Diversity of pathogenic Fusarium populations associated with asparagus roots in decline soils in Spain and the UK

Asparagus decline is a disease associated with several species of Fusarium . In order to assess the relative significance of causative species, single-stranded conformational polymorphism (SSCP) analysis of the ITS2 (internal transcribed sequence) region of the ribosomal DNA was used to rapidly and objectively identify the fusarial populations associated with the roots of two intensively sampled asparagus crops, one in the UK and the other in Spain. Over 360 fusarial isolates were obtained from fields showing symptoms of asparagus decline, and most were easily differentiated by SSCP into four principal species, F. oxysporum f. sp. asparagi , F. proliferatum , F. redolens and F. solani . Fusarium oxysporum f. sp. asparagi (Foa) was most frequently isolated from the UK site (69%), whilst Foa and F. proliferatum were found in similar proportions overall (40 and 39%, respectively) from the Spanish site, although individual fields showed considerable intraregional variation. Other minor populations, such as F. culmorum , were also found. Most isolates were highly pathogenic to asparagus in vitro , although F. solani isolates comprised both pathogenic and nonpathogenic populations. Two populations of Foa were distinguished by a single ITS2 base transition, and the dominance of these two populations differed between Europe and the USA. Fusarium proliferatum was more abundant in Spain than in the UK. Phylogenetic analysis using EF1α sequences indicated that isolates of F. oxysporum pathogenic to asparagus are spread across a number of clades within the species complex, supporting the hypothesis that pathogenicity to asparagus in this species is a relatively unspecialized trait.     

北京地区西洋参根腐病病原鉴定及其致病性

根腐病是北京地区栽培西洋参的主要病害之一,为了明确引起该类病害的病原菌,对北京怀柔区罹病西洋参参根进行了病原菌的分离及鉴定。结合形态学以及rDNA ITS区、转录延伸因子TEF 1α区的分子序列特征,对分离菌株的鉴定结果显示,分离频率较高的11个菌株分属于茄镰孢（Fusarium solani）、尖镰孢（F.oxysporum）、三线镰孢（F. tricinctum）、多育镰孢（F. proliferatum）、毁灭柱孢（Cylindrocarpon destructans）。通过柯赫氏法则验证,F. solani和F. oxysporum是引起北京地区栽培西洋参根腐病的主要病原菌;致病性测定结果发现F. solani不同菌株的致病性及致病力表现出明显差异。     

Combining nonpathogenic strains of Fusarium oxysporum with sodium chloride to suppress fusarium crown rot of asparagus in replanted fields

Nonpathogenic strains of Fusarium oxysporum (NPFO) were tested alone, and in combination with rock salt (NaCl), for their ability to suppress fusarium crown and root rot of asparagus in replanted fields. Five NPFO strains were first screened with and without NaCl in the glasshouse using autoclaved asparagus soil mixed with potting mix. Asparagus roots were treated with NPFO suspensions or water, then inoculated 1 week later with pathogenic F. oxysporum ; then after 1 week pots received 100 mL 1% NaCl or water. After 12 weeks the NPFO strain CS-20 increased root weight when compared with controls. Rock salt did not affect root growth. Root lesions were reduced relative to the control by all NPFO strains and by NaCl applications, but there were no interactions between NPFO and NaCl. The NPFO strains CS-20, CWB 314 and CWB 318 were compared with no treatment in two 4-year replanted field studies in Hamden, and Windsor, CT, USA. Crowns were treated with NPFO at planting, and NaCl was broadcast at 280 kg ha⁻¹, and at 560 kg ha⁻¹ in later years. CWB 318 reduced disease at both sites and increased marketable yield in Hamden. Treatment with NaCl increased yield by 29% over controls in Windsor, but did not increase yield in Hamden. Recovered root isolates of F. oxysporum categorized by heterokaryon tests revealed that, in the glasshouse, CS-20 colonized roots better than other strains, while CWB 318 was the only strain found on roots from field plots after 4 years. Applications of NaCl did not hinder the suppressive ability of the nonpathogenic strains. The perennial nature of asparagus may require that nonpathogenic strains possess superior survival traits along with biocontrol traits.     

