Disease In Nursery Research Articles

First Report of Downy Mildew Caused by Plasmopara halstedii on Black-eyed Susan (Rudbeckia fulgida cv. 'Goldsturm') in Maryland.

The North American perennial black-eyed Susan (Rudbeckia fulgida cv. Goldsturm) is an important nursery crop, prized by gardeners and landscapers for its persistent bloom and ease of cultivation. In September 2013, disease symptoms characteristic of downy mildew were observed from multiple R. fulgida plants at two commercial nurseries in the Maryland counties of Howard and Anne Arundel. Over 100 R. fulgida were affected by this disease in both nurseries, rendering the plants unmarketable and causing a substantial financial loss. Plants exhibited dark necrotic lesions on the adaxial leaf surface, and sporulating masses of white mycelium on the abaxial leaf surface and on the adaxial in extreme infections. Plants were stunted with a reduced number of blooms. Microscopic visualization showed coenocytic mycelium, hyaline sporangiophores (length 261 to 904 μm; = 557 μm; n = 20) that were straight and monopodially branched at right angles with several terminal branchlets. Sporangia were hyaline, ovoid to elliptical with smooth surfaces ( = 31 × 28 μm; n = 50). Based on morphological data, the organism was identified as Plasmopara halstedii (Farl.) Berl. & De Toni in Sacc (2). Voucher specimens were deposited in the U.S. National Fungus Collections (BPI 892792 to 892794). Molecular identification was conducted by extracting genomic DNA from sporangiophores and mycelium tweezed from the surface of three infected plants, with extractions performed using the QIAGEN Plant DNA kit (QIAGEN, Gaithersburg, MD). The large subunit of the nuclear rDNA was amplified by PCR using primers LROR and LR7 (3) and sequenced bidirectionally. BLASTn searches of NCBI GenBank showed that the resultant rDNA sequences (accessions KF927152 to KF927154) shared 99% nucleotide identity with curated P. halstedii sequences, consistent with morphological identification. To confirm pathogenicity, three 3.78-liter (1 gallon) containerized R. fulgida cv. Goldsturm plants were inoculated with a sporangial suspension of 2.4 × 104 sporangia/ml and sprayed until both the upper and lower surface of the leaves were completely covered. One negative control plant was sprayed with deionized water. Plants were placed in clear plastic bags in a growth chamber (20°C, 12-h photoperiod). Disease symptoms were observed 3 days post inoculation on all plants. The control plant was symptomless. Morphological features of the pathogen on the surface of inoculated plants were identical to those observed from the original infected plants. Although P. halstedii on R. fulgida cv. Goldsturm has been previously reported in Virginia in 2006 and Florida in 2004, to our knowledge, this is the first report on R. fulgida cv. Goldsturm in Maryland (1). Black-eyed Susans are widely distributed throughout Maryland's landscape and are a staple plant for gardeners, nurserymen and landscape professionals. Given the destructive nature of this disease, downy mildew has the potential to cause considerable economic losses to the state's ornamental crop industry.

Cylindrocladium quinqueseptatum leaf and twig blight of Eucalyptus species

Cylendrocladium quinqueseptatum, a soil born pathogen, cause leaf and twig blight disease in nurseries and plantation of Eucalyptus often aggravates during monsoon season to epidemic proportions resulting in large scale mortality of young seedling in nurseries and extensive defoliation of young plants and young coppice shoots in plantations. CLB caused by Cylindrocladium quinqueseptatum was first serious disease to be reported by Sehgal et al.,(1969) that affected Eucalyptus seedlings in nurseries and plantation. Cylindrocladium is known to cause foliage as well as root diseases of Eucalyptus in Kerala (Sharma et al. 1984). Different diseases of this pathogen has been reported to cause Eucalyptus diseases in south India. However, in northern part of India only one species, Cylendrocladium quinqueseptatum is encountered causing seedling blight in Eucalyptus nurseries.

