Rapid Screening for Relative Salt Tolerance among Chile Pepper Genotypes

Chile peppers are economically important crops in southern regions of the United States. Limited information is available on irrigation management with low-quality water or on salt-affected soils. The objective of this study was to determine the relative salt tolerance of 20 genotypes of chile peppers. In Expt. 1, seeds of selected pepper types (Anaheim, Ancho, Cayenne, Paprika, Jalapeño, Habanero, and Serrano) were germinated in potting mix and seedlings were grown in 2.6-L pots. Six weeks after sowing, salinity treatments were initiated by irrigating plants with nutrient solutions of different electrical conductivities (ECs): 1.4 (control), 3.0, or 6.0 dS·m−1. After 1 month of initiating treatments, shoots were harvested and dry weights were determined. All plants survived and no visual salt injury was observed regardless of pepper variety and treatment. There were no statistical differences between control and saline solution treatments in final height and shoot dry weight of Habanero 1, ‘Early Jalapeño’, ‘AZ-20’, ‘NuMex Joe E. Parker’, and ‘NuMex Sandia’. In Expt. 2, seeds of 20 genotypes were directly sown in 2.6-L containers filled with loamy sand. Saline water irrigation was initiated 37 days after sowing by irrigating plants either with saline (nutrient solution based, similar to Expt. 1) or nutrient solution (control). More than half the genotypes did not have 100% survival in the salinity treatment. Ancho 1, Ancho 2, Cayenne 1, ‘Early Jalapeño’, and ‘AZ-20’ had 100% survival regardless of salinity treatment. No plants of ‘TAM Mild Habanero’ survived when irrigated with saline water and less than half of the plants survived in the control. The relative tolerance of chile genotypes to salinity varied with substrate in some genotypes. From the combined results of the two experiments, the 20 pepper genotypes were ranked for salt tolerance based on seedling survival, visual quality, and growth. ‘Early Jalapeño’ and ‘AZ-20’ were relatively tolerant to salinity among the 20 genotypes, whereas ‘TAM Mild Habanero’ and ‘Ben Villalon’ were sensitive. Ancho 1, Ancho 2, Cayenne 1, and Cayenne 2 also had relatively high tolerance based on survival and visual quality, although shoot growth was reduced significantly.

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Seedling Emergence, Growth, and Mineral Nutrition of Ornamental Chile Peppers Irrigated with Saline Water

HortScience ◽

10.21273/hortsci.47.11.1653 ◽

2012 ◽

Vol 47 (11) ◽

pp. 1653-1657 ◽

Cited By ~ 1

Author(s):

Genhua Niu ◽

Pedro Osuna ◽

Youping Sun ◽

Denise S. Rodriguez

Keyword(s):

Salt Tolerance ◽

Saline Solution ◽

Saline Water ◽

Seedling Emergence ◽

Electrical Conductance ◽

Dry Weight ◽

Salt Damage ◽

Shoot Dry Weight ◽

Plant Salt Tolerance ◽

Chile Peppers

Ornamental chile peppers are popular bedding plants. As high-quality water supply becomes limited in many parts of the world, alternative waters such as municipal reclaimed water is encouraged to be used for landscape irrigation. The purpose of this study was to assess the relative salt tolerance of 10 cultivars of ornamental chile peppers by irrigating the mature plants with saline solutions and germinating seeds in saline substrate in a greenhouse. In the mature plant salt tolerance experiment, plants were irrigated with nutrient solution (no addition of salts, control) or saline solution at electrical conductance (EC) of 4.1 dS·m−1 or 8.1 dS·m−1 for 8 weeks. Plants in the EC of 4.1 dS·m−1 treatment did not have any foliar salt damage regardless of cultivar. At EC of 8.1 dS·m−1, ‘NuMex Memorial Day’ had the most severe foliar salt damage, whereas ‘NuMex April Fool’s Day’, ‘NuMex Cinco de Mayo’, ‘NuMex Thanksgiving’, and ‘NuMex Twilight’ had little or no foliar damage. Shoot dry weight (DW) reduction at EC of 8.1 dS·m−1 compared with control was smallest in ‘NuMex Thanksgiving’ (15%), whereas ‘NuMex Memorial Day’ had the greatest reduction of 74% followed by ‘NuMex Christmas’ of 61%. The highest shoot DW reduction in ‘NuMex Memorial Day’ coincided with lowest visual score, indicating that this cultivar was the least tolerant to salinity. The leaf Na+ and Cl− concentrations increased dramatically with increasing EC of the irrigation water in all cultivars. The highest Na+ concentration of 10.9 mg·g−1 DW at EC of 8.1 dS·m−1 was observed in ‘NuMex Christmas’. The highest Cl− concentration at EC of 8.1 dS·m−1 was found in ‘NuMex Memorial Day’ with 64.8 mg·g−1 DW, which was four times higher than the control. In the seedling emergence experiment, seeds of the 10 cultivars were germinated in substrate either moistened with reverse osmosis water (EC ≈0) or saline solution at EC of 17.1 dS·m−1. ‘NuMex Christmas’ and ‘NuMex Memorial Day’ had the lowest relative seedling emergence index, indicating that these two cultivars were the least tolerant to salinity during the seedling emergence stage. ‘NuMex Thanksgiving’ and ‘NuMex Cinco de Mayo’ had the highest relative seedling emergence index. Combining the results from both experiments, we concluded that ‘NuMex Cinco de Mayo’ and ‘NuMex Thanksgiving’ were the most tolerant cultivars, whereas ‘NuMex Christmas’ and ‘NuMex Memorial Day’ were the least tolerant ones.