Prevalence and pathogenicity of spinach root pathogens of the genera Aphanomyces, Phytophthora, Fusarium, Cylindrocarpon, and Rhizoctonia in Sweden

In a 4-year disease survey in commercial spinach fields, pathogens were isolated from spinach root pieces placed on selective agar media. Aphanomyces cladogamus was the most abundant pathogen, followed by Phytophthora. cryptogea and Fusarium oxysporum. Rhizoctonia solani was found only occasionally. Other pathogens isolated were F. redolens, F. sambucinum and Cylindrocarpon destructans. P. cryptogea was the most severe pathogen, causing death of most plants, but A. cladogamus also caused severe root damage. Isolates of F. oxysporum ranged from highly pathogenic, i.e. P. oxysporum f.sp. spinaciae race 1. to moderately pathogenic and non-pathogenic, Rhizoctonia solani isolates also varied widely in their pathogenicity. Only a small number of the F. redotens and F. sambucinum isolates were pathogenic and most C. destructans isolates were weakly pathogenic. Isolation frequencies were relatively stable from year to year, but P. cryptogea was isolated more frequently in autumn than in spring. No clear relationships were found between pathogen prevalence and disease severity index of surveyed field plants, between pathogen prevalence and plant developmental stage, or between prevalence of the different pathogens isolated.     

浙江省荸荠上镰刀菌种类及其致病性

作者从浙江省11个县市采集病、健株及球茎标样共669个,经分离、纯化、鉴定,获以下7个镰刀菌种;尖孢镰刀菌(Fusarium oxysporum)、锐顶镰刀菌(F.acuminatum)、半裸镰刀菌(F.semitectum)、层出镰刀菌(F.proliferatum)、茄病镰刀菌(F.solani)、禾谷镰刀菌(F.graminearum)和木贼镰刀菌(F.equiseti)。田间观察和接种试验证明:F.oxysporum是引起荸荠茎秆枯萎的主要病原,这是一种新的危险性病害。病株矮小、黄化、基腐、后期倒伏。1986～1990年少数县市发病率达40％～50％。人工接种证明:F.acuminatum也属强致病性的种类。而F.proliferatum和F.solani则是引起贮藏期球茎腐烂的主要病原。其余种类仅具微弱或无致病性。     

Pathogenic, Genetic and Molecular Characterisation of Fusarium oxysporum f.sp. lilii

Isolates of Fusarium oxysporum from lily were screened for pathogenicity, vegetative compatibility and DNA restriction fragment length polymorphisms, and compared to reference isolates of F. oxysporum f.sp. gladioli and F. oxysporum f.sp. tulipae to justify the distinction of F. oxysporum f.sp. lilii. Twenty-four isolates from different locations in The Netherlands (18 isolates), Italy (4 isolates), Poland and the United States (1 isolate each) shared unique RFLP patterns with probes D4 and pFOM7, while hybridization did not occur with a third probe (F9). Except for a self-incompatible isolate, these 24 isolates all belonged to a single vegetative compatibility group (VCG 0190). Isolates belonging to VCG 0190 were highly pathogenic to lily, but not to gladiolus or tulip, except for a single nonpathogenic isolate. Six saprophytic isolates of F. oxysporum from lily were nonpathogenic or only slightly aggressive to lily, gladiolus and tulip, belonged to unique VCGs and had distinct RFLP patterns. Three pathogenic isolates previously considered to belong to F. oxysporum f.sp. lilii were identified as F. proliferatum var. minus; all three belonged to the same VCG and shared unique RFLP patterns. These three isolates were moderately pathogenic to lily and nonpathogenic to gladiolus and tulip. The reference isolates of F. oxysporum f.sp. tulipae were pathogenic to tulip, but not to lily and gladiolus; they shared a distinct RFLP pattern, different from those encountered among pathogenic and saprophytic isolates from lily, and formed a separate new VCG (VCG 0230). Reference isolates of F. oxysporum f.sp. gladioli belonging to VCG 0340 proved pathogenic to both gladiolus and lily, but not to tulip. These isolates, as well as isolates belonging to VCGs 0341, 0342 and 0343 of F. oxysporum f.sp. gladioli, had RFLP patterns different from those encountered among the isolates from lily or tulip. These findings identify F. oxysporum f.sp. lilii as a single clonal lineage, distinct from F. oxysporum f.sp. gladioli and f.sp. tulipae.     