In-vitro Evaluation of Some Selected Fungicides against Pestalotio psis clavispora and Pseudocochliobolus eragrostidis Isolated from Vitellaria paradoxaSeedlings.

Preliminary investigation into the nursery diseases of Vitellaria paradoxa in the Nigerian Institute fo r Oil Palm Research substation, Bida, Niger state, revealed mainly the presence of leaf pustules and leaf blight disease. Symptoms of the leaf pustules disease results in a total defoliation of the leaves of the plant while that of the leaf blight is characterized by fire burnt edges around most of the leaves in a single seedlings or shrub. Mic roorganisms isolated from the infected leaf samples using the direct plating techniques on potato dextrose agar include Pestalotiopsis clavispora and Botryodiplodia (Lasiodiplodia spp) from the leaf pustules while P. clavisp ora and Pseudocochliobolus eragrostidis were isolated from the leaf blight. The result of the effect of the fungici des captan, benlate dithane M-45and difolatan at 500 and 250ppm on the in-vitro mycelia growth of P. clavispo ra and P. eragrostidis for a test period of nine days shows benlate at 500ppm to completely inhibit the mycelia g rowth of P. clavispora and P. eragrostidis. The fungicide dithane did not completely inhibit the pathogens at sa me concentration of 500 and 250ppm. The mode of action of the fungicide benlate on P. clavisopra and P. eragr ostidis was fungicidal. It's therefore recommended that the fungicide benlate be used for field trials.

Bahasa Indonesia

Leaf spot disease of oil palm caused by Curvularia sp. is the major disease in nursery. Implementation of best nursery practices is the key to prevent it. Generally, fungicides are used only if epidemic of leaf spot diseases occur in the field. The objectives of this research were to determine causal agent of leaf spot disease of oil palm and the potential alternative weed host around the nursery, to select suitable fungicides, and to study the effect of fungicides rotation to disease incidence. The results showed that the causal agent of leaf spot disease of oil palm was Curvularia lunata. The fungus was also found on grasses, Cyperus rotundus and Imperata cylindrica. Difeconazol, copper oxide, and propineb suppressed leaf spot disease in nursery. Application of fungicide by rotation between difeconazol and copper oxide with frequency every 10 days suppressed the development of leaf spot disease of oil palm in the nursery.

Calonectria diseases on ornamental plants in Europe and the Mediterranean basin: an overview.

Species of Calonectria and their cylindrocladium-like asexual morphs are important plant pathogens of agronomic and forestry crops, especially in the tropical and subtropical regions of the world. Calonectria species have been associated with a wide range of disease symptoms on a large number of plant hosts. On horticultural crops, most records of Calonectria species come from the Northern Hemisphere, where they occur mainly in gardens and ornamental nurseries. In Europe and the Mediterranean basin, several species are widespread in nurseries and cause extensive damage to ornamental plants. In the past, identification of species was based on phenotypic characters and sexual compatibility using standardised media. More recently, morphological characteristics, phylogenetic studies (DNA sequence data of the β-tubulin, histone H3 and translation elongation factor-1α gene regions) and mating studies have revealed the presence of several cryptic species complexes that were formerly treated as single Calonectria species. These studies resulted in the introduction of several new species. Other studies aimed at understanding environmental sustainability focused attention on soil solarisation and biological control as means for controlling these pathogens. The potential use of biological control agents (BCAs) and chemicals for controlling Calonectria-induced diseases has recently been addressed. In this review we discuss the Calonectria species detected in Europe and the Mediterranean basin, and the disease management strategies. In view of the mandatory implementation of integrated pest management (IPM) for all European countries by 2014, this paper provides basic information as a platform for the adaptation of more sustainable integrated measures to control Calonectria diseases in European nurseries.

