Diversity and Sources of Multiple Disease Resistance in Hordeum spontaneum
Fetch, T G Jr
ABSTRACT
Fetch, T. G., Jr., Steffenson, B. J., and Nevo, E. 2003. Diversity and sources of multiple disease resistance in Hordeum spontaneum. Plant Dis. 87:1439-1448.
Hordeum spontaneum, the progenitor of cultivated barley, is known to be a rich source of disease resistance genes. The objective of this study was to assess the diversity of H. spontaneum accessions from Israel and Jordan for their reaction to six fungal pathogens of importance to cultivated barley in the United States and Canada. Overall, a high level of macro-scale (across collection sites) and micro-scale (within a collection site) diversity for disease reaction was found in the 116 accessions of H. spontaneum evaluated at the seedling stage. Additionally, genetic heterozygosity for resistance loci was common in H. spontaneum. The frequency of resistance in accessions from Jordan and Israel was high for Septoria speckled leaf blotch (77 and 98%, respectively), leaf rust (70 and 90%), net blotch (72 and 68%), and powdery mildew (58 and 70%); intermediate for spot blotch (53 and 46%); and low for stem rust (2 and 26%). The level of disease resistance in H. spontaneum was not strongly correlated with any of the weather variables (temperature, precipitation, and humidity) monitored near the collection sites. However, in general, resistance was more often found in germ plasm from mesic (e.g., Mediterranean coast) than in xeric (e.g., Negev Desert) areas. Two H. spontaneum accessions (Shechem 12-32 and Damon 11-11) were resistant to all six pathogens and may be useful parents in programs breeding barley for multiple disease resistance. The high level of diversity and heterozygosity for disease reaction found in this study indicates that H. spontaneum is an extraordinarily rich and largely untapped source of unique disease resistance alleles for cultivated barley improvement.
Additional keywords: Hordeum vulgare, wild barley
The Upper Midwest region of the United States and the central provinces of Canada comprise one of the largest barley (Hordeum vulgare L.) growing regions in North America. Diseases that commonly impact barley production in this region include stem rust (caused by Puccinia graminis Pers.:Pers. f. sp. tritici Eriks. & E. Henn.), net blotch (caused by Pyrenophora teres Drechs f. teres Smedeg. [anamorph: Drechslerei teres f. teres (Sacc.) Shoem.]), spot blotch (caused by Cochliobolus sativus (Ito & Kurib.) Drechs. ex Dastur [anamorph: Bipolaris sorokiniana (Sacc.) Shoem.]), Septoria speckled leaf blotch (SSLB) (caused by Septoria passerinii Sacc.), and to a lesser degree leaf rust (caused by Puccinia hordei G. Otth) and powdery mildew (caused by Blumeria [=Erysiphe] graminis (DC.) E.O. Speer f. sp. hordei Em. Marchai) (39,41). The deployment of host resistance is often the preferred method of control for these diseases because it is an effective, economical, and environmentally sound strategy.
Although sources of resistance to these diseases have been identified in H. vulgare, they do not represent the greatest potential diversity available for breeding purposes. An alternative and rich source of genetic diversity for disease resistance is the wild barley species H. spontaneum (syn. H. vulgare L. subsp. spontaneum (C. Koch) Thell.). H. spontaneum is native to the Fertile Crescent (32,50), a large region extending from Israel and western Jordan to southeastern Turkey and arcing down through eastern Iraq and western Iran (12). In this region, the host and its pathogens have co-evolved over thousands of years (3), resulting in a high degree of diversity for both resistance in the host and virulence in the pathogen. Indeed, previous investigators have identified numerous sources of resistance in H. spontaneum to powdery mildew (11,15,31), leaf rust (17,27), net blotch (14,22,38), SSLB (22,28,45), and leaf scald (1).
