Striga is a major constraint to food production in Africa. Most technologies developed for the eradication of Striga asiatica from the United States are not adaptable to Africa. Imazapyr and pyrithiobac coated imidazolinone-resistant (IR)-resistant maize seed prior to planting at rates of 30 to 45 g ha−1 provide near season long control of Striga and can increase maize yields three- to fourfold if supplied with fertilizer. Slow release seed coatings reduce maize injury when post-planting rains are sparse and improve Striga control when there is excessive rainfall early in the season. Models suggest that herbicide resistance may not be a significant threat in short season maize, but vigilance in removing flowering Striga plants that are not controlled is recommended due to the known risk of evolution of resistance to these herbicides. Stacking the IR gene with glyphosate resistance and using imazapyr treated seed and applying glyphosate mid-season would provide season long Striga control and delay the evolution of resistance to both herbicides. To date, adoption of this technology has been limited by a number of factors. However, it should be included as one component of a multi-factor approach to increasing maize productivity in areas of Africa where Striga is problematic.

Parasitic weeds such as Phelipanche and Orobanche are obligate holoparasites that attack roots of almost all economically important crops in semiarid regions of the world. A wide variety of parasitic weed control strategies (chemical, biological, cultural, and resistant crops) has been tried. Unfortunately, most are partially effective and have significant limitations. The current mini review will discuss the needs for alternative methods and will summarize current and new biotechnology-based approaches for broomrape control. At present, we have generated transgenic tobacco plants expressing a cecropin peptide (sarcotoxin IA), under the control of the inducible HMG2 promoter. Transgenic lines enhanced host resistance to the parasitic weed; transgenes showed higher numbers of aborted parasitization events, reduced Phelipanche biomass, and increased host biomass. Sarcotoxin IA had no obvious effect on growth and development of transgenic host plants. Mannitol content in the parasite is regulated by Mannose 6-Phosphate Reductase (M6PR) gene, an essential process to broomrape species for water and nutrient uptake from the host. In our study, we used the inverted repeat technique to silence the parasite target gene, M6PR. In this study it was shown that the endogenous M6PR mRNA from P. aegyptiaca tubercles or shoots grown on transgenic tomato plants harboring the M6PR silencing construct was reduced by 60 to 80%. The number of dead tubercles was also increased significantly on transgenic plants as compared with the control plants. The strategies presented here are potentially superior to other methods in that they are effective, have a low cost of implementation for producers, and are safe for the environment.

The Parasitic Plant Genome Project has sequenced transcripts from three parasitic species and a nonparasitic relative in the Orobanchaceae with the goal of understanding genetic changes associated with parasitism. The species studied span the trophic spectrum from free-living nonparasite to obligate holoparasite. Parasitic species used were Triphysaria versicolor, a photosynthetically competent species that opportunistically parasitizes roots of neighboring plants; Striga hermonthica, a hemiparasite that has an obligate need for a host; and Orobanche aegyptiaca, a holoparasite with absolute nutritional dependence on a host. Lindenbergia philippensis represents the closest nonparasite sister group to the parasitic Orobanchaceae and was included for comparative purposes. Tissues for transcriptome sequencing from each plant were gathered to identify expressed genes for key life stages from seed conditioning through anthesis. Two of the species studied, S. hermonthica and O. aegyptiaca, are economically important weeds and the data generated by this project are expected to aid in research and control of these species and their relatives. The sequences generated through this project will provide an abundant resource of molecular markers for understanding population dynamics, as well as provide insight into the biology of parasitism and advance progress toward understanding parasite virulence and host resistance mechanisms. In addition, the sequences provide important information on target sites for herbicide action or other novel control strategies such as trans-specific gene silencing.

The witchweeds, members of the genus Striga, are noxious and persistent pests in farmers' fields and serious constraints to crop productivity throughout Africa, India, and Southeast Asia. Among the primary hosts for Striga are the major cereals (maize, sorghum, rice, and millet) and grain legumes (cowpea) that are important food staples worldwide. The negative impact of parasitic plants on crop productivity increases globally each year, and their potential for affecting domestic agriculture looms larger as the movement of seed resources expands on a global scale. At the present time there is a limited understanding of how Striga and other parasitic plants select a suitable host and overcome the innate defense responses of the host in order to complete their life-cycle. In the grasses most reported resistance to Striga appears to be polygenic with a large genotype by environment interaction. In contrast, resistance to S. gesnerioides in cowpea is conferred by single dominant genes functioning in a race-specific manner suggesting that a gene-for-gene mechanism similar to effector-triggered immunity (ETI) described in other host–pathogen interactions is likely operating in these parasite-host associations. A hallmark of ETI is the direct or indirect recognition of parasite-derived avirulence (Avr) factors and other effectors that interfere with plant innate immunity by host sensors (or R proteins) leading to activation of defense responses. The recent cloning and functional characterization of a race-specific R gene from cowpea encoding a canonical coiled-coil (CC)-nucleotide binding site (NBS)-leucine-rich repeat (LRR) type R-protein opens the door for further exploration of the mechanism of host resistance and provides a focal point for studies aimed at uncovering the molecular and genetic factors underlying parasite virulence and host selection. The potential for the development of novel strategies for parasite control and eradication based on parasite virulence factors is discussed.

Broomrapes (Orobanche and Phelipanche spp.) are obligate root parasites that spend most of their life cycle in the soil subsurface, making them hard to detect. In these underground developmental stages, broomrapes are highly sensitive to herbicides, and therefore knowledge of the dynamics of their parasitism is essential to precisely apply herbicide for their control. To address these complexities, two approaches have been proposed: (1) estimating the temporal variation in parasitism dynamics and predicting broomrape parasitism on its host by thermal time; (2) characterizing the spatial variation in infestation within and between fields by using a geographical information system and a global positioning system. In addition, the use of molecular markers to identify broomrape infestation (species and amount) in the field can contribute to determining its spatial distribution, which can then be used for site-specific weed management. In this paper, we discuss how technology can be optimized for control of the root-parasitic broomrapes. Special attention is given to the development of integrative approaches. An example of a decision support system for the rational management of Egyptian broomrape in processing tomato is given.