶切表型均相同;用Ava I酶切后,产生了2种酶切表型。所测孢囊线虫群体与GenBank收录的大豆孢囊线虫序列相似度高达99%以上。     

Interaction of Fusarium solani f. sp. glycines and Heterodera glycines in Sudden Death Syndrome of Soybean

ABSTRACT Sudden death syndrome (SDS) of soybean is caused by the soilborne Fusarium solani f. sp. glycines (synonym F. virguliforme). In a sequential approach, two multifactor factorial-design microplot experiments were conducted to investigate the effects of fungal infestation levels and soil moisture on both root necrosis and foliar SDS severity, and the interaction between F. solani f. sp. glycines and Heterodera glycines in fumigated versus nonfumigated soil. In 2003, soybean cv. Spencer was grown in nonfumigated or methyl bromide-fumigated soil and infested with increasing levels of F. solani f. sp. glycines, either under rainfall or irrigated after growth stage V6/R1. In 2004, interactions between F. solani f. sp. glycines and H. glycines were explored in a factorial inoculation design in fumigated or nonfumigated soil, planted to Williams 82 or Cyst-X20-18. In both years, higher levels of foliar SDS severity and root necrosis were found in F. solani f. sp. glycines-infested soils with H. glycines than in soils without the nematode on the soybean cultivars susceptible to both pathogens. Both natural infestations of H. glycines in 2003 and artificially amended populations of H. glycines in 2004 contributed to higher foliar SDS severity. More severe foliar SDS symptoms always were associated with more root necrosis, but elevated levels of root necrosis did not predict severe leaf symptoms. In contrast to the critical role of H. glycines, increasing fungal infestation levels had no significant effects on increasing either foliar SDS symptoms or root necrosis. Effects of moisture regime and fungal infestation levels also were examined in factorial greenhouse and growth chamber experiments. High soil moisture resulted in higher levels of SDS root necrosis. In the greenhouse, root necrosis increased at a higher rate in low soil moisture than the rate in high soil moisture. The two pathogens acted as a complex and the disease development was strongly dependent on high soil moisture.     

Virulence Genes in Heterodera glycines: Allele Frequencies and Ror Gene Groups Among Field Isolates and Inbred Lines

ABSTRACT Genetic variation in field populations of Heterodera glycines is a key issue for both resistance gene deployment and basic understanding of virulence-gene flow in populations. In this study, we examined phenotypically defined genes for virulence under selection from host resistance. We separated the most common H. glycines genotypes in the United States into two virulence groups, based on their reproductive abilities on the resistant soybean plant introduction (PI) 88788. These groups correspond to previously identified virulence genes in the nematode, as follows: the dominant gene in H. glycines to PI88788, and the recessive genes to PI90763 and Pickett/Peking. Virulence allele frequencies and virulence genotype frequencies of selected field isolates were investigated by testing the host range of single-female-derived lines, which were developed through single-female inoculation on the standard susceptible soybean 'Lee 68'. By comparing virulence genotype frequencies between the original field isolates and their single-female-derived lines, we were able to determine allele frequencies in the field populations. The results suggest that tremendous variation in H. glycines virulence genes exists among field populations. Potential mechanisms of selection which could cause virulence genotype frequency increases are discussed as related to population genetics equilibrium theory.     

几种除草剂对大豆孢囊线虫存活和孵化的影响

在实验室内寄主体外测定了8种大豆田常用除草剂对大豆孢囊线虫（SCN）(Heterodera glycines Ichinohe)2龄幼虫（J2）存活率及卵孵化的影响,同时测定了乙草胺和2,4滴丁酯+异丙草胺对温室和田间SCN繁殖的影响。结果表明,所有供试除草剂对大豆孢囊线虫J2均有不同程度的致死作用,其中以乙草胺、2,4滴丁酯+异丙草胺和嗪草酸甲酯处理效果最好,24 h死亡率分别达99.33%、99.33%和89.33%;乙草胺、2,4滴丁酯+异丙草胺、嗪草酸甲酯和精噁唑禾草灵处理7 d,J2的死亡率均达100%,精喹禾灵、精吡氟氯禾草灵和三氟羧草醚处理的死亡率分别为91.33%、81.33%和79.33%;所有除草剂对孢囊孵化都有较强的抑制作用,其中2,4滴丁酯+异丙草胺的抑制作用最强,孢囊孵化率仅为1.25%;乙草胺和2,4滴丁酯+异丙草胺对温室和田间SCN的繁殖均有较强的抑制作用,对大豆根系生长均无不良影响。不同除草剂对大豆种子萌发有一定影响。     

