Virulence of Rhizoctonia oryzae on Wheat and Barley Cultivars from the Pacific Northwest

Economic Loss from Rhizoctonia Bare Patch in Rainfed Winter Wheat and Spring Barley in Oregon

Economic loss from Rhizoctonia bare patch, caused by Rhizoctonia solani AG-8, was estimated in two 50-ha fields on a single farm. A winter wheat crop was managed as a conventionally cultivated 2-year wheat/fallow rotation and a spring barley crop was managed as a no-till annual crop. Aerial photographs revealed that the patch-affected area was nearly double in barley (17%) compared with wheat (9%). Yield inside patches was reduced by 73 and 68% for wheat and barley, respectively. Grain produced on each field was reduced more for winter wheat (21.6 metric tons [t], valued at US$5,080) than for spring barley (16.8 t, valued at US$2,784). More precise estimates of economic damage and more robust management practices for Rhizoctonia bare patch must be developed.

Waitea circinata var. zeae Causing Root Rot of Cabbage and Oilseed Rape

Cabbage, a widely used and popular vegetable, and oilseed rape, the second most valuable oilseed crop in the world, are two important species from the Brassicaceae family. Two geographically separated outbreaks of cabbage and oilseed rape root rot with estimated incidence of 15 and 20%, respectively, were recorded during 2017 in the Vojvodina region, Serbia. Twelve hyphal-tip isolates were obtained from symptomatic cabbage and oilseed rape plants and identified as Waitea circinata var. zeae based on morphological and molecular features. This indicates that W. circinata var. zeae has expanded its host range to the Brassicaceae family. Sequence analyses of internal transcribed spacer (ITS) and large subunit of the ribosomal DNA, RPB2, and β-tubulin genes revealed the highest similarity with multiple W. circinata var. zeae. Neighbor-joining analyses of ITS sequences resulted in a phylogenetic tree with one well-defined branch of W. circinata var. zeae, with two separate groups. All Serbian isolates and the majority of isolates originating from natural infection of dicotyledonous plants grouped together in group I. Following artificial inoculation, W. circinata var. zeae isolates caused mild to medium root necrosis of seedlings of 2 monocotyledonous and 12 dicotyledonous plant species, implying a wider host range than was known for W. circinata var. zeae. Additionally, this is the first occurrence of W. circinata var. zeae on dicotyledonous host plants in Europe. Because cabbage and oilseed rape are important crops grown worldwide, the occurrence of this new soilborne pathogen with a broad host range imposes the necessity for changes in routine disease control practices, particularly crop rotation.

Seed Treatment Effects on Maize Seedlings Coinfected with Rhizoctonia solani and Pratylenchus penetrans

The roots of maize seedlings typically are attacked by a complex of organisms that includes fungal pathogens and plant-parasitic nematodes but few studies have examined the effects of these organisms in combination. Rhizoctonia solani can be an important component of the seedling disease complex; like other fungi, its effect on the plant may be influenced by the activity of nematodes such as the root-lesion nematode Pratylenchus penetrans. In this study, we assessed the impact of seed treatments, including fungicide-nematicide combinations, on maize seedlings exposed to R. solani and P. penetrans alone or in combination. In growth-chamber and greenhouse experiments, seed treated with various active ingredient combinations were planted in an autoclaved sand-soil mixture with or without inoculum of R. solani. In some treatments, a suspension of P. penetrans adults and juveniles was added to the sand-soil mixture. In the greenhouse experiments, infection by R. solani caused dramatic reductions in root length, volume, surface area, and numbers of root tips and root forks, whereas P. penetrans infestation alone reduced only shoot fresh weight. Statistical interactions between the effects of the two organisms were not significant, although fungal infestation significantly reduced the numbers of nematodes extracted from roots. Seed treatments significantly improved most root development variables, and the combination that included four fungicides, thiamethoxam, and abamectin was the best treatment for most variables. Results were similar in the growth-chamber experiments, where R. solani caused significant reductions in nearly all shoot and root development measurements, and seed treatment with sedaxane, alone or combined with abamectin, consistently provided the best results. R. solani was more damaging to seedlings than P. penetrans, and the combination of the two organisms did not cause more damage than R. solani alone. Seed-treatment active ingredients that specifically targeted R. solani (sedaxane) and P. penetrans (abamectin) had large positive effects on seedling health, causing significant improvements in root and shoot growth and development compared with untreated seedlings exposed to these pathogens.

