PEST MANAGEMENT: DISEASES. Blast Vulnerability Detected in Novel Blast-Resistant Germplasm

Transcription

1 PEST MANAGEMENT: DISEASES Blast Vulnerability Detected in Novel Blast-Resistant Germplasm F.N. Lee and G.C. Eizenga ABSTRACT Previous research in artificially inoculated greenhouse tests and field nurseries identified new rice germplasm accessions as being resistant to the common blast (Pyricularia grisea) races found in Arkansas (IB-1, IB-49, IC-17, IE-1, IE-1k, IG-1, and IH-1) and eliminated those accessions with major blast-resistance genes (Pi-b, Pi-ta). currently utilized in U.S. varieties, thus identifying accessions with novel blast-resistant genes that could be incorporated into resistant varieties for Arkansas producers. Subsequent testing revealed an unanticipated blast vulnerability to archived P. grisea races IB-45, IB-54, and ID-13, which predominated during the 1950s and 1960s, in several of the resistant accessions. When inoculated with race ID-13, very-susceptible-type 7 to 8 leaf lesions developed in all tests of very closely related accessions 4607, 4611, 4612, 4632, 4642, 4593, 4594, 4633, and 4641(1); and R 312. Also, Guang 6ai-4, Wab P18-hb, and Wab450-I-B-P-62-hb developed susceptible-type 5 to 6 lesions in one or more tests inoculated with race ID-13. Susceptible-type 4 to 7 leaf lesions developed on all the aforementioned accessions in two of the four tests inoculated with race IB-54. A limited vulnerability to race IB-45 was also noted for all accessions and to race IB-17 for Wab P18-hb, Wab450-I-B-P-62-hb, and R 312. Most importantly, with the exception of a single IB-54 test, two accessions, Kechengnuo No. 4 and Shufeng 117, tested resistant to all the archived blast races to date. INTRODUCTION The University of Arkansas rice pathology research, funded in part by the Rice Research and Promotion Board (RRPB), has the overall objective of developing rice 139

2 AAES Research Series 550 disease-control strategies. This research effort requires a dedicated search for new disease-resistant germplasm obtained from various sources world wide. Once quarantined to eliminate unknown diseases, the new accessions are thoroughly tested in inoculated greenhouse and field disease nurseries. These tests are routinely conducted at the University of Arkansas Rice Research and Extension Center (UA-RREC) Stuttgart, Ark., and at the University of Arkansas Pine Tree Experiment Station (UA-PTES) located near Colt, Ark. Through joint collaborations with the USDA-ARS Dale Bumpers National Rice Research Center (DB NRRC) scientists conducting molecular analysis for DNA markers, it is now possible to identify many known disease-resistance genes and enhance identification of new resistance sources. The overall emphasis of the project is detection and identification of major resistance for common fungal diseases of rice occurring in the U.S., particularly rice sheath blight caused by Rhizoctonia solani and rice blast caused by Pyricularia grisea. This project recently reported the discovery of novel blast-resistant (R-) genes in germplasm accessions that are currently not utilized in contemporary U.S. varieties (Eizenga et al., 2004; Eizenga et al., 2006; Lee et al., 2003). These newly identified blast-resistance genes are currently being incorporated into new varieties by the UA breeding programs funded by the RRPB. This report presents data on an unexpected blast susceptibility in this very desirable germplasm to previously untested P. grisea races that were prevalent during the 1950s and 1960s PROCEDURES The germplasm accessions that tested resistant to contemporary blast races (IB-1, IB-49, IC-17, IE-1, IE-1k, IG-1, and IH-1) and lacked the major blast-resistance genes (Pi-b, Pi-ta) in previous tests were selected and evaluated in four different three-replication tests in the greenhouse facilities located at the UA-RREC. Check varieties were Drew, Wells, and Zhe 733. Standardized greenhouse procedures were used to test entry reaction to archived P. grisea races IB-17, IB-45, IB-54, or ID-13 that predominated during the 1950s and 1960s (Marchetti, 1994). Contemporary check race IE-1k was also included. Moderately drought-stressed plants at the 4-leaf (V4) growth stage were inoculated with an atomized spore suspension (2 x10 5 spores/ml) of individual races. Test plants were immediately placed in a 100% humidity chamber for 12 to 24 hours, moved to greenhouse benches, and grown under upland conditions for approximately 7 days when leaf-lesion severity was estimated using the standard visual 0 to 9 rating scale (Lee et al., 2003). Leaf-lesion categories were: 0 = no lesions; 2 to 3 = small, closed center lesions with brown borders indicate plant-restriction lesion development; 4 = slightly larger, usually elongated susceptible lesions with ash-grey centers and reddish brown borders; 5 to 6 = elongated blast-susceptible lesions with well-developed ashgrey centers and reddish-brown borders; 7 to 8 = larger susceptible type lesions with little if any evidence of border discoloration or other plant resistance response; and 9 = very large susceptible-type lesions with no evidence of plant response to infection that typically result in rapid leaf death. 140