Fungi on roots and stem bases of asparagus in the Netherlands: species and pathogenicity

A survey was made to identify the most important soilborne fungal pathogens of asparagus crops in the Netherlands. Ten plants were selected from each of five fields with a young (1–4 y) first planting, five fields with an old (6–13 y) first planting and five fields with a young replanting. The analysis included fungi present in the stem base and the roots of plants with symptoms of foot and root rot or showing growth decline without specific disease symptoms. Isolates of each species were tested for pathogenicity to asparagus on aseptically grown plantlets on Knop's agar. Symptoms were caused byFusarium oxysporum, F. culmorum, Botrytis cinerea, Penicillium verrucosum var.cyclopium, Cylindrocarpon didymum, Phialophora malorum, Phoma terrestris andAcremonium strictum. F. oxysporum was by far the most common species and was isolated from 80% of the plants. Not all of its isolates were pathogenic to asparagus. Symptoms were caused by 67%, 78% and 93% of the isolates obtained from young first plantings, old first plantings and replantings, respectively.F. culmorum was isolated from 31% of the plants. Two other notorious pathogens of asparagus,F. moniliforme andF. proliferatum, did not occur in our samples.Species causing symptoms in the vitro test that were found on more than 5% of the plants were additionally tested for their pathogenicity in pot experiments.F. oxysporum f.sp.asparagi caused severe foot and root rot, significantly reduced root weights and killed most of the plants.F. culmorum caused lesions on the stem base often resulting in death of the plant.P. terrestris, a fungus only once reported as a pathogen of asparagus, caused an extensive root rot, mainly of secondary roots that became reddish. The fungus was isolated in only a few samples and is not to be regarded as an important pathogen in Dutch asparagus crops.P. malorum caused many small brown lesions on the stem base and incidentally also on the upper part of small main roots. This is the first report of its pathogenicity to asparagus. The fungus is one of the organisms inciting spear rust and it reduced crop quality rather than crop yield.P. verrucosum var.cyclopium andC. didymum did not cause symptoms in pot experiments.Because of its predominance on plants with foot and root rot and its high virulence,F. oxysporum f.sp.asparagi was considered to be the main soilborne pathogen of asparagus in the Netherlands.     

安徽省玉米穗腐病主要致病镰孢菌的分离与鉴定

为明确安徽省玉米穗腐病主要致病镰孢菌的种类,采用单孢分离、形态学鉴定和分子生物学鉴定的方法,对2017年采集自安徽省6个玉米主产区——合肥市、蚌埠市、淮北市、阜阳市、亳州市和宿州市的玉米穗腐病病样进行了分离鉴定。在获得的455株镰孢菌中,拟轮枝镰孢菌、禾谷镰孢复合种、层出镰孢菌、尖孢镰孢菌和黄色镰孢菌的分离频率分别为59.13%、21.28%、13.68%、5.12%和0.79%。其中拟轮枝镰孢菌在各地的分布最广,属于安徽省的优势致病镰孢菌。     

Examination of the relationships between vegetative compatibility groups and races in Fusarium oxysporum f.sp. dianthi

The feasibility of identifying races of Fusarium oxysporum f.sp. dianthi by tests for vegetative compatibility type was investigated. Nitrate non-utilizing nitl and NitM mutants were generated from 51 isolates of F. oxysporum f.sp. dianthi , 18 isolates of f. oxysporum from Dianthus spp. not belonging to f.sp. dianthi and, for comparison, 11 isolates of F. proliferatum from Dianthus spp. Vegetative compatibility groups (VCGs) among the isolates were identified by pairing all nitl with all NitM mutants.
Vegetative compatibility was found between isolates of F. oxysporum f.sp. dianthi races 1 and 8 (VCG 0022), races 2, 5 and 6 (VCG 0021) and race 4 (VCG 0020), and wilt-causing isolates previously classified as F. redolens from D. caryophyllus (VCG 0023) and D. barbatus (VCG 0024), Three self-compatible wilt-causing isolates were vegetatively incompatible with all other isolates (VCGs 0025,0026 and 0027), Two VCGs were found among isolates of F. oxysporum from D. caryophyllus not belonging to f.sp. dianthi ; six non-pathogenic isolates were self-compatible but vegetatively incompatible with all other isolates. The foot-rot-associated isolates of F. proliferatum from D. caryophyllus constituted a separate VCG.
Virulence analyses revealed at least four new races among VCGs 0023 to 0027, New Isolates could be categorized as races as a result of VCG analysis and VCG classification correctly indicated that the race identities previously ascribed to two old isolates had been incorrect. Vegetative compatibility tests offer the prospect for rapid identification of races, although inoculation tests continue to be necessary to differentiate races that belong to a single VCG.     