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Salt Tolerance and Growth of 13 Avocado Rootstocks Related Best to Chloride Uptake

HortScience ◽

10.21273/hortsci13198-18 ◽

2018 ◽

Vol 53 (12) ◽

pp. 1737-1745 ◽

Cited By ~ 2

Author(s):

Nydia Celis ◽

Donald L. Suarez ◽

Laosheng Wu ◽

Rui Li ◽

Mary Lu Arpaia ◽

...

Keyword(s):

Salt Tolerance ◽

Major Ions ◽

Growth Parameters ◽

Saline Water ◽

Block Design ◽

The United States ◽

Persea Americana ◽

Trunk Diameter ◽

Salinity Levels ◽

High Concentrations

Avocado (Persea americana Mill.) is one of the most salt-sensitive crops and one of the highest value crops per acre. In the United States, avocados are grown primarily in California, in regions experiencing both scarcity of freshwater and salinization of available water supplies. Thus, our objectives were to evaluate avocado rootstocks for salt tolerance and evaluate the relationship between leaf ion concentrations, trunk diameter, leaf burn, and fruit yield. Our field experiment evaluated the salt tolerance of the Hass scion grafted onto 13 different avocado rootstocks using the Brokaw clonal rootstock technique. The experiment consisted of 156 trees arranged in a randomized complete block design with six replications of each saline [electrical conductivity (EC) = 1.5 dS·m–1, Cl– = 4.94 mmol·L–1] and nonsaline (EC= 0.65 dS·m–1, Cl– = 0.73 mmol·L–1) irrigation water treatment. We collected soil samples and leaves, then analyzed them for major ions. The rootstocks R0.06, R0.07, PP14, and R0.17, which had high concentrations of Cl and Na in the leaves, were the least salt tolerant, with 100% mortality in the rows irrigated with saline water for 23 months. The rootstocks R0.05, PP40, R0.18, and Dusa, which had low concentrations of Cl ions in the fully expanded leaves, were least affected by salinity, and these rootstocks exhibited the greatest yields, largest trunk diameters, and greatest survival percentages in the saline treatment. Yield and growth parameters correlated well with leaf Cl concentration, but not Na, indicating that salt damage in avocado is primarily a result of Cl ion toxicity. Under arid inland environments, no variety performed satisfactorily when irrigated with an EC = 1.5 dS·m–1 water (Cl– = 4.94 mmol·L–1). However, the more tolerant varieties survived at soil salinity levels that would apparently be fatal to varieties reported earlier in the literature.

