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Crop Science 43:701-709 (2003)
© 2003 Crop Science Society of America

PLANT GENETIC RESOURCES

Evaluation of Spring Cereal Grains and Wild Triticum Germplasm for Resistance to Rhizoctonia solani AG-8

J. D. Smitha, K. K. Kidwell*,a, M. A. Evansb, R. J. Cookc and R. W. Smileyd

a Dep. of Crop and Soil Sci., Washington State Univ., Pullman, WA 99164-6420
b Program in Statistics, Washington State Univ., Pullman, WA 99164-3144
c Dep. of Plant Pathology, Washington State Univ., Pullman, WA 99164-6430
d Dep. of Botany and Plant Pathology, Oregon State Univ., Columbia Basin Agric. Res. Center, Pendleton, OR 97801-0370

* Corresponding author (kidwell{at}mail.wsu.edu)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
Rhizoctonia root rot, caused by Rhizoctonia solani Kühn AG-8 (Anastomosis Group 8), is a yield-limiting disease of direct-seeded cereals in the Pacific Northwest (PNW) region of the USA, and to date, no resistant Triticum germplasm has been identified. The objective of this research was to identify potential sources of genetic resistance among selected members of the primary, secondary, and tertiary gene pools of wheat (Triticum aestivum L.) for use in cultivar improvement. Members of the primary gene pool {spring wheat germplasm, synthetic hexaploids, triticale (x Triticosecale spp.), Triticum turgidum var. durum (Desf.) Bowden [= T. turgidum subsp. durum (Desf.) Husn.]}, secondary gene pool [Aegilops cylindrica Host, Dasypyrum villosum (L.) P. Candargy], and tertiary gene pool (Hordeum vulgare L.) of wheat were screened for disease response to two isolates (C1 and D2) of R. solani AG-8 in controlled environment assays. Variation for magnitude of susceptibility to both isolates was detected, but no sources of resistance were identified among primary or tertiary gene pool members. Isolate D2 generally produced more severe disease than isolate C1, with the exception of the most susceptible accessions for which both isolates caused equally severe disease. All accessions of D. villosum exhibited some level of seedling resistance to isolate C1, and 73% also displayed a resistance response to isolate D2. All D. villosum/durum amphiploids, as well as Chinese Spring/D. villosum addition lines, were susceptible to both isolates. Differences in disease response between the two isolates suggest that varying levels of virulence, or aggressiveness, exist among R. solani AG-8 isolates.

Abbreviations: AG, anastomosis group • CIMMYT, International Maize and Wheat Improvement Center • PNW, Pacific Northwest • WGRC, Wheat Genetics Research Center


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
RHIZOCTONIA ROOT ROT, caused by the soilborne fungal pathogen R. solani AG-8, was first diagnosed as a problem in direct-seeded wheat and barley in the PNW in the mid-1980s (Weller et al., 1986). Wheat is generally less severely affected than barley, and spring-seeded crops are more vulnerable to infection than fall-sown crops (Smiley, 1996). Yield losses (Smiley et al., 1990; Kelly and Dube, 2000) associated with Rhizoctonia root rot are especially pronounced in direct-seed systems (Pumphrey et al., 1987; Cook, 1990, 1991; Cotterill, 1990; Smiley, 1996). Despite the dramatic effect of R. solani AG-8 on cereal production, little effort has been made to develop tolerant or resistant cultivars for commercial use. Disease prevention through the use of genetically resistant cultivars can increase the viability of direct-seed production systems by reducing the risk of low crop yields associated with Rhizoctonia root rot.