Characterizing and Mapping Resistance in Synthetic-Derived Wheat to Rhizoctonia Root Rot in a Green Bridge Environment

Root rot caused by Rhizoctonia spp. is an economically important soilborne disease of spring-planted wheat in growing regions of the Pacific Northwest (PNW). The main method of controlling the disease currently is through tillage, which deters farmers from adopting the benefits of minimal tillage. Genetic resistance to this disease would provide an economic and environmentally sustainable resource for farmers. In this study, a collection of synthetic-derived genotypes was screened in high-inoculum and low-inoculum field environments. Six genotypes were found to have varying levels of resistance and tolerance to Rhizoctonia root rot. One of the lines, SPBC-3104 ('Vorobey'), exhibited good tolerance in the field and was crossed to susceptible PNW-adapted 'Louise' to examine the inheritance of the trait. A population of 190 BC₁-derived recombinant inbred lines was assessed in two field green bridge environments and in soils artificially infested with Rhizoctonia solani AG8. Genotyping by sequencing and composite interval mapping identified three quantitative trait loci (QTL) controlling tolerance. Beneficial alleles of all three QTL were contributed by the synthetic-derived genotype SPCB-3104.

Molecular Characterization, Morphological Characteristics, Virulence, and Geographic Distribution of Rhizoctonia spp. in Washington State

Rhizoctonia root rot and bare patch, caused by Rhizoctonia solani anastomosis group (AG)-8 and R. oryzae, are chronic and important yield-limiting diseases of wheat and barley in the Inland Pacific Northwest (PNW) of the United States. Major gaps remain in our understanding of the epidemiology of these diseases, in part because multiple Rhizoctonia AGs and species can be isolated from the same cereal roots from the field, contributing to the challenge of identifying the causal agents correctly. In this study, a collection totaling 498 isolates of Rhizoctonia was assembled from surveys conducted from 2000 to 2009, 2010, and 2011 over a wide range of cereal production fields throughout Washington State in the PNW. To determine the identity of the isolates, PCR with AG- or species-specific primers and/or DNA sequence analysis of the internal transcribed spacers was performed. R. solani AG-2-1, AG-8, AG-10, AG-3, AG-4, and AG-11 comprised 157 (32%), 70 (14%), 21 (4%), 20 (4%), 1 (0.2%), and 1 (0.2%), respectively, of the total isolates. AG-I-like binucleate Rhizoctonia sp. comprised 44 (9%) of the total; and 53 (11%), 80 (16%), and 51 (10%) were identified as R. oryzae genotypes I, II, and III, respectively. Isolates of AG-2-1, the dominant Rhizoctonia, occurred in all six agronomic zones defined by annual precipitation and temperature within the region sampled. Isolates of AG-8 also were cosmopolitan in their distribution but the frequency of isolation varied among years, and they were most abundant in zones of low and moderate precipitation. R. oryzae was cosmopolitan, and collectively the three genotypes comprised 37% of the isolates. Only isolates of R. solani AG-8 and R. oryzae genotypes II and III (but not genotype I) caused symptoms typically associated with Rhizoctonia root rot and bare patch of wheat. Isolates of AG-2-1 caused only mild root rot and AG-I-like binucleate isolates and members of groups AG-3, AG-4, and AG-11 showed only slight or no discoloration of the roots. However, all isolates of AG-2-1 caused severe damping-off of canola, resulting in 100% mortality. Isolates of Rhizoctonia AG-8, AG-2-1, AG-10, AG-I-like binucleate Rhizoctonia, and R. oryzae genotypes I, II, and III could be distinguished by colony morphology on potato dextrose agar, by PCR with specific primers, or by the type and severity of disease on wheat and canola seedlings, and results of these approaches correlated completely. Based on cultured isolates, we also identified the geographic distribution of all of these Rhizoctonia isolates in cereal-based production systems throughout Washington State.