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Relative Salt Tolerance of Five Herbaceous Perennials

HortScience ◽

10.21273/hortsci.41.6.1493 ◽

2006 ◽

Vol 41 (6) ◽

pp. 1493-1497 ◽

Cited By ~ 22

Author(s):

Genhua Niu ◽

Denise S. Rodriguez

Keyword(s):

Salt Tolerance ◽

Osmotic Potential ◽

Tap Water ◽

Visual Quality ◽

Dry Weight ◽

Salinity Treatment ◽

Herbaceous Perennials ◽

Elevated Salinity ◽

Leaf Osmotic Potential ◽

Salinity Levels

Use of recycled water to irrigate urban landscapes may be inevitable, because the freshwater supply has been diminishing and the population continues to grow in the arid and semiarid southwestern United States. However, little information exists on the performance of landscape plants irrigated with nonpotable water. Two greenhouse studies were conducted during the summer and the fall to characterize the relative salt tolerance of five herbaceous perennials by irrigating the plants with a saline solution at an electrical conductivity (EC) of 0.8 dS·m–1 (tap water), 2.0 dS·m–1, or 4.0 dS·m–1. In the summer study, after 10 weeks of treatment, Achillea millefolium L., Gaillardia aristata Foug., and Salvia coccinea Juss ex J. had an aesthetically acceptable appearance for landscape performance (visual quality scores of 4 points or more), whereas Agastache cana (Hook.) Woot. & Standl. and Echinacea purpurea (L.) Moench had relatively low tolerance to salinity. Dry weight of shoots of A. millefolium, A. cana, and G. arstata was lower at elevated salinity levels. In the fall study, A. millefolium, E. purpurea, G. arstata, and S. coccinea had acceptable growth and visual quality at elevated salinity levels, whereas A. cana had lower quality and reduced growth. Dry weight of shoots was lower in G. arstata and A. millefolium at an EC of 2.0 dS·m–1 or 4.0 dS·m–1. Leaf osmotic potential of all species in the summer experiment was significantly lower at higher salinity compared with the control. In the fall experiment, leaf osmotic potential in A. millefolium, E. purpurea, and G. aristata at 4 dS·m–1 was lower compared with lower salinity treatment and the control. Leaf osmotic potential in the fall was higher than that of the same species at the same salinity level in the summer experiment, indicating that plants in the fall were less stressed than in the summer. Combined the results from both experiments, the authors concluded that A. millefolium, G. arstata, and S. coccinea had a relatively high salt tolerance (as much as 4 dS·m–1 of irrigation water under greenhouse conditions) among the tested species, whereas A. cana and E. purpurea were not tolerant to salt and should not be irrigated with low-quality water.

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Sustainable Irrigation Management of Ornamental Cordyline Fruticosa “Red Edge” Plants with Saline Water

Sustainability ◽

10.3390/su11133751 ◽

2019 ◽

Vol 11 (13) ◽

pp. 3751

Author(s):

Blanca M. Plaza ◽

Juan Reca ◽

Juan Martínez ◽

Francisco Alex ◽

Maria Teresa Lao

Keyword(s):

Experimental Data ◽

Nutrient Solution ◽

Saline Water ◽

Irrigation Management ◽

Combination Method ◽

Control Treatment ◽

Saline Stress ◽

Red Edge ◽

Cordyline Fruticosa ◽

Sustainable Irrigation

The aim of this work was to analyze the influence of the salinity of the nutrient solution on the transpiration and growth of Cordyline fruticosa var. “Red Edge” plants. A specific irrigation management model was calibrated with the experimental data. An experiment was performed with four treatments. These treatments consisted of the application of four nutrient solutions with different electrical conductivity (ECw) levels ranging from 1.5 dS m−1 (control treatment) to 4.5 dS m−1. The results showed that day-time transpiration decreases when salt concentration in the nutrient solution increases. The transpiration of the plant in the control treatment was modelled by applying a combination method while the effect of the salinity of the nutrient solution was modelled by deriving a saline stress coefficient from the experimental data. The results showed that significant reductions in plant transpiration were observed for increasing values of ECw. The crop development and yield were also affected by the increasing salinity of the nutrient solution. A relationship between the ECw and the relative crop yield was derived.

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Growth, Visual Quality, and Morphological Responses of 12 Viburnum Taxa to Saline Water Irrigation

HortScience ◽

10.21273/hortsci14940-20 ◽

2020 ◽

Vol 55 (8) ◽

pp. 1233-1241

Author(s):

Youping Sun ◽

Ji Jhong Chen ◽

Haifeng Xing ◽

Asmita Paudel ◽

Genhua Niu ◽

...

