Biology, ecology and management of parasitic weeds: Current status and new insights

Plant–plant parasitism is a well-known and studied biological phenomenon with a wide range of life forms. Parasitic plants depend on other plants for the entire or partial fulfilment of their life-cycle or enhancement of their reproduction (Shen et al., 2006). Indeed, plant–plant parasitism ranges from relative independence of the parasite on the host plant, that is, the facultative and the hemi-parasites, to complete dependence, that is, the holo-parasites. This distinction is based on the presence of chlorophyll in the parasite's stem and leaves (Depamphilis & Palmer, 1990) and their dependence on host presence for seed germination and early growth (Matthies, 1997; Riopel & Timko, 1995). Parasitic plants are further subdivided according to whether they infect aboveground parts of their host, that is, stem parasites, or belowground parts, that is, root parasites. What all parasitic plants have in common is their ability to produce a haustorium, a unique root structure that enables the penetration of host-plant (root or stem) tissue and establishment of a vascular bridge between parasite and host (Joel, 2013; Kokla & Melnyk, 2018). It is through this vascular connection that parasitic plants can extract water and nutrients from the host while a range of macromolecules like metabolites, proteins, mRNA's and pathogens are exchanged in both directions (Yoshida et al., 2016). As parasitic plants extract water, nutrients and organic carbon from the host and modify the host plant growth regulator balance and photosynthesis rate, the host plants are often severely negatively impacted by this infection (Frost et al., 1997; Kabiri et al., 2021; Mauromicale et al., 2008). Infection by facultative parasitic plants (Kabiri et al., 2017) or obligate hemi-parasitic plants (Press & Stewart, 1987) seems to have equal or even more destructive effects on the host plant as infection by holo-parasitic plants (Lins et al., 2007). Parasitic plants are omnipresent, from the temperate zones to the tropics, and from humid to arid climates (Press & Graves, 1995). Around the world, parasitic plants lead to severe economic losses, in particular when they parasitize food crops, as they can significantly reduce both quantity and quality of the harvestable products (Rodenburg et al., 2016; Vurro et al., 2010). Species of parasitic plants that represent severe weed problems, henceforth referred to as parasitic weeds, are found across many agro-climatic zones and cropping systems and represent the whole array of possible parasitism (Parker & Riches, 1993). The most important root parasitic weeds are Broomrapes (Orobanche spp. and Phelipanche spp.) in temperate and Mediterranean zones (Rubiales, 2020), and Witchweeds (Striga spp. and Alectra spp.) and Rice Vampireweed (Rhamphicarpa fistulosa) in the tropics, mainly in Africa (Musselman & Rodenburg, 2023). Research on parasitic weeds dates back nearly one century but started to gain momentum in the 1950s with the discovery of Striga asiatica in North and South Carolina (Eplee, 1973). Since then, a progressive increase in parasitic weed research efforts has led to an exponential increase in the number of scientific publications. Major advances have been made in understanding of the biology and ecology of parasitic weeds, as well as identifying entry points and technologies for their management (e.g., Kavuluko et al., 2021; Rodenburg & Bastiaans, 2024a; Shimels et al., 2024; Vurro, 2024; Yoder & Scholes, 2010; Zwanenburg & Blanco-Ania, 2018). The current research efforts on parasitic weeds range from a focus on the molecular and biochemical underpinning of parasite–host interactions at the cell and plant level (e.g., Fishman & Shirasu, 2021; Yoneyama & Bennett, 2024; Yoshida & Kee, 2021) to modelling and experimental agronomy and agroecological work at the field, farm and even landscape and climate level (e.g., Cartry et al., 2021; Kimathi et al., 2023; Rodenburg & Bastiaans, 2024b; Rubiales & Fernandez-Aparicio, 2012; Zhang et al., 2022). Research on management strategies currently focus on agroecological applications such as crop diversification (Scott & Freckleton, 2022), suppression through the soil microbiome (Masteling et al., 2019), means to capitalize on the suicidal germination principle through the application of synthetic analogues of germination stimuli (Kountche et al., 2019), advanced techniques for resistance breeding (e.g., Irafasha et al., 2023; Mutinda et al., 2023; Runo et al., 2024), and novel biocontrol strategies based on enhanced plant pathogens (Baker et al., 2024; Lüth et al., 2024). This Special Issue of Weed Research is dedicated to the fascinating world of parasitic weeds. Submissions focusing on all aspects-from biology, physiology and ecology to management-with relevance to weedy species of parasitic plants were welcomed. The call for this Special Issue resulted in seven papers. Five papers presented studies on Striga hermonthica, probably the most widespread and devastating parasitic weed of the African continent. Two of the