Early evaluations of small grains for resistance to R. solani AG-8 in Australia included germplasm from oat (Avena sativa L.), barley (McDonald and Rovira, 1985), wheat, triticale, and rye (Secale cereale L.; Neate, 1989). This germplasm displayed varying levels of susceptibility to the disease rather than resistance (Neate, 1989). An evaluation of winter wheat cultivars in Oregon identified five lines with similar yield potential in research plots artificially infested with R. solani AG-8 and adjacent, noninfested control plots, suggesting the existence of tolerance (the host's ability to yield equally well under disease pressure) to this disease in the common wheat gene pool (R.W. Smiley and P.K. Zwer, 1993, unpublished data). In a previous study, several spring wheat genotypes adapted to the PNW also appeared to be more tolerant to pressure from R. solani AG-8 than were others (Smith et al., 2003). Although differences in tolerance were detected among genotypes, none demonstrated true resistance to the pathogen. In an investigation of the USDA National Small Grains barley collection (1214 accessions), a range of variation for disease reaction to R. solani AG-8 was detected among Hordeum germplasm (Jitkov, 1997). Twenty-four percent of the lines exhibited some level of resistance. These reports indicate that variation for disease reaction among cereal accessions exists; however, no specific germplasm line has been identified as a potential gene donor for resistance to R. solani AG-8. Durum and triticale have not been widely examined for reaction to R. solani AG-8 to date.

Aegilops tauschii Coss., the putative D genome progenitor of wheat, has been a useful source of resistance for several plant pathogens (Yildrim et al., 1995; Hussien et al., 1997). Synthetic hexaploid germplasm (AABBDD), created by the International Maize and Wheat Improvement Center (CIMMYT) through hybridization of durum (AABB) and A. tauschii (DD), has proven to be another valuable resource for genetic resistance to diseases and pests (Del Blanco et al., 2000). Synthetic hexaploids provide a gene transfer bridge between A. tauschii and T. aestivum, and therefore, may be beneficial genetic resources in wheat breeding programs.

Aegilops cylindrica (2n = 4x = 28; CD; primary gene donor) is a major weed in the wheat-production areas of the PNW. It is an allotetraploid that shares the D genome with bread wheat, and interspecific hybrids between the two species occur in the field (Zemetra et al., 1998), suggesting that A. cylindrica may be a viable gene donor for wheat improvement.

Dasypyrum villosum (L.) P. Candargy (2n = 14, VV; secondary gene donor) is a Mediterranean annual grass with potential for use in wheat improvement (Blanco et al., 1988) since it contains genes that confer resistance to a variety of pathogens (Leske, 1979; Heun and Mielke, 1983; Murray et al., 1984; Pei et al., 1986). Preliminary data from R.J. Cook and S.S. Jones (1998, unpublished data) suggest that D. villosum is a potential donor of genes for resistance to Rhizoctonia root rot. Amphiploids of T. aestivum and D. villosum have been obtained; however, F1 hybrids are male sterile and only partially female fertile since Triticum chromosomes do not pair with D. villosum chromosomes (Maan, 1987).

The objective of this research was to identify potential sources of genetic resistance to Rhizoctonia root rot among spring wheat germplasm and relatives of wheat for use in cultivar improvement.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 REFERENCES
 
Germplasm and Genetic Populations
Thirteen spring wheat and 12 spring barley cultivars evaluated in direct-seeded field trials (Kidwell et al., 1998) were screened for levels of susceptibility to R. solani AG-8 isolates C1 and D2. Cultivars were chosen based on preliminary fiber degradation results that reflect decomposition rates of crop residue (Stubbs, 2000), another potentially useful trait for direct-seed systems. CIho 8342 (Jitkov, 1997) was included as a resistant barley control. In addition, two spring cultivars of T. turgidum var. durum and four triticale lines were screened with R. solani AG-8. The hard red spring cultivar, ‘Scarlet’ (Kidwell et al., 1999), was included as a susceptible control in each evaluation (Smith et al., 2003).

Seventy-two synthetic wheat lines created at CIMMYT were obtained for evaluation from the Wheat Genetics Research Center (WGRC, Kansas State University). Pedigree information for these lines can be found at the WGRC website (http://www.ksu.edu/wgrc/Germplasm/synthetics.html). Six accessions of A. cylindrica collected in eastern Washington and Oregon (provided by F. Young, USDA-ARS, Pullman, WA) were assayed for reaction to R. solani AG-8.