Resistance Reaction of Conifer Species (European Larch, Norway Spruce, Scots Pine) to Infection by Selected Necrotrophic Damping-off Pathogens

Three conifer species (European larch, Norway spruce, Scots pine) were investigated for their resistance to five damping-off pathogens (Rhizoctonia solani, Fusarium solani, F. oxysporum, F. culmorum, F. avenaceum). Inoculation of the primary roots of seedlings with these pathogens caused host cell death which did not prevent the invasive growth of these fungi; seedlings that had formed secondary and tertiary roots could overcome the infection to a significant degree. Infections with R. solani caused significant mortality to all the conifer species. In contrast, the tree species expressed different levels of resistance when challenged with the Fusarium isolates, with Norway spruce being the most resistant compared to uninoculated controls. Some of the Fusarium isolates were more pathogenic to certain hosts than others; F. oxysporum for European larch, F. avenaceum for Scots pine, F. solani for European larch; only F. culmorum was significantly pathogenic to Norway spruce. No significant differences in disease severity were observed at different soil pH (4.3–7.5). Disease progression was delayed at lower (10–15°C) rather than higher temperatures (20–25°C).     

Local and Systemic Effects of NaCl on Root Composition, Rhizobacteria, and Fusarium Crown and Root Rot of Asparagus

ABSTRACT The role of NaCl in suppression of Fusarium crown and root rot of asparagus was investigated in split root culture so the direct effects of NaCl on the root and rhizosphere could be separated from effects that were translocated to the nontreated root side. One side of the root system was exposed to 100 ml of 0, 0.5, or 1.0% NaCl, while the other side received deionized water. Both sides of the root system were inoculated with conidial suspensions of the pathogens Fusarium oxysporum and F. proliferatum. When plants were harvested and assayed, root lesions and CFUs of F. oxysporum or F. proliferatum per centimeter of root from both exposed and nonexposed roots decreased as the NaCl rate increased to 1.0%, but the reduction relative to the control was significantly greater on roots that were directly exposed to NaCl (51% reduction in root lesions) than on adjacent nonexposed roots (31% reduction in root lesions). On both sides of the root systems, disease suppression with NaCl was associated with increases in the rhizosphere densities of fluorescent pseudomonads and Mn-reducing bacteria in the rhizosphere soil. In addition, as the NaCl rate increased, root tissues had marked reductions in malic acid and amino acids while concentrations of Cl and Mn increased in equal proportions on both sides of the root system. Chloride ions were absorbed in greater amounts than Na ions, and were more mobile in the plant than Na. Plants treated with 1% NaCl (171 meq of Cl(-) per liter) had soil leachates 1 week later of 47 meq of Cl(-) per liter from pots exposed to NaCl, but in the adjacent nonexposed pots, the amount of Cl in the leachates slowly increased over the course of the study to 20 meq/liter, presumably through the root exudation. These findings suggest that suppression of Fusarium crown and root rot of asparagus with NaCl may be due to multiple mechanisms. Maximum suppression occurs when NaCl is directly applied to roots, but suppression still occurs on distal non-treated roots resulting from systemic mechanisms. The latter mechanism may be associated with a root-mediated alteration in the rhizobacteria.     

