Abstract. In the doubled haploid barley population Gobernadora/CMB643, genes conferring the traits lateral florets and anther extrusion were mapped via molecular markers in the centromeric region of chromosome 2. The quantitative trait locus (QTL) with the greatest effect on head scab resistance (Type II) was previously mapped in the same region, with MWG503 as the common and closest molecular marker for all three characters. In this paper the potential of using morphological markers in breeding barley for head scab resistance is discussed.

Introduction. Head scab caused by Fusarium graminearum affects wheat and barley production in parts of China, Canada, Mexico, and the Andean and Southern Cone regions of South America in years with high precipitation. Scab reduces yield and produces toxins that can adversely affect grain quality. To provide farmers with effective and affordable protection against the disease, the ICARDA/CIMMYT Barley Program in Mexico is breeding barley for fusarium resistance. This effort has resulted in the release of three fusarium resistant barley varieties by China (Gobernadora = Zhenmai-1) and Ecuador (Atahualpa and Shyri). Barley is a staple crop for peasant farmers in the Ecuadorian highlands. If farmers did not have access to resistant varieties, they might consume toxin contaminated grain, raising food safety concerns.

Materials and Methods. Transferring head scab resistance genes into malting barley lines involves three phases. In phase 1, F1 seed from a Gobernadora x CMB643 cross was produced in Mexico and sent to Oregon State University, Corvallis, Oregon, where it was used for doubled haploid (DH) production applying the Hordeum bulbosum technique. Part of the Latin American Barley Network, OSU has contributed DH production and mapping scab resistance genes in the Gob/CMB643 DH population using molecular markers. DH lines were screened for scab resistance in North Dakota, USA, Toluca, Mexico, and Shanghai, China, in 1996-97. Local cultures of F. graminearum were used for field inoculations at all locations, though inoculation methods varied among locations. Primary infection (Type I) data from all three countries, fungal spread (Type II) data from Mexico, and DON content data from the USA were used by the North American Barley Genome Mapping Project to map head scab resistance genes (Zhu et al. 1999).

Results and Discussion. Mapping of head scab resistance genes in the Gob/CMB643 population showed several QTLs distributed in all chromosomes except chromosome 7 (Zhu et al. 1999). However, the QTL with the greatest effect on resistance (Type II) was mapped in the centromeric region of chromosome 2.

The association between a morphological marker (lateral floret) and scab resistance was reported by Gilchrist et al. (1996) based on observations of barley spikes. The molecular marker for lateral floret coincides with Type II resistance genes located in the same region of chromosome 2 (Table 1). Correlations between lateral floret and scab resistance (Type II) and DON content were 0.63 and 0.54, respectively (Zhu et al. 1999). Although given the small size of the DH population we could not determine whether these associations were due to linkage or pleiotropy, indirect selection for scab resistance in barley is possible using this morphological marker. During the last three years, the ICARDA/CIMMYT Barley Program was able to do rapid visual selection using this trait as the selection criterion. However, the trait can only be used to select two-rowed populations, not six-rowed barley.

Phase two included the development at OSU of a new set of DH lines named RECLA (the Spanish acronym for Latin American Barley Network). The parents in this complex cross were Orca (from Oregon), CMB643, GOBE-DH24, GOBE-DH96, NE175 (from Uruguay), and AF9216 (from Brazil). The last two lines were chosen for their malting quality and their Southern Cone adaptation.

Preliminary screening of RECLA DHs for scab resistance (Types I and II) was conducted in the greenhouse at El Batan, Mexico, in the fall of 1998. A correlation (r = 0.51) between lateral floret size and the area under the disease progress curve for Type II resistance was found in this population.

In phase three, scab resistant RECLA DHs were crossed with two varieties having low grain protein content, Logan (North Dakota) and Aquario (Chile). There are plans to use the single-seed descent method for generation advance and lateral floret as a selection criterion to form two populations by divergent selection. F5 populations will be field screened using artificial inoculation of F. graminearum in Toluca, Mexico.

Table 1. Quantitative trait loci (QTLs) detected in the Gobernadora x CMB643 doubled haploid population.

In Japan, anther extrusion is used as a selection criterion in breeding for scab resistance (T. Ban, personal communication); spikes that do not extrude anthers are considered resistant. Data on anther extrusion in the GOBE and RECLA DHs were collected at CIANO Experiment Station, northwestern Mexico, during the 1998-99 winter. Lines with lateral florets had more anther extrusion than those with no lateral florets. Anther extrusion in the GOBE population was mapped in the same centromeric region of chromosome 2 with the closest molecular marker MWG 503 (Table 1). This result indicates that anther extrusion is a morphological trait that can be used to select both two- and six-rowed barley for head scab resistance.

Understanding the different resistance mechanisms and their epidemiological role is necessary for breeding head scab resistant barley. In 1998 several head scab resistant sources were compared to determine their reaction to different scab resistance mechanisms (Types I and II, and DON content), and grain yield losses were estimated by comparing yields in protected and non-protected plots.

Since unacceptably high toxin content is the main reason for rejecting grain in the developed world, it is interesting that several two- and six-rowed hull-less cultivars showed zero toxin content (DON). Low DON values are in agreement with studies by Clear et al. (1997), who reduced grain toxin content (DON) 59% by threshing susceptible hull-less cultivars to eliminate the lema and palea, which contains a large portion of the toxin. It is possible that the "hull-less" trait, recessively inherited (n) and located on chromosome 7, could be used as a morphological marker in selecting for reduced toxin content in the grain, perhaps the most critical aspect of breeding for head scab resistance.

The development of head scab resistant, two-rowed, hull-less barley is more advanced than resistant, six-rowed, hull-less barley in the ICARDA/CIMMYT breeding program in Mexico. However, advanced six-rowed hull-less lines with head scab resistance derived from Chevron and, possibly, another source will be tested in Toluca during the summer of 1999 to confirm their resistance.

In 1998 the El Niño phenomenon caused higher than normal rains in Toluca: 1200 mm during the growing cycle. Scab symptoms in resistant cultivars were considerably increased to levels not observed in previous years, due to genotype x environment interaction. The use of scab resistant varieties to solve this disease problem will require the application of appropriate agronomic practices, such as plowing to incorporate plant residues and using suitable crop rotations. However, given the increasing trend towards no tillage in the world, the problem of head scab will continue to haunt us into the next century.

References

Clear, R.M., Patrick, S. K., Nowicki, T., Gaba, D., Edney, M. and Babb, J.C. 1997. The effect of hull removal and pearling on Fusarium species and trichothecenes in hull-less barley. Can. J. Plant Sci. 77:161-166.

Gilchrist, L., Mujeeb-Kazi, A., Vivar, H. and Crossa, J. 1996. Progress in Fusarium Scab in Wide Crosses and Barley Programs. In Proceedings of the 1996 Regional Fusarium Scab Forum. Oct. 1996. Winnipeg, Manitoba.

Zhu, H., Gilchrist, L., Hayes, P., Kleinhofs, A., Kudrna. D., Liu, Z., Steffenson. B., Toojinda, T. and Vivar, H. 1999. Does function follow form? Principal QTLs for Fusarium Head Blight (FHB) resistance are coincident with plant architecture trait in a doubled haploid population of barley. Theo. Adv. Genetics (in press).