Antibacterial, antibiofilm, and antivirulence potential of the main diterpenes from Copaifera spp. oleoresins against multidrug-resistant bacteria

To evaluate the antibacterial, antibiofilm and antivirulence potential of the main diterpenes from Copaifera spp. oleoresins against multidrug-resistant (MDR) bacteria. Antimicrobial assays included determination of the Minimum Inhibitory Concentration (MIC), Minimum Bactericidal Concentration (MBC), Minimum Inhibitory Concentration of Biofilm (MICB50), as well as synergistic and antivirulence assays for eight diterpenes against MDR. The tests revealed that two diterpenes (named 1 and 5) showed the best results, with MIC and MBC between 12.5 and 50 μg/mL against most MDR bacteria. These diterpenes exhibited promising MICB50 in concentration between 3.12-25 μg/mL but showed no synergistic antimicrobial activity. In the assessment of antivirulence activity, diterpenes 1 and 5 inhibited only one of the virulence factors evaluated (Dnase) produced by some strains of S. aureus at subinhibitory concentration (6.25 μg/mL). Results obtained indicated that diterpenes isolated from Copaifera oleoresin plays an important part in the search of new antibacterial and antibiofilm agents that can act against MDR bacteria.

Genome wide association study identifies novel single nucleotide polymorphic loci and candidate genes involved in soybean sudden death syndrome resistance

Fusarium virguliforme is a soil borne root pathogen that causes sudden death syndrome (SDS) in soybean [Glycine max (L.) Merrill]. Once the fungus invades the root xylem tissues, the pathogen secretes toxins that cause chlorosis and necrosis in foliar tissues leading to defoliation, flower and pod drop and eventually death of plants. Resistance to F. virguliforme in soybean is partial and governed by over 80 quantitative trait loci (QTL). We have conducted genome-wide association study (GWAS) for a group of 254 plant introductions lines using a panel of approximately 30,000 SNPs and identified 19 single nucleotide polymorphic loci (SNPL) that are associated with 14 genomic regions encoding foliar SDS and eight SNPL associated with seven genomic regions for root rot resistance. Of the identified 27 SNPL, six SNPL for foliar SDS resistance and two SNPL for root rot resistance co-mapped to previously identified QTL for SDS resistance. This study identified 13 SNPL associated with eight novel genomic regions containing foliar SDS resistance genes and six SNPL with five novel regions for root-rot resistance. This study identified five genes carrying nonsynonymous mutations: (i) three of which mapped to previously identified QTL for foliar SDS resistance and (ii) two mapped to two novel regions containing root rot resistance genes. Of the three genes mapped to QTL for foliar SDS resistance genes, two encode LRR-receptors and third one encodes a novel protein with unknown function. Of the two genes governing root rot resistance, Glyma.01g222900.1 encodes a soybean-specific LEA protein and Glyma.10g058700.1 encodes a heparan-alpha-glucosaminide N-acetyltransferase. In the LEA protein, a conserved serine residue was substituted with asparagine; and in the heparan-alpha-glucosaminide N-acetyltransferase, a conserved histidine residue was substituted with an arginine residue. Such changes are expected to alter functions of these two proteins regulated through phosphorylation. The five genes with nonsynonymous mutations could be considered candidate SDS resistance genes and should be suitable molecular markers for breeding SDS resistance in soybean. The study also reports desirable plant introduction lines and novel genomic regions for enhancing SDS resistance in soybean.