The center of origin for wild progenitor species is often the place where the greatest genetic diversity may be found. With respect to H. spontaneum, the region of the Fertile Crescent bounded by Israel and Jordan may contain the greatest genetic diversity for the species (4,32). Disease resistance is common in H. spontaneum; thus, it may be possible to identify individual accessions with resistance to all of the important diseases in the Upper Midwest region of the United States and central provinces of Canada. These accessions could then be crossed with advanced barley breeding lines to simultaneously transfer genes for multiple disease resistance and also increase allelic diversity. Thus, the objective of this study was to assess the diversity of H. spontaneum accessions from Israel (including Palestinian authority lands of the West Bank and the Gaza Strip) and Jordan for their reaction to six fungal pathogens and to identify sources of multiple disease resistance. A preliminary report of this research has been published (10).
MATERIALS AND METHODS
Plant materials. One hundred and sixteen H. spontaneum accessions from the Institute of Evolution, University of Haifa, Israel, were evaluated for resistance to six foliar pathogens in the greenhouse. Each accession was derived from a single plant and was arbitrarily selected from 57 collection sites in Israel and Jordan (Fig. 1). The accessions at each site were collected no closer than 1 m apart along a transect of 25 to 200 m. Collection sites were ecogeographically diverse and varied greatly with respect to humidity (34 to 65% mean annual relative humidity (RH) at 1400 hours), precipitation (71 to 1,600 mm annually), temperature (1 to 16°C and 20 to 330C mean annual temperature for January and August, respectively), and elevation (-350 to 1,530 m below/above sea level) (Tables 1 and 2). Standard susceptible and resistant checks and differential host lines (if available) also were included in the experiments to verify the infection type (IT)/infection response (IR) and virulence pattern, respectively, to each pathogen (8,18,21,40,45,52).
Pathogen isolates. Information on the isolate designation, source, and pathotype (if known) of each fungal pathogen used is given in Table 3. All pathogen isolates were collected from infected barley plants in the field with the exception of the B. graminis f. sp. hordei isolate, which was taken from an infected plant in the greenhouse. Pathogens were stored as dried urediniospores at -80°C (rusts) or as conidiospores using a silica gel method (9), with the exception of the B. graminis f. sp. hordei isolate, which was from fresh plants. The pathogen isolates used represent common virulence spectra found in the Upper Midwest region of the United States. Two pathotypes of the stem rust pathogen were used in this study to more fully characterize the resistance spectra present in H. spontaneum. Pathotype QCCJ possesses virulence for the barley stem rust resistance gene Rpg1 and avirulence for rpg4, while pathotype MCCF possesses avirulence for both Rpg1 and rpg4 (18). Inoculation protocols. For each disease evaluation, five seeds of each H. spontaneum accession were planted either in plastic pots (10 cm square) or in cones (3.8 cm diameter and 21 cm depth) filled with a peat mossiperlite (3:1 ratio) potting mixture. Fertilizer was applied at planting with water-soluble (15-0-15, N-P-K) and controlled release (14-14-14, N-P-K) formulations. Plants were grown in a greenhouse at 22 ± 3°C with a 13-h photoperiod provided by sunlight and metal halide bulbs (530 to 710 µE·m^sup -2^·s^sup -1^). For leaf and stem rust evaluations, 7-day-old seedlings (primary leaf fully expanded) were inoculated with heat-shocked urediniospores suspended in a lightweight mineral oil (4 mg of spores in 0.7 ml of oil per 100 plants). Plants were then placed in mist (generated by ultrasonic humidifiers) chambers and incubated at 20°C for 16 h in the dark at near 100% RH. After the mist period, leaf rust inoculated plants were allowed to slowly dry for at least 3 h before being placed in a growth chamber at 19 to 20°C. Stem rust inoculated plants were exposed to light (120 to 160 µE·m^sup -2^·s^sup -1^ and a temperature ramp increasing from 20 to 24°C) after the mist period and allowed to slowly dry for at least 3 h before being placed into one of two growth chamber environments (19 to 20°C or 27 to 28°C). The two incubation environments were used because some stem rust resistance genes are known to be temperature sensitive (18). For net and spot blotch evaluations, inoculum was produced on agar plates seeded with silica gel crystals containing adsorbed conidia. seedlings were inoculated (5 to 6 × 10^sup 4^ conidia per ml of H2O, 1.7 × 10^sup -1^ ml per plant) when the third leaf was fully expanded. Plants were placed in mist chambers overnight at 20°C and then incubated in a greenhouse at 22 ± 3°C. For powdery mildew evalualions, seedlings were inoculated at the primary leaf stage by shaking mildewinfected plants (previously infected with a single pustule isolate) over the H. spontaneum accessions and then incubating at ambient greenhouse temperature (23 ± 2°C) (41). For SSLB evaluations, accessions were inoculated (5 × 10^sup 5^ pycnidiospores per ml of H2O, 5.0 × 10^sup -1^ ml per plant) when the second leaf was fully expanded and then subjected to a 72-h mist period at 21°C/dark and 25°C/light (45). The first 40 h were in darkness, followed by a photoperiod of 5 h for the remainder of the mist period. Plants were then incubated in a greenhouse at 22 ± 3°C. All experiments were repeated once.