Eleven accessions of D. villosum (from Greece and Italy; provided by S.S Jones and T.D. Murray, Washington State University) and two sets of disomic addition lines of D. villosum also were tested. E.R. Sears (1953) created one addition line set in a ‘Chinese Spring’ background using two accessions of D. villosum as V chromosome donors. This set is missing the addition line for the 3V chromosome. The second set of addition lines, provided by A. Lukaszewski (UC Riverside), is complete, and was created in a Chinese Spring background using a single Sicilian accession of D. villosum as the V chromosome donor parent (Lukaszewski, 1988). D. villosum/T. turgidum var. durum amphiploids, (2n = 6x = 42, AABBVV), also were screened for resistance to R. solani AG-8. In total, cultivars or accessions from nine distinct germplasm groups were assayed for resistance in this study.

Inoculum Production
Two isolates of R. solani AG-8 were used to assess disease reactions among genotypes. Isolate C1 (USDA-ARS Root Disease and Biological Control Research Unit, Pullman, WA) was originally isolated from spring barley grown near Clyde, WA, in Walla Walla County in 1984 (Weller et al., 1986). The D2 isolate (Oregon State Univ., Columbia Basin Agricultural Research Center, Pendleton, OR) was obtained near Pendleton, OR, in Umatilla County and was originally isolated from winter wheat in crop year 2000. Inoculum of both isolates was produced as previously described (Smith et al., 2003).

Growth Chamber Assays
Two separate growth chamber experiments were constructed to assess the reaction of cereal grain cultivars, amphiploids, addition lines, and accessions of D. villosum at the seedling stage to the presence and absence of R. solani AG-8. The first of these experiments consisted of a completely randomized experimental design with a two-way treatment structure (genotype crossed with the presence/absence of R. solani AG-8 isolate C1) with specific genotypes replicated 10 times per treatment combination within each growth chamber. Each growth chamber represented a complete experiment with genotype nested within a respective growth chamber (Steele et al., 1997).

The second growth chamber experiment was implemented to evaluate the reaction of remaining genotypes to the C1 and D2 isolates of R. solani AG-8. In an effort to better control variation between growth chambers and for improved ability to compare responses among genotypes across chambers, a randomized complete block design with a two-way treatment structure (genotype crossed with the presence/absence of R. solani AG-8 isolate C1) was used. Three replicates of each genotype were randomly placed in each of four growth chambers (blocks) and one of two treatments were applied: (i) pasteurized soil infested with R. solani AG-8 as oat grain inoculum; and (ii) noninfested pasteurized soil (control). The entire block design was repeated for the D2 isolate, resulting in a split-plot experimental design structure (isolate representing the whole-plot structure and the block design representing the subplot structure). Treatment preparation and plant establishment protocols are described in Smith et al. (2003).

Evaluations were performed according to the procedure described by Ogoshi et al. (1990), with the following modifications: growth chambers were programmed with a 14-h photoperiod, a daytime temperature of 23°C, and a nighttime temperature of 11°C (Smiley and Uddin, 1993); and humidity was maintained at 95% to decrease plant transpiration and evaporative losses from the soil. Roots were washed with a high-power stream of water, and then scored based on a 0 (no lesions evident) to 8 (all roots completely severed within 1 cm of the seed) scale as described (Smith et al., 2003). The presence of R. solani AG-8 in infected root tissue was confirmed by isolation on an agar medium (Smith et al., 2003).

Data Analyses
Data were analyzed using SAS (version 6.12; SAS Institute, Cary, NC). Evaluations of disease reaction among genotypes to R. solani AG-8 isolate C1 for the first growth chamber experiment was analyzed as a completely randomized design with genotype nested within growth chamber. Analysis of variance was conducted to test for significance of genotype and inoculum main effects, and to identify significant interactions between main effects (Steele et al., 1997). Variation due to chamber was controlled by including this effect in the ANOVA. For the second growth chamber experiment, the evaluation of disease reaction among the remaining genotypes to R. solani AG-8 isolates C1 and D2 was analyzed as a split-plot experimental design. The whole-plot main effects consisted of the R. solani AG-8 isolates C1 and D2 with blocks (chambers) acting as whole plot replicates. The subplot main effects consisted of genotype and inoculum (Steele et al., 1997).

