Genetic manipulation through variation of the Ph locus for introgressing stress resistance in wheat.

A. Mujeeb-Kazi, S. Cano, and R. Delgado.

Alien species with stress resistances other than the conventional T. aestivum variability are of special interest in breeding programs, and we at the International Wheat and Maize Improvement Center (CIMMYT) are making an effort to incorporate and exploit resistance genes in a wheat background using diverse alien sources in order to pyramid genes. The objectives of this study are to elucidate the exploitation of Ae. variabilis as a resistance gene donor for Karnal bunt and Cochliobolus sativus through some genetic transfer strategies. The germplasm developed, together with its cytological validation and C. sativus screening data under Mexican conditions, also may serve other objectives.

Amphiploids provide the means of generating stress and disease diagnostic data. For biotic/abiotic stress resistances with an ill-defined mode of inheritance, use of disomic alien addition lines may be one classical and lengthy methodology to utilize for subsequently effecting alien transfers. However, when the meiotic process does not facilitate recombination-based alien transfers, other gene transfer strategies become essential. A few have been described. Such associations were observed earlier by others for `ph wheat/Aegilops species' hybrids, but a constraint was identified in producing BC1 derivatives from these. We have used the ph-based F1 hybrid of the cross `wheat/Ae. variabilis' and successfully backcrossed it twice to produce BC1- and BC2-selfed derivatives. This germplasm offers a means of screening for C. sativus and Karnal bunt, with current emphasis on bread wheat improvement.

Considering the strength of molecular diagnostics, we are of the opinion that maximizing alien transfers at the F1 stage may be advantageous in the quest to produce practical products. Producing new ph-based F1 hybrids is one option where back- or top-crossing can rapidly build up valuable germplasm. Alternatively, where ph-based hybrids are difficult to produce or advance by backcrossing, utilization of `perennial wheat (Ph)/alien' F1 combinations (produced and maintained at CIMMYT over several years by clonal propagation) may be a solution. Such Ph F1 male-sterile hybrids become maternal parents amenable for trigeneric (e.g., `F1 x Ae speltoides') product generation aimed at suppressing the Ph locus and promoting alien introgression. Further, the existing Ph F1 wheat/alien species hybrids can be crossed with the ph wheat, yielding heterozygous Ph ph BC1 derivatives. Polyhaploids from these BC1 derivatives (BC1 x maize, millet, or Tripsacum) segregate for the dominant Ph or ph recessive loci and are cytologically identified, and the appropriate high pairing ph combination is advanced further.

In efforts to screen for the two stresses, three hybrids of durum wheat and 10 of bread wheat with Ae. variabilis were produced that were somatically normal (2n = 4x = 28; 2n = 5x = 35). The hybrids have meiotic associations that do not facilitate recombination exchange products.

Amphiploids were induced that selectively demonstrate the desired stress resistances, both in T. aestivum and T. turgidum combinations. Their C. sativus screening results are presented in Table 13.

Reaction of Triticum turgidum L. var. durum to artificial inoculation of Karnal bunt.

G. Fuentes-Davila, O. Abdalla, and W.H. Pfeiffer.

Karnal bunt, caused by Tilletia indica (syn. Neovossia indica (Mitra) Mundkur), is a disease that generally affects part of the wheat grain. Yield loss is not economically important. However, adverse effects can occur on grain quality and by-products, especially when the percentage of infected grains is high. In addition, quarantines in many countries Insert Table 14. Page 150.

Insert Table 15. Page 151.

Insert Table 16. Page 152.

against T. indica limit the exchange and distribution of wheat and triticale germplasm. The main objective of the CIMMYT Karnal bunt research program has been to identify sources of resistance; incorporate resistance to suitable genotypes; and develop advanced lines. Tolerance to Karnal bunt has been detected in durum wheat and triticale since early artificial screening. Therefore, since 1987-88, the wheat program has given more emphasis to testing T. aestivum and synthetic lines looking for other sources of resistance.

Reported here are the results of three, four, and five cycles of artificial testing of some durum wheat lines, that showed consistently low levels of Karnal bunt infection. Those lines were planted in nurseries on 4 November, 13 November, and 4 December, 1987, at Cd. Obregon, Sonora, Mexico, and they were planted on November 8, 18, and 28 during 1988-91. Durum wheat advanced lines were evaluated by artificial inoculation with a sporidial suspension of 10,000/ml, injecting 1 ml per head during the boot stage after 4 pm. High relative humidity was generated and maintained by an overhead sprinkler irrigation during the period of inoculation. Percent infection was calculated from the number of infected and healthy grains measured on 10 inoculated heads/entry after hand-threshing. The susceptible bread wheat check `Seri M82' averaged 58.8 % infection in 1988, whereas the susceptible cultivar WL-711 had 75.4 % in 1989, 46.8 % in 1990, 73 % in 1991, and 76.8 % in 1992.

