Assessment of resistance to Plasmodiophora brassicae in swedes

A method is described in which swede seedlings inoculated with standardized concentrations of resting spores of specific populations of Plasmodiophora brassicae can be evaluated for resistance in different environments. Similar ranking for resistance of four swede cultivars inoculated with one pathogen population was obtained from seedling tests in a glasshouse, from young plants in a polythene tunnel, and from mature plants in field trials. Differential resistance of the four cultivars to two pathogen populations evident in glasshouse seedling tests was more clearly demonstrated in the field where there was a highly significant cuitivar × pathogen population interaction (p<0 001) for both root fresh weight and mean disease category.
The inclusion of uninoculated control plants in field trials enabled a direct comparison of yield with that from inoculated plants. Cultivars were defined as resistant in terms of yield if they did not suffer any crop loss in comparison with uninoculated plants, even though some plants showed restricted gall development. The distribution of fresh weight to galls, roots and shoots could be used to characterize the relative resistance of cultivars; in the most susceptible cultivars there was rapid gall development but little increase in root or shoot fresh weight between 6 and 12 weeks after inoculation. Differential response was determined during the 6 weeks following inoculation; the implications of this observation are discussed in relation to growth stage and rate of development of host and pathogen.     

Colonization of lettuce cultivars and rotation crops by Fusarium oxysporum f. sp. lactucae,the cause of fusarium wilt of lettuce

The severity of fusarium wilt is affected by inoculum density in soil, which is expected to decline during intervals when a non‐susceptible crop is grown. However, the anticipated benefits of crop rotation may not be realized if the pathogen can colonize and produce inoculum on a resistant cultivar or rotation crop. The present study documented colonization of roots of broccoli, cauliflower and spinach by Fusarium oxysporum f. sp. lactucae, the cause of fusarium wilt of lettuce. The frequency of infection was significantly lower on all three rotation crops than on a susceptible lettuce cultivar, and the pathogen was restricted to the cortex of roots of broccoli. However, F. oxysporum f. sp. lactucae was isolated from the root vascular stele of 7·4% of cauliflower plants and 50% of spinach plants that were sampled, indicating a greater potential for colonization and production of inoculum on these crops. The pathogen was also recovered from the root vascular stele of five fusarium wilt‐resistant lettuce cultivars. Thus, disease‐resistant plants may support growth of the pathogen and thereby contribute to an increase in soil inoculum density. Cultivars that were indistinguishable based on above‐ground symptoms, differed significantly in the extent to which they were colonized by F. oxysporum f. sp. lactucae. Less extensively colonized cultivars may prove to be superior sources of resistance to fusarium wilt for use in breeding programmes.     

The potential for the Rapid Screening of Potato Cultivars (Solanum tuberosum) for Resistance to Powdery Scab (Spongospora subterranea) using a Laboratory Bioassay

Powdery scab of potato, once established in a field, is difficult to control because of the longevity of the resting spores (cystosori) of the causal organism, Spongospora subterranea f.sp. subterranea. Host resistance is likely to be the most efficient in a long-term control strategy for preventing build-up of field inoculum and spread of the disease. Resistance screening of potato cultivars is mostly done in laborious field trials where disease development is likely to be unpredictable. A bioassay with potato tissue cultured plantlets and cystosori as inoculum is described and was tested for its potential to screen potato cultivars at an early stage for their relative susceptibility to powdery scab by comparing the lab results with field data. With cystosori inoculum of Swiss origin, the laboratory test showed clear differences between the potato cultivars in the severity of zoosporangial root infection which correlated better with ranked tuber infection data, compared to root galling. There are apparent differences in the relative trends in susceptibility between roots and tubers of five selected cultivars when using naturally infested soil instead of prepared cystosori as inoculum in the lab bioassay. Furthermore, differences in the severity of zoosporangial root infection of two selected cultivars were found when cystosori from different countries where used as inoculum. A possible host genotype × pathogen interaction is discussed. The bioassay has the potential to screen and select for resistant material at an early breeding stage thus making field trials not unnecessary but more economical. It will allow the use of a standard set of pathogen collections and facilitate testing for inoculum virulence in infested soils.     