Disease assessment. Disease reactions were assessed after specific incubation periods using published assessment guides for each respective pathogen. Leaf and stem rust reactions were rated 10 to 13 days postinoculation using scales modified from Levine and Cherewick (24) and Miller and Lambert (29), respectively. Powdery mildew was rated 9 to 10 days postinoculation using the 0 to 4 scale of Torp et al. (44). Net and spot blotch reactions were rated 10 to 12 days postinoculation using the 1 to 9 scales of Tekauz (43) and Fetch and Steffenson (9), respectively. SSLB was rated 17 to 18 days postinoculation using a 0 to 5 scale (45). For the rusts, powdery mildew, and SSLB, ITs/IRs of 0 or 1 were considered indicative of resistance, 2 of intermediate resistance, and 3 or higher of susceptibility. For net and spot blotch, IRs of 1 to 3 were considered indicative of resistance, 4 to 5 of intermediate resistance, and 6 to 9 of susceptibility.
Statistical analyses. Phenotypic diversity for disease reaction in H. spontaneum was estimated using the Shannon information index, where hs·j = -[summation operator]p^sub i^, ln(p^sub i^), with maximum h^sub s·j^ = 1.099 (16). For this analysis, the ITs/IRs for each disease were separated into the general reaction classes of resistant, intermediate, and susceptible as previously described. Shannon’s information index (h^sub s·j^) was determined based on the frequency (p^sub i^) of each class. Chi-square values were calculated using contingency tables (2 × 3) to estimate the homogeneity between populations derived from Israel and Jordan for each disease. Pearson correlation coefficients (PROC CORR, SAS version 8, SAS Institute, Gary, NC) were used to assess the relationship between the average infection response (IR/IT) of each accession and the environmental parameters (i.e., those most likely to affect disease pressure) at each H. spontaneum collection site. For net and spot blotch, the actual IR was used for correlation, while for the other diseases the IR/IT was adjusted from a 0 to 4 scale to a 1 to 5 scale (by adding 1) for statistical purposes.
RESULTS
The reactions of control and differential lines were consistent with previously published results (8,18,21,40,45,52). Accessions of H. spontaneum exhibited similar, but not identical ITs/IRs in the two experiments. The most common ITs/IRs observed on H. spontaneum across the two experiments are presented in Tables 4 and 5.
The highest frequency of disease resistance found in H. spontaneum was to S. passerinii. Over 98% of accessions from Israel (Table 4; all but Eyzariya 15-29) and 77% of accessions from Jordan (Table 5) exhibited IRs of 0 to 2 to S. passerinii. Most accessions (83% from Israel and 57% from Jordan) exhibited an IR of 0, indicating a very high level of resistance. Resistant and susceptible accessions were found within individual collection sites in both Israel (Eyzariya) and Jordan (N. Shuna, Wadi Mujib, Talal, Karak, Laban, Tafila, S. Shuna, and Jarash). This indicates that micro-scale diversity exists for S. passerinii reaction at some locations.