None of the lines was immune to the inoculation. However, the lowest infection level of 0.04 % was found in `MEMO `S'/MEXI75' (Table 14). This line had only two counts of infection out of 12 inoculation tests. Lines tested during five cycles showed a mean range from 0.69 to 1.26 % infection, whereas the overall range was 0-4.36 % (Table 14). Lines tested during four cycles showed a mean range from 0.04 to 1.26 % infection, and the overall range was 0-4.49 % (Table 15). The mean range for lines tested during the three cycles was 0.40 to 0.84 % infection, whereas the overall range was 0-3.83 % (Table 16).

The durum wheat lines shown in Tables 14-16, and had low percentage of infection during several cycles of artificial testing, could be utilized as a source in breeding programs to enhance Karnal bunt resistance.

ITEMS FROM NEPAL

NEPAL AGRICULTURAL RESEARCH COUNCIL

National Wheat Research Program, Pupandehi, Bhairahawa, Nepal.

M.R. Bhatta, R.N. Devkota, D.R. Pokharel, and B.R. Thapa.

Crop season.

The 1994-95 wheat crop was planted in 634,000 hectares with a total production of 915,000 metric tones and a productivity of 1,443 kg/ha. Compared to the 1993-94 wheat season, total wheat area, production, and productivity increased by 1.8, 4.3, and 2.3 %, respectively. On the whole, the wheat season was favorable, with frequent rains late in the season and an absence of desiccating, dry hot winds during the reproductive stage. The steady increase in wheat area is partly because of shifts in eating habits and a greater demand for wheat flour by an increasing number of wheat-based bakeries in Nepal. The rice-wheat cropping system comprises nearly 84 % of the total wheat area. The low productivity of wheat is because of poor soil fertility; inadequate irrigation facilities; late-season heat; drought stress; low seed replacement by wheat growers; and diseases, particularly leaf rust and foliar blights of Bipolaris sorokiniana and Pyrenophora tritici repentis.

The major wheat cultivars currently popular are UP 262, Nepal 297, BL 1022, Nepal 251, and Triveni in the Terai region, and Annapurna-1, Annapurna-3, and RR 21 in the hills. Recently released wheat cultivars, Bhrikuti, BL 1135, and Annapurna-4, are becoming popular among wheat growers because of their high-yield potential and resistance to major diseases.

Wheat breeding activities.

The major objective of the wheat breeding and varietal improvement program is to develop wheat cultivars that fit well into a rice-wheat cropping pattern. These cultivars will have a high yield potential; resistance to multiple diseases (leaf and stripe rusts, major foliar blight pathogens, and loose smut); tolerance to sterility; and post-anthesis heat and drought stress. To create genetic variability for desirable traits a modest hybridization program makes 150 to 200 crosses per year among selected parents. Some specific crosses also are made available each year through the CIMMYT bread wheat program, in addition to advanced nurseries and yield trials. This year, as many as 1,779 advanced/fixed lines were evaluated in the form of screening nurseries and yield trials at 14 different testing sites. A total of 2,449 segregating plant/progenies was evaluated, and some 1,910 progenies were selected for the next cycle. New genotypes identified for farmers' field testing are:

BL1496 (PRL `S'/Toni//Chil `S'), BL 1473 (NL 297/NL 352), NL 713 (CPAN 169/HD 2204), NL 716 (Lira `S'/PRL `S'//Toni), NL 724 (K8101/K68), NL 729 (VEE/K8020), NL 731 (K78/DSN 72//HUW 206), and NL 728 (HUW 202//K7537/HD 2160M) for the Terai region and NL 665 (Lira/Fufan//VEE#5 `S'), WK 810 (VEE/4/PVN/CBB//CNO/3/JAR/ORZ), NL 714 (HUW 206/HD 2189//HUW 202), and BL 1530 (Chil/PVN) for the hills.

Disease situation and breeding strategy.

Leaf rust and the foliar blight complex seriously reduce yield in the Terai areas, whereas stripe rust and loose smut affect yield in the hills. Two species of foliar blight pathogens, B. sorokiniana and P. tricici repentis, cause blight symptoms in wheat. This year, leaf and stripe rust incidence was less and appeared late in the season even in susceptible cultivars. Helminthosporium leaf blight has remained a serious problem in all wheat-growing environments of the Terai. Yield loss attributed to this disease has been estimated from 9 to 27 % in susceptible cultivars. Recently released wheat cultivars BL 1135 and Bhrikuti possess a moderate degree of resistance to the foliar blight complex pathogens. Virulence to Lr26 and Yr9 has been detected at a moderate level. The importance of the adult plant resistance gene Lr34 in combination with other genes has been great in controlling leaf rust epidemics. Our present breeding strategy is to combine these durable resistance genes into a high-yielding, more adaptive, foliar blight-resistant, genetic background. CIMMYT wide cross advanced lines with Th. curvifolium (CHRIYA 1, CHRIYA 3, and CHRIYA 7) in addition to some Chinese, Brazilian, and Zambian lines exhibit a moderate to high degree of foliar blight resistance under Bhairahawa conditions. However, some have exhibited a very good level of resistance on foliar tissues, but have black-point infection on grains. A few of these lines have been utilized widely in a hybridization program and have given an adequate level of resistance in segregating populations.