Contrasted responses of Botrytis cinerea isolates developing on tomato plants grown under different nitrogen nutrition regimes

The nutritional status of a plant is known to influence its susceptibility to pathogens. In the case of Botrytis cinerea, the role of nitrogen fertilization of various host plants on disease development appears to be variable. This study was carried out to characterize possible variability associated with isolates and inoculum density of B. cinerea in its ability to infect leaf‐pruning wounds and to develop stem lesions on tomato plants, as affected by the nitrogen input. Six isolates differing in their aggressiveness to tomato were compared. They all had similar reaction patterns in vitro in response to differential nitrogen levels. In tests on plants grown with contrasted regimes of nitrate fertilization, overall disease severity was lower for all isolates on plants with higher nitrogen inputs, regardless of inoculum concentration. However, differences among isolates were observed in the effect of plant nitrogen nutrition on infection and on lesion expansion. Disease onset was delayed on all plants with higher nitrogen inputs, but the response was greater for isolates with lower aggressiveness on tomato. The highest contrast among isolates was observed with the colonization of stems. The daily rate of stem lesion expansion decreased with increasing nitrogen fertilization levels for the more aggressive isolates, while it increased for the less aggressive isolate. Hypotheses to explain these results are discussed in light of the possible physiological effects of nitrogen fertilization on nutrient availability for the pathogen in the host tissue and of possible production of defence metabolites by the plant.     

Effect of genotype unit number and spatial arrangement on severity of yellow rust in wheat cultivar mixtures

Pure stands of a yellow rust-susceptible wheat cultivar, pure stands of a resistant cultivar, and a 1 : 1 random mixture of resistant and susceptible cultivars were compared to populations in which strips or hills of the cultivars were alternated to attain genotype units (units of the same host genotype) that were larger in area than that of a single wheat plant. These four host populations were grown in plots of different sizes in order also to alter the number of units per host population. The goal was to determine if increasing the number of genotype units in mixed populations of large genotype units improved disease control relative to pure-line populations by increasing the amount of inoculum exchange among genotype units. Random mixtures of the two cultivars always provided better disease control than did alternating strips or hills. Evidence for an effect of genotype unit number on the efficacy of mixtures for rust control was found in only one of three experiments. Random mixtures of the two cultivars increased grain yield relative to the pure stand mean, but alternating strips did not.     

Development of a field biotest using artificial inoculation to evaluate resistance and yield effects in sugar beet cultivars against <Emphasis Type="Italic">Cercospora beticola</Emphasis>

Cercospora beticola resistance and disease yield loss relationships in sugar beet cultivars are best characterised under field conditions with heavy natural infection; this does not occur regularly under German climatic conditions. Since Cercospora resistance reduces the rate of pathogen development, high yield loss was observed in studies using artificial inoculation. Our study, therefore aimed to optimise inoculum density to obtain cultivar differentiation, which correlates to natural infection. In 2005 and 2006, field trials were carried out to determine the effect of different inoculum densities on Cercospora resistance of three sugar beet cultivars possessing variable resistance. The epidemic progress and white sugar yield loss (WSY_loss) were determined and their relationship evaluated. An optimal inoculum concentration range (between 10,000–20,000 infectious Cercospora units ml⁻¹ inoculum suspension) was determined which allowed maximum resistance parameter differentiation in terms of C. beticola disease severity (DS), area under the disease progress curve (AUDPC) and WSY_loss. The correlation between AUDPC and WSY_loss was identical for all cultivars independent of the resistance level, demonstrating that tolerant reactions of the cultivars under study were not detectable. This study provides evidence that even under optimal inoculum levels necessary to obtain maximum differentiation between cultivars, climatic conditions are important for disease management, but remain unpredictable, indicating that artificial inoculation needs to be optimised, but that single field locations are not sufficient and reliable to evaluate Cercospora resistance.     

Factors Affecting Pseudomonas savastanoi pv. savastanoi Plant Inoculations and Their Use for Evaluation of Olive Cultivar Susceptibility

ABSTRACT Pseudomonas savastanoi pv. savastanoi causes olive knot disease, which is present in most countries where olive trees are grown. Although the use of cultivars with low susceptibility may be one of the most appropriate methods of disease control, little information is available from inoculation assays, and cultivar susceptibility assessments have been limited to few cultivars. We have evaluated the effects of pathogen virulence, plant age, the dose/response relationship, and the induction of secondary tumors in olive inoculation assays. Most P. savastanoi pv. savastanoi strains evaluated were highly virulent to olive plants, but interactions between cultivars and strains were found. The severity of the disease in a given cultivar was strongly dependent of the pathogen dose applied at the wound sites. Secondary tumors developed in noninoculated wounds following inoculation at another position on the stem, suggesting the migration of the pathogen within olive plants. Proportion and weight of primary knots and the presence of secondary knots were evaluated in 29 olive cultivars inoculated with two pathogen strains at two inoculum doses, allowing us to rate most of the cultivars as having either high, medium, or low susceptibility to olive knot disease. None of the cultivars were immune to the disease.     