Accessions of H. spontaneum with resistance (ITs of 0 to 2) to powdery mildew and leaf rust also were very common in both Israel (Table 4; 70 and 90%, respectively) and Jordan (Table 5; 58 and 70%, respectively). Highly resistant (IR of 0 or IT of 0;) accessions comprised 41 and 44% of the total from Israel and 36 and 13% of the total from Jordan for the two respective diseases. Resistant and susceptible accessions were found at a number of collection sites in both Israel (Gadot, Shechem, Bar Giyyora, Eyzariya, Tel Shoket, Bor Mashash, Yeroham, Wadi QiIt, Ein-Zukim, and Avedat) and Jordan (Irbid, East Irbid, West Mafraq, Amman, Madaba, Mt. Nebo, Wadi Mujib, Talal, Laban, Tafila, Wadi Musa, Safi, South Shuna, Salt, Sakib, and S. Irbid), indicating micro-scale diversity for reaction to the two diseases. Plants resistant and susceptible to powdery mildew were found within several accessions from Israel (Gadot 6-35, Eyzariya 15-29, and Ein-zukim 32-05). The same was true for leaf rust with accessions Yeroham 1932, Sede Boqer 20-22, and Machtesh SI13. This result suggests heterozygosity for powdery mildew and leaf rust resistance loci in these single plant-derived accessions.
Resistance to net blotch also was frequent in H. spontaneum, as about 70% of accessions from both Israel (Table 4) and Jordan (Table 5) exhibited IRs of 1 to 5. Approximately one-third of the accessions from both countries were highly resistant (IRs of 1 to 2). Resistant and susceptible accessions were found at 11 of the 30 collection sites in Israel and 15 of the 27 collection sites in Jordan. This indicates that micro-scale diversity for net blotch reaction exists across a number of locations in both countries, especially Jordan. Resistant and susceptible plants were detected within three accessions (Laban 15-09, Dana 17-48, and Ajlun 25-35) from Jordan, suggesting heterozygosity at net blotch resistance loci.
Resistance to spot blotch was less common in H. spontaneum, as only 46 and 53% of accessions from Israel (Table 4) and Jordan (Table 5), respectively, exhibited IRs of 1 to 5. Only six highly resistant (IRs of 1 to 2) accessions were found from the two countries. Resistant and susceptible accessions were found at 14 of the 30 collection sites in Israel and 10 of the 27 collection sites in Jordan, again indicating micro-scale diversity for spot blotch reaction across a number of locations. Resistant and susceptible plants also were detected within two accessions (Tabigha 7-12 and Eyzariya 15-29) from Israel, suggesting heterozygosity at spot blotch resistance loci.
The lowest frequency of disease resistance in H. spontaneum was to stem rust. Only 26 and 11% of the accessions from Israel exhibited ITs of O; to 2+ to pathotypes MCCF and QCCJ, respectively, at the low incubation temperature (Table 4). The resistance in most of these accessions was temperature sensitive, as only 4 of 16 and 1 of 7 accessions remained resistant to pathotypes MCCF and QCCJ, respectively, at the high incubation temperature. Shechem 12-37 was the only highly resistant (IT = O;) accession identified (pathotype MCCF at low temperature). Accessions Mt. Hermon 1-46, Gadot 6-35, Zefat 8-05, Damon 11-11, Shechem 12-32, Tel Shoket 16-46, and Revivum 18-04 exhibited some resistance to both pathotypes at the low incubation temperature. Resistance to stem rust was rare in H. spontaneum accessions from Jordan, as only three accessions (Zarqa 6-21, Madaba 8-32, Wadi Wala 10-05) exhibited ITs of 2^sup -^ to 2^sup +^ to pathotype MCCF or QCCJ under either of the two incubation environments (Table 5). Zarqa 6-21 was the only accession that was resistant to both pathotypes (low incubation temperature only). Resistant and susceptible accessions to at least one of the pathotypes were found at 12 of the 30 collection sites in Israel and three of the 27 collection sites in Jordan. Resistant and susceptible plants also were detected within five accessions (Rosh Pinna 5-23, Shechem 12-37, Tel Shoket 16-46, Revivum 18-04, and Yeroham 19-32) from Israel to at least one of the two pathotypes, suggesting heterozygosity at stem rust resistance loci.
