Parasitic¯owering weeds of the genus Striga (Scrophulariaceae) cause substantial losses in sorghum [Sorghum bicolor (L.) Moench] production in sub-Saharan Africa. Striga-resistant sorghum cultivars could be a major component of integrated striga management, if resistance was available in adapted, productive germplasm. In this paper we review methodologies for breeding striga-resistant sorghums. The agar-gel assay is an excellent tool to screen host genotypes in the laboratory for low production of the striga seed germination stimulant. Further laboratory assays are needed which allow the non-destructive, rapid and inexpensive evaluation of individual plants for additional resistance mechanisms. Field screening for striga resistance is hampered by high microvariability in African soils, heterogeneity of natural infestations, and concomitant large environmental effects on striga emergence. An improved ®eld testing methodology should include one or several of the following practices: ®eld inoculation with striga seeds; appropriate experimental design including elevated replication number; speci®c plot layout; use of appropriate susceptible and resistant checks; evaluation in adjacent infested and uninfested plots; and the use of selection indices derived from emerged striga counts, striga vigor, and grain yield or a host plant damage score. Due to the extreme variability of the parasite and signi®cant genotypeÂenvironment interaction effects, multi-locational screening is recommended to obtain materials with stable performance. Additional strategies include: careful de®nition of the target environments; determination of the most important selection traits in each target environment; characterization of crop germplasm and improvement of available sources of resistance for better agronomic performance; transfer and pyramiding of resistance genes into adapted, farmer-selected cultivars; development of striga-resistant parent lines for hybrid or synthetic cultivars; and development of random-mating populations with multiple sources of resistance. The development of markerassisted selection techniques for broad-based, polygenic striga resistance is underway. This approach is particularly promising because striga resistance tests are dif®cult, expensive, and sometimes unreliable; the parasite is quarantined; and some resistance genes are recessive. Transgenic, herbicide-tolerant sorghums could contribute to an immediate, cost-effective control of striga by herbicides, but such cultivars are not yet available. The selection of sorghum cultivars with speci®c adaptation to integrated striga management approaches could contribute to sustainable sorghum production in striga-infested areas of sub-Saharan Africa. #

Molecular markers for resistance of sorghum to the hemi-parasitic weed Striga hermonthica were mapped in two recombinant inbred populations (RIP-1, and -2) of F(3:5) lines developed from the crosses IS9830 x E36-1 (1) and N13 x E36-1 (2). The resistant parental lines were IS9830 and N13; the former is characterized by a low stimulation of striga seed germination, the latter by "mechanical" resistance. The genetic maps of RIP-1 and RIP-2 spanned 1,498 cM and 1,599 cM, respectively, with 137 and 157 markers distributed over 11 linkage groups. To evaluate striga resistance, we divided each RIP into set 1 (116 lines tested in 1997) and set 2 (110 lines evaluated in 1998). Field trials were conducted in five environments per year in Mali and Kenya. Heritability estimates for area under the striga number progress curve (ASNPC) in sets 1 and 2 were respectively 0.66 and 0.74 in RIP-1 0.81 and 0.82 in RIP-2. Across sites, composite interval mapping detected 11 QTL (quantitative trait loci) and nine QTL in sets 1 and 2 of RIP-1, explaining 77% and 80% of the genetic variance for ASNPC, respectively. The most significant RIP-1 QTL corresponded to the major-gene locus lgs (low stimulation of striga seed germination) in linkage group I. In RIP-2, 11 QTL and nine QTL explained 79% and 82% of the genetic variance for ASNPC in sets 1 and 2, respectively. Five QTL were common to both sets of each RIP, wtih the resistance alleles deriving from IS9830 or N13. Since their effects were validated across environments, years and independent RIP samples, these QTL are excellent candidates for marker-assisted selection.

No abstract

Breeding of sorghum (Sorghum bicolor L. Moench) for resistance to the parasitic weed Striga hermonthica (Del.) Benth. has been hampered by the difficulty of evaluating host resistance in the field and lack of reliable screening techniques. Therefore, we investigated the value of various indirect and direct measures of Striga resistance as selection traits. Two sorghum recombinant inbred populations of 226 F 3:5 lines each were developed from the crosses (1) IS 9830  E 36-1 and (2) N 13  E 36-1. Strigaresistant line IS 9830 is characterized by low stimulation of Striga seed germination, whereas Striga-susceptible line E 36-1 produces germination stimulants in abundance. Line N 13 possesses ''mechanical'' resistance and probably also an antibiosis mechanism. Resistance was assessed in 1997 and 1998 using in vitro agar-gel assays with Striga seeds from Kenya, Mali, and Niger, pot trials in the respective three countries, and field experiments in Kenya and Mali. The agar-gel assay proved to be a useful, precise and fast indirect selection method to screen for sorghum entries with the low-stimulant character. However, correlation analysis showed that this resistance mechanism was ineffective in some environments, especially in Kenya, pointing to the necessity of field evaluation. Because of low heritability estimates and moderate to low correlations to Striga resistance under field conditions, pot screening appeared to be of limited use in breeding programs. The field trials confirmed the effectiveness of several direct measures of Striga resistance in sorghum: emerged Striga counts, Striga severity index, and area under the Striga number or severity progress curves. A two-row plot field layout with an empty row between plots, coupled with artificial infestation of test rows, lattice design and six replications offered an improved screening procedure that achieved high heritability. Significant genotype  environment interactions in the field experiments stress the importance of multi-locational trials to achieve stable Striga resistance.