© 2010 Plant Management Network.
Soybean Resistance to Field Populations of Heterodera glycines in Selected Geographic Areas
Jamal Faghihi, Research and Extension Nematologist, Department of Entomology, Purdue University, West Lafayette IN 47907; Patricia A. Donald, Research Plant Pathologist, USDA-ARS, West Tennessee Research and Education Center, Jackson, TN 38301; Gregory Noel, Research Plant Pathologist, USDA-ARS, University of Illinois, Urbana, IL 61801; Tom W. Welacky, Research Biologist, Agriculture and Agri-Food Canada, Harrow, ON NOR 1GO, Canada; and Virginia R. Ferris, Professor, Department of Entomology, Purdue University, West Lafayette IN 47907
Faghihi, J., Donald, P. A., Noel, G., Welacky, T. W., and Ferris, V. R. 2010. Soybean resistance to field populations of Heterodera glycines in selected geographic areas. Online. Plant Health Progress doi:10.1094/PHP-2010-0426-01-RS.
Data were collected 2006 through 2008 from 527 soil samples to determine the current effectiveness of PI 88788 and other sources of Heterodera glycines resistance in three geographically separated areas of soybean production: Tennessee and Indiana/Illinois, USA, and Ontario, Canada. In Tennessee where PI 88788 source of resistance has been used since 1978, 93% of field populations reproduced on PI 88788 (≥ 10% of susceptible cultivar), and no HG Type 0 populations were found. In Indiana and Illinois, where resistance was used since the mid-1980s, from 56 to 88% of the populations reproduced on PI 88788 (≥ 10%). PI 548402 (Peking), PI 90763, and PI 437654 had low reproduction (≤ 10%) unlike Tennessee where 78% of the populations reproduced on PI 548402 (≥ 10%) and 93% reproduced on PI 90763 (≥ 10%). In Ontario, where cultivars with PI 88788 resistance were used after 1989, PI 88788 in 73% of the field populations had ≤ 10% reproduction. But 15% of Ontario populations reproduced on PI 548402 (≥ 10%) and 6% reproduced on PI 90763 (≥ 10%), two sources of resistance not generally present in commercial cultivars grown in Ontario.
Soybean cyst nematode (Heterodera glycines) (SCN) is a serious pest of soybean (Glycine max) in Canada and the United States (19,20). Economic yield loss due to H. glycines in the United States varies with the growing season and the price of soybean. The loss estimate was > 300 million bushels from 2003 to 2005 (16). The true extent of losses in the North Central region of the United States may be underestimated because lack of stress on soybeans during the growing season often masks yield loss prior to harvest. Considerable success for the last three decades in managing the predominant H. glycines "race 3" (7) with cultivar resistance derived mostly from PI 88788 has diminished concern about H. glycines throughout the industry. Resistance in PI 88788 has been effective and for most part durable. It is estimated to be present in about 97% of cultivars resistant to H. glycines. A number of factors lead to use of this single source of resistance including the documentation that selection on PI 88788 reduced the index of parasitism on PI 90763 and PI 89772 suggesting linkage of resistance alleles (11,21). Recently, cultivars derived with PI 88788 resistance supported high H. glycines egg population density (4,9,12,13,15,17,23) because dependence on a single source of resistance selected for virulent H. glycines phenotypes (3). Other available sources of resistance have not been incorporated widely into soybean. We characterized H. glycines populations in areas where initial use of resistant cultivars ranged from 1978 to 1989. Our research was undertaken to compare SCN field populations exposed to PI 88788 source of resistance for different lengths of time. We considered also the possibility that widely separated regional field populations might differ in their innate responses to SCN resistance genes. We used the HG Type test for this characterization (14). Goals of this research were to: (i) determine the current effectiveness of PI 88788 as a source of resistance to H. glycines in Tennessee and Indiana/Illinois, USA, and Ontario, Canada; and (ii) determine the reaction to other sources of H. glycines resistance in areas where resistance derived from PI 88788 may not be effective.