A high level of diversity (h^sub s·j^ = 0.993 to 1.025) for reaction to powdery mildew, leaf rust, net blotch, and spot blotch was found in the combined analysis for accessions from both Israel and Jordan (Table 6). The lowest level of diversity found was for reaction to SSLB and stem rust (pathotype MCCF) (h^sub s·j^ = 0.615 and 0.519, respectively). Chi-square analysis using 2 × 3 contingency tables revealed that the frequency of general disease reactions was similar between accessions from Israel and Jordan for powdery mildew ([chi]^sup 2^ = 1.64, P = 0.45, NS), net blotch ([chi]^sup 2 ^ = 0.84, P > 0.95, NS), and spot blotch ([chi]^sup 2^ = 1.67, P = 0.45, NS). However, significant differences in class frequencies were found between accessions from the two countries for SSLB ([chi]^sup 2^ = 13.04, P
The relationship between environmental parameters near the collection sites and the general disease response of H. spontaneum accessions is presented in Table 7. In general, no consistent and statistically significant relationships were detected between the environmental parameters of altitude, temperature, rainfall, or humidity and the disease response of H. spontaneum. A low but significant correlation was found between the level of resistance to Puccinia hordei and the amount of rainfall and level of humidity in both Israel and Jordan. In Israel, resistance to B. graminis f. sp. hordei was associated with high humidity and cooler daytime temperatures in August.
DISCUSSION
Overall, a high level of macro-scale diversity (i.e., across collection sites) for disease reaction was found in the 116 accessions of H. spontaneum evaluated. This result confirms previous studies documenting diversity for reaction to different diseases (e.g., powdery mildew, leaf rust, and leaf scald) in this wild species (1,15,30). Other studies reporting a high level of macro-scale diversity in H. spontaneum have included assays of DNA polymorphism (5,6,46), allozymic variation in proteins (33,35), growth physiology (48), and agronomic traits (34). The level of macroscale diversity to individual pathogens varied greatly in this study as measured by Shannon’s information index. In the combined analysis from both Israel and Jordan, the highest level of diversity was observed for reaction to B. graminis f. sp. hordei (1.025) and Pyrenophora teres f. teres (1.021), and the lowest to S. passerinii (0.615) and Puccinia graminis f. sp. tritici (0.519). The low level of diversity found for reaction to 5. passerinii and Puccinia graminis f. sp. tritici was due to a high frequency of resistance and susceptibility, respectively. In addition to macro-scale diversity, we also detected micro-scale diversity within individual collection sites where accessions were gathered no closer than l m apart along transects of 25 to 200 m. Micro-scale diversity within individual collection sites also was found for powdery mildew and leaf rust reaction by Jana and Nevo (15), who studied accessions of H. spontaneum from Iran, Syria, and Turkey. Resistant and susceptible plants were identified within individual accessions (originally derived from single plants) to all diseases except SSLB, indicating that genetic heterozygosity exists for many resistance loci. This phenomenon appears to be a common feature of gramineous species at their center of origin (53). The disease evaluations in this investigation were all done on seedling plants to just one or two pathotypes of each pathogen. As such, these evaluations assess the effects of seedling resistance genes with major effects and therefore represent only a small fraction of the potential diversity for disease resistance that may be important in agriculture. A wide spectrum of diversity for adult plant resistance (both qualitative and quantitative) also exists in H. spontaneum (3), but was not considered in this investigation. With the high level of macro- and micro-scale diversity for disease reaction, extensive genetic heterozygosity for resistance loci, and largely uncharacterized mechanisms of adult plant resistance, H. spontaneum is an incredibly rich and largely untapped source of unique disease resistance alleles for cultivated barley improvement.