Effects of inoculum density,leaf wetness duration and nitrate concentration on the occurrence of lettuce leaf spot

In Ehime Prefecture, Japan, lettuce leaf spot (Septoria lactucae) caused huge losses in marketable lettuce yields. To explore potential measures to control disease outbreaks, the effects of inoculum density, leaf wetness duration and nitrate concentration on the development of leaf spot on lettuce (Lactuca sativa) were evaluated. Conidia were collected from diseased plants in an infested field by single-spore isolation and were used to inoculate potted lettuce plants with different conidial concentrations. Lesions developed on inoculated lettuce plants at inoculum concentrations from 10⁰ to 10⁶ conidia/ml. The disease was more severe when the inoculum exceeded 10² conidia/ml, and severity increased with increasing concentrations. Assessment of the relationship between disease development and the duration of postinoculation leaf wetness revealed that symptoms appeared when the inoculated plants remained wet for 12 h or longer. The number of lesions and total nitrogen content in the lettuce leaves both increased when nitrate was applied.     

Effects of inoculum density,plant age and temperature on disease severity caused by pythiaceous fungi on several plants

Alfalfa, maize, sorghum and sugarbeet plants were inoculated with zoospores ofPhytophthora andPythium species in order to assess the effects of inoculum density, plant age and temperature on disease severity. Seedlings were grown axenically in test tubes and inoculated with zoospore suspensions. Disease severity was assessed by measuring the root growth and discoloration of treated and control seedlings. The incremental root length of all plants decreased and root discoloration increased as inoculum concentration of the pathogen increased. Changes were more intensive among low levels of zoospore concentrations and no significant differences in disease severity were found for inoculum densities higher than 10⁴ zoospores ml^-1. Disease severity was negatively related to plant age. Disease development on sugarbeet seedlings infected withPythium andPhytophthora species was affected by temperature, but the pattern of response was determined by the pathogen’s temperature preferences. The incremental root length decreased as temperature increased up to 25°C. The effect ofPythium dissimile andPhytophthora cactorum on root length was significantly lower at 35°C than at 25°C, whereasPythium aphanidermatum andPhytophthora nicotianae caused significant damage to roots even at 35°C. http://www.phytoparasitica.org posting Dec. 3, 2001.     

Virulence and inoculum density‐dependent interactions between clubroot resistant canola (Brassica napus) and Plasmodiophora brassicae

To mitigate the impact and dissemination of clubroot in western Canada, canola (Brassica napus) producers have relied on clubroot resistance traits. However, in 2013 and 2014, new strains of the clubroot pathogen, Plasmodiophora brassicae, emerged that are virulent on most clubroot‐resistant (CR) canola genotypes. Novel strains of the pathogen were inoculated onto two susceptible canola cultivars, one resistant line and six CR cultivars. Although all cultivars/lines showed a susceptible response to inoculation with the new strains of P. brassicae, the severity of disease reaction, root hair infection rates and the amount of P. brassicae DNA present in each canola genotype varied depending on the strain. In addition, the effect of inoculum density on disease severity and gall formation was recorded for one of these new strains on a universally susceptible Chinese cabbage cultivar and one susceptible and 10 resistant canola genotypes. Although root galls were observed at an inoculum density of 10³ spores per mL of soil, clear differentiation of susceptible and resistant reactions among canola cultivars/lines was not observed until the inoculum density reached 10⁵ spores mL^?1. At a spore density of 10⁶ spores mL^?1 and above, all cultivars/lines developed susceptible reactions, although there was some differentiation in the degree of reaction. This study shows the potential to develop a unique disease profile for emergent clubroot pathotypes and shows a useful range of spore densities at which to study new P. brassicae strains.     