Stunted Patches in Onion Bulb Crops in Oregon and Washington: Etiology and Yield Loss

Onion stunting caused by Rhizoctonia spp. is an important soilborne disease on very sandy soils in the Columbia Basin of Oregon and Washington. From 2010 to 2013, 251 isolates of Rhizoctonia or Rhizoctonia-like spp. were obtained from soil and onion plant samples collected from inside and outside patches of stunted plants in 29 onion fields in the Columbia Basin. Sequence analysis of the internal transcribed spacer (ITS) region was used to identify the isolates, with 13 anastomosis groups (AGs) or subspecies detected. The most frequent was Waitea circinata var. circinata (25%), followed by Rhizoctonia solani AG 3 (17%), R. solani AG 4 (14%), Ceratobasidium sp. AG A (10%), R. solani AG 8 (7%), Ceratobasidium sp. AG K (6%), R. solani AG 2-1 (6%), W. circinata var. zeae (6%), R. solani AG 5 (4%), Ceratobasidium sp. AG G (2%), R. solani AG 11 (2%), and R. solani AG 1-1B and AG 10 (each <1%). However, the distribution of AGs and subspecies varied depending on whether soil or onion plants samples were collected within or adjacent to patches of stunted onion plants. In an attempt to predict the risk of onion stunting for a field prior to planting, DNA concentrations of AG 2-1, AG 3, AG 4, and AG 8 were quantified from bulk soil samples collected from each of nine growers' fields approximately 1 month before onion sowing in 2012. The preplant DNA concentrations did not show a significant association with the amount of stunting observed in the fields during the growing season. In contrast, the frequency of isolation and DNA concentration of R. solani AG 8 detected in soil samples collected during the growing season were greater from inside patches of stunted onion plants than from adjacent healthy areas of an onion crop sampled in 2012, but not for soil samples collected similarly from an onion crop in 2013. AG 2-1, AG 3, and AG 4 DNA concentrations did not differ significantly in soil sampled inside versus outside stunted patches in the fields sampled in 2012 and 2013. Relationships between the number of bulbs harvested or bulb weight versus severity of stunting were defined using correlation and regression analyses for six onion cultivars grown in seven fields surveyed in 2012 and 2013. Onion stunting reduced the average marketable bulb yield by 25 to 60% within stunted patches of the six cultivars. Stunting did not reduce onion plant stand but consistently reduced the size of bulbs, and yield reduction increased with increasing disease severity.

Molecular and genetic aspects of controlling the soilborne necrotrophic pathogens Rhizoctonia and Pythium

The soilborne necrotrophic pathogens Rhizoctonia and Pythium infect a wide range of crops in the US and worldwide. These pathogens pose challenges to growers because the diseases they cause are not adequately controlled by fungicides, rotation or, for many hosts, natural genetic resistance. Although a combination of management practices are likely to be required for control of Rhizoctonia and Pythium, genetic resistance remains a key missing component. This review discusses the recent deployment of introduced genes and genome-based information for control of Rhizoctonia, with emphasis on three pathosystems: Rhizoctonia solani AG8 and wheat, R. solani AG1-IA and rice, and R. solani AG3 or AG4 and potato. Molecular mechanisms underlying disease suppression will be addressed, if appropriate. Although less is known about genes and factors suppressive to Pythium, pathogen genomics and biological control studies are providing useful leads to effectors and antifungal factors. Prospects for resistance to Rhizoctonia and Pythium spp. will continue to improve with growing knowledge of pathogenicity strategies, host defense gene action relative to the pathogen infection process, and the role of environmental factors on pathogen-host interactions.

Interactions between the root pathogen Rhizoctonia solani AG-8 and acetolactate-synthase-inhibiting herbicides in barley

BACKGROUND

The widespread acceptance of reduced-tillage farming in cereal cropping systems in the Pacific Northwest of the United States has resulted in increased use of herbicides for weed control. However, soil residual concentrations of widely used imidazalone herbicides limit the cultivation of barley, which is more sensitive than wheat. In addition, increased severity of the root rot disease caused by Rhizoctonia solani is associated with reduction in tillage. Many crops exhibit altered disease responses after application of registered herbicides. In this study, the injury symptoms in barley caused by sublethal rates of two acetolactate synthase (ALS)-inhibiting herbicides, imazamox and propoxycarbazone-sodium, were assessed in factorial combinations with a range of inoculum concentrations of the root rot pathogen Rhizoctonia solani AG-8.

RESULTS

Both herbicides and pathogen had negative impacts on plant growth parameters such as root and shoot dry weight, shoot height and first leaf length, and interactions between pathogen and herbicide were detected.

CONCLUSIONS

The results suggested that sublethal rates of herbicides and R. solani could alter severity of injury symptoms, possibly owing to the herbicide predisposing the plant to the pathogen.