Keyword(s):

Salinity Stress ◽

Saline Solution ◽

Saline Water ◽

Morphological Characteristics ◽

Visual Quality ◽

The United States ◽

Growth Responses ◽

Salt Damage ◽

Salinity Levels ◽

The Impact

Viburnums are widely used in gardens and landscapes throughout the United States. Although salinity tolerance varies among plant species, research-based information is limited on the relative salt tolerance of viburnum species. The morphological and growth responses of 12 viburnum taxa to saline solution irrigation were evaluated under greenhouse conditions. Viburnum taxa included Viburnum ×burkwoodii, V. cassinoides ‘SMNVCDD’, V. dentatum ‘Christom’, V. dentatum var. deamii ‘SMVDLS’, V. dilatatum ‘Henneke’, V. ×‘NCVX1’, V. nudum ‘Bulk’, V. opulus ‘Roseum’, V. plicatum var. tomentosum ‘Summer Snowflake’, V. pragense ‘Decker’, V. ×rhytidophylloides ‘Redell’, and V. trilobum. A nutrient solution at an electrical conductivity (EC) of 1.3 dS·m−1 (control) or saline solutions at ECs of 5.0 and 10.0 dS·m−1 were applied eight times over a 9-week period. Growth, visual quality, and morphological characteristics were quantified at the 4th week and 8th–9th week to assess the impact of salinity stress on the viburnum taxa. Saline solution irrigation imposed detrimental salinity stress on viburnum plant growth and visual quality, and the degree of salt damage was dependent on the salinity levels of irrigation solution and the length of exposure to salinity stress as well as viburnum taxa. Viburnum ×burkwoodii and V. ×‘NCVX1’ had little foliar salt damage during the entire experiment, except those irrigated with saline solution at an EC of 10.0 dS·m−1 exhibited slight to moderate foliar salt damage at the eighth week. Viburnum dilatatum ‘Henneke’, V. plicatum var. tomentosum ‘Summer Snowflake’, and V. trilobum irrigated with saline solution at an EC of 5.0 dS·m−1 had slight and severe foliar salt damage at the 4th and 8th week, respectively. Plants irrigated with saline solution at an EC of 10.0 dS·m−1 exhibited severe foliar salt damage at the 4th week, and all died by the 8th week. Other viburnum taxa also showed various foliar salt damage, especially at an EC of 10.0 dS·m−1. The shoot dry weights of V. ×burkwoodii and V. ×‘NCVX1’ irrigated with saline solution at ECs of 5.0 and 10.0 dS·m−1 were similar to those in the control at both harvest dates. However, the shoot dry weight of other tested viburnum taxa decreased to some extent at the 9th week. A cluster analysis concluded that V. ×burkwoodii and V. ×‘NCVX1’ were considered the most salt-tolerant viburnum taxa, whereas V. dilatatum ‘Henneke’, V. plicatum var. tomentosum ‘Summer Snowflake’, and V. trilobum were sensitive to salinity levels used in this study. This research may guide the green industry to choose relatively tolerant viburnum taxa for landscape use and nursery production where low-quality water is used for irrigation.

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Assessing Tolerance to Sodium Chloride Salinity in Fourteen Floriculture Species

HortTechnology ◽

10.21273/horttech.21.5.539 ◽

2011 ◽

Vol 21 (5) ◽

pp. 539-545 ◽

Cited By ~ 23

Author(s):

Gonzalo H. Villarino ◽

Neil S. Mattson

Keyword(s):

Sodium Chloride ◽

Salt Tolerance ◽

Irrigation Water ◽

Saline Water ◽

Dry Weight ◽

The United States ◽

Greenhouse Production ◽

Saline Irrigation ◽

Bedding Plants ◽

Agronomic Crops

The use of saline irrigation water may be inevitable in the future since the freshwater supply is decreasing over time. In some regions of the United States, producers of both ornamental and agronomic crops are already facing a limited supply of high-quality water. Therefore, it is necessary to determine the salt tolerance of commonly used greenhouse bedding plants to minimize potential salt damage before use of nonpotable water sources is mandated. Research screening several bedding plants has not taken place for more than two decades. Therefore, we undertook experiments to screen popular bedding plants for salt tolerance during greenhouse production. Transplants were exposed to 0 (control), 20, 40, 60, or 80 mm sodium chloride (NaCl) in the irrigation water for 5 weeks resulting in average substrate pour-thru electrical conductivity (EC) values of 4.0 (control), 7.0, 9.8, 12.1, or 14.2 dS·m−1, respectively. Pansy (Viola tricolor) and zinnia (Zinnia angustifolia), the most sensitive species examined, exhibited 100% mortality when exposed to an EC of 14.2 dS·m−1. The least affected species for dry weight (DW) was snapdragon (Antirrhinum majus) with a 54% reduction as EC increased from 4.0 to 14.2 dS·m−1. Only fuchsia (Fuchsia hybrida) and snapdragon were unaffected by an EC of 7.0 dS·m−1, whereas at 9.8 dS·m−1 all of the species had a significantly reduced DW as compared with control plants. Verbena (Verbena ×hybrida), petunia (Petunia ×hybrida), coleus (Solenostemon scutellarioides), and begonia (Begonia hiemalis) were the only species that did not undergo a significant height reduction in comparing 9.8 dS·m−1 to control. A classification of the 14 species is created here on the basis of plant DW to provide guidance as to which species could be irrigated with more saline water while not compromising plant growth and quality.