Effects of Irrigation and Tillage on Temporal and Spatial Dynamics of Sclerotinia minor Sclerotia and Lettuce Drop Incidence

ABSTRACT The temporal and spatial dynamics of Sclerotinia minor sclerotia and the resulting incidence of lettuce drop were studied under furrow irrigation with conventional tillage and subsurface-drip irrigation with minimum tillage during 1993-95. Lettuce crops were grown each year during the spring and fall seasons. All plants were inoculated immediately after thinning in the spring of 1993. Grids of 24 contiguous quadrats (1 by 1 m(2)) were demarcated in the centers of each 150-m(2) plot. Lettuce drop incidence in each quadrat was evaluated each season prior to harvest. One soil sample (100 cm(3)) was collected from each quadrat at harvest and after tillage prior to planting of the next crop for both spring and fall crops and assayed for S. minor sclerotia using wet sieving. Lloyd's index of patchiness, the beta-binomial distribution, and variance of moving window averages were used to evaluate the spatial patterns of sclerotia and lettuce drop incidence under the two irrigation systems and associated tillage treatments. Disease incidence remained significantly higher under furrow irrigation than under subsurface-drip irrigation throughout the study period, and was significantly higher on fall crops than on spring crops. Under furrow irrigation, the number of sclerotia at the end of a crop season increased significantly over that at the beginning of the season, but no significant changes were detected over years. In contrast, the number of sclerotia within a single season did not increase significantly under subsurface drip irrigation, nor was year-to-year accumulation of sclerotia statistically significant. The degree of aggregation of sclerotia increased significantly during a cropping season under furrow irrigation, but not under subsurface drip irrigation. The conventional tillage after harvest under furrow irrigation decreased the degree of aggregation of sclerotia after each season, but the distribution pattern of sclerotia under subsurface-drip irrigation changed little by the associated minimum tillage. Spatial pattern analyses suggested that the aggregation of S. minor sclerotia occurred at a scale of no more than 1 m, and distribution of diseased lettuce plants was random at a scale larger than 1 m. The combination of fewer sclerotia produced by each crop and its unaltered distribution under subsurface drip irrigation and associated minimum tillage makes it a valuable cultural practice for lettuce drop management.     

Depth Distribution of Rotylenchulus reniformis Under Different Tillage and Crop Sequence Systems

ABSTRACT The population density of the reniform nematode, Rotylenchulus reniformis, was monitored at depths of 0 to 30, 30 to 60, 60 to 90, and 90 to 120 cm in a tillage and crop sequence trial in south Texas in 2000 and 2001. Main plots were subjected to three different tillage systems: conventional tillage (moldboard plowing and disking), ridge tillage, and no-tillage. Subplots were planted with three different crop sequences: spring cotton and fall corn every year; spring cotton and fall corn in one year, followed by corn for two years; and cotton followed by corn and then grain sorghum, one spring crop per year. The population density of R. reniformis on corn and grain sorghum was low throughout the soil profile. In plots planted with spring cotton and fall corn every year, fewer nematodes were found at depths of 60 to 120 cm in the no-tillage and ridge tillage systems than in the conventional tillage system. Population densities were lower at depths of 0 to 60 cm than at 60 to 120 cm. Soil moisture and cotton root length did not affect nematode population densities in the field. When soil was placed in pots and planted with cotton in the greenhouse, lower population densities developed in soil taken from depths of 0 to 60 cm than in soil from depths of 60 to 120 cm. Final nematode populations were similar in size in soil from the different tillage systems, but reproductive factors were higher in soil from plots with reduced-tillage systems than in soil from plots with conventional tillage. Reduced-tillage practices lowered the risk of increases in R. reniformis populations and reduced population densities following 2 years of non-hosts throughout soil depths, but population densities resurged to the same high levels as in soil planted with cotton every year during one season of cotton.     

大豆根渗出物对大豆孢囊线虫4号生理小种卵孵化的影响

 本文研究了中国小黑豆抗源灰皮支黑豆和元钵黑豆根渗出物对大豆孢囊线虫4号生理小种越冬孢囊、新鲜孢囊和离体卵孵化的影响。结果表明:抗源品种根渗出物诱导越冬孢囊孵化幼虫数目一开始就显著高于去离子水对照,孵化8 d后显著低于感病对照鲁豆1号;根渗出物诱导新鲜孢囊孵化幼虫数和离体卵孵化率在各材料间的差异表现一致,即抗源品种始终显著低于感病对照,孵化中后期与去离子水对照的差异达显著水平。     