The environment can be a key component contributing to the range of genetic diversity found in various organisms (49). Patterns of genetic variation in H. spontaneum, including disease resistance, are influenced by environmental factors such as temperature and humidity and are not random (33,36). Environmental conditions that favor high disease pressure should increase selection for disease resistance. This implies that possible locations of resistance sources can be predicted using environmental and ecological information. However, in this study, there generally was not a relationship between environment and disease resistance. Low but significant correlations were found between the level of resistance to Puccinia hordei and the amount of rainfall and level of humidity at collection sites in both Israel and Jordan. Higher levels of leaf rust resistance were associated with higher rainfall and humidity. These significant correlations are not surprising given that Puccinia hordei requires moist conditions for infection and that high rainfall and humidity will increase disease pressure. However, S. passerinii, Pyrenophora teres f. teres, C. sativus, and Puccinia graminis f. sp. tritici also require moisture for infection, but no significant correlations were found to environmental parameters for these pathogens. There are several possible reasons for this result. For S. passerinii and Puccinia graminis f. sp. tritici, the lack of correlation between resistance and environmental parameters is likely related to the low degree of diversity found in the accessions for reaction to the two pathogens. Another possibility is that the general environmental data taken from weather stations near the collection sites may not be indicative of the micro-scale environment where H. spontaneum is subject to pathogen infection (e.g., in valleys or other water courses where moisture accumulates). It is also likely that the critical periods for pathogen infection and spread occur within short and specific time intervals, and that the environmental conditions that favor infection during these periods are not reflected in the values (e.g., average annual rainfall or humidity at 1400 hours, January temperature) that were used in this analysis. Environmental parameters such as leaf wetness duration, relative humidity, and temperature measured at the time of year that corresponds to critical plant growth stages (tillering to heading) may give a better indication of environmental influence on disease pressure, which is more likely to correlate to evolution of disease resistance. Due to the overall lack of correlation between resistance and environmental parameters assessed in this study, it would be difficult to reliably identify specific sites where possible sources of resistance to specific diseases might be found. However, this may be possible at a regional level. The frequency of resistance in H. spontaneum was generally higher in areas of greater precipitation. Thus, one may be more likely to find resistant H. spontaneum accessions in environments that are mesic (e.g., along the Mediterranean coast and Jordan River) than in those that are xeric (e.g., Negev Desert).
The frequency (77 to 98%) and level (most with IR of 0) of resistance to SSLB in H. spontaneum was the highest among all diseases examined in this study. This was especially true for accessions from Israel. Resistance to SSLB appears to be very common in H. spontaneum, as many resistant accessions have been reported from other countries such as Iraq, Lebanon, Syria, and Turkey (22,28,45). S. passerinii has been reported on cultivated barley in Israel (19), but its role in the coevolution of H. spontaneum resistance is not known. It is possible that a higher level of variation for reaction to SSLB exists in H. spontaneum but was simply not detected by the isolate of S. passerinil used in this study. Additional studies using isolates of S. passerinil from the Middle East and other regions should be conducted to obtain a more comprehensive assessment of the diversity in H. spontaneum to SSLB. It was interesting to note that accessions with SSLB resistance were found at several collection sites (e.g., Bor Mashash, Revivum, Yeroham, and Sede Boqer) in the Negev Desert, where precipitation is less than 200 mm per year. This result was unexpected given that S. passerinil is a splash-dispersed pathogen and requires moist weather conditions for infection and spread.