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The Impact of Salt Stress on Plant Growth, Mineral Composition, and Antioxidant Activity in Tetragonia decumbens Mill.: An Underutilized Edible Halophyte in South Africa

Horticulturae ◽

10.3390/horticulturae7060140 ◽

2021 ◽

Vol 7 (6) ◽

pp. 140

Author(s):

Avela Sogoni ◽

Muhali Jimoh ◽

Learnmore Kambizi ◽

Charles Laubscher

Keyword(s):

Antioxidant Activity ◽

Plant Growth ◽

Mineral Composition ◽

Nutrient Solution ◽

Crop Production ◽

Saline Water ◽

Total Yield ◽

Control Treatment ◽

Antioxidant Power ◽

The Impact

Climate change, expanding soil salinization, and the developing shortages of freshwater have negatively affected crop production around the world. Seawater and salinized lands represent potentially cultivable areas for edible salt-tolerant plants. In the present study, the effect of salinity stress on plant growth, mineral composition (macro-and micro-nutrients), and antioxidant activity in dune spinach (Tetragonia decumbens) were evaluated. The treatments consisted of three salt concentrations, 50, 100, and 200 mM, produced by adding NaCl to the nutrient solution. The control treatment had no NaCl but was sustained and irrigated by the nutrient solution. Results revealed a significant increase in total yield, branch production, and ferric reducing antioxidant power in plants irrigated with nutrient solution incorporated with 50 mM NaCl. Conversely, an increased level of salinity (200 mM) caused a decrease in chlorophyll content (SPAD), while the phenolic content, as well as nitrogen, phosphorus, and sodium, increased. The results of this study indicate that there is potential for brackish water cultivation of dune spinach for consumption, especially in provinces experiencing the adverse effect of drought and salinity, where seawater or underground saline water could be diluted and used as irrigation water in the production of this vegetable.

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Early Growth Stage Characterization and the Biochemical Responses for Salinity Stress in Tomato

Plants ◽

10.3390/plants10040712 ◽

2021 ◽

Vol 10 (4) ◽

pp. 712

Author(s):

Md Sarowar Alam ◽

Mark Tester ◽

Gabriele Fiene ◽

Magdi Ali Ahmed Mousa

Keyword(s):

Salt Tolerance ◽

Salinity Tolerance ◽

Salinity Treatment ◽

Salt Tolerant ◽

Improvement Program ◽

Biochemical Basis ◽

Elevated Salinity ◽

Tolerance Indices ◽

Promising Source ◽

Tomato Genotypes

Salinity is one of the most significant environmental stresses for sustainable crop production in major arable lands of the globe. Thus, we conducted experiments with 27 tomato genotypes to screen for salinity tolerance at seedling stage, which were treated with non-salinized (S1) control (18.2 mM NaCl) and salinized (S2) (200 mM NaCl) irrigation water. In all genotypes, the elevated salinity treatment contributed to a major depression in morphological and physiological characteristics; however, a smaller decrease was found in certain tolerant genotypes. Principal component analyses (PCA) and clustering with percentage reduction in growth parameters and different salt tolerance indices classified the tomato accessions into five key clusters. In particular, the tolerant genotypes were assembled into one cluster. The growth and tolerance indices PCA also showed the order of salt-tolerance of the studied genotypes, where Saniora was the most tolerant genotype and P.Guyu was the most susceptible genotype. To investigate the possible biochemical basis for salt stress tolerance, we further characterized six tomato genotypes with varying levels of salinity tolerance. A higher increase in proline content, and antioxidants activities were observed for the salt-tolerant genotypes in comparison to the susceptible genotypes. Salt-tolerant genotypes identified in this work herald a promising source in the tomato improvement program or for grafting as scions with improved salinity tolerance in tomato.