Optimum Timing of Preplant Applications of Glyphosate to Manage Rhizoctonia Root Rot in Barley

Rhizoctonia root rot, caused by Rhizoctonia solani AG-8 and R. oryzae, is considered one of the main deterrents for farmers to adopt reduced-tillage systems in the Pacific Northwest. Because of the wide host range of Rhizoctonia spp., herbicide application before planting to control weeds and volunteer plants is the main management strategy for this disease. To determine the effect of timing of glyphosate applications on the severity of Rhizoctonia root rot of barley, field experiments were conducted in 2007, 2008, and 2009 in a field naturally infested with a high level of both R. solani and R. oryzae. Crop volunteer plants and weeds were allowed to grow over the winter and plots were sprayed with glyphosate at 42, 28, 14, 7, and 2 days prior to planting. As the herbicide application interval increased, there were significant increases in shoot length, length of the first true leaf, and number of healthy seminal roots and a decrease in disease severity. Yield and the number of seminal roots did not show a response to herbicide application interval in most years. The activity of R. solani, as measured by toothpick bioassay and real-time polymerase chain reaction, declined over time in all treatments after planting barley. The herbicide application interval required to meet 80 and 90% of the maximum response (asymptote) for all plant and disease measurements ranged from 11 to 27 days and 13 to 37 days, respectively. These times are the minimum herbicide application intervals required to reduce disease severity in the following crop.

Scarlet-Rz1, an EMS-generated hexaploid wheat with tolerance to the soilborne necrotrophic pathogens Rhizoctonia solani AG-8 and R. oryzae

The necrotrophic root pathogens Rhizoctonia solani AG-8 and R. oryzae cause Rhizoctonia root rot and damping-off, yield-limiting diseases that pose barriers to the adoption of conservation tillage in wheat production systems. Existing control practices are only partially effective, and natural genetic resistance to Rhizoctonia has not been identified in wheat or its close relatives. We report the first genetic resistance/tolerance to R. solani AG-8 and R. oryzae in wheat (Triticum aestivum L. em Thell) germplasm 'Scarlet-Rz1'. Scarlet-Rz1 was derived from the allohexaploid spring wheat cultivar Scarlet using EMS mutagenesis. Tolerant seedlings displayed substantial root and shoot growth after 14 days in the presence of 100-400 propagules per gram soil of R. solani AG-8 and R. oryzae in greenhouse assays. BC(2)F(4) individuals of Scarlet-Rz1 showed a high and consistent degree of tolerance. Seedling tolerance was transmissible and appeared to be dominant or co-dominant. Scarlet-Rz1 is a promising genetic resource for developing Rhizoctonia-tolerant wheat cultivars because the tolerance trait immediately can be deployed into wheat breeding germplasm through cross-hybridization, thereby avoiding difficulties with transfer from secondary or tertiary relatives as well as constraints associated with genetically modified plants. Our findings also demonstrate the utility of chemical mutagenesis for generating tolerance to necrotrophic pathogens in allohexaploid wheat.

Influence of glyphosate, crop volunteer and root pathogens on glyphosate-resistant wheat under controlled environmental conditions

BACKGROUND

The herbicide glyphosate has a synergistic effect on root disease because of increased susceptibility from reduced plant defenses resulting from the blockage of the shikimic acid pathway. Could glyphosate-resistant (GR) wheat cultivars and glyphosate application in-crop increase the risk of damage from soil-borne pathogens? Growth chamber experiments were conducted with two GR wheat lines and their corresponding glyphosate-sensitive (GS) parents and four pathogens (Rhizoctonia solani Kühn R. oryzae Ryker & Gooch, Gaeumannomyces graminis (Sacc.) v. Arx & J. Olivier var. tritici J. Walker and Pythium ultimum Trow). Treatments consisted of different herbicide timings and planting of crop volunteer to mimic management practices in the field.

RESULTS

GR cultivars were not inherently more susceptible to root pathogens than GS cultivars, and application of glyphosate did not increase root disease. When crop volunteer was grown in close proximity to GR cultivars, the timing of glyphosate application had a profound effect. In general, the longer the crop volunteer was left before killing with glyphosate, the greater was the competitive effect on the planted crop. Both R. solani and G. graminis var. tritici reduced plant height, number of tillers and root length of the GR cultivars in the presence of crop volunteer with glyphosate application.

CONCLUSION

To minimize the damaging effects of these pathogens, producers should apply glyphosate at least 2-3 weeks before planting GR wheat, as currently advised for GS cereals.