Quantification of within‐season focal spread of wheat take‐all in relation to pathogen genotype and host spatial distribution

Within season gradient of wheat take‐all was measured in field experiments according to line or random sowing host spatial distribution, and two Gaeumannomyces graminis var. tritici (Ggt) isolates (G1i and G2i), representative of the G1 and G2 genotype groups in terms of aggressiveness. Root disease incidence and severity were assessed at six dates from early March to late June on plants located at regular distances from the inoculum sources. Simple models relating disease intensity at different levels of hierarchy fitted observed data well, and indicated a strong disease aggregation both within and among plants. Disease severity on source plants placed nearby the inoculum source increased over time, ranging from 5 to 46% at the first assessment, and from 55 to 98% at the last assessment, being in general larger for G2i than G1i. In line‐sown plots, disease progressed steadily along the line but did not extend beyond 20 cm, seldom reaching the neighbour line. Disease rarely reached 25 cm in the direct‐seeded crop stands. These results indicate that Ggt intensifies but does not spread to a large extent during a cropping season. Distance from the source, pathogen genotype and assessment date had a significant effect on disease severity according to mixed model analyses, disease spread being larger for G2i than G1i. However, no significant effect of host spatial distribution could be detected. Yield loss within 20 cm of the source plant ranged between 20 and 40%, and was not significantly affected by pathogen genotype or host spatial distribution.     

Interactions between Fusarium oxysporum f. sp. tracheiphilum and Meloidogyne spp. in Vigna unguiculata. 1. Effects of different inoculum densities on Fusarium wilt

The effects of chlamydosporesandconidia of Fusarium oxysporum f sp. tracheiphilum at different initial spore concentrations were compared in the wilt-susceptible cowpea cultivar California Blackeye No. 5 (CB5). In glasshouse experiments with one inoculum density of either Meloidogyne incognita or M javanica, chlamydospores resulted in greater incidence and severity of Fusarium wilt than conidia at the same inoculum densities. Wilt symptoms also increased on wilt-resistant cultivar CB3 as inoculum densities of M. javanica were increased. When three cultivars were infested with moderate or high densities of both F. o. tracheiphilum and M. javanica. only CB5 developed sere wilt at either inoculum density. The wilt-tolerant cultivar Grant had mild wilt symptoms in most plants at moderate inoculum densities, and a tenfold increase in inoculum did not increase wilt ratings. CB3. however, had higher incidence and severity of Fusarium wilt symptoms at high inoculum densities, although 60% of the plants survived for 9 weeks.     

Population Dynamics of Fusarium Oxysporum f. Sp. Radicis-lycopersici in Relation to the Onset of Fusarium Crown and Root Rot of Tomato

Fusarium oxysporum f. sp. radicis-lycopersici the causal agent of crown and root rot in tomato comprises two overlapping separate phases: monocyclic and polycyclic. Oversummering inoculum is the source of primary infection (the monocyclic phase) and the spread from plant to plant via root-to-root contact is the source of the secondary infection (the polycyclic phase). In the present work, relationships between initial inoculum density, population dynamics of the pathogen in the root zone of diseased plants, and disease onset were studied. For the monocyclic phase, 55.1% of the variance of disease onset was attributed to the rate of pathogen proliferation in the root zone of plants, and only 12.8% of the variance was attributed to the amount of initial inoculum density. For the polycyclic phase, disease onset was not related to either initial inoculum density or the rate of pathogen proliferation in the root zone. At disease onset, the inoculum density of the pathogen in the root zone of plants infected from oversummering inoculum reached an average of 4.08 log cfu g soil^–1. The inoculum density of the pathogen in the root zone of plants infected by their diseased neighbors was 3.23 log cfu g soil^–1. A large variation in pathogen proliferation rate in the root zone was found among individual plants, suggesting that differences in the level of soil suppressiveness may occur not only between fields, but even in the same field over short distances.     

Fusarium crown and root rot of tomatoes in the UK

Fusarium crown and root rot caused by Fusarium oxysporum f. sp. radicis-lycopersici was found in the UK in 1988 and 1989 mainly in rockwool-grown tomato crops. Up to 14% of plants were affected in individual crops. In experiments, leaf and stem symptoms did not appear until the time of first fruit harvest even when the plants were inoculated at planting, first flowers or fruit set. Conidial inoculum at 10⁶ spores/plant applied at seed sowing killed 70–83% of tomato seedlings, whereas similar levels of inoculum applied to young plants caused root and basal stem decay, and eventually death but only after fruit harvest began. Disease incidence and symptom severity increased with inoculum concentration. Experimentally, the disease was more severe in peat- or compost-grown plants than in rockwool. Disease spread was only a few centimetres in 50 days in experimental rockwool-grown plants. All tomato cultivars tested were highly susceptible. Prochloraz-Mn was highly effective against the pathogen in vitro and controlled the disease in the glasshouse, but only when applied preventively. Non-pathogenic Fusarium oxysporum isolates and Trichoderma harzianum also reduced FCRR disease levels.     