Classification and Identification of Xanthomonas translucens Isolates, Including Those Pathogenic to Ornamental Asparagus

ABSTRACT In order to confirm and refine the current classification scheme of Xanthomonas translucens and to identify novel strains from ornamental asparagus, a collection of field and reference strains was analyzed. Rep-polymerase chain reaction (PCR) genomic fingerprint profiles were generated from 33 isolates pathogenic to asparagus as well as 61 X. trans-lucens reference strains pathogenic to cereals and grasses. Amplified ribo-somal gene restriction analysis profiles were obtained from most of these and 29 additional Xanthomonas reference strains. Rep-PCR genomic fingerprint profiles of all strains were compared with those in a large Xanthomonas database using computer-assisted analysis. Rep-PCR ge-nomic fingerprinting facilitated the characterization and discrimination of X. translucens, including the pathovars arrhenatheri, graminis, phlei, phleipratensis, and poae, as well as a number of strains received as X. translucens pv. cerealis. Strains received as pathovars hordei, secalis, translucens, undulosa, and other cerealis strains were grouped in two subclusters that correspond to the recently redefined pathovars X. trans-lucens pvs. undulosa and translucens. All 33 novel isolates from ornamental asparagus (tree fern; Asparagus virgatus) were identified as X. translucens pv. undulosa. Moreover, a unique amplified small subunit ribosomal gene MspI/AluI restriction profile specific for all X. translucens strains tested, including those pathogenic to asparagus, allowed discrimination from all other Xanthomonas spp. Although phage tests were inconclusive, the classification of the asparagus strains within the X. translucens complex was supported by pathogenicity assays in which all the isolates from ornamental asparagus induced watersoaking on wheat. Surprisingly, several X. translucens reference strains affected asparagus tree fern as well. That the novel asparagus isolates belong to X. translucens pv. undulosa is extraordinary because all hosts of X. translucens pathovars described to date belong only to the families Gramineae and Poaceae, whereas asparagus belongs to the phylogenetically distant family Liliaceae.     

Pathogenicity of fungi associated with winter loss and injury in white lupin

Fungi isolated from diseased white lupin plants were tested for pathogenicity in standardized glasshouse tests to assess their potential as causes of plant death in winter in autumn-sown crops. Pleiochaeta setosa and, to a slightly lesser extent, Fusarium avenaceum caused lesions or plant death when inoculated onto hypocotyls of lupin seedlings. Disease was more severe when the hypocotyls were wounded before inoculation. F. avenaceum also caused injury when inoculated onto roots; wounding the roots had no effect. Some isolates of F. solani also caused injury, especially after root inoculation, but isolates of F. oxysporum and Cylindrocarpon destructans were only slightly or non-pathogenic. Isolates of Fusarium and C. destructans , but not of P. setosa , differed in pathogenicity. There were differences in susceptibility to disease among three lupin cultivars.     

云南灯盏花根腐病的2种新病原鉴定及防治药剂的室内筛选

引起云南省灯盏花根腐病的病原为茄病镰刀菌(Fusarium solani)、尖孢镰刀菌(F.oxysporum)和半裸镰刀菌(F.semitectum)3种真菌,茄病镰刀菌和尖孢镰刀菌为灯盏花根腐病的2种新病原,其中茄病镰刀菌为主要致病菌,其分离率为42.86%,茄病镰刀菌有伤接种发病率为70%,无伤接种发病率为56.7%。采用室内生长速率法测定了50%多菌灵可湿性粉剂、70%甲基硫菌灵可湿性粉剂、64%恶霜·锰锌可湿性粉剂、10%苯醚甲环唑水分散粒剂、10亿活芽胞/g 枯草芽胞杆菌·荧光假单胞杆菌可湿性粉剂5种供试杀菌剂对灯盏花根腐病主要病原茄病镰刀菌的毒力。结果表明：5种供试药剂对茄病镰刀菌都有抑制作用。对茄病镰刀菌的有效中浓度（EC50）分别为0.000 4、0.050 7、0.053 5、0.081 3和8.624 0 mg/mL,其中50%多菌灵可湿性粉剂有效中浓度最低,相对抑制效果最好。5种药剂的毒力相关系数均在0.89以上,表明药剂质量浓度与抑制作用呈较高相关性。     

Genetic Diversity of Pathogenic and Nonpathogenic Populations of Fusarium oxysporum Isolated from Carnation Fields in Argentina