3 B.R. Wells Rice Research Studies 2006 RESULTS AND DISCUSSION An unanticipated blast susceptibility was identified in novel germplasm accessions inoculated with archived isolates (Table 1). The nine very closely related germplasm accessions 4607, 4611, 4612, 4632, 4642, 4593, 4594, 4633, and 4641(1); and the more distantly related R 312 developed very-susceptible 7 to 8-type lesions in all tests when inoculated with race ID-13. In addition, accessions Guang 6ai-4, Wab P18-hb, and Wab450-I-B-P-62-hb developed susceptible type 5 to 6 lesions from race ID-13 in one or more tests. Susceptible-type 4 to 7 lesions developed on all the aforementioned accessions in two of the four tests inoculated with race IB-54. Accessions Guang 6ai-4 and Wab P18-hb developed type 4 to 5 lesions in three of four tests inoculated with race IB-45. Although remaining accessions were more tolerant, a vulnerability to race IB-45 was noted for novel germplasm accessions 4607, 4632, 4594, 4633, 4641(1), and R 312, which exhibited type 4 to 6 ratings in individual tests. Most accessions were resistant to race IB-17 although some vulnerability was noted with Wab P18-hb, Wab450-I-B-P-62-hb, and R 312 in individual tests. Check-entry Zhe 733 and germplasm accessions Kechengnuo No. 4 and Shufeng 117 were resistant, with the exception of a single IB-54 test, to the archived blast races tested. The standard 0 to 9 visual evaluation of leaf lesions provides a crude assay of plant resistance to specific blast races under controlled conditions. Errors in individual tests originate with plant growth conditions, environmental conditions in the greenhouse and inoculation chambers, viability and quality of inoculum, and evaluator skill. However, data from multiple tests generally estimate varietal resistance under typical production-field conditions. Type 0 to 3 lesions usually indicate a resistant variety. Type 5 to 6 lesions are typical for varieties with good to excellent field resistance when grown under high soil moisture or flooded field conditions but subject to severe leaf and rotten neck blast if moisture stressed. Varieties with type 7 and higher lesions characteristically require intense management practices or are unacceptable for general production fields. A consistent lesion type over multiple replicated tests generally estimates varietal performance. Inconsistent susceptible rating in individual tests, such as those recorded for races IB-54 and IB-45, provide an indication of varietal vulnerability as field conditions vary from optimal to stress conditions. If these classifications are correct, data presented in Table 1 indicate the accessions 4607, 4611, 4612, 4632, 4642, 4593, 4594, (1) and R 312 with type 7 to 8 lesions will be especially susceptible to race ID-13 in grower fields, comparable to known blast-susceptible varieties M-201 and Frances. Also, accessions Guang 6ai- 4, Wab P18-hb, and Wab450-I-B-P-62-hb will require growers to carefully manage cultural practices in the presence of race ID-13. Cultural practices will also determine susceptibility of all test entries to races IB-45 and IB-54. The uniformly high susceptibility to blast race ID-13 (Table 1) confirms previous molecular research (Eizenga et al., 2006) showing a close genetic relationship (Table 1) between accessions 4607, 4611, 4612, 4632, 4642, 4593, 4594, 4633, and 4641(1), which are placed in DNA cluster group 3 and have the same genetic background (K3). 141