S. hermonthica papers focus on host-plant resistance responses (Kunguni et al., 2023; Odero et al., 2024), one on host-plant tolerance (Ahmed et al., 2025), one on farm conditions and crop management (Rusinamhodzi et al., 2024) and one on the application of the host-plant growth regulator cytokinin to trigger suicidal germination (Jamil et al., 2023). In another paper, a methodology for strigolactone-analogue testing in soils is developed (Wallach et al., 2024). The seventh paper presents a study on re-infection of tomato by Phelipanche ramosa, one of the most important and widely distributed broomrape species in Europa (Leman et al., 2024). S. hermonthica is one of the most widely distributed and problematic parasitic weed species in Africa (Musselman & Rodenburg, 2023). The most affected crops are maize, sorghum, millet, and rice. What infested farms frequently have in common, apart from the geographic overlap with S. hermonthica, is relatively low soil fertility (Kamara et al., 2014), erratic rainfall (Dossou-Aminon et al., 2016), a dominance of intensified monocropping (Dugje et al., 2008) and limited crop production and crop protection resources, including information and support from agricultural extension services (Schut et al., 2015). In this issue, Rusinamhodzi et al. (2024) examine the relationship between farm resources, management decisions, S. hermonthica infestation, and maize productivity in western Kenya. The authors employed surveys, field assessments, and soil analysis to evaluate how resource endowment affects land use, soil fertility, and Striga infestation. This study revealed a clear relation between poverty and Striga incidence. The more resource-poor farmers were associated with poor crop and soil management, which led to low maize yields and high Striga infection levels. While this is often assumed, it has rarely been demonstrated in the literature before. Due to limited financial and physical resources, the adoption of advanced control strategies was very low. The most frequently practiced Striga control methods were hand and hoe weeding, corroborating earlier observations across African smallholder farms (Rodenburg et al., 2019; Tippe et al., 2017). Strong correlations between Striga infestation and soil fertility (mainly soil organic carbon) and between Striga infection and (mineral or organic) fertiliser inputs were also revealed, similar to findings by Ayongwa et al. (2010) and Samake et al. (2006) in West Africa. This led to the conclusion that improving soil fertility is key to mitigating Striga infestation and boosting maize productivity. The authors proposed tailored, low-cost, and context-specific solutions, such as legume intercropping and enhanced manure management, for effective and sustainable Striga control in smallholder farming systems. As observed before in Kenya, farmers would require guidance from extension for the implementation of effective Striga management strategies (Roobroeck et al., 2023). Co-learning approaches, engaging farmers and researchers in iterative cycles of application and evaluation through participatory workshops, would be another powerful means to achieve changes in farm management leading to improved soil fertility and reduced Striga infestation (Marinus et al., 2021). Crop varieties with improved levels of parasitic weed resistance or tolerance are prominent components of an integrated Striga management approach (e.g., Abdullahi et al., 2022; Randrianjafizanaka et al., 2018). Resistance reduces the number of parasitic weed infections while tolerance mitigates the negative effects of such infections on the host plant (Rodenburg & Bastiaans, 2011). Both defence mechanisms are genetically determined, heritable traits. A wide range of parasitic weed resistant and tolerant crop varieties has been identified in crops like sunflower (e.g., Molinero-Ruiz et al., 2006), cowpea (e.g., Kondi et al., 2024; Sadda et al., 2021), sorghum (e.g., Abate et al., 2017; Rodenburg et al., 2005), rice (e.g., Rodenburg et al., 2017) and, to a lesser extent, maize (e.g., Dossa et al., 2024; Gasura et al., 2019). Resistance mechanisms are expressed before or after parasite attachment to the host root. The first mechanism is associated with one of the main Achilles' heels of obligate root parasitic weeds, that is, their dependence on host-plant derived compounds for seed germination. So-called pre-attachment resistance occurs in cultivars that produce and release reduced quantities or less potent qualities of Striga germination stimulating compounds in the rhizosphere (e.g., Jamil et al., 2011; Serghini et al., 2001; Shimels et al., 2024), leading to fewer germinated parasitic weed seeds. Resistance occurring beyond host–parasite contact is referred to as post-attachment resistance and this is based on a range of phenomena at the root tissue level that prevent the parasite from establishing a structural vascular connection with the host (Mwangangi et al., 2021). Post-attachment resistance may occur at the root epidermis or cortex following a hypersensitive response (Mohamed et al., 2003), at or beyond the root endodermis