Improved methodologies for breeding striga-resistant sorghums

Parasitic flowering weeds of the genus Siriga (Scrophulariaceae) cause subs tantial losses in sorghum [Sorghum bicolor (L.) Moench] production in sub-S aharan Africa. Striga-resistant sorghum cultivars could be a major componen t of integrated striga management, if resistance was available in adapted, productive germplasm. In this paper we review methodologies for breeding st riga-resistant sorghums. The agar-gel assay is an excellent tool to screen host genotypes in the laboratory for low production of the striga seed germ ination stimulant. Further laboratory assays are needed which allow the non -destructive, rapid and inexpensive evaluation of individual plants for add itional resistance mechanisms. Field screening for striga resistance is ham pered by high microvariability in African soils, heterogeneity of natural i nfestations, and concomitant large environmental effects on striga emergenc e. An improved field testing methodology should include one or several of t he following practices: field inoculation with striga seeds; appropriate ex perimental design including elevated replication number: specific plot layo ut; use of appropriate susceptible and resistant checks; evaluation in adja cent infested and uninfested plots; and the use of selection indices derive d from emerged striga counts, striga vigor, and grain yield or a host plant damage score. Due to the extreme variability of the parasite and significa nt genotype x environment interaction effects, multi-locational screening i s recommended to obtain materials with stable performance. Additional strat egies include: careful definition of the target environments; determination of the most important selection traits in each target environment; charact erization of crop germplasm and improvement of available sources of resista nce for better agronomic performance; transfer and pyramiding of resistance genes into adapted, farmer-selected cultivars: development of striga-resis tant parent Lines for hybrid or synthetic cultivars; and development of ran dom-mating populations with multiple sources of resistance. The development of marker-assisted selection techniques for broad-based, polygenic striga resistance is underway. This approach is particularly promising because str iga resistance tests are difficult, expensive, and sometimes unreliable; th e parasite is quarantined; and some resistance genes an recessive. Transgen ic, herbicide-tolerant sorghums could contribute to an immediate, cost-effe ctive control of striga by herbicides, but such cultivars are not yet avail able. The selection of sorghum cultivars with specific adaptation to integr ated striga management approaches could contribute to sustainable sorghum p roduction in striga-infested areas of sub-Saharan Africa. (C) 2000 Elsevier Science B.V. All rights reserved.