Automated image processing framework for analysis of the density of fruiting bodies of Leptosphaeria maculans on oilseed rape stems

Understanding the transmission of plant pathogen inoculum during the periods when the host plants are not present is crucial for predicting the initiation of epidemics and optimizing mitigation strategies. However, inoculum production at the end of the cropping season, survival during the intercrop period, and the emergence or release of inoculum can be highly variable, difficult to assess, and generally inferred indirectly from symptom data. As a result, a lack of large datasets hampers the study of these epidemiological processes. Here, inoculum production was studied in Leptosphaeria maculans, the cause of phoma stem canker of oilseed rape. The fungus survives on stubble left in the field, from which ascospores are released at the beginning of the next cropping season. An image processing framework was developed to estimate the density of fruiting bodies produced on stem pieces following incubation in field conditions, and a quality assessment of the processing chain was performed. A total of 2540 standardized RGB digital images of stems were then analysed, collected from 27 oilseed rape fields in Brittany over four cropping seasons. Manual post-processing removed 16% of the pictures, e.g. when moisture-induced darkening of the oilseed rape stems caused overestimation of the area covered with fruiting bodies. The potential level of inoculum increased with increasing phoma stem canker severity at harvest, and depended on the source field and the cropping season. This work shows how image-based phenotyping generates high-throughput disease data, opening up the prospect of substantially increased precision in epidemiological studies.     

Effect of planting density of maize on the progress and spread of northern leaf blight fromExserohilum turcicum infested residue source

Effect of plant density (37,037, 44,444 and 55,555 plants/ha) on the increase of northern leaf blight in time and space on two maize cultivars planted at spacings of 90, 75 and 60 between rows and 30 cm within rows was investigated.Exserohilum turcicum infested maize residue was used as an inoculum source. Maize density did not significantly affect the disease indices, but significantly influenced the grain yield of the two cultivars. In contrast, the two cultivars differed significantly (P0.01) in disease severity, but not grain yield. Higher disease severities and grain yields were associated with higher plant densities, whereas the apparent infection rate was lower in higher plant density. Distance from the maize residue (inoculum source) significantly influenced disease severity. The percentage leaf area blighted, area under disease progress curve and disease progress curve intercept decreased with distance from the maize residue area, but the apparent infection rate on EV8429-SR appeared to increase with distance. Disease gradients (b) were higher in closely planted maize and flattened with time in one location only.     

Relationships Between Verticillium dahliae Inoculum Density and Wilt Incidence, Severity, and Growth of Cauliflower

ABSTRACT Microplot and field experiments were conducted to evaluate the effects of inoculum density on Verticillium wilt and cauliflower growth. Soil containing Verticillium dahliae microsclerotia was mixed with various proportions of fumigated soil to establish different inoculum densities (fumigated soil was used as the noninfested control). Seven inoculum density treatments replicated four times were established, and the treatments were arranged in a randomized complete block design. Soil was collected from each microplot immediately after soil infestation for V. dahliae assay by plating onto sodium polypectate agar (NP-10) selective medium using the Anderson sampler technique. Five-week-old cauliflower was transplanted into two beds within each 1.2- by 1.2-m microplot. At the same time, several extra plants were also transplanted at the edge of each bed for destructive sampling to examine the disease onset (vascular discoloration) after planting. Cauliflower plants were monitored for Verticillium wilt development. Stomatal resistance in two visually healthy upper and two lower, diseased leaves in each microplot was measured three times at weekly intervals after initial wilt symptoms occurred. At maturity, all plants were uprooted, washed free of soil, and wilt incidence and severity, plant height, number of leaves, and dry weights of leaves and roots were determined. The higher the inoculum density, the earlier was disease onset. A density of 4 microsclerotia per g of dry soil caused 16% wilt incidence, but about 10 microsclerotia per g of soil caused 50% wilt incidence. Both wilt incidence and severity increased with increasing inoculum density up to about 20 microsclerotia per g of soil, and additional inoculum did not result in significantly higher disease incidence and severity. A negative exponential model described the disease relationships to inoculum levels under both microplot and field conditions. Stomatal resistance of diseased leaves was significantly higher at higher inoculum densities; in healthy leaves, however, no treatment differences occurred. The height, number of leaves, and dry weights of leaves and roots of plants in the fumigated control were significantly higher than in infested treatments, but the effects of inoculum density treatments were variable between years. Timing of cauliflower infection, crop physiological processes related to hydraulic conductance, and wilt intensity (incidence and severity) were thus affected by the inoculum density. Verticillium wilt management methods used in cauliflower should reduce inoculum density to less than four micro-sclerotia per g of soil to produce crops with the fewest number of infected plants.     