'MN1606SP' by 'Spencer' filial soybean population reveals novel quantitative trait loci and interactions among loci conditioning SDS resistance

KEY MESSAGE

Four novel QTL and interactions among QTL were identified in this research, using as a parent line the most SDS-resistant genotype within soybean cultivars of the US early maturity groups. Soybean sudden death syndrome (SDS) reduces soybean yield in most of the growing areas of the world. The causal agent of SDS, soilborne fungus Fusarium virguliforme (Fv), releases phytotoxins taken up by the plant to produce chlorosis and necrosis in the leaves. Planting resistant cultivars is the most successful management practice to control the disease. The objective of this study was to identify quantitative trait loci (QTL) associated with the resistance response of MN1606SP to SDS. A mapping population of F 2:3 lines created by crossing the highly resistant cultivar 'MN1606SP' and the susceptible cultivar 'Spencer' was phenotyped in the greenhouse at three different planting times, each with three replications. Plants were artificially inoculated using SDS infested sorghum homogeneously mixed with the soil. Data were collected on three disease criteria, foliar disease incidence (DI), foliar leaf scorch disease severity (DS), and root rot severity. Disease index (DX) was calculated as DI × DS. Ten QTL were identified for the different disease assessment criteria, three for DI, four for DX, and three for root rot severity. Three QTL identified for root rot severity and one QTL for disease incidence are considered novel, since no previous reports related to these QTL are available. Among QTL, two interactions were detected between four different QTL. The interactions suggest that resistance to SDS is not only dependent on additive gene effects. The novel QTL and the interactions observed in this study will be useful to soybean breeders for improvement of SDS resistance in soybean germplasm.

Soybean Sudden Death Syndrome Caused by Fusarium virguliforme is Impaired by Prolonged Flooding and Anaerobic Conditions

High soil moisture usually favors soybean sudden death syndrome (SDS), caused by Fusarium virguliforme (Fv), but the effects of the duration of the flooding period and accompanying anaerobic conditions on the soybean-Fv interaction are not clear. Greenhouse studies were conducted using susceptible and resistant cultivars exposed to the following treatments: 3, 5, or 7 days of continuous flooding, repeated short-term flooding of 8 h/week for 3 weeks, and a no-flood check treatment. At 7, 14, and 21 days after flooding (DAF), seedlings in the no-flood, 3-day, and repeated short-term treatments showed the highest root rot and foliar symptom severity, whereas seedlings in the 7-day treatment showed the lowest severity. Fv inoculum density in soil was lowest in the 7-day flooding treatment. In a hydroponic system, the steady transcript levels of soybean defense genes and Fv candidate virulence genes were measured in response to different oxygen levels using qPCR. Fv-infected roots exposed to 12 h of anaerobic conditions showed down-regulation of the defense-related soybean genes Laccase, PR3, PR10, PAL, and CHS, and the Fv virulence genes pectate lyase (PL), and Fv homolog of the pisatin demethylase (PDA). Our study suggests that short-term flooding tends to increase SDS, while prolonged flooding negatively impacts SDS due to reduction of Fv density in soil. Moreover, anaerobic conditions down-regulate both soybean defense genes and Fv candidate virulence genes.

Identification of Highly Variable Supernumerary Chromosome Segments in an Asexual Pathogen

Supernumerary chromosome segments are known to harbor different transposons from their essential counterparts. The aim of this study was to investigate the role of transposons in the origin and evolution of supernumerary segments in the asexual fungal pathogen Fusarium virguliforme. We compared the genomes of 11 isolates comprising six Fusarium species that cause soybean sudden death syndrome (SDS) or bean root rot (BRR), and identified significant levels of genetic variation in A+T-rich repeat blocks of the essential chromosomes and in A+T-neutral regions of the supernumerary segments. The A+T-rich repeat blocks in the essential chromosomes were highly variable between F. virguliforme and non-F. virguliforme isolates, but were scarcely variable between F. virguliforme isolates. The A+T-neutral regions in the supernumerary segments, however, were highly variable between F. virguliforme isolates, with a statistically significant number (21 standard deviations above the mean) of single nucleotide polymorphisms (SNPs). And supernumerary sequence types and rearrangement patterns of some F. virguliforme isolates were present in an isolate of F. cuneirostrum but not in the other F. virguliforme isolates. The most variable and highly expressed region in the supernumerary segments contained an active DNA transposon that was a most conserved match between F. virguliforme and the unrelated fungus Tolypocladium inflatum. This transposon was absent from two of the F. virguliforme isolates. Furthermore, transposons in the supernumerary segments of some F. virguliforme isolates were present in non-F. virguliforme isolates, but were absent from the other F. virguliforme isolates. Two supernumerary P450 enzymes were 43% and 57% identical to their essential counterparts. This study has raised the possibility that transposons generate genetic variation in supernumerary chromosome segments by frequent horizontal transfer within and between closely related species.