Geographic Areas of Focus
We chose Tennessee and Indiana and Illinois in the United States, and Ontario, Canada, because these areas vary markedly with respect to years of planting cultivars resistant to H. glycines. Tennessee has four decades of planting resistant cultivars with resistance derived from PI 548402 (cv. Peking) beginning in the 1960s followed by PI 88788 resistance available in cv. Bedford (Maturity Group V) in 1978. Illinois and Indiana did not have widespread planting of resistance until the mid 1980s when cv. Fayette (MGIII; PI 88788 source of resistance) was widely available. In Ontario planting of cv. Bell (MGI; PI 88788 source of resistance) began in 1989.
We sampled 20 to 100 H. glycines-infested fields in each region resulting in 32 to 269 soil samples (Table 1). Sampling methods varied among areas. Personnel at each research institution processed their own soil samples for detection of H. glycines. Tennessee soil samples were 50 cores (2.5 cm diameter by 15 cm deep) per 10 ha from random soybean fields collected primarily by University of Tennessee Extension agriculturalists at the request of producers. Indiana samples included those submitted by growers to the nematology laboratory following Purdue guidelines (6) of soil collected in a zig-zag pattern within the field to a depth of 10 to 15 cm until 500 cm³ of soil had been collected from several areas of the field with a soil probe. Illinois samples were collected from randomly selected fields along county roads. A diamond pattern within the field was used consisting of five cores in the center of each 1.3 ha resulting in approximately 120 cores per field collected per field from which 100 cm³ were used for cyst determination. Soil samples from Ontario were randomly collected by seed company agricultural representatives from their respective fields. Fifty to 120 soil cores were collected in a "X" pattern from 4 to 12 ha per sample location.
Table 1. Frequency of HG Types for Heterodera glycines populations characterized for virulence phenotypes from Tennessee and Illinois/Indiana, USA, and Ontario Canada.
x HG Type has replaced the race designation in scientific communication of virulence phenotypes for SCN (14).
Greenhouse HG Type Characterizations
Soil samples which contained cysts when determining cyst/egg population density were placed in pots in a greenhouse and planted with a cultivar which had no phenotypic expression of resistance. However, HG Type characterization was done directly if the egg population density was greater than 700 cysts. For some samples the egg population density was increased for multiple generations on susceptible cv. Hutcheson, Williams 82, Lee 74, or Kenwood 94. After 1 month, soil was removed from the roots by hand and roots were examined visually for development of females. If females were not observed, seeds were planted into the same soil and seedlings allowed to grow for one additional month prior to checking the roots for cysts. If or when new females were found on roots, the entire root ball and soil were washed, and females were collected on a 250-µm-pore sieve (Fig. 1).
Tennessee soil samples containing ≥ 6 cysts in the field sample were retained for greenhouse bioassay. Cysts were extracted from the soil using a semiautomatic elutriator (2), cysts were crushed (5) and an average of 2,500 eggs/ml placed near the seeds. Plants were grown in 8-cm-diameter pots containing 200 cm³ of steam sterilized soil [three parts sand: one part sandy loam soil (3:1 v:v)]. Each pot (Fig. 2) contained three seeds of one soybean line of the seven HG Type indicator lines: PI 548402, PI 88788, PI 90763, PI 437654, PI 209332, PI 89772, and PI 548316 (cv. Cloud) (United States Regional Soybean Laboratory, Urbana, IL) (14). Positive results for reproduction on PI 437654 were repeated prior to inclusion in this study due to past inconsistent results with this accession. There were three replications of each line. Susceptible controls included PI 548658 and cvs. Hutcheson and Essex. The air temperature was 27°C with heat supplied directly under the bench. Water was supplied as needed and heated to 20°C from October through April prior to application to the pots. Plants were fertilized with Miracle Gro 2 and 3 weeks after planting. After 30 to 45 days females were dislodged from roots with a strong water spray and collected on nested 850-µm-pore and 250-µm-pore sieves. Females were counted with a dissecting microscope.