4 AAES Research Series 550 In addition, this suggests these accessions share a common resistance gene. Accession R 312 also is highly susceptible to race ID-13, although placed in DNA cluster group 8, and also shares some of the same genetic background (K3). With respect to the race ID-13-susceptible accessions, it should be noted resistant accessions Shufeng 117 (DNA cluster group 8 with K3 and K7 genetic backgrounds) and Kechengnuo No. 4 (DNA cluster group 7 with genetic backgrounds in K3, K5 and K7) share the common genetic background K7, which may contain different R-genes. The new germplasm identifies one or more unique blast-resistance genes not utilized in current U.S. varieties. Unfortunately, the results presented here show some accessions are also vulnerable to older blast races predominating in varieties grown during the 1950s and 1960s. Due to P. grisea s inherent ability to adapt to the resistance genes present in the predominant rice varieties, re-appearance of the archived races or equally virulent races is anticipated if these new R-genes become widely utilized in Arkansas fields. In spite of this possibility, these new R-genes are extremely valuable. Although molecular markers to identify these new R-genes are not currently available, breeders can proactively pyramid resistance genes using greenhouse assays along with markers specific for R-genes such as Pi-ta, Pi-b, Pi-k h, and Pi-k s, which confer resistance to archived races IB-45, IB-54, and ID-13 (Eizenga et al., 2006). SIGNIFICANCE OF FINDINGS Rice growers depend upon proper conservation, discovery, and manipulation of resistance genes as a basic component of rice disease-control strategies. Defining disease liabilities of new blast-resistant R-genes provides plant breeders with information required to successfully incorporate that resistance source into modern rice varieties having higher yielding and quality characteristics. This research also alerts us to a potential problem of unexpected losses to rice blast such as those experienced with the variety Newbonnet during the 1980s or more recently with Banks during 2004 to ACKNOWLEDGMENTS Funding for this research was provided in part by rice grower check-off funds through the Arkansas Rice Research and Promotion Board. Hazel Mullins evaluated lesion severity. LITERATURE CITED Eizenga, G.C., F.N. Lee, and Y. Jia Identification of blast resistance genes in wild relatives of rice (Oryza spp.) and newly introduced rice (O. sativa) lines. In: R.J. Norman, J.-F. Meullenet, and K.A.K. Moldenhauer (eds.). B.R. Wells Rice Research Studies University of Arkansas Agricultural Experiment Station Research Series 517: Fayetteville, Ark. 142

5 B.R. Wells Rice Research Studies 2006 Eizenga, G.C., H.A. Agrama, F.N. Lee, W. Yan, and Y. Jia Twelve rice germplasm accessions identified with novel blast resistance for U.S. rice variety improvement. In: R.J. Norman, J.-F. Meullenet, and K.A.K. Moldenhauer (eds.). B.R. Wells Rice Research Series University of Arkansas Agricultural Experiment Station Research Series 540: Fayetteville, Ark. Lee, F.N., W.G. Yan, J.W. Gibbons, M.J. Emerson, and S.D. Clark Rice blast and sheath blight evaluation results for newly introduced rice germplasm. In: R.J. Norman and. J.-F. Meullenet (eds.). B.R. Wells Rice Research Studies University of Arkansas Agricultural Experiment Station Research Series 504: Fayetteville, Ark. Marchetti, M.A Race-specific and rate-reducing resistance to rice blast in U.S. rice cultivars. pp In: R.S. Zeigler, S. Leong, and P.S. Teng (eds.). Rice Blast Disease. Commonweath. Agric. Bur. Int., Willingford, U.K. 143

6 AAES Research Series 550 Table 1. Leaf lesion type observed when archived U.S. blast races in two (IE-1k) or four Germplasm accessions were grouped into clusters and Tests of inter- Accession IB-17 IB-45 IBname Shufeng Kechengnuo no Guang 6ai Wab P18-hb Wab450-I-B-P-62-hb (1) R DREW WELLS Zhe z Data taken from Eizenga et al (2006). 144

7 B.R. Wells Rice Research Studies 2006 blast-resistant germplasm was inoculated with (IB-17, IB-45, IB-54, or ID-13) different replicated tests. the genetic background according to DNA (SSR) markers. national blast races DNA Genetic -54 ID-13 IE-1k cluster back group z ground(s) z K3, K K3, K5, K K5, K K K K K K K K K K K K K3, K K K