CIMMYT, 06600 Mexico, DF, Mexico

CIMMYT is dedicated to helping the poor in low-income countries. This is achieved through increasing productivity and sustainability of wheat and maize farming systems. Protection of the natural resource base as part of the sustainability of the system is critical. Crop protection research in the Wheat Program at CIMMYT focuses principally on genetic control of diseases and on the integration of crop management. Since a large part of the wheat in the developing world is derived from CIMMYT germplasm, it is incumbent upon CIMMYT to develop environmentally friendly and cost-effective methods of disease control while maximizing genetic diversity.

The CIMMYT Wheat Program depends mostly on time-proven methods of breeding. Principally single and three-way crosses are used and when appropriate, limited back-crosses are made. Population management is mostly through modified bulk breeding systems. Crosses are targeted for mega-environments as defined in CIMMYT publications. The high yield potential, broad adaptation and disease resistance of the wheat germplasm are incorporated with the aid of international nursery data. International multi-location testing helps to obtain longer-lasting resistance.

More than 20 years of research and observation indicate that rate-limiting or slow-rusting resistance for the rusts is generally longer lasting than major gene or hypersensitive resistance. CIMMYT's use of 'Frontana'-derived resistance to leaf rust and yellow rust, and 'Hope'-derived resistance to stem rust, are the most appropriate examples of durable resistance. It appears that minor additive genes play a significant role in determining durability of resistance. CIMMYT searches for this type of resistance and is attempting to combine different sources for increased stability.

CIMMYT is also diversifying resistance by utilizing alien genes in its search for more durable resistance. Genes such as Sr 31 in conjunction with Sr 2 complex add increased insurance against breakdown of stem rust resistance. Furthermore, wheat progenitors such as Aegilops squarrosa are the basis of novel germplasm known as synthetic hexaploid wheats and appear to provide new diversity for scab, rusts, spot blotch, Karnal bunt and Septoria tritici leaf blotch resistance, as well as increased yield potential.

Where appropriate, novel techniques are being used to identify and to introgress new genes. At present, double haploid methods are being tested to speed the breeding process. Use of QTLs is being studied for marking durable resistance to leaf rust. Marker-assisted selection is being analysed for incorporation of alien resistance to barley yellow dwarf luteovirus, and transgenic wheat with constitutive resistance genes is being investigated for resistance to various diseases.

Management systems may play a major role in the development of diseases. One system where CIMMYT has a major effort in supporting sustainability is the rice-wheat system of South Asia. This cropping system covers ca 10 million ha in Bangladesh, India, Nepal and Pakistan. Management issues relating to yield stagnation and crop protection are extremely important and are being studied in partnership with national agricultural systems scientists. Issues include turn-around time between crops, tillage, rotations, soil fertility and structure. All these may have effects on root rots and foliar blights in the cropping system.