ABSTRACT In order to elucidate the origin of Fusarium oxysporum f. sp. dianthi in Argentina, the genetic diversity among pathogenic isolates together with co-occurring nonpathogenic isolates on carnation was investigated. In all, 151 isolates of F. oxysporum were obtained from soils and carnation plants from several horticultural farms in Argentina. The isolates were characterized using vegetative compatibility group (VCG), intergenic spacer (IGS) typing, and pathogenicity tests on carnation. Seven reference strains of F. oxysporum f. sp. dianthi also were analyzed and assigned to six different IGS types and six VCGs. Twenty-two Argentinean isolates were pathogenic on carnation, had the same IGS type (50), and belonged to a single VCG (0021). The 129 remaining isolates were nonpathogenic on carnation and sorted into 23 IGS types and 97 VCGs. The same VCG never occurred in different IGS types. Our results suggest that the pathogen did not originate in the local populations of F. oxysporum but, rather, that it was introduced into Argentina. Given the genetic homogeneity within Argentinean isolates of F. oxysporum f. sp. dianthi, either IGS type or VCG can be used for the identification of the forma specialis dianthi currently in Argentina.     

Use of random amplified polymorphic DNA (RAPD) to identify races 1, 2, 4 and 8 of Fusarium oxysporum f. sp. dianthi in Italy

The RAPD fingerprinting procedure was used in combination with pathogenicity assays on differential cultivars to characterize a representative collection of 72 Fusarium spp. isolates of different geographic origin collected from diseased carnation. In F. oxysporum f. sp. dianthi, isolates were grouped according to the physiologic race: group 1 included isolates of race 4; group 2 was formed by isolates of race 2 and single representatives of races 5 and 6; group 3 included isolates of races 1 and 8. No correlation was found between RAPD data and geographic origin of the isolates tested: representatives of race 2 isolated in Italy, Israel and Japan had the same amplification profile. Three isolates which showed a low level of pathogenicity on all carnation cultivars tested shared an identical amplification pattern and are probably saprophytic F. oxysporum. Finally, two F. redolens isolates from Japan and seven non-pathogenic isolates of F. proliferatum collected from diseased carnation in Italy, Israel and The Netherlands were clearly distinguishable according to their RAPD fingerprint. The results are discussed in relation to previous studies on the genetic diversity of F. oxysporum f. sp. dianthi and to the development of forma specialis- and pathotype-specific diagnostic tools.     

甘肃枸杞镰孢菌根腐病病原鉴定及优势病原菌生物学特性

为了明确甘肃省枸杞根腐病病原种类、生态分布和优势病原菌的生物学特性,于2013年和2014年采集甘肃省各枸杞主产区根腐病患病植株样品,分离纯化后进行形态和分子鉴定,统计分离到的各主产区的病原种类,并对优势病原菌的适宜生长温度、光照条件、pH和碳氮源进行了研究。结果共分离到260株镰孢菌菌株,其中GSJT7-2和GSJY8-1分离频率分别为66.15%和26.92%,属优势种;将2株镰孢菌进行EF-1α基因序列分析及对比,菌株GSJT7-2(登录号:KM457089)与腐皮镰孢菌(Fusarium solani),GSJY8-1(登录号:KM457093)与尖镰孢菌(F.oxysporium)同源性达99%,在系统发育树中聚在一起,分别将它们的分类地位初步确定为腐皮镰孢菌和尖镰孢菌。甘肃各产区病原菌分离种类和频率有较大差异,靖远县以尖镰孢菌为主,占82.89%;其余采集地均以腐皮镰孢菌为主,均占80%以上。腐皮镰孢菌GSJT7-2和尖镰孢菌GSJY8-1在10~40℃内均能生长,最适温度分别为35℃和25℃,最适pH为8~9和8,均在12 h光暗交替条件下生长最快,供试碳氮源中GSJT7-2和GSJY8-1对蔗糖和Na NO3利用效果最好。     

Incidence of soil-borne plant pathogens isolated from barley and winter wheat, and other crops in the rotation, on Prince Edward Island