For both experiments, least square mean comparisons were utilized to determine significant differences among genotypes (P < 0.05; Steele et al., 1997). In addition, single degree of freedom contrasts were used to make comparisons between germplasm types based on mean disease reaction across genotypes within each germplasm group.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
REFERENCES
 
For both experiments, a significant genotypic effect was detected through ANOVA procedures (Table 1). In addition, a significant interaction between inoculum and genotype was detected for both analyses. This interaction can be attributed to the variable disease reaction among germplasm types included in the evaluation (Table 1). In particular, no statistical difference was detected among genotypes when the R. solani AG-8 isolates were not present (data not shown). Because of the importance of comparing genotypes in the presence of the R. solani AG-8 isolates, and the statistically significant interaction between genotype and the inoculum treatment, only the least square means comparisons for genotypes in the presence of the R. solani AG-8 isolates are shown (Tables 2–4).


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  		Table 1. Analysis of variance results for mean disease ratings of Triticum, Dasypyrum, Aegilops and Hordeum vulgare genotypes screened in growth chamber evaluations for reaction to Rhizoctonia solani AG-8 isolates C1 and D2 in completely randomized (CR) or split-plot (SP) design experiments.

 

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  		Table 2. Mean disease score based on a 0 to 8 scale, (0 indicates no disease, and 8 indicates all roots severed), standard deviation, and replicate number of spring wheat, Dasypyrum, and spring barley genotypes screened in completely randomized growth chamber evaluations for reaction to Rhizoctonia solani AG-8 isolate C1.

 

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  		Table 4. Mean disease score based on a 0 to 8 scale (0 indicates no disease, and 8 indicates all roots severed), standard deviation, and replicate number of a spring wheat, Dasypyrum, Triticum, and spring barley genotypes screened in growth chamber evaluations for reaction to Rhizoctonia solani AG-8 isolate D2 in split-plot assays.

 

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  		Table 3. Mean disease score based on a 0 to 8 scale (0 indicates no disease, and 8 indicates all roots severed), standard deviation, and replicate number of Triticum and Aegilops germplasm screened in growth chamber evaluations for reaction to Rhizoctonia solani AG-8 isolate C-1 in split-plot assays.

 
On average, higher disease ratings were observed with isolate D2 than for C1 (D2 = 5.5, C1 = 4.9), with the exception of ratings for the addition lines (C1 = 6.6; D2 = 5.5) and barley cultivars tested (C1 = 6.1; D2 = 5.8; Tables 2–4). Differences in disease response between isolates suggest that varying levels of virulence may exist among R. solani AG-8 isolates. Direct statistical comparisons between genotypes within germplasm groups, in terms of isolate effects, cannot be made due to experimental design differences between assays for each isolate.

Genotypes evaluated for disease reaction were grouped into nine categories based on disease ratings: highly resistant (0–0.9); resistant (1.0–3.0); moderately resistant (3.1–4.0); moderately susceptible (4.1–5.0); susceptible (5.1–6.0); and highly susceptible (6.1–8.0) (modified from Jitkov et al., 1996). The noninfested pasteurized soil treatment always received a score of 0 (data not shown). Scarlet, the spring wheat cultivar included as a control in each evaluation, was rated moderately susceptible to D2 (Table 4) and susceptible to C1 (Table 2), which agrees with previous disease ratings for this cultivar (Smith et al., 2003).

All spring wheat, barley, durum, and triticale cultivars were moderately to highly susceptible (4.4–7.1) to both isolates of R. solani AG-8 (Tables 2–4). Even though all of the spring wheat cultivars tested were susceptible (C1 = 5.5, D2 = 5.6; range 4.4–6.6) to R. solani AG-8, variation in disease reaction within isolate treatments was detected among cultivars based on least square means comparisons (Tables 2–4), which agrees with previous spring wheat screening results for isolate C1 (Smith et al., 2003). This also was the case for the spring barley cultivars (C1 = 6.1, D2 = 5.8; range 5.3–7.1) and the synthetic hexaploids (C1 = 5.0, D2 = 5.7; range 4.2–6.2).