Several previous studies have found H. spontaneum to be a rich source of powdery mildew resistance (7,15). Our data confirmed these findings, as 58 and 70% of accessions from Jordan and Israel, respectively, exhibited resistant infection responses. In this study, resistance was more common in mesic environments, although several highly resistant accessions were also identified in the xeric environments of Revivum and Machtesh-Gadol in the Negev Desert of Israel. This result is in contrast to previous research where powdery mildew resistance was absent at sites near the Negev Desert (31). The contrasting results found between this and the other two studies may be due to the different pathogen cultures used or perhaps a limited host sample size. Nevertheless, this indicates that sources of powdery mildew resistance can be found in areas where the environment might be considered unfavorable for the development of the disease. Fischbeck and Jahoor (11) studied the genetics of powdery mildew resistance in H. spontaneum accessions obtained from Israel and found that the resistance response was quantitative when challenged by local isolates of B. graminis f. sp. hordei, but qualitative (hypersensitive reactions) when challenged by European pathogen isolates. This finding has important implications regarding the choice of pathogen isolates used in disease screening tests. Future evaluations of this germ plasm should be made to local pathotypes isolated at or near sites where the H. spontaneum accessions were collected.
The percentage of H. spontaneum accessions exhibiting resistance to Puccinia hordei was high (70%) and very high (90%) in Jordan and Israel, respectively. The alternate host of Puccinia hordei is Ornithogalum (Star of Bethlehem lily), which is endemic to Israel and co-exists with H. spontaneum in many regions of the country (3). Strong co-evolutionary forces between the host and pathogen (as mediated by the alternate host) have been operating for several millennia and likely contributed to the great diversity found in H. spontaneum for leaf rust reaction. Leaf rust resistance was more common in accessions from collection sites with higher rainfall and humidity, as revealed in the correlation analysis. Anikster et al. (2) found that leaf rust resistance was common in H. spontaneum accessions from central and northern Israel, but not in the arid regions of the south. Similarly, Moseman et al. (30) found leaf rust resistance common in accessions from the Coastal Plains of Israel, where the environmental conditions are more conducive for disease development (i.e., higher rainfall and humidity) and Ornithogalum is present. Manisterski et al. (27) speculated that H. spontaneum originating from dry regions in Israel would likely be universally susceptible to leaf rust. However, in this study, we identified highly resistant accessions (IT of 0;) from the arid sites of Revivum and Sede Boqer in the northern Negev Desert. This finding highlights the great diversity for disease resistance in H. spontaneum across the center of origin (51). Our study also confirms and extends previous studies (17,27,30) documenting the large reservoir of unexploited leaf rust resistance in this wild progenitor species. Indeed, four of the seven most recently described leaf rust resistance genes in Hordeum (RphlO, Rph11, Rph15, and Rph16) were derived from H. spontaneum (13). Future leaf rust research on H. spontaneum should include evaluations to more widely virulent cultures of Puccinia hordei and genetic tests to determine the allelic relationships of resistance genes to those already described.
About two-thirds of the H. spontaneum accessions evaluated in this study exhibited intermediate to high levels of resistance to Pyrenophora teres f. teres. Similar results were found for accessions from the Middle East by several other research groups (14,22,28,38). Kenneth et al. (20) reported a high degree of diversity for net blotch reaction in H. spontaneum and for virulence in Pyrenophora teres f. teres in Israel. It is therefore likely that Israel is an important center of diversity for the pathogen and that co-evolutionary forces are operating in this pamosystem. In this study, accessions with high levels of net blotch resistance were found in the higher moisture areas of the Mediterranean coast to the arid region of the Negev Desert. It was interesting to note that the number of accessions that were susceptible to net blotch was high in the Golan Heights-a region where leaf rust and powdery mildew resistance was common.