following deposition of phenolic compounds (Martín-Sanz et al., 2020) or formation of other mechanical barriers, presumably by enhanced lignification (Cissoko et al., 2011; Mbuvi et al., 2017), or a yet unexplained incompatibility response (Amusan et al., 2008; Gurney et al., 2006; Mutinda et al., 2018). As the cited studies show, such resistance mechanisms are demonstrated to exist in a range of crop species. While resistance mechanisms are extensively studied and characterised, tolerance remains more elusive, partly because this mechanism is often confounded with resistance. In tolerant host plant genotypes (i.e., rice, maize and sorghum), photosynthesis and related processes are demonstrated to be less impacted by Striga infection than those of sensitive ones (Gurney et al., 2002; Rodenburg et al., 2008, 2017), but the underlying mechanisms are not fully understood yet. Four studies, published in the current issue, shed more light on host plant resistance and tolerance against parasitic weeds. Odero et al. (2024) investigated the existence of pre- and post-attachment resistance against S. hermonthica among a group of maize hybrids. They identified two hybrids, that is, H614D and H629, that exhibit both pre- and post-attachment resistance comparable to the resistant check, that is, landrace KSTP'94. Other hybrids were identified that either had good pre- or post-attachment resistance. These are valuable findings as resistance in maize is not as common a feature as in other cereal hosts, such as rice and sorghum. Another study on S. hermonthica resistance focuses on finger millet (Eleusine coracana); again a significant contribution as it represents the first published work on Striga resistance in this crop. Kunguni et al. (2023) observed post-attachment resistance in a cultivated (OKHALE1) and a wild (LESK10) genotype of finger millet. Striga seedlings, grown from pre-germinated seeds, were unable to form vascular connections with host plants of these genotypes. Apart from the novel crop type, this study also underscores the potential of wild relatives as reservoirs of durable resistance traits. In yet another crop, tomato, and with another parasitic weed species, the holo-parasitic Phelipanche ramosa, Leman et al. (2024) explored reinfection dynamics. They reveal that in cultivar Moneymaker a first infection results in the development of a systemic defence reducing susceptibility to subsequent infection attempts. This was not observed in another tomato cultivar, Zuckertraube. The observations could be explained by advanced (host) plant aging at the time of reinfection or by induced resistance. Upregulation of lignin biosynthesis genes was observed in Moneymaker upon initial infection, and not in Zuckertraube, which suggests this response to be a cultivar-specific resistance build up. This is a novel finding with potential relevance for tomato growers as the authors suggest that with their methods, they could induce resistance in cuttings of tomato plants used for production purposes. The fourth study on host-plant defence mechanisms against parasitic weeds focuses on tolerance against S. hermonthica (Ahmed et al., 2025). Within a selection of 20 sorghum genotypes, clear differences in resistance were found. Within groups of comparable resistance levels, there were also clear differences in the extent to which Striga infection reduced host plant biomass, height, and leaf physiological traits such as photosynthesis. This can be explained by genetic differences in physiological tolerance (as demonstrated before by Rodenburg et al., 2008, and Gurney et al., 2002). Six sorghum genotypes, among which known tolerant Framida, were identified with relatively high levels of tolerance. Differences in tolerance against Striga were then compared to differences in strigolactone quantities. The authors observed that root exudates of tolerant sorghum genotypes had proportionally higher levels of orobanchol and lower levels of 5-deoxystrigol or sorgomol. Orobanchol is therefore suggested to play a role in tolerance against Striga. This is a novel finding which could potentially provide a boost to the research on Striga tolerance and the development of Striga-tolerant cultivars. The parasite's dependence on germination stimuli is not only targeted through the use of pre-attachment resistant cultivars. The application of exogenous, synthetic analogues of such germination stimuli (such as strigolactones) prior to crop establishment could lead to parasitic weed seeds germinating in vain. If germination is not followed by successful host attachment and a viable vascular connection in a timely manner (a matter of days) the parasitic weed seedling dies (Musselman, 1980). This principle, called suicidal germination, could deplete or at least reduce the parasitic weed seedbank (Takeuchi et al., 1995). Hence, while in resistant cultivars the aim is to reduce seed germination, following lower host-plant production of germination stimuli, in the suicidal germination principle the aim is to maximize seed germination by (artificially) increasing the levels of germination stimuli. The