Slow Sand Filters as a part of integrated protection of seedlings against disease in forest nurseries

Slow Sand Filters (SSF) are a biological method used to protect nursery plants, from pathogen infections which can cause serious diseases in many forest tree species. Thanks to SSF application the number of phytopathogens in nurseries can be significantly reduced, as demonstrated by many field and greenhouse experiments (e.g. in Polish nurseries, and for horticultural crops in Germany and The Netherlands). In this study, the effect of pollution from fertilizers and fungicides used in agriculture (e.g. PCNB) on the efficiency of SSFs was assessed. A quantitative analysis was performed of the copiotrophic and oligotrophic bacterial composition colonizing SSF biofilms. The efficiency with which selected Oomycete strains belonging to the genus Phytophthora (P. alni, P. cactorum, P. plurivora) were removed from water was determined based on genetic material (DNA of the organisms) found in the SSF filtrate. Specific primers and TaqMan probes (qPCR) appeared to be the most sensitive molecular methods. Moreover, the microbiological analysis of SSF biofilm performed with selective media allowed the growth of copiothrophic and oligothrophic bacteria to be estimated. The influence of fungicide (PCNB) and N-fertilizer on the number of bacteria in each biofilm was also evaluated. The pollution of water with fertilizer (being used for plant irrigation) was demonstrated to reduce the efficiency of filtration more than fungicide addition (the amount of DNA from those investigated pathogens in the water decreased with time). The amount of bacteria in SSF biofilm readily increased after application of N-fertilizer in contrast to fungicide (PCNB) addition.

Emerging Hazelnut Cultivation and the Severe Threat of Bacterial Blight in Chile

AbstractSevere attacks of bacterial blight were observed on young plants throughout the hazelnut growing areas in Chile. The incidence of the disease in nurseries and fields ranged from 60–90%. The causal agent was identified as Xanthomonas arboricola pv. corylina, based on phenotypic and genetic tests.

First report of Nectria haematococca causing root rot disease of physic nut (Jatropha curcas) in China

A serious root rot disease in nurseries and plantations of J. curcas has been observed in Luodian County, Guizhou Province, China, since 2006. The symptoms were yellowing, wilting and leaf fall as well as blackening and rotting of the roots and collars. Laboratory examination and greenhouse pathogenicity tests showed that the causal agent of the disease was Nectria haematococca. This is the first report of N. haematococca, as the cause of root rot of Jatropha curcas L. in China.

Phytophthora in Finnish nurseries

International trade and travel have facilitated the spread of harmful organisms around the world. Human-mediated movement of plants and plant products is now generally accepted to be the primary mode of introduction of pathogens and pests. Species of Phytophthora, a genus of plant pathogens, are commonly spread in this way and have caused severe epidemics in silviculture, horticulture and natural systems all over the world. The objective of the study was to gather information on the occurrence of Phytophthora spp. in Finnish nurseries. Furthermore, the aim was to produce information for risk assessments for these Phytophthora spp. and establish molecular means for their detection. Phytophthora cactorum was found to persist in natural waterbodies and results suggest that irrigation water might be a source of inoculum for the disease in nurseries. In addition to P. cactorum, isolates from ornamental Rhododendron in nurseries yielded three species new to Finland: P. ramorum, P. plurivora and P. pini. The only species with quarantine status, P. ramorum, was able to persist in the nursery in spite of an annual European Commission sanitation protocol. Phytophthora plurivora and the closely related P. pini had more hosts among Nordic tree species and other plants abundant in Finnish nurseries and forest ecosystems. They were also found to have higher infectivity rates compared to P. ramorum and P. cactorum. All four species survived two weeks in -5 °C , and thus soil survival of these Phytophthoras in Finland is likely under current climatic conditions. The most common tree species in Finnish nurseries, Picea abies, was highly susceptible to P. plurivora and P. pini in pathogenicity trials. In a histological examination of P. abies shoot tissues inoculated with P. plurivora zoospores, fast necrotrophic growth was observed in nearly all tissues of the fresh shoot. The production of propagules in P. abies shoot tissue was only weakly indicated. In this study, a PCR–DGGE technique was developed for simultaneous detection and identification of Phytophthora spp. It reliably detected Phytophthora in plant tissues and could discriminate most test species as well as indicate instances of multiple-species infections. It proved to be a useful detection and identification tool either applied alone or in concert with traditional isolation culture techniques. The limitations of the method are also discussed. In conclusion, all of the introduced species of Phytophthora had properties that promote a high risk of establishment and spread in Finland. The efficient transport of Phytophthora via commercial traffic and favorable conditions due to expected climate change will increase the incidence and establishment of Phytophthora in new areas. Thus, it is probable that pathogens of this genus will be introduced and become established in Finland and other Nordic countries unless efficient phytosanitary control becomes standard practice in the international plant trade.