In contrast to the diseases described above, the frequency of resistance to spot blotch in H, spontaneum was lower (46 to 53%). Other investigators have reported a low frequency of spot blotch resistance in this species. This result is in agreement with previous investigations. Jana and Bailey (14) evaluated 291 H. spontaneum accessions to C. sativus and found only 13 with resistance (4.5% of total). Metcalfe et al. (28) and Legge et al. (22) evaluated a collection of 43 accessions from Turkey, Iraq, Lebanon, and Syria, but found no resistance. In this study, six accessions exhibited high levels of spot blotch resistance (IR = 2), four from Israel (Gadot 6-35, Bet Shean 21-46, Mehola 22-42, and Avedat 33-28) and two from Jordan (Shuna N. 1-43 and Irbid 2-14). All but one (Avedat 33-28) of these accessions was from northern Israel and Jordan.
Resistance to stem rust was extremely rare in H. spontaneum accessions from Israel and Jordan. This result is in agreement with those of Metcalfe et al. (28) and Legge et al. (22), who found no promising levels of stem rust resistance in H. spontaneum from the Middle East. The alternate hosts of Puccinia graminis f. sp. tritici (i.e., Berberis and Mahonia spp.) originated in Asia (3,23); thus, strong co-evolutionary forces were apparently not operating between the pathogen and H. spontaneum in this region of the Fertile Crescent. Accessions from Damon, Gadot, and Shechem in Israel did exhibit high levels of resistance (ITs = 0;1 to 120;) to pathotypes MCCF and QCCJ, but only at the low-temperature environment. These accessions should be evaluated in the field to determine whether they carry broad-based adult plant resistance against Puccinia graminis f. sp. tritici pathotypes commonly found in North America.
One of the primary objectives of this research was to identify accessions of H. spontaneum that possessed high levels of resistance to all six pathogens of importance in the Upper Midwest region of the United States and central provinces of Canada. Such accessions could then be crossed with adapted breeding lines to simultaneously transfer genes for multiple disease resistance, possibly by marker-assisted selection. Two of the 116 accessions (Shechem 12-32 and Damon 11-11) evaluated were resistant to all of the pathogens tested. These accessions were collected from fairly divergent sites in north-eastern and north-central Israel. Only a small fraction (~3%) of the available accessions from the Institute of Evolution was evaluated in this study; thus, it is likely that additional sources of multiple disease resistance will be identified. Crosses have been made between the two H. spontaneum resistance sources and advanced barley breeding lines to elucidate the number, effect, and chromosomal position of the resistance genes by molecular mapping techniques. Identification of molecular markers closely linked to the resistance genes will facilitate their introgression in breeding lines by marker-assisted selection. Six-rowed malting barley cultivars in the Upper Midwest region of the United States have adequate levels of spot blotch resistance and stem rust resistance (except to pathotype QCCJ). The incorporation of SSLB, powdery mildew, leaf rust, and net blotch resistance will allow producers to attain more stable yields in their barley cultivars without the threat of disease losses.
ACKNOWLEDGMENTS
The authors gratefully acknowledge the assistance of Hala Toubia-Rahme for evaluating H. spontaneum accessions for resistance to Septoria speckled leaf blotch. We also thank Yongliang Sun, Renee Senn, Firas Abu-El Samen, and Faisal Nimer for their technical assistance in this study. A portion of this research was conducted at North Dakota State University by the first two authors. This research was funded in part by the American Malting Barley Association, the Lieberman-Okinow Endowment at the University of Minnesota, and Agriculture and Agri-Food Canada.
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T. G. Fetch, Jr., Cereal Research Centre, Agriculture and Agri-Food Canada, Winnipeg, Manitoba R3T 2M9, Canada; B. J. Steffenson, Department of Plant Pathology, University of Minnesota, St. Paul, MN 55108, USA; and E. Nevo, Institute of Evolution, University of Haifa, Mt. Carmel, Haifa 31905, Israel
Corresponding author: T. G. Fetch, Jr.
E-mail: tfelch@agr.gc.ca
Published as paper number 1850 of the contribution series from the Cereal Research Centre of Agriculture and Agri-Food Canada.
Accepted for publication 14 July 2003.
Copyright American Phytopathological Society Dec 2003
Provided by ProQuest Information and Learning Company. All rights Reserved
