ICARDA Annual Report 2001

Shortage of water is one of the most serious constraints crop production in Mediterranean-type dryland environments around the world. Tolerance to drought has been a difficult trait to characterize and quantify, and current understanding is largely based on comparative physiology. The use of molecular markers to complement field evaluation can advance researchers' understanding of drought tolerance as well as improve their ability to select for this trait in crop improvement programs.

ICARDA is working with Australian scientists to develop barley germplasm with improved adaptation to low-rainfall environments. Current research focuses on identifying molecular markers for key traits associated with drought tolerance, including novel alleles from wild relatives and landraces of barley. Recent studies were based on a population derived from crossing a Syrian barley landrace ('Arta') with an improved ICARDA line ('Harmal-02'). The two parental lines are well adapted to low-rainfall, exhibit similar time to maturity, and represent a wide genetic diversity. Last year the collaborative experimental sites of ICARDA—two in Syria and four in Jordan—were exposed to significant drought stress, allowing the scientists to measure a range of associated traits including growth habit, early vigor, tiller number, leaf chlorophyll content, plant height, days to heading, biomass, kernel weight, grain yield, and harvest index. A number of lines outyielded both the parental lines and well adapted local varieties, although the differences were not associated with maturity effects, i.e. were due to true differences in drought resistance. A molecular map was constructed with 247 molecular markers, including 73 simple sequence repeats (SSRs) and 174 amplified fragment length polymorphisms (AFLPs). This was used to genotype 94 random inbred lines from this population.

The barley stem-gall-midge (Myetiola hordei) is a destructive insect pest causing extensive damage to barley crops in North Africa. It is estimated that 30% of the crop in Tunisia and 50% in Libya is infested, while up to 35% of the grain yield in Morocco may be lost to stem-gall-midge. Identifying sources of resistance and developing resistant varieties are therefore important objectives of the ICARDA barley improvement program.

Since the problem occurs mainly in North Africa, screening of germplasm has been undertaken by the National Institute of Agricultural Research in Settat, Morocco. More than 5000 barley lines have been evaluated during the past four years, under artificial infestation in the greenhouse and at two locations in the field. In all, 99 lines have been selected for their tolerance to this pest, 55 of which performed very well under the dry conditions of the 2001 season.

The foliar disease—barley yellow rust—also known as barley stripe rust, is caused by the fungus Puccinia striiformis f.sp. hordei. A highly virulent race of this pathogen was introduced into the Americas in 1975. From Colombia, it spread to Peru, Bolivia, Chile, Argentina, and Mexico, and it can now be found in the USA and Canada. Until the early 1980s, most barley cultivars planted in the Andes were highly susceptible local landraces, leading to yield losses of around 50% in Bolivia, and 70% in Colombia.

A quantitative trait locus (QTL) for biological yield was detected at one of the two sites in Syria (Tel Hadya) and at the four sites in Jordan. This locus had a detectable, but statistically not significant, association with grain yield at all six locations (Fig. 2).

The positive allele was derived from the ICARDA improved line 'Harmal-02.’ A positive allele was also conferred by the Syrian landrace 'Arta.' In this case, a QTL was detected at the same location on chromosome 1H at each of the five lowest rainfall sites, but not at Tel Hadya (Fig. 3). Quantitative trait loci for grain yield have previously been reported at the hor1 locus on the telomeric region of the short arm of chromosome 1H, and on the telomeric region of the long arm of chromosome 1H. Further work to confirm the effect of the 1H QTL will help breeders to improve their efficiency in selecting lines with higher yields in drought-stressed environments.

In 1990 scientists began extensive field screening of barley germplasm for resistance to yellow rust. Over 44,000 accessions have now been screened, and the resulting information is available through the online database of the Germplasm Resource Information Network (GRIN). Joint research by ICARDA and CIMMYT led to the development and adoption of germplasm with resistance to the most prevalent pathotypes in the Americas. Another collaborative project with Oregon State University was initiated in the early 1990s, with the goal of providing agronomically competitive, disease-resistant varieties through systematic characterization and introgression of unique sources of resistance. These activities have led to a better understanding of the genetic basis of the disease, and have accelerated the advancement of selected resistant lines.

Recently, changes in the resistance patterns of well-known genotypes were reported in Ecuador and Peru, and these were observed by ICARDA scientists during a visit in 2001. It is possible that a new race of the fungus has appeared, to which few sources of resistance currently exist. In response to this threat, ICARDA distributed yellow rust differentials and trap nurseries to the region. These are collections of barley lines with different disease reactions, and they can help to identify when a new race is present. New lines selected in the presence of the possible new race are also being incorporated in barley breeding programs, to guard against a future outbreak. Continuing international cooperation will be essential to understanding and combating this serious disease of barley.

Powdery mildew (Erisiphe graminis f.sp. hordei) is another important foliar disease of barley. It is particularly problematic in semi-arid regions, where early infection can affect the stand establishment of the barley crop and greatly reduce the number of tillers. Molecular mapping of resistance genes for powdery mildew at ICARDA is aimed at the selection of appropriate lines for use by barley breeding programs.

Two barley lines, 'WI2291' and 'Tadmor', both widely grown in the Mediterranean region, were studied. The European differential tester set was also evaluated (Fig. 4). Six cultures of powdery mildew, developed from single spore isolates, were tested. Four of them originated from Syrian barley and two from European barley lines. Interestingly, 'WI2291' exhibited the same disease reaction to different isolates as the line 'Villa,’ which is known to carry the Mlg powdery mildew resistance gene on chromosome 4H (Fig. 5). The same gene may well be present in 'WI2291.

The next step was to investigate the progeny. The parental lines and 71 doubled haploid (DH) lines originating from the cross were infected with all six isolates of powdery mildew. The isolate TH1 was found to differentiate best between the parents and DH lines (x2=0.18) and the disease reactions it caused were used for linkage mapping. To identify markers that are tightly linked with the Mlg resistance locus, segregation analysis was performed with restriction fragment length polymorphism (RFLP), random amplified polymorphic DNA (RAPD), SSR (simple sequence repeats) and AFLP (amplification fragment length polymorphism) markers on the DH lines. Two AFLP markers with distances of 2 and 2.4 cM from the Mlg gene were identified and mapped on chromosome 4H. Identifying these highly specific markers represents an important addition to our knowledge of sources of resistance to powdery mildew disease.

Seed-borne diseases such as barley leaf stripe, loose smuts and covered smuts are common in many barley-growing areas of the CWANA region. They can be particularly problematic for small-scale farmers who cannot afford or obtain chemical seed treatment. ICARDA scientists are screening for broad-spectrum resistance to the most economically damaging diseases.

A study of fixed barley genotypes within the ICARDA advanced breeding nurseries revealed several sources of resistance to seed-borne and a combination of seed-borne and foliar diseases. A total of 245 accessions showed combined resistance to smuts and barley leaf stripe. These were tested for resistance to various isolates of the pathogens. Sixteen fixed genotypes maintained a high level of resistance to smuts and nine of these showed combined resistance to smuts and barley leaf stripe (Table 2). After three years of testing, significant progress has been made in selecting resistant lines. These can now be used as parental material and can be exploited directly by national programs in areas where seed-borne diseases are a problem, for example, Central and West Asia, the Nile Valley, and the Red Sea region.