Identification of a soybean rust resistance gene in PI 567104B

KEY MESSAGE

Using a combination of phenotypic screening and molecular, statistical, and linkage analyses, we have mapped a dominant soybean rust resistance gene in soybean PI 567104B. Asian soybean rust (SBR), caused by the fungus Phakopsora pachyrhizi Syd. and P. Syd., is one of the most economically important diseases that affect soybean production worldwide. A long-term strategy for minimizing the effects of SBR is the development of genetically resistant cultivars. The objectives of the study were to identify the location of a rust-resistance (Rpp) gene(s) in plant introduction (PI) 567104B, and to determine if the gene(s) in PI 567104B was different from previously mapped Rpp loci. The progeny of the cross of 'IAR 2001 BSR' × PI 567104B was phenotyped from field assays of the F 2:3 and F 4:5 generations and from a growth chamber assay of 253 F 5:6 recombinant inbred lines (RILs). For the growth chamber, the phenotyping was conducted by inoculation with a purified 2006 fungal isolate from Mississippi. A resistance gene locus on PI 567104B was mapped to a region containing the Rpp6 locus on chromosome 18. The high level of resistance of F 1 plants from two other crosses with PI 567104B as one of the parents indicated that the gene from PI 567104B was dominant. The interval containing the gene is flanked by the simple sequence repeat (SSR) markers Satt131 and Satt394, and includes the SSR markers BARCSOYSSR_18_0331 and BARCSOYSSR_18_0380. The results also indicated that the resistance gene from PI 567104B is different from the Rpp1 to the Rpp4 genes previously identified. To determine if the gene from PI 567104B is different from the Rpp6 gene from PI 567102B, additional research will be required.

Genotypic and Phenotypic Characterization of Fungi in the Fusarium oxysporum Species Complex from Soybean Roots

Isolates in the Fusarium oxysporum species complex (FOSC) from soybean range from nonpathogenic to aggressive pathogens causing seedling damping-off, wilt, and root rot. The objective of this research was to characterize the genotype and phenotype of isolates within the FOSC recovered predominantly from soybean roots and seedlings. Sequence analyses of the translation elongation factor (tef1α) gene and the mitochondrial small subunit (mtSSU), polymerase chain reaction restriction fragment length polymorphism (PCR-RFLP) analysis of the intergenic spacer (IGS) region, and identification of the mating type loci were conducted for 170 isolates. Vegetative compatibility (VC) tests were conducted for 114 isolates. Isolate aggressiveness was tested using a rolled towel assay for 159 isolates. Phylogenetic analysis of the tef1α and mtSSU and PCR-RFLP analysis of the IGS region separated the FOSC isolates into five clades, including F. commune. Both mating type loci, MAT1-1 or MAT1-2, were present in isolates from all clades. The VC tests were not informative, because most VC groups consisted of a single isolate. Isolate aggressiveness varied within and among clades; isolates in clade 2 were significantly less aggressive (P < 0.0001) when compared with isolates from the other clades and F. commune. The results from this study demonstrate the high levels of genotypic and phenotypic diversity within the FOSC from soybean but further work is needed to identify characteristics associated with pathogenic capabilities.