Barley and winter wheat were sampled over 3 years to characterize soil-borne organisms involved with cereal crown and root disease complexes. Winter wheat crowns were infected more often by Fusarium avenaceum and F. sambucinum than barley crowns. Bipolaris sorokiniana and F. graminearum were more common in crown tissue of barley underseeded with clover than in winter wheat crowns. In roots, the incidence of F. avenaceum was highest in winter wheat. In soil, populations of Rhizoctonia solani and F. sambucinum were higher in winter wheat than barley underseeded with ryegrass. Stunt nematodes (Tylenchorhynchus spp.) were greatest in winter wheat and barley underseeded with ryegrass. The incidence of F. avenaceum in roots of winter wheat correlated positively with the severity of crown and root rot symptoms. The severity of barley crown and root rot correlated positively with the incidence of R. solani in crowns, F. avenaceum in crowns and roots, B. sorokiniana in roots, and numbers of stunt nematodes in the soil. The incidence of soil-borne organisms was also recorded in annual ryegrass. soybean, potato, pea, and clover which were grown in rotation with barley and winter wheat. The incidence of R. solani and F. avenaceum was highest in clover crown tissue, and F. sambucinum was prevalent in soybean hypocotyls. Soil population levels of R. solani, F. avenaceum , and stunt nematodes were greatest in ryegrass and pea soil just prior to ploughing down or harvesting these crops, respectively.     

Potential role for salicylic acid in induced resistance of asparagus roots to Fusarium oxysporum f.sp. asparagi

Pre-inoculation of asparagus ( Asparagus officinalis ) roots with selected nonpathogenic isolates of Fusarium oxysporum (np Fo ) has previously been shown to induce systemic resistance against infection by F. oxysporum f.sp. asparagi ( Foa ) through activation of plant-defence mechanisms. To elucidate the putative np Fo -mediated defence pathways, the effect of salicylic acid (SA) was examined in a split-root system of asparagus where one half of the seedling root system was drenched with SA and the activation of defence responses was measured subsequently on the remaining roots. SA-treated plants exhibited enhanced systemic resistance, with a significant reduction in disease severity of the roots inoculated with Foa , compared with untreated plants. SA activated peroxidase and phenylalanine ammonia-lyase, as well as lignification, upon Foa attack, in a manner similar to that observed with np Fo pretreatment. In addition, application of diphenyleneiodonium, an SA biosynthesis inhibitor, led to failure of np Fo to induce lignin deposition and systemic resistance. Treatment of fungal spores with SA did not affect germination and growth of either np Fo or Foa in in vitro antifungal assays. Production of SA at the site of np Fo infection may be involved in the induction of Foa resistance in asparagus roots.     

Physiological Races and Vegetative Compatibility Groups of Fusarium oxysporum f. sp. lactucae Isolated from Crisphead Lettuce in Japan

One hundred and sixteen isolates of Fusarium oxysporum f. sp. lactucae obtained from 85 fields in three crisphead lettuce-producing areas in Nagano Prefecture, Japan were typed for races using differential cultivars Patriot, Banchu Red Fire and Costa Rica No. 4. They were also grouped into vegetative compatibility groups (VCGs) using complementation tests with nitrate non-utilizing (nit) mutants. Two California strains reported as F. oxysporum f. sp. lactucum, a type culture of F. oxysporum f. sp. lactucae, and 28 avirulent isolates of F. oxysporum obtained from crisphead lettuce were included for comparison. Among Nagano isolates, 66 isolates were identified as race 1, and 50 as race 2. Race 1 strains derived from Shiojiri and Komoro cities and race 2 from Kawakami village and Komoro city. All isolates of race 2 were biotin auxotrophs, and the race could be distinguished based on its requirement for biotin on minimal nitrate agar medium (MM). Pathogenic isolates were classified into two VCGs and three heterokaryon self-incompatible isolates. Strong correlations were found between race and VCG. All the race 1 strains were assigned to VCG 1 except self-incompatible isolates, and all the race 2 strains to VCG 2. The 28 avirulent isolates of F. oxysporum were incompatible with VCG 1 and VCG 2. California strains was vegetatively compatible with VCG 1, and they were assigned to race 1. Based on vegetative compatibility, these two races of F. oxysporum f. sp. lactucae may be genetically distinct, and F. oxysporum f. sp. lactucae race 1 is identical to F. oxysporum f. sp. lactucum. Received 7 May 2002/ Accepted in revised form 6 September 2002