Fungal diseases in forest nurseries in Finland

Norway spruce (Picea abies), Scots pine (Pinus sylvestris) and silver birch (Betula pendula) are the major tree species grown in Finnish forest nurseries where 99% of the seedlings are grown in containers first in plastic-covered greenhouses and later outdoors. The main diseases on conifer seedlings are Scleroderris canker (Gremmeniella abietina), Sirococcus blight and cankers (Sirococcus conigenum), snow blights (Herpotrichia juniperi and Phacidium infestans) and needle casts (Lophodermium seditiosum and Meria laricis). Also grey mould (Botrytis cinerea) and birch rust (Melampsoridium betulinum) are among the diseases to be controlled with fungicides. During last years Scleroderris canker has been a problem on Norway spruce, which has been since 2000 the most common species produced in Finnish nurseries. Root die-back (uninucleate Rhizoctonia sp.) on container-grown spruce and pine was a problem in the 1990s. Today the disease has become less common in modern nurseries due to improvements in hygiene and cultivation practice. Since 1991 stem lesions and top dying caused by Phytophthora cactorum has been a problem on birch. The ongoing climate change has already had effect on rusts and powdery mildews as well as other fungi infecting leaves. All diseases, which gain high precipitation and warm and long autumns. For same reasons winter stored seedlings need sprayings against grey mold. Fungal infections are also possible during short-day (SD) treatment, that is necessary for summer and autumn plantings and a beneficial step prior freezing temperatures outside or in freezer storage. Growers are encouraged to use cultural and integrated pest management techniques such as better nursery hygiene, including removing plant debris in nursery growing areas and hot water washing of containers plus removal of diseased, spore-producing seedlings and trees around the nursery.

Vertical spread ofPuccinia psidiiurediniospores and development of eucalyptus rust at different heights

Rust, caused by Puccinia psidii, is an important disease in eucalyptus nurseries and plantations in Brazil. In the field, eucalyptus plants taller than 3–4 m are not infected by the pathogen. We hypothesised that microclimatic conditions above 3 m do not favour either pathogen establishment or disease development. Therefore, we evaluated the effect of the height gradient on rust incidence, related to leaf wetness duration and the number of airborne urediniospores. Young eucalyptus plants were maintained at heights from 0.5 to 4.0 m, and disease was assessed weekly at all heights in five plant sets. Rust incidence reduced as height increased. The values of area under rust progress and rust incidence were negatively correlated (P ≤ 0.05) with height and positively correlated (P ≤ 0.05) with the number of both hours with leaf wetness and airborne urediniospores. Thus establishment and progress of eucalyptus rust were affected by leaf wetness duration and concentration of airborne urediniospores, which decreased as the height increased.

First report of the pitch canker fungus Fusarium circinatum affecting Pinus taeda seedlings in Uruguay

In recent years Pinus spp. plantations have become an important resource that is continuously increasing in Uruguay as well as in other countries. Fungal diseases in nurseries are common, and among them, Fusarium circinatum (the pitch canker fungus) was found associated with Pinus taeda seedling, representing the first record of this pathogen in Uruguay.