development of applications for parasitic weed management based on this principle needs to address a number of challenges regarding identification, stability (in the soil), costs, and effective delivery of compounds (Vurro, 2024; Vurro et al., 2016). Two articles in the current Special Issue focus on this aspect. The studies by Wallach et al. (2024) and Jamil et al. (2023), published in this Special Issue, underscore both the promise and the challenges of employing synthetic strigolactones (such as GR24) and alternative germination inducers (like cytokinins) as part of a parasitic weed management approach. One of the main challenges to the employment of the suicidal germination principle in parasitic weed management is that the germination stimulating compounds are quickly broken down in soils (Kgosi et al., 2012). Wallach et al. (2024) developed a method to study the fate of strigolactones, as the most prominent class of parasitic weed seed germination stimulants, in the soil. The bioassay that Wallach et al. developed could be used for further studies on strigolactones and soil microbiome effects on the residual time of these compounds. The study by Jamil et al. (2023), in this issue, addresses the efficiency and applicability of suicidal germination as a parasitic weed management strategy. They explored cytokinins like thidiazuron, with or without fluridone, to induce Striga germination. The addition of fluridone departed from studies demonstrating that this carotenoid inhibitor enhances the germination stimulating effects of plant hormones such as strigolactones and gibberellic acids (Chae et al., 2004; Kusumoto et al., 2006). Jamil et al. (2023) reported that a combination of thidiazuron and fluridone achieved up to 93% germination under laboratory conditions. Greenhouse trials by Jamil et al. also showed an 86%–100% reduction in Striga emergence when these compounds were applied prior to sowing of a Striga-susceptible rice variety (IAC-165), compared to treatments without these compounds. Pending field verification, this finding increases the range of products that could be used for the implementation of the suicidal germination method as a parasitic weed management approach. Evidently, studying interactions between cytokinins and soil microbial communities, assessing non-target impacts of cytokinins, and determining effects on crop yields would be crucial other steps to take. Parasitic weeds are quite different from any other plant considered a weed because of their unique biological and physiological systems. They require intimate interactions with crop plants throughout their life-cycle and result in high levels of crop losses. Besides environmental factors that could influence the parasitic weed life-cycle and the level of infection, the interactions with the crop plant determine their development and the level of impact they have on the crop. The complexity of these interactions and changes in the environment and the genetics of the parasite make durable parasitic weed management a challenging task. The challenging socio-economic and environmental contexts where parasitic weeds are most problematic further hamper progress in management strategies. However, the advent of novel technologies gives researchers new tools to investigate and understand the interactions between hosts and parasites, and the research progress made in the last years is opening new and potentially effective solutions for managing and controlling parasitic weeds. The continuous research progress provided a rationale to produce a Special Issue of Weed Research, collecting some of the most recent interesting advances in this important research field. This Special Issue on parasitic weeds presents new insights as well as convincing confirmations with relevance for integrated management approaches. The importance of soil fertility management on S. hermonthica infested farms has been confirmed as one of the backbones to integrated Striga management. Other studies contributed to the application potential of another important pillar of parasitic weed management, that is, cultivars with enhanced defence mechanisms. New Striga resistance and tolerance mechanisms are identified in genotypes of important host species. Advances in understanding Striga tolerance in sorghum are made, and induced Phelipanche resistance has been demonstrated in tomato. Other research advances were presented on a relatively new Striga management strategy, that is, application of the suicidal germination principle. A bioassay has been developed for testing the suicidal germination activity of exogenously applied compounds. These could be strigolactones or other classes of plant hormones, such as cytokinins, as demonstrated here. Altogether this Special Issue marks a significant milestone in research on parasitic weed management. As most of this work was done under controlled or semi-controlled conditions, validation from fields and actual farms would be an important future step. The peer review history for this article is available at https://www.webofscience.com/api/gateway/wos/peer-review/10.1111/wre.70015.