Identification and quantification of Rhizoctonia solani and R. oryzae using real-time polymerase chain reaction

Rhizoctonia solani and R. oryzae are the principal causal agents of Rhizoctonia root rot in dryland cereal production systems of the Pacific Northwest. To facilitate the identification and quantification of these pathogens in agricultural samples, we developed SYBR Green I-based real-time quantitative-polymerase chain reaction (Q-PCR) assays specific to internal transcribed spacers ITS1 and ITS2 of the nuclear ribosomal DNA of R. solani and R. oryzae. The assays were diagnostic for R. solani AG-2-1, AG-8, and AG-10, three genotypes of R. oryzae, and an AG-I-like binucleate Rhizoctonia species. Quantification was reproducible at or below a cycle threshold (Ct) of 33, or 2 to 10 fg of mycelial DNA from cultured fungi, 200 to 500 fg of pathogen DNA from root extracts, and 20 to 50 fg of pathogen DNA from soil extracts. However, pathogen DNA could be specifically detected in all types of extracts at about 100-fold below the quantification levels. Soils from Ritzville, WA, showing acute Rhizoctonia bare patch harbored 9.4 to 780 pg of R. solani AG-8 DNA per gram of soil.. Blastn, primer-template duplex stability, and phylogenetic analyses predicted that the Q-PCR assays will be diagnostic for isolates from Australia, Israel, Japan, and other countries.

Effect of inoculum density and soil tillage on the development and severity of rhizoctonia root rot

Rhizoctonia spp. cause substantial yield losses in direct-seeded cereal crops compared with conventional tillage. To investigate the mechanisms behind this increased disease, soils from tilled or direct-seeded fields were inoculated with Rhizoctonia spp. at population densities from 0.8 to 250 propagules per gram and planted with barley (Hordeum vulgare). The incidence and severity of disease did not differ between soils with different tillage histories. Both R. solani AG-8 and R. oryzae stunted plants at high inoculum densities, with the latter causing pre-emergence damping-off. High inoculum densities of both species stimulated early production of crown roots in barley seedlings. Intact soil cores from these same tilled and direct-seeded fields were used to evaluate the growth of Rhizoctonia spp. from colonized oat seeds. Growth of R. oryzae was not affected by previous tillage history. However, R. solani AG-8 grew more rapidly through soil from a long-term direct-seeded field compared to tilled soils. The differential response between these two experiments (mixed, homogenized soil versus intact soil) suggests that soil structure plays a major role in the proliferation of R. solani AG-8 through soils with different tillage histories.

Characterization of Rhizoctonia-Like Fungi Isolated from Agronomic Crops and Turfgrasses in Mississippi

Twenty-three isolates of Rhizoctonia spp. from agronomic crops and turfgrasses were characterized by cytological and pathological methods in order to establish the identity, pathogenicity, and virulence of Rhizoctonia spp. and anastomosis groups that occur on these hosts in Mississippi. Twelve isolates were identified as R. solani, including the five anastomosis groups (AGs) AG-1-IB, AG-2-2, AG-4, AG-5, and AG-13. Rhizoctonia zeae, R. oryzae, and eight binucleate Rhizoctonia sp., including R. cerealis, also were identified. R. solani AG-4 isolates were consistently the most virulent isolates on all hosts in pathogenicity evaluations. Pathogenicity of AG-2-2 and AG-5 isolates, binucleate Rhizoctonia spp., and R. oryzae varied between hosts. Two AG-2-2 isolates from bermudagrass or wheat were determined to be clonal isolates, with numerous self-anastomosis reactions. R. solani (AG-1-IB) was pathogenic on all graminaceous hosts. R. cerealis produced sharp eyespot symptoms on wheat and corn and minimal symptoms on cotton and soybean. This is a first report of R. cerealis as a pathogen of corn. R. zeae isolates were pathogenic on all hosts, including cotton and soybean. These results indicate that a diverse group of Rhizoctonia spp. occurs as pathogens on a wide variety of agronomic crops and turfgrasses in Mississippi.

Reduction of Rhizoctonia Bare Patch in Wheat with Barley Rotations

Rhizoctonia bare patch caused by Rhizoctonia solani AG-8 is a major fungal root disease in no-till cropping systems. In an 8-year experiment comparing various dryland no-till cropping systems near Ritzville, WA, Rhizoctonia bare patch first appeared in year 3 and continued unabated through year 8. Crop rotation had no effect on bare patch during the first 5 years. However, from years 6 to 8, both soft white and hard white classes of spring wheat (Triticum aestivum L.) grown in a 2-year rotation with spring barley (Hordeum vulgare L.) had an average of only 7% of total land area with bare patches compared with 15% in continuous annual soft white wheat or hard white wheat (i.e., monoculture wheat). In years 6 to 8, average grain yield of both soft white wheat and hard white wheat were greater (P < 0.001) when grown in rotation with barley than in monoculture. Although both classes of wheat had less bare patch area and greater grain yield when grown in rotation with barley, monoculture hard white wheat was more severely affected by Rhizoctonia than soft white wheat. Soil water levels were higher in bare patches, indicating that roots of healthy cereals did not grow into or underneath bare patch areas. This is the first documentation of suppression of Rhizoctonia bare patch disease in low-disturbance no-till systems with rotation of cereal crops.