Management of Alternaria leaf blight of Dalbergia sissoo

Leaf blight in the seedlings and young trees of Dalbergia sissoo, an epidemic disease in nurseries and plantations, caused by Alternaria alternata is recorded from western Uttar Pradesh. The symptoms of leaf blight started from margins and progressively to the midrib and giving a brown and crispy appearance to the infected leaves. To find out effective chemical fungicide for disease management, eight fungicides viz. Indofil M-45 [Mancozeb] (0.2%), Chlorothalonil (0.2%), Cuman-L [Ziram] (0.2%), Ridomil [Metalaxyl] (0.2%), Indofil Z-78 [Zineb] (0.2%), Copper oxychloride (0.2%), Jkstein [Methyl Benzimidazole Carbonate] (0.1%) and Topsin M [Thiophanate Methyl] (0.1%) were sprayed on D. sissoo seedlings in nursery. The results indicated that all the fungicides significantly reduced the disease incidence as compared to control. However, Indofil M-45 treatment was found to be most effective which exhibited 80.87 per cent disease control over control experiment.

First report of Xanthomonas axonopodis pv. poinsettiicola, the bacterial leaf spot pathogen on Euphorbia pulcherrima in Austria

In September 2007 symptoms typical of bacterial leaf spot were observed on pot-grown Euphorbia pulcherrima in a commercial plant nursery in Carinthia, Austria. Leaf symptoms were dark to rust-brown spots with yellow haloes that sometimes coalesced to larger areas. Isolation of bacteria was performed on yeast agar (YPGA) and King's B Agar and colonies were more or less yellow and glossy. A strain of X. axonopodis pv. poinsettiicola (LMG849) from the Belgian Coordinates Collection of Microorganisms and a strain of X. campestris pv. vesicatoria (DSMZ 50861) from the German Collection of Microorganisms and Cell Cultures were used as controls. Identification was performed using two PCR assays, which combined allowed precise identification of the pathogen. Three pairs of oligonucleotide primers specific for different hrp gene regions of several pathovars from the X. campestris/axonopodis cluster were used for identification to species level (Leite et al., 1994). The 1075 bp amplicon described by Leite et al. (1994) was digested with HaeIII and Sau3AI, producing restriction patterns similar to those of X. campestris pv. vesicatoria and X. axonopodis pv. poinsettiicola. Additionally the PCR amplicons (1075 bp) of the isolated strain and the reference strain (LMG849) were sequenced. Phylogenetic sequence comparison grouped the sequence from the isolated strain with that from strain LMG849, lending further support to identification as X. axonopodis pv. poinsettiicola. A second PCR assay specific for X. campestris pv. vesicatoria designed to amplify the rhs family gene (Park et al., 2007) was negative. We therefore concluded that the bacteria isolated from diseased E. pulcherrima plants was X. axonopodis pv. poinsettiicola. Pathogenicity was confirmed by inoculating young E. pulcherrima plants with bacterial suspensions. After two weeks bacteria were re-isolated from newly emerged leaves. Isolates were identified as X. axonopodis pv. poinsettiicola using the molecular methods described above. Combining these two PCR assays, X. axonopodis pv. poinsettiicola isolates could be identified from leaves by amplification of DNA extracted from diseased material. This is the first report of X. axonopodis pv. poinsettiicola in Austria. An outbreak of this disease in nurseries would have a major economical impact, since E. pulcherrima is produced in yearly quantities of approximately 3·5 million units. The authors would like to thank Mr. Kelvin Hughes for his helpful comments on this manuscript.

Identification and Investigation on Antagonistic Effect of Trichoderma spp. on Tea Seedlings White Foot and Root Rot (Sclerotium rolfsii Sacc.) in vitro Condition