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Anaerobic Mineralization of Recirculating Aquaculture Drum Screen Effluent for Use as a Naturally-Derived Nutrient Solution in Hydroponic Cropping Systems

Conservation ◽

10.3390/conservation1030013 ◽

2021 ◽

Vol 1 (3) ◽

pp. 151-167

Author(s):

Joseph Tetreault ◽

Rachel Fogle ◽

Todd Guerdat

Keyword(s):

Nutrient Solution ◽

Agricultural Waste ◽

The United States ◽

Anaerobic Treatment ◽

Production Costs ◽

Effluent Treatment ◽

Limiting Factors ◽

Fertilizer Treatment ◽

Recirculating Aquaculture ◽

Recirculating Aquaculture Systems

Operation and effluent treatment costs are limiting factors for the success of recirculating aquaculture systems (RAS) in meeting seafood demand in the United States. Adopting a capture-and-reuse waste management model similar to terrestrial agriculture farmers would allow RAS farmers to monetize effluent and offset production costs. The moisture content and nutrient profile of RAS effluent makes it a potential option for use as a hydroponic fertilizer. Treatment of RAS waste is needed to mineralize particulate-bound nutrients before becoming a viable hydroponic nutrient solution. Anaerobic treatment (AT), a method used by municipal and agricultural waste treatment facilities to reduce total solids, has been shown to successfully mineralize particulate-bound nutrients from RAS effluent. Continuously mixed anaerobic batch bioreactors were used to evaluate the degree to which AT may mineralize particulate-bound nutrients in solid RAS waste. Concentrations of twelve different macro- and micro-nutrients were analyzed in the waste before and after treatment. Effluent samples were analyzed to determine the fraction of each nutrient in the solid and aqueous forms. This study showed that AT is an effective method to mineralize particulate-bound nutrients in RAS effluent and the mineralization rate data may be used to design a pilot-scaled flow-through RAS effluent treatment system.

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Pathogenicity and Virulence of Pythium Species Obtained from Forest Nursery Soils on Douglas-Fir Seedlings

Plant Disease ◽

10.1094/pdis-09-12-0895-re ◽

2013 ◽

Vol 97 (6) ◽

pp. 744-748 ◽

Cited By ~ 23

Author(s):

Jerry E. Weiland ◽

Bryan R. Beck ◽

Anne Davis

Keyword(s):

Pseudotsuga Menziesii ◽

Seedling Survival ◽

The United States ◽

Douglas Fir ◽

Tree Seedlings ◽

Pythium Species ◽

Forest Nursery ◽

Damping Off ◽

Greenhouse Study ◽

Forest Nurseries

Pythium species are common soilborne oomycetes that occur in forest nursery soils throughout the United States. Numerous species have been described from nursery soils. However, with the exception of P. aphanidermatum, P. irregulare, P. sylvaticum, and P. ultimum, little is known about the potential for other Pythium species found in nursery soils to cause damping-off of tree seedlings. A greenhouse study was conducted to evaluate the pathogenicity and virulence of 44 Pythium isolates representing 16 species that were originally recovered from soil at three forest nurseries in Washington and Oregon. Seeds of Douglas-fir (Pseudotsuga menziesii) were planted into soil infested with each of the isolates. Seedling survival, the number of surviving seedlings with necrotic root lesions, and taproot length were evaluated 4 weeks later. Responses of Douglas-fir to inoculation varied significantly depending on Pythium species and isolate. Eight species (P. dissotocum, P. irregulare, P. aff. macrosporum, P. mamillatum, P. aff. oopapillum, P. rostratifingens, P. sylvaticum, and P. ultimum var. ultimum) significantly reduced the number of surviving seedlings compared to the noninoculated treatment. However, all Pythium species caused a greater percentage of seedlings to develop root lesions (total mean 40%) than was observed from noninoculated seedlings (17%). Taproot length varied little among Pythium treatments and was not a useful character for evaluating pathogenicity. Results confirm the ability of P. irregulare, P. mamillatum, and P. ultimum var. ultimum to cause damping-off of Douglas-fir seedlings, and are indicative that other species such as P. dissotocum, P. aff. macrosporum, P. aff. oopapillum, P. rostratifingens, and P. sylvaticum may also be responsible for seedling loss.

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