Usefulness of 10 genomic regions in soybean associated with sudden death syndrome resistance

Sudden death syndrome (SDS) is an important soybean [Glycine max (L) Merrill] disease caused by the soilborne fungus Fusarium virguliforme. Currently, 14 quantitative trait loci (QTL) had been confirmed associated with resistance or tolerance to SDS. The objective of the study was to evaluate usefulness of 10 of these QTL in controlling disease expression. Six populations were developed providing a total of 321 F2-derived lines for the study. Recombinant inbred lines (RIL) used as parents were obtained from populations of 'Essex' × 'Forrest' (EF), 'Flyer' × 'Hartwig' (FH), and 'Pyramid' × 'Douglas' (PD). Disease resistance was evaluated in the greenhouse at three different planting times, each with four replications, using sorghum infested with F. virguliforme homogeneously mixed in the soil (Luckew et al., Crop Sci 52:2215-2223, 2012). Four disease assessment criteria-foliar disease incidence (DI), foliar leaf scorch disease severity (DS), area under the disease progress curve (AUDPC), and root rot severity-were used. QTL were identified in more than one of the disease assessment criteria, mainly associated with lines in the most resistant categories. Five QTL (qRfs4, qRfs5, qRfs7, qRfs12, and Rfs16) were associated with at least one of the disease assessments across multiple populations. Of the five, qRfs4 was associated with DI, AUDPC, and root rot severity, and Rfs16 with AUDPC and root rot severity. The findings suggest it may be possible for plant breeders to focus on stacking a subset of the previously identified QTL to improve resistance to SDS in soybean.

Aggressiveness of Fusarium species and impact of root infection on growth and yield of soybeans

Fusarium spp. are commonly isolated from soybean roots but the pathogenic activity of most species is poorly documented. Aggressiveness and yield impact of nine species of Fusarium were determined on soybean in greenhouse (50 isolates) and field microplot (19 isolates) experiments. Root rot severity and shoot and root dry weights were compared at growth stages V3 or R1. Root systems were scanned and digital image analysis was conducted; yield was measured in microplots. Disease severity and root morphology impacts varied among and within species. Fusarium graminearum was highly aggressive (root rot severity >90%), followed by F. proliferatum and F. virguliforme. Significant variation in damping-off (20 to 75%) and root rot severity (<20 to >60%) was observed among F. oxysporum isolates. In artificially-infested microplots, root rot severity was low (<25%) and mean yield was not significantly reduced. However, there were significant linear relationships between yield and root symptoms for some isolates. Root morphological characteristics were more consistent indicators of yield loss than root rot severity. This study provides the first characterization of aggressiveness and yield impact of Fusarium root rot species on soybean at different plant stages and introduces root image analysis to assess the impact of root pathogens on soybean.

Investigation of the Fusarium virguliforme fvtox1 mutants revealed that the FvTox1 toxin is involved in foliar sudden death syndrome development in soybean

The soil borne fungus, Fusarium virguliforme, causes sudden death syndrome (SDS) in soybean, which is a serious foliar and root rot disease. The pathogen has never been isolated from the diseased foliar tissues; phytotoxins produced by the pathogen are believed to cause foliar SDS symptoms. One of these toxins, a 13.5-kDa acidic protein named FvTox1, has been hypothesized to interfere with photosynthesis in infected soybean plants and cause foliar SDS. The objective of this study is to determine if FvTox1 is involved in foliar SDS development. We created and studied five independent knockout fvtox1 mutants to study the function of FvTox1. We conducted Agrobacterium tumefaciens-mediated transformation to accomplish homologous recombination of FvTox1 with a hygromycin B resistance gene, hph, to generate the fvtox1 mutants. Approximately 40 hygromycin-resistant transformants were obtained from 10(6) conidial spores of the F. virguliforme Mont-1 isolate when the spores were co-cultivated with the A. tumefaciens EHA105 but not with LBA4044 strain carrying a recombinant binary plasmid, in which the hph gene encoding hygromycin resistance was flanked by 5'- and 3'-end FvTox1 sequences. We observed homologous recombination-mediated integration of hph into the FvTox1 locus among five independent fvtox1 mutants. In stem-cutting assays using cut soybean seedlings fed with cell-free F. virguliforme culture filtrates, the knockout fvtox1 mutants caused chlorophyll losses and foliar SDS symptoms, which were over twofold less than those caused by the virulent F. virguliforme Mont-1 isolate. Similarly, in root inoculation assays, more than a twofold reduction in foliar SDS development and chlorophyll losses was observed among the seedlings infected with the fvtox1 mutants as compared to the seedlings infected with the wild-type Mont-1 isolate. These results suggest that FvTox1 is a major virulence factor involved in foliar SDS development in soybean. It is expected that interference of the function of this toxin in transgenic soybean plants will lead to generation of SDS-resistant soybean cultivars.