The aim of this two years research work is to recognize bio control agents and its antagonistic efficiency on Sclerotium rolfsii in vitro condition. Sclerotium rolfsii Sacc. is a causal agent of white foot and root rot disease in many plants. This fungi causing damage in tea nurseries of tea cultivated countries especially in Iran. Due to importance this disease in tea nurseries of Iran and impossibility using of chemical control against this damaging agent In this research, five species of Trichoderma viride, Trichoderma harzianum, Trichoderma hamatum, Trichoderma longibrachiatum and Trichoderma parceramosum as antagonist fungi against Sclerotium rolfsii were collected and identified from tea nurseries and seedlings in Iran. Then efficiency of these bio control agent against Sclerotium rolfsii were investigated in vitro condition. Results showed that these species with different mechanism such as lysis of sclerotia, inhibited mycelial growth of Sclerotium rolfsii with volatile metabolites producing and parasitized the hyphal trends of disease agent were showed its antagonistic effects against causal agent of white foot and root rot in tea seedling. Trichoderma viride, Trichoderma harzianum and Trichoderma hamatum after 30 days destructed and lysis the sclerotia 98.5, 86.5 and 85%, respectively. Producing of volatile metabolites after 72 h reciprocal growth of Trichoderma viride, Trichoderma harzianum and Trichoderma longibrachiatum till 60.8, 54.8 and 54.4% prevented mycelium growth of Sclerotium rolfsii, respectively. All identified species caused parasitize (to hyphal contact method) pathogen fungal hypha that the first three species showed more parasite effect.

Biological control of seedling diseases in forest nurseries in Kerala.

Trichoderma harzianum, T. viride , and Psendomonas fluorescens were screened in forest nursery in two consecutive years to study the efficacy against the seedling damping-off pathogens, Rhizoctonia solani and Cylindrocladium quinqueseptatum . Solarization of nursery beds by tarping the moistened soil by thick polythene sheets was also carried out as a part of the experiment. The antagonists were introduced in the nursery beds by soil amendment and seed coating either singly or in different combinations using Eucalyptus tereticornis and E. grandis as the host plants. Solarization increased the temperature in the top layer of soil (2.5-5 cm depth) up to 51°C and reduced the pathogen inoculum considerably. Among various treatments, T. harzianum as soil application was the most effective one against damping-off followed by the combination of soil solarization and T. harzianum soil application.

Evaluation of fungicides and biocontrol agents against grey blight disease of tea in the field

Grey blight disease of tea has gained importance recently due to the extensive usage of harvesting machines in tea plantations. The study on the impact of different harvesting practices on disease development showed the highest disease incidence in continuously shear-harvested fields and least in hand-plucked ones. Fungicides and biocontrol agents with different modes of action were screened against the disease in the field. The systemic fungicides thiophanate methyl and carbendazim at 0.05% were most effective, followed by the contact fungicides mancozeb (dithane M45) and copper oxychloride (COC) at 0.3%. Biocontrol agents such as Trichoderma, Gliocladium and Pseudomonas gave satisfactory control when applied along with a food base. Fungicides also gave satisfactory protection against the disease in nurseries. Dipping shears in solutions of fungicide, detergent or biocontrol agents failed to protect the disease spread. The present study revealed that the disease can be effectively controlled using fungicides with different modes of action, and that biocontrol agents can also be incorporated in the spraying schedule.

Control of black foot disease in grapevine nurseries

Black foot disease of grapevines is a decline and dieback disease caused by a soilborne pathogen complex including Cylindrocarpon liriodendri, C. macrodidymum, Campylocarpon fasciculare and Campyl. pseudofasciculare. These pathogens cause primary infections of roots and basal ends of grafted cuttings in nursery soils. Thirteen fungicides were screened in vitro for mycelial inhibition of these pathogens. Prochloraz manganese chloride, benomyl, flusilazole and imazalil were the most effective fungicides tested, and were subsequently included in semi‐commercial field trials. Basal ends of grafted cuttings were dipped in various chemical and biological treatments prior to planting in open‐rooted nurseries. Black foot pathogens were not isolated from grafted cuttings prior to planting. Additional treatments involved soil amendments with Trichoderma formulations and hot water treatment of dormant nursery grapevines. Field trials were evaluated after eight months. Isolations from uprooted plants revealed low levels of black foot pathogens in the roots of untreated control plants, and significantly higher levels in basal ends of rootstocks. The incidence of black foot pathogens, as well as that of Petri disease pathogens, was not significantly and/or consistently reduced by the majority of chemical or biological treatments. However, these pathogens were not isolated from uprooted plants that were subjected to hot water treatment. It is therefore recommended that hot water treatment of dormant nursery plants be included in an integrated strategy for the proactive management of these diseases in grapevine nurseries.