河北省大豆孢囊线虫分子鉴定及其分布

2003年,在河北省邯郸、邢台、石家庄、保定、秦皇岛、唐山、张家口等地采集大豆根际土壤样品65份,应用大豆孢囊线虫特异性引物对采集的孢囊线虫进行了分子鉴定,结果表明从邯郸的永年、磁县、峰峰,邢台的南和、广宗、清河,保定的南大园、西马池、望都、定州,石家庄的栾城、鹿泉、平山及石家庄市郊,秦皇岛的抚宁、昌黎,唐山的迁西、迁安、丰润的大豆和张家口的红云豆上采集的孢囊线虫的孢囊及其二龄幼虫均为大豆孢囊线虫;卢龙和滦县的样品中没有发现大豆孢囊线虫的孢囊及其二龄幼虫,且孢囊的检出率与二龄幼虫的检出率的趋势一致。     

黄土高原旱地不同保护性耕作措施对马铃薯田土壤水温效应及产量的影响

设计实施了黄土高原西部旱农区马铃薯传统耕作(T)和五种保护性耕作措施:垄上覆膜沟内覆草摆种(PDSS)、垄上覆膜沟内覆草浅播(PDSSH)、垄上覆草摆种(RSS)、平作覆草摆种(FSS)和垄上不覆膜沟内覆草摆种(DSS)的大田对比试验,对不同耕作措施下各层次土壤水分、地温及产量进行了研究.结果表明:马铃薯免耕秸秆覆盖能明显改善0～80 cm土壤贮水量,特别是对0～30 cm土壤贮水量的影响尤为突出;与传统耕种相比,免耕秸秆覆盖可以降低各个土壤测定层的日均积温(≥0℃),并能在晴天14∶00时有效降低结薯层的温度,结薯层的温度FSS为27～29℃,PDSSH为21～28.5℃,较传统耕种的27～32℃略低,降低了高温对马铃薯生长的不利影响;免耕秸秆覆盖除RSS外,出苗率和产量均高于传统耕种,且FSS的产量最高,达27 468.3 kg/hm2,较传统耕种增产30.05%.     

Isolation, Characterization, and Distribution of a Biocontrol Fungus from Cysts of Heterodera glycines

ABSTRACT Seventy-six populations of Heterodera glycines were collected from 33 counties in 10 states of the United States along the Mississippi and Missouri Rivers in 1992 and 1993. A sterile hyphomycete fungus of an unnamed taxon, designated ARF18 and shown to be a parasite of eggs of H. glycines, was isolated from eggs and cysts of 10 of the populations from Kentucky, Louisiana, Mississippi, and Tennessee. Ten isolates of ARF18 obtained in this study and seven isolates obtained in earlier studies were characterized for cultural morphology on several growth media, the ability to produce sclerotium-like structures (SLS) on cornmeal agar, growth rates, pathogenicity to eggs of H. glycines in vitro, and mitochondrial (mt) DNA restriction fragment length polymorphisms (RFLPs). All 17 isolates of ARF18 readily grew on potato dextrose agar, cornmeal agar, and nutrient agar. Based on colony morphology and SLS appearance on cornmeal agar, the isolates could be grouped into two morphological phenotypes. Isolates that produced SLS that were composed of a compact mass of hyphae were designated ARF18-C, whereas isolates that produced SLS composed of a mass of loosely clumped hyphae were designated ARF18-L. Only minor differences in growth rates were detected among the ARF18-C and ARF18-L isolates. All 10 ARF18-C isolates, which were from Arkansas, Louisiana, Mississippi, and Tennessee, belonged to a single mtDNA RFLP haplotype. The seven ARF18-L isolates shared many comigrating mtDNA restriction fragments with one another, but belonged to three distinct mtDNA RFLP haplotypes. Ability to infect eggs of H. glycines in vitro varied considerably among the various isolates of ARF18. In particular, several of the ARF18-C isolates were consistently able to infect over 50% (mean = 70.0%, standard deviation = 16%) of the eggs of H. glycines, whereas ARF18-L infected eggs to a lesser degree (mean = 25%, standard deviation = 27%). ARF18-C was isolated only from H. glycines populations from below 37 degrees N latitude. The presence of ARF18 was associated with soils with Mg levels <314 kg/ha, cyst numbers >4.5 per 100 cm(3), and iron levels >203.5 kg/ha; or with Mg levels >314 kg/ha and Na levels <121 kg/ha. The widespread distribution of ARF18 and the ability of some isolates to aggressively colonize eggs of H. glycines are indications that it has potential as a biological control agent for H. glycines.