First Report of Fusarium commune Causing Damping-off, Seed Rot, and Seedling Root Rot on Soybean (Glycine max) in the United States

During 2007 to 2009, symptomatic and asymptomatic soybean plants were collected from fields in 18 Iowa counties. Fusarium isolates were recovered from surface-sterilized root tissue on peptone PCNB agar (2). Single-spore isolates were transferred to synthetic low nutrient agar (SNA) overlain with pieces (1 × 2 cm) of sterile filter paper, and to potato dextrose agar (PDA), and placed in the dark for 10 to 14 days for morphological identification (4). Twenty-three isolates were identified as Fusarium commune K. Skovg., O'Donnell & Nirenberg, previously in the F. oxysporum species complex (4). Colonies on PDA had white, fluffy, aerial mycelium with magenta to violet pigmentation in the medium. On SNA, macroconidia, chlamydospores, and microconidia on monophialides and polyphialides were consistent with the species description (4). Identification of all 23 isolates was confirmed by DNA sequencing of the translation elongation factor (EF1-α) gene, using ef1 and ef2 primers, and the mitochondrial small subunit (mtSSU), using primers MS1 and MS2 (4) [GenBank accessions for two representative isolates: EF1-α (JX289892 and JX289893), and mtSSU (JX289894 and, JX289895)]. Pathogenicity of two representative isolates of F. commune was tested on soybean (cv. AG2403) in a greenhouse, in water baths set at 18°C, using autoclaved soil mixed with infested sand-cornmeal inoculum (3). The experiment entailed a completely randomized design (CRD) with five replications (single plant/150 ml cone) per treatment, and was conducted three times. Dry root and shoot weights, and root rot severity (visual estimate of percent root rot on the entire root system) were evaluated after 6 weeks. Mean seedling emergence in soil infested with F. commune was 47 and 40% for the two isolates; in contrast, non-inoculated control plants had 100% emergence. There were significant differences in root (P < 0.0001) and shoot (P < 0.0001) weights, and root rot severity (P < 0.0001), between inoculated and non-inoculated plants. Seedlings that emerged were severely stunted and had dark brown lesions. F. commune was reisolated from infected roots of inoculated plants, but not from non-inoculated plants. Pathogenicity of both isolates to soybean (cv. MN1805) was also tested using a petri dish assay, in which eight seeds were placed on a plate with a 4-day-old culture growing on 2% water agar (1). Plates were rated 7 days later for seed germination, seed rot, and lesion development, using an ordinal scale (1). The experiment entailed a CRD with three replicate plates/treatment, and was conducted three times. Germination of inoculated seeds ranged from 37.5 to 75.0%, and germinated seedlings had dark brown lesions on the taproots. There was a significant difference between isolates in the petri dish assay (P = 0.0030); one isolate was less aggressive, but both isolates resulted in significantly more disease than on the non-inoculated control plants, which had 100% germination and no symptoms (P < 0.0001). F. oxysporum is a known soybean pathogen (1), but isolates of F. commune may have been misidentified as F. oxysporum in previous studies. To our knowledge, this is the first report of F. commune as a pathogen on soybean in the U.S.A. References: (1) K. E. Broders et al. Plant Dis. 91:727, 2007. (2) J. F. Leslie and B. A. Summerell. The Fusarium Laboratory Manual. Blackwell Publishing, Oxford, UK, 2006. (3) G. P. Munkvold and J. K. O'Mara. Plant Dis. 86:143, 2002. (4) K. Skovgaard et al. Mycologia. 94:630, 2003.