Molecular characterization of plant growth promoting rhizobacteria that enhance peroxidase and phenylalanine ammonia-lyase activities in chile (Capsicum annuum L.) and tomato (Lycopersicon esculentum Mill.)

Pythium and Phytophthora species are associated with damping-off diseases in vegetable nurseries and reduce seedling stand and yield. In this study, bacterial isolates were selected on the basis of in vitro antagonism potential to inhibit mycelial growth of damping-off pathogens along with plant growth properties for field assessment in wet and winter seasons. We demonstrate efficacy of bacterial isolates to protect chile and tomato plants under natural vegetable nursery and artificially created pathogen-infested (Pythium and Phytophthora spp.) nursery conditions. After 21 days of sowing, chile and tomato plants were harvested and analysed for peroxidase and phenylalanine ammonia-lyase activities. Pseudomonas sp. strains FQP PB-3, FQA PB-3 and GRP(3 )were most effective in increasing shoot length (P > 0.05%) in both artificial and natural field sites. For example, Pseudomonas sp. FQA PB-3 treatment increased shoot length by 40% in the artificial Pythium 4746 infested nursery site in chile plants in the wet season. The bacterial treatments significantly increased the activity of peroxidase and phenylalanine ammonia-lyase in chile and tomato plant tissues, which are well known as indicators of an active lignification process. Thus, we conclude that treatment with potential bacterial plant growth promoting agents help plants against pathogen invasion by modulating plant peroxidase and phenylalanine ammonia-lyase activities.

Molecular Characterization of Fusarium oxysporum and Fusarium commune Isolates from a Conifer Nursery

ABSTRACT Fusarium species can cause severe root disease and damping-off in conifer nurseries. Fusarium inoculum is commonly found in most container and bareroot nurseries on healthy and diseased seedlings, in nursery soils, and on conifer seeds. Isolates of Fusarium spp. can differ in virulence; however, virulence and colony morphology are not correlated. Forty-one isolates of Fusarium spp., morphologically indistinguishable from F. oxysporum, were collected from nursery samples (soils, healthy seedlings, and diseased seedlings). These isolates were characterized by amplified fragment length polymorphism (AFLP) and DNA sequencing of nuclear rDNA (internal transcribed spacer including 5.8S rDNA), mitochon-drial rDNA (small subunit [mtSSU]), and nuclear translation elongation factor 1-alpha. Each isolate had a unique AFLP phenotype. Out of 121 loci, 111 (92%) were polymorphic; 30 alleles were unique to only highly virulent isolates and 33 alleles were unique to only isolates nonpathogenic on conifers. Maximum parsimony and Bayesian analyses of DNA sequences from all three regions and the combined data set showed that all highly virulent isolates clearly separated into a common clade that contained F. commune, which was recently distinguished from its sister taxon, F. oxysporum. Interestingly, all but one of the nonpathogenic isolates grouped into a common clade and were genetically similar to F. oxysporum. The AFLP cladograms had similar topologies when compared with the DNA-based phylograms. Although all tested isolates were morphologically indistinguishable from F. oxysporum based on currently available monographs, some morphological traits can be plastic and unreliable for identification of Fusarium spp. We consider the highly virulent isolates to be F. commune based on strong genetic evidence. To our knowledge, this is the first reported evidence that shows F. commune is a cause of Fusarium disease (root rot and dampingoff) on Douglas-fir seedlings. Furthermore, several AFLP genetic markers and mtSSU sequences offer potential for development of molecular markers that could be used to detect and distinguish isolates of F. oxysporum nonpathogenic to conifers and highly virulent isolates of F. commune in forest nurseries.