A New Method for the Quantification of Rhizoctonia solani and R. oryzae from Soil

Rhizoctonia solani anastomosis group (AG) 8 and R. oryzae are important root pathogens on wheat and barley in the dryland production areas of the inland Pacific Northwest. R. solani AG-8 is difficult to isolate from root systems and quantify in soil because of slow growth and low population densities. However, both pathogens form extensive hyphal networks in the soil and can grow a considerable distance from a food base. A quantitative assay of active hyphae was developed, using wooden toothpicks as baits inserted into sample soils. After 2 days in soil, toothpicks were placed on a selective medium, and the numbers of colonies that grew after 24 h were counted under a dissecting microscope. R. solani and R. oryzae could be distinguished from other fungi based on hyphal morphology. This method was tested in natural soils amended with known inoculum densities of R. solani AG-8 and R. oryzae. Regressions were used to compare the inoculum density or toothpick colonization curves to a predicted curve based on the volume of the toothpicks. The slopes and y intercept of log-log transformed regressions did not differ from the predicted curves in most cases. This technique was used to assess the hyphal activity of R. solani AG-8 and R. oryzae from soil cores taken from various positions in and around Rhizoctonia bare patches at two locations. Activity of R. solani was highest in the center and inside edge of the patch, but there was no effect of patch position on R. oryzae. This simple and inexpensive technique can be used for detection and diagnosis in grower fields and to study the ecology and epidemiology of Rhizoctonia spp.

Virulence of Pythium Species Isolated from Wheat Fields in Eastern Washington

Although Pythium root rot in wheat (Triticum aestivum) is well documented, limited information is available concerning which species of Pythium are most responsible for disease damage. The objective of this study was to examine the variation in virulence on wheat among isolates of Pythium collected from cereal grain fields in eastern Washington. Isolates of nine Pythium species were tested for virulence on spring wheat cultivars Chinese Spring and Spillman. Cultivars were planted in pasteurized soil infested with Pythium isolates and placed in a growth chamber maintained at a constant 16°C and ambient humidity. Plant height, length of the first true leaf, and number of seminal roots were recorded, and roots were digitally scanned to create computer files that were analyzed using WinRhizo software. Pythium isolates caused a significant reduction (P < 0.05) in the number of root tips, root length, and length of the first leaf. Differences in virulence were detected among species and among isolates within species. Isolate Pythium debaryanum 90136 and P. ultimum 90038 were the most virulent and may prove useful in future disease screening assays of Triticum germ plasm.

Evaluation of Adapted Wheat Cultivars for Tolerance to Pythium Root Rot

Genetic resistance in wheat (Triticum aestivum) against Pythium species would be an efficient means of control of this major root fungal pathogen, but so far no source has been identified. In addition, no long-term, sustainable options for controlling Pythium root rot are available; therefore, identifying and then incorporating genetic resistance into wheat cultivars would create an ideal method of control for this disease. The objective of this study was to examine the level of tolerance to Pythium root rot among a diverse set of wheat germ plasm collected from all major wheat production regions in the United States. Pythium debaryanum isolate 90136 and P. ultimum isolate 90038, previously identified as the most virulent Pythium isolates on wheat, were used to infest pasteurized soil, which was seeded with wheat genotypes and placed in a growth chamber maintained at a constant 16°C with a 12-h photoperiod and ambient humidity. Length of the first leaf and plant height measurements were recorded, and roots were digitally scanned to create computer files that were analyzed using WinRhizo software for length and number of tips. Significant (P < 0.05) differences in plant variables were detected among wheat genotypes in the presence of both Pythium species, and a significant (P < 0.0001) correlation between plant stunting and root loss was detected. Based on both shoot and root measurements, Caledonia, Chinese Spring, MN97695, and OR942504 appear to be highly susceptible to Pythium root rot, whereas genotypes KS93U161, OH708, and Sunco were the most tolerant to this disease.