First Report of Fusarium armeniacum Causing Seed Rot and Root Rot on Soybean (Glycine max) in the United States

In a survey for Fusarium root rot, soybean plants were sampled from eight counties across Iowa in 2008 to 2009. Fusarium isolates were recovered from surface-sterilized symptomatic and asymptomatic root tissue by culturing on peptone PCNB agar (2). Single-spore isolates were transferred to carnation leaf agar (CLA) and potato dextrose agar (PDA) for morphological identification; 11 isolates were identified as F. armeniacum (Forbes, Windels, and Burgess) Burgess and Summerell (previously F. acuminatum ssp. armeniacum) (2). Colonies on PDA produced white aerial mycelium, red to apricot pigment in agar, and bright orange sporodochia in the center of the culture. Some isolates produced a pionnotal form of slow-growing colonies with little aerial mycelium and abundant orange sporodochia. On CLA, macroconidia in orange sporodochia on carnation leaves and chlamydospores formed abundantly, but microconidia were absent (2). Species identity for the 11 isolates was confirmed by sequencing of the elongation factor gene (EF1-α) using ef1 and ef2 primers (4) (reference sequences deposited in GenBank JX101763 and JX101764). Pathogenicity of seven F. armeniacum isolates was tested using surface-sterilized soybean seed, cv. AG2403, in a petri dish assay with 3-day-old cultures on 2% water agar (1). Germination, seed rot, and lesion development were scored 7 dai using an ordinal scale (1). The experiment was a completely randomized design (CRD), had three replicate plates per isolate, and was conducted twice. All seven isolates were pathogenic on soybean, though variation in aggressiveness was observed among isolates (P < 0.0001) related to colony morphology on PDA. Seed germination was 0 to 40% when inoculated with four isolates showing white fluffy aerial mycelium on PDA. Seedlings were severely stunted with dark brown lesions covering a majority of the root system. When inoculated with three isolates showing the pionnotal form of slow-growing mycelium, germination was 70 to 100%, with few small brown lesions (~5 to 10 mm) on the roots. Noninoculated controls showed 100% germination and no symptoms. Pathogenicity was also tested in a growth chamber assay at 18°C using autoclaved soil mixed with an infested sand-cornmeal inoculum (3). Data for dry root and shoot weights and root rot severity (visually scored on a % scale) were collected at 6 weeks. The CRD experiment had five replications (single plant in a cone containing 150 ml infested soil), and was conducted twice. Root symptoms and similar variation in aggressiveness among isolates (based on colony morphology) was observed in inoculated plants. Isolates differed significantly for effects on root weight (P = 0.0125), shoot weight (P = 0.0035), and root rot severity (P = 0.0158). F. armeniacum was reisolated from infected root tissue, but not from noninoculated controls. Recovered isolates maintained their original colony morphology. F. armeniacum was previously reported in Minnesota on symptomless corn (2), but it has not been reported on soybean and its pathogenicity has not been established on any crop. To our knowledge, this is the first report of F. armeniacum as a pathogen on soybean in the United States. References: (1) K. E. Broders et al. Plant Dis. 91:727, 2007. (2) J. F. Leslie and B. A. Summerell. The Fusarium Laboratory Manual. Blackwell Publishing, Oxford, UK, 2006. (3) G. P. Munkvold and J. K. O'Mara. Plant Dis. 86:143, 2002. (4) K. O'Donnell et al. Proc. Natl. Acad. Sci. 95:2044, 1998.

