M. van Ginkel, L. Gilchrist, and C. Velazquez

In the CIMMYT bread wheat breeding effort we generate materials intended for higher rainfall areas in the developing world, among other mega-environments. In high rainfall environments, the major wheat production constraints are diseases plus certain abiotic stresses, such as waterlogging, sprouting-prone conditions and, sometimes, nutrient imbalances (both deficiency and toxicity). However, the most observable stresses are the biotic ones. Of these, yellow or stripe rust (Puccinia striiformis) and leaf or brown rust (P. recondita) are often obvious, in particular the first, plus such foliar blights as Septoria tritici, tan spot (Pyrenophora tritici-repentis), and very occasionally Septoria nodorum and Fusarium nivale on the leaves. The main virus disease is BYDV.

Among diseases affecting the spike, Fusarium head scab (FHS), induced by various Fusarium species, is the number one problem, and seems to be expanding. The recent increase in this disease globally is probably due to the expansion of what are ironically called (from a disease standpoint) more sustainable production methods, such as zero, minimum, or reduced tillage, plus the intensification of rotations, in particular those including corn (maize), an alternate host of Fusarium spp.

As FHS spreads and causes damage by reducing the amount of harvested seed and contaminating the grain with toxins, joint efforts to combat this scourge have increased. This meeting is witness to such efforts. Key among approaches to control the disease is the incorporation of genetic resistance.

The CIMMYT program requests, receives, and specifically develops genetically diverse germplasm with resistance to FHS. Various reports documenting these sources are available (Gilchrist et al., 1997a, 1997b; 1999). Also, genetic studies aimed at determining modes of inheritance have been carried out and published (Singh et al., 1995; Van Ginkel et al., 1996). In recent years efforts by the pathology group have concentrated on differentiating germplasm in regard to the four types of resistance commonly applied in FHS (I, II, III, and IV). Our breeding strategy has focused on combining different resistances in adapted backgrounds (Singh and van Ginkel, 1997).

Two areas of recent research on FHS are reported here.

Three crosses were made among three resistance sources considered likely to be different based on their genealogy. We chose two parents (1 and 2, below) whose pedigrees contain no Chinese germplasm. The three parents were:

1. Gov/Az//Mus/3/Dodo/4/Bow
2. Bau/Milan
3. Catbird