In Illinois, cysts were extracted by hand using nested 850-µm-pore and 250-µm pore sieves. Heterodera glycines populations were increased on PI 548658 in field soil diluted with an equal part of sand (1:1 v:v) to obtain a final volume of 400 cm³/pot. Additional increases were done in 100% sand. Characterizations were done with 280 cm³ sand in a 7-cm-diameter pot. Females were removed from roots as described above. Debris was removed and females were concentrated with centrifugal flotation (10) and then crushed in a tissue grinder. Eggs were collected on a 25-µm-pore sieve and diluted to 4,000 eggs/ml for inoculation. Seeds of the differentials were germinated on paper towels for 48 to 72 h and one seedling of each differential transplanted into a pot. All differentials were replicated four times. The greenhouse in Illinois had radiant heat supplied under benches. Soil temperature was maintained at 24 to 26°C during the winter and 29 to 34°C during the summer.
In Indiana, seeds of each soybean differential were germinated in sand. When seedlings were several inches tall, sand was washed from the roots. Each seedling was placed in a 2.5-cm cell of a seedling tray partially filled with a steamed soil:sand (1:3) mixture. One ml of inoculum (2,000 to 3,000 eggs/ml) was pipetted over the roots, and additional soil/sand mixture added to the cell. Three replicates of each soybean differential were used. Plants were grown for 8 to 10 weeks at a temperature of about 27°C. When development of the second generation of females was produced, roots were dipped in water to remove soil and sand. These experiments were carried out throughout the year based on availability of inoculum.
In Ontario field soil samples were increased on susceptible Kenwood 94 in 360-cm³ pots until approximately 160,000 eggs were available. Eggs were collected on a 250-µm-pore sieve. HG Type line indicators were planted with one plant per cone-tainer and replicated four times. Seeds were germinated and seedlings transplanted when radicals were 2.5 cm long into 19 × 3.5-cm cone-tainers of sterilized 50% washed masonry and 50% beach sand. When transplanted, each plant was inoculated with 4,000 eggs placed in the planting hole. Susceptible controls included PI 548658, cv. Pickett, Essex, and Kenwood 94. Thirty cone-tainers were placed in 3-liter pails and submerged in a water control table. The water control table had continuous circulation with the temperature maintained at 27°C. As needed, water of 22°C was added by trickle system and hand application to cone-tainers. Plants were fertilized with 20-20-20 mixture of 4 g/liter water and applied at 5 ml per plant 2 weeks after planting. After 40 to 45 days females were dislodged from roots with a strong water spray and collected on nested 600-µm-pore and 250-µm-pore sieves. Females were counted with the aid of a dissecting microscope. A Female Index (FI) (7) was calculated where FI = (number of cysts on indicator line/number of cysts on susceptible control)*100.
In Tennessee no HG Type 0 populations were found. One population was characterized as HG Type 7. Most Tennessee populations were HG Type 18.104.22.168 (Table 1). Of 1,400 soil samples, 882 (63%) were infested but only 26 (3%) had over 100 cysts within 60 days of greenhouse bioassay initiation. Of the 94 Tennessee populations characterized, 93 (99%) produced a susceptible reaction on PI 88788 (≥ 10%), whereas 74 (79%) produced a susceptible reaction on PI 548402 (≥ 10%).
Thirty-two H. glycines populations tested from Illinois were HG Types 0 (31%) and 2.5.7 (38%). The three remaining HG Types characterized constituted less than 16% individually (Table 1).
From Indiana 269 tested populations had reactions similar to Illinois populations, which shares a similar soybean cropping history. Twenty-eight percent of the Indiana H. glycines populations were rated as HG Type 0 and 34% as HG Type 2.5.7. There were fewer than 24 populations of each remaining HG Type (Table 1). No H. glycines populations were found with a virulent phenotype on PI 548402, PI 90763, or PI 437654. More than half of the populations showed a positive female index (≥10%) (7) on PI 88788 (Table 2).