First Report of Fusarium proliferatum Causing Root Rot on Soybean (Glycine max) in the United States

Fusarium spp. are widespread soilborne pathogens that cause important soybean diseases such as damping-off, root rot, Fusarium wilt, and sudden death syndrome. At least 12 species of Fusarium, including F. proliferatum, have been associated with soybean roots, but their relative aggressiveness as root rot pathogens is not known and pathogenicity has not been established for all reported species (2). In collaboration with 12 Iowa State University extension specialists, soybean roots were arbitrarily sampled from three fields in each of 98 Iowa counties from 2007 to 2009. Ten plants were collected from each field at V2-V3 and R3-R4 growth stages (2). Typical symptoms of Fusarium root rot (2) were observed. Symptomatic and asymptomatic root pieces were superficially sterilized in 0.5% NaOCl for 2 min, rinsed three times in sterile distilled water, and placed onto a Fusarium selective medium. Fusarium colonies were transferred to carnation leaf agar (CLA) and potato dextrose agar and later identified to species based on cultural and morphological characteristics. Of 1,230 Fusarium isolates identified, 50 were recognized as F. proliferatum based on morphological characteristics (3). F. proliferatum isolates produced abundant, aerial, white mycelium and a violet-to-dark purple pigmentation characteristic of Fusarium section Liseola. On CLA, microconidia were abundant, single celled, oval, and in chains on monophialides and polyphialides (3). Species identity was confirmed for two isolates by sequencing of the elongation factor (EF1-α) gene using the ef1 and ef2 primers (1). Identities of the resulting sequences (~680 bp) were confirmed by BLAST analysis and the FUSARIUM-ID database. Analysis resulted in a 99% match for five accessions of F. proliferatum (e.g., FD01389 and FD01858). To complete Koch's postulates, four F. proliferatum isolates were tested for pathogenicity on soybean in a greenhouse. Soybean seeds of cv. AG2306 were planted in cones (150 ml) in autoclaved soil infested with each isolate; Fusarium inoculum was applied by mixing an infested cornmeal/sand mix with soil prior to planting (4). Noninoculated control plants were grown in autoclaved soil amended with a sterile cornmeal/sand mix. Soil temperature was maintained at 18 ± 1°C by placing cones in water baths. The experiment was a completely randomized design with five replicates (single plant in a cone) per isolate and was repeated three times. Root rot severity (visually scored on a percentage scale), shoot dry weight, and root dry weight were assessed at the V3 soybean growth stage. All F. proliferatum isolates tested were pathogenic. Plants inoculated with these isolates were significantly different from the control plants in root rot severity (P = 0.001) and shoot (P = 0.023) and root (P = 0.013) dry weight. Infected plants showed dark brown lesions in the root system as well as decay of the entire taproot. F. proliferatum was reisolated from symptomatic root tissue of infected plants but not from similar tissues of control plants. To our knowledge, this is the first report of F. proliferatum causing root rot on soybean in the United States. References: (1) D. M. Geiser et al. Eur. J. Plant Pathol. 110:473, 2004. (2) G. L. Hartman et al. Compendium of Soybean Diseases. 4th ed. The American Phytopathologic Society, St. Paul, MN, 1999. (3) J. F. Leslie and B. A. Summerell. The Fusarium Laboratory Manual. Blackwell Publishing, Oxford, UK, 2006. (4) G. P. Munkvold and J. K. O'Mara. Plant Dis. 86:143, 2002.

Germination and Sporulation of Colletotrichum acutatum on Symptomless Strawberry Leaves

ABSTRACT The germination and sporulation of Colletotrichum acutatum were characterized over time on strawberry leaves (cv. Tristar) and plastic coverslips incubated at 26 degrees C under continuous wetness. Conidia germinated within 3 h after inoculation and formed melanized appressoria with pores by 9 h after inoculation. Host penetration was not observed up to 7 days after inoculation. Production of secondary conidia on conidial and hyphal phialides began within 6 h after inoculation. Secondary conidiation was responsible for up to a threefold increase in the total number of conidia within 7 days after inoculation. Primary conidia and hyphae began to collapse 48 h after inoculation, whereas melanized appressoria remained intact. These findings suggest that appressoria and secondary conidia of C. acutatum produced on symptomless strawberry foliage may be significant sources of inoculum for fruit infections.