In contrast to a previous report (Jitkov, 1997), the putative resistant control CIho 8342 (C1 = 6.1, D2 = 5.5) was scored as susceptible in our study. Other field (R.W. Smiley, 1999, personal communication) and greenhouse (O. Ziv and R.J. Cook, 2000, personal communication) disease screening studies also were unable to confirm Clho 8342 as resistant to R. solani AG-8. However, in a recent field screening of barley genotypes exposed to low- and high-inoculum levels of R. solani AG-8, Clho 8342 was rated as having a low percentage of diseased roots and had similar grain yields across high- and low-inoculum treatments, suggesting that it has some tolerance or resistance to this disease (Wesselius, 2001). While more studies using different environmental conditions and levels of disease pressure are obviously needed to clarify the reaction type of this line, these seemingly contradictory results reveal the difficulties associated with detecting subtle, and possibly environmentally sensitive, levels of resistance or tolerance to this disease.

Variation was detected between durum lines in response to the D2 isolate only (C1 = 4.7, D2 = 5.9; range 4.6–6.3). No significant variation for disease reaction was detected among triticale lines (C1 = 4.8, D2 = 5.2; range 4.6–5.3), or among accessions of A. cylindrica (C1 = 4.4, D2 = 5.4; range 4.2–5.9) within isolate treatments. This may be associated, in part, with the low number of accessions evaluated for each of these germplasm types.

On average, significantly lower disease ratings (P < 0.05; Tables 2–4) were obtained in response to both isolates among all eleven accessions of D. villosum compared with spring wheat, spring barley, durum, triticale, synthetic wheat, D. villosum addition lines, D. villosum/durum amphiploids and A. cylindrica accessions. In particular, seminal roots of D. villosum accessions exhibited fewer lesions than the other types of germplasm, especially when infected with C1, and roots of D. villosum genotypes also maintained their white, healthy appearance when exposed to either C1 or D2. In response to the D2 isolate, one accession of D. villosum was resistant, 63% were moderately resistant, and the remaining accessions (27%) were moderately susceptible (range 3.3–4.6), providing more evidence that isolate D2 may be more aggressive or virulent than isolate C1.

All five D. villosum/durum amphiploids were susceptible to both isolates of R. solani AG-8 (C1 = 5.4; D2 = 6.2; Tables 2 and 4). In addition, the disomic Chinese spring/D. villosum addition lines screened in an attempt to locate the resistance gene or genes within D. villosum to a chromosome all were susceptible to R. solani AG-8 (Tables 2 and 4). This absence of resistance in derivatives of D. villosum may be attributed to one of the following factors: (i) The accessions used to make these lines may have been susceptible to R. solani AG-8; or (ii) resistance in the diploid may be masked when hybridized with tetraploid and hexaploid wheat parents. Similar results were obtained in a survey of wheat germplasm for resistance to take-all [caused by Gaeumannomyces graminis (Sacc.) Arx & D. Olivier var. tritici], where increased tissue blackening caused by the pathogen was observed in the durum/A. tauschii amphiploids compared with the original resistant A. tauschii parent (Eastwood et al., 1993). Loss of gene function is common when transferring genes from lower to higher ploidy levels (Kerber and Green, 1980) and may be due to the influence of one or more intergenomic suppressor loci in the wheat genomes (Innes and Kerber, 1994).

Thus far, moderate levels of resistance have been identified solely among accessions of the wild relative D. villosum, a member of the secondary Triticum gene pool. This germplasm may be useful as a gene donor source for disease resistance to R. solani AG-8 in wheat cultivar improvement programs.


   ACKNOWLEDGMENTS
 
We thank David Weller, USDA-ARS, Pullman, WA, for his technical assistance with R. solani AG-8 pathology and experimental development. We also thank the following researchers for providing germplasm utilized in this study: Adam Lukaszewski, UC-Riverside; Stephen Jones and Timothy Murray, Washington State University; Frank Young, USDA-ARS Pullman, WA; and Bikram Gill, Kansas State University. Funding for this project was provided by the O.A. Vogel Fund (project no. 3568), O.A. Vogel Fellowship, and the College of Agriculture and Home Economics at Washington State University. This is scientific paper no. 0106-23 from the Dep. of Crop and Soil Sciences, Washington State University, Pullman, WA.

Received for publication February 11, 2002.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 





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