Table 2. Relative reproduction (FR, female indexx) of Heterodera glycines populations from Tennessee and Illinois/Indiana, USA, and Ontario, Canada, on seven Glycine max and Glycine spp. host indicator lines.
x Female index = (number of cysts on indicator line/number of cysts on susceptible control)*100.
y Populations with FI ≥ 10%
PI 88788 did not allow greater than 10% reproduction in many field populations in greenhouse trials. A larger than expected number of Ontario populations reproduced on PI 548402 and PI 90763, two sources of resistance not generally found in cultivars grown in Ontario. PI 88788 was susceptible to 36 (27%) of the Ontario H. glycines populations (Table 2). PI 548402 was susceptible to 20 (15%) populations. Of the Ontario H. glycines populations, 57% were determined to be HG Type 0 and 7, while none of the remaining 22 HG Types accounted for more than 14 populations and only one population was found for most of them (Table 1).
The current practice of using plant resistance to manage H. glycines may have limited value in the future unless changes are made to diversify the gene base for resistance to this nematode. Reproduction of H. glycines populations on PI 88788-derived cultivars has become a concern in areas where this source of resistance has been used. There was less evidence of adaptation to this source of resistance in Illinois, Indiana, and Ontario than in Tennessee where soybean resistance to SCN was available as early as 1960s (8). PI 548316 was susceptible to populations from all areas and should not be used in resistance breeding. This observation was also made by Mitchum et al. (13). Although PI 548316 is one of the indicator lines for determination of HG type (14) we found it to be of little value in HG-typing of our H. glycines populations as did others (13,23) because all populations were able to reproduce on this source of resistance. PI 437654 allowed very little reproduction of populations from all the areas, followed by PI 90763. These two may now be better sources of resistance to H. glycines in all locations than is PI 88788, one of the earliest known sources of SCN resistance. There is ample evidence of adaptation over time by SCN to this source of resistance.
There are over 120 identified sources of resistance to H. glycines (18) with similar cellular responses among some sources (3). There is still ample G. max germplasm to use to reduce the effect of H. glycines. In addition, there is demonstrated resistance in other soybean relatives (1). Both private and public breeders should deploy known resistance sources to a greater degree to provide growers with more options. Pyramiding different resistance genes or using greater resistance gene diversity within soybean is the best defense against this pest (1,15). PI 548402 allowed little reproduction on populations from Indiana and Illinois, but did not inhibit reproduction in characterized populations in Tennessee. In Ontario, virulent phenotype of some H. glycines populations on PI 548402 was unexpected because soybean cultivars with this source of resistance have not been widely grown in Ontario thus far. Use of plant resistance can be a driving force for virulent phenotype distribution but other factors including movement of soil can influence distribution.
Diversity among HG Types in these H. glycines populations from Ontario contrasted with less diverse populations found in areas further south, and might create a tough challenge for the development of resistant cultivars and management of H. glycines throughout Ontario and in nearby areas in northern states of the United States. These results are consistent with findings in Minnesota (23). Zheng and Chen suggest that this distribution is similar to that seen in China and may be due to climatic conditions (23). However, SCN populations in Tennessee and Missouri were more homogeneous in later than earlier studies (13,22).
Further monitoring and research is needed on H. glycines reaction variability to known resistance genes. The diversity of H. glycines soybean resistance in the United States is narrow because few sources of resistance have been used, partial/incomplete multigenic resistance is difficult to breed, resistance traits are linked to undesirable agronomic traits, and parents (or, resistance sources) are susceptible to other pathogens. Our research demonstrates that H. glycines diversity varies among areas and that choice of resistance sources should be expanded in cultivar development and use.
We thank the North Central Soybean Research Program for partial support of this research.
The use of trade, firm, or corporation names in this publication is for the information and benefit of the reader. Such use does not constitute an official endorsement or approval by the USDA Agricultural Research Service of any product or service to the exclusion of others that may be suitable.
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