Canopy Size and Induced Resistance in Stylosanthes scabra Determine Anthracnose Severity at High CO(2)

ABSTRACT This study examines the relative importance of canopy size and induced resistance to Colletotrichum gloeosporioides at 350- and 700-ppm atmospheric CO(2) concentrations on susceptible Stylosanthes scabra 'Fitzroy' from two studies in a controlled environment facility (CEF) and in the field. Plants were grown at the two CO(2) concentrations in a repeated experiment in the CEF and inoculated at 6, 9, or 12 weeks of age. Although the physiological maturity of plants was at a similar stage for all three ages, the number of lesions per plant increased with increasing plant age at both CO(2) concentrations. At 350 ppm, the increase was associated with canopy size and increasing infection efficiency of the pathogen, but at 700 ppm, it was associated only with canopy size, because infection efficiency did not change with increasing age. A level of resistance was induced in plants at 700 ppm CO(2). In a second study, plants were raised for 12 to 14 weeks at the two CO(2) concentrations in the CEF and exposed to C. gloeosporioides inoculum in replicated field plots under ambient CO(2) over three successive years. Fitzroy developed a dense and enlarged canopy, with 28 to 46% more nodes, leaf area, and aboveground biomass at high CO(2) than at low CO(2). Up to twice as many lesions per plant were produced in the high CO(2) plants, because the enlarged canopy trapped many more pathogen spores. The transient induced resistance in high CO(2) plants failed to operate when exposed to pathogen inoculum under ambient CO(2) in the field. These results highlight the need to consider both canopy size and host resistance in assessing the influence of elevated CO(2) on plant disease.     

Quantifying Aphanomyces euteiches in Alfalfa with a Fluorescent Polymerase Chain Reaction Assay

ABSTRACT A polymerase chain reaction (PCR) assay using a set of specific primers and a dual-labeled probe (TaqMan) was developed to quantify the amount of Aphanomyces euteiches DNA in alfalfa plants exhibiting varying levels of disease severity. The study included isolates of race 1 and race 2 of A. euteiches. The assay also discriminated between alfalfa populations for resistance based on analysis of DNA extracted from bulked plant samples. Analysis of individual plants and bulked plant samples of standard check populations with both pathogen isolates resulted in Spearman rank correlations between pathogen DNA content and disease severity index ratings that were greater than 0.75 and highly significant (P < 0.0005). In experiments with a race 1 isolate, the amount of pathogen DNA present in the resistant check WAPH-1 was significantly less than in the susceptible check Saranac. In experiments with a race 2 isolate, the amount of pathogen DNA in the resistant check WAPH-5 was significantly less than in either of the susceptible checks, Saranac and WAPH-1. Discrimination between commercial cultivars based on quantitative PCR analysis of bulked plant samples was similar to classification based on visual assessment of disease severity.     

Temporal dynamics of root and foliar severity of soybean sudden death syndrome at different inoculum densities

Temporal dynamics of soybean sudden death syndrome (SDS) root and foliar disease severity were studied in growth chamber experiments on susceptible plants exposed to different inoculum densities (0, 10⁰, 10¹, 10², and 10³ conidia g⁻¹ soil) of Fusarium virguliforme. The monomolecular model provided the best fit to describe the progress of root and foliar disease severity over time. Disease severity and area under disease progress curve (AUDPC) both increased in response to increasing inoculum density (P < 0.01), particularly for foliar symptoms. Rate of disease progress increased as inoculum densities increased for both root and foliar disease severities. The incubation period for root and foliar disease severity ranged from 9 to 18 and 15 to 25 days, respectively. Significant differences in root rot severity were most easily detected during the early stages of infection, whereas root rot and foliar severities were only weakly correlated when both were assessed simultaneously at later stages of disease development. Root rot severity assessments performed 15 to 20 days after inoculation (DAI) were most highly correlated (r > 0.9, P < 0.01) with foliar disease severity assessments performed 30 to 50 DAI. Root biomass was reduced by up to 67% at the three highest inoculum densities, indicating the aggressiveness that F. virguliforme possesses as a root rot pathogen on soybeans.