Improved methodologies for breeding striga-resistant sorghums

Abstract

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. #

Screening for resistance to Striga gesnerioides and estimation of yield loss among Cowpea (Vigna unguiculata (L.) Walp.) progenies in the Upper East Region of Ghana

“…Field screening under artificial infestation is not always practical due to the fact that it can cause Striga seeds spreading to novel regions and it is moreover not consistent because breeders do not have any control of the parasite density and distribution (Haussmann et al, 2000). Pot screening has been operative as an alternative technique to confirm uniform infestation of Striga seeds.…”

Striga

“…In sorghum, several cultivars and breeding lines have been reported to be resistant and/or tolerant to Striga Rodenburg et al 2006;Ejeta, 2007). Most of these donor sources have been summarized in Haussmann et al (2000) and include Dobbs, Framida (SRN 4841), 555, N 13, SRN 6496, and SRN 39 (Ramaiah, 1987;Ezeaku and Gupta, 2004). Tesso et al (2007) reported three sorghum cultivars (P-9401, P-9403 and PSL85061) to express resistance to Striga.…”

“…Relatively few cultivars with good level of Striga resistance have been identified in millet crops, mainly sorghum and pearl millet Kountche et al, 2013). Improved breeding approaches that promise good prospects for developing cultivars with resistance to Striga have been suggested to take advantage of the ever-growing knowledge of the complex nature of the host-parasite interactions Haussmann et al, 2000;Omanya et al 2004). Adoption of these strategies has resulted in the development of appropriate breeding populations, and deployment of deliberate selection for resistance to Striga.…”

“…Benth. and S. asiatica (L.) Kuntze (henceforth denoted as Striga) are deemed to be the most economically important weeds parasitizing cereals (Berner et al, 1995;Cochrane and Press, 1997;Haussmann et al, 2000;Gressel et al, 2004;Gethi et al, 2005;Aly, 2007;Spallek et al, 2013). Striga infest millets, such as sorghum, pearl millet, finger millet, foxtail millet, little millet, proso millet, fonio, teff and barnyard millet (Chemisquy et al, 2010;Olivier, 1995;Parker, 2012;Atera et al, 2012), which represent the staple food and fodder crops of millions of poor rural families in the tropical and semi-arid regions of their cultivation, therefore are of utmost significance to subsistence farmers.…”

Integrated control of <em>Striga hermonthica</em> (Del.) Benth. in Burkina Faso through host plant resistance, biocontrol and fertilizers

“…The following observations were made in the two central rows of each plot (10 hills, or a total area of 1.92 m²): (1) symptoms of fungal disease on sorghum, (2) weekly counts of emerged Striga plants from four to twelve weeks after the first emergence of Striga in the experiment, (3) visual estimation of vigour of emerged Striga plants (Haussmann et al, 2000) at 85 DAS, (4) weight of Striga dry biomass at sorghum harvest and (5) sorghum yield. Striga count data were used to calculate area under the Striga number progress curve (ASNPC, Haussmann et al, 2000).…”