Rhamphicarpa fistulosa, a widespread facultative hemi-parasitic weed, threatening rice production in Africa

Rhamphicarpa fistulosa is a facultative hemi-parasitic plant of the Orobanchaceae family, adapted to wet soils. Apart from tropical Australia, it is only found in sub-Saharan Africa, where it is considered a minor weed in cereal crops such as rice. Due to this status, the species has received only sporadic attention. Recent field observations and encounters with rice farmers in several African countries showed that R. fistulosa is, however, a more serious and increasing production constraint than previously thought. Results from a systematic literature review and a global herbarium study support this. The species has a broad distribution over Africa (at least 35 countries from Madagascar to Senegal and from Sudan to South Africa) and a wide range in altitude (0–2150 m a.s.l.) and environment (waterlogged swamps to moist free-draining uplands). Rhamphicarpa fistulosa is relatively independent and persistent because of the presumably wide host range, the facultative nature of its parasitism and its prolific seed (estimated 100 000 seeds m−2 under moderate infestation levels). Finally, R. fistulosa causes severe yield losses (average 60%) and high regional annual economic losses (estimated US $175 million), while effective control options are scant and awareness of the species among important R&D stakeholders is almost absent. An integrated approach is advocated to assist the rice sector to reduce current R. fistulosa-inflicted losses and to prevent further spread of the species into new areas

Host interactions, pathology, economic impacts and management of the facultative parasitic weed Rhamphicarpa fistulosa

Rhamphicarpa fistulosa (Hochst.) Benth., is a facultative parasitic plant causing persistent weed problems in smallholder rice crops across Africa. It was first identified as a rice weed in the 1990's in West Africa. The objective of this review was to comprehensively inform the crop protection research community and extension services on the newly gained insights on R. fistulosa since the last review in 2014. Following a systematic literature search in Web of Science on papers published after September 2014, the cut-off date of the last review, the resulting publications are reviewed. Parasitizing Rhamphicarpa fistulosa plants gain considerably in biomass and reproductive output, compared to independently growing plants. The host incurs severe reductions in photosynthetic efficiency and biomass production. Host-plant assimilates primarily benefit parasite growth and reproduction. Rice yield losses caused by R. fistulosa infestations range from 24 to 73% depending on infestation density and cultivar. Annual crop losses in sub-Saharan Africa were estimated at 204,000 tons of milled rice, with associated annual economic losses of US $82M, affecting >140,000 rural households. Rice cultivars NERICA-L-40 and -31 were identified as resistant and high yielding under R. fistulosa infested conditions. Early sowing decreases R. fistulosa infection. Contrary to the obligate parasitic weed Striga spp., the facultative R. fistulosa is not controlled by fertilisers. The past decade of research generated valuable insights in the genetics, germination biology, environmental and ecological contexts, distribution and impacts on crop yields and economies, and disclosed feasible management options

Differences in the ecology of witchweed and vampireweed: implications for rice farming in Africa

Parasitic weeds in African rice systems affect household-level food security and income generation in Africa. Most affected farmers are smallholders with limited weed management capacities, crop production infrastructure and resources. The facultative parasite Rhamphicarpa fistulosa (rice vampireweed) has become a major problem in rainfed lowland rice whereas the obligate parasites Striga asiatica and S. hermonthica(witchweed) are problematic in rainfed upland rice. Striga asiatica, S. hermonthica and Rhamphicarpa fistulosaare all root hemiparasitic plants from the same family, Orobanchaceae, with similar distribution and host-plant effects. The differences in biology (facultative vs obligate parasitism) and ecology (lowland vs upland) between these weed species appear more important than their similarities regarding the effectiveness of management strategies in rice production systems

Understanding and optimizing species mixtures using functional–structural plant modelling

Plant species mixtures improve productivity over monocultures by exploiting species complementarities for resource capture in time and space. Complementarity results in part from competition avoidance responses that maximize resource capture and growth of individual plants. Individual organs accommodate to local resource levels, e.g. with regard to nitrogen content and photosynthetic capacity or by size (e.g. shade avoidance). As a result, the resource acquisition in time and space is improved and performance of the community as a whole is increased. Modelling is needed to unravel the primary drivers and subsequent dynamics of complementary growth responses in mixtures. Here, we advocate using functional–structural plant (FSP) modelling to analyse the functioning of plant mixtures. In FSP modelling, crop performance is a result of the behaviour of the individual plants interacting through competitive and complementary resource acquisition. FSP models can integrate the interactions between structural and physiological plant responses to the local resource availability and strength of competition, which drive resource capture and growth of individuals in species mixtures. FSP models have the potential to accelerate mixed-species plant research, and thus support the development of knowledge that is needed to promote the use of mixtures towards sustainably increasing crop yields at acceptable input levels

Fertilization benefits the facultative parasitic plant Rhamphicarpa fistulosa while gains by the infected host Oryza sativa are marginalized

• Background and Aims. Rhamphicarpa fistulosa (Hochst.) Benth. is an annual facultative parasitic plant adapted to hydromorphic soils. In sub-Saharan Africa it causes high crop losses as a weed in rainfed lowland rice (Oryza sativa L.). Fertilisers are often proposed as control measure against hemiparasitic weeds but understanding of nutrient effects on R. fistulosa is currently still elusive. • Methods. In two greenhouse pot experiments, conducted in 2016 in the Netherlands and in 2019 in the UK, host plants (O. sativa, cv IR64) and parasitic plants (R. fistulosa) were grown alone or combined, and subjected to different nutrient availability levels. Biomass measurements were used to assess if and how nutrient availability effects are expressed in the host and parasite. • Key Results. Compared to parasite-free host plants, the biomass of parasite-infested plants was severely reduced and nutrient effects on host plant biomass were less pronounced. Inversely, increased nutrient availability did not have an effect on parasitic plants when grown alone, but when grown with a host the parasitic plant biomass proportionally increased. Grown together, the combined biomass of host plant and parasite was substantially lower than that of the host plant grown alone. The ratio in biomass between host plant and parasite was unaffected by nutrient availability. • Conclusions. Fertilization benefits to rice plants are severely reduced but not completely nullified by R. fistulosa infection. The parasite's production and reproduction benefits accrued from increased nutrient availability are restricted to conditions in presence of a host plant. Host presence and nutrient effects are thus observed to be synergetic; R. fistulosa plants parasitizing a suitable host respond strongly to increasing levels of nutrients. This is associated with an increased root biomass of the parasitic plant itself but more likely resulting from exploitation of the nutrient uptake capacity of the host plant it parasitizes on

Combining host plant defence with targeted nutrition: key to durable control of hemiparasitic Striga in cereals in sub-Saharan Africa?

Host plant defence mechanisms (resistance and tolerance) and plant nutrition are two of the most widely proposed components for the control of hemiparasitic weeds of the genus Striga in tropical cereal production systems. Neither of the two components alone is effective enough to prevent parasitism and concomitant crop losses. This review explores the potential of improved plant nutrition, being the chemical constituent of soil fertility, to fortify the expression of plant inherent resistance and tolerance against Striga. Beyond reviewing advances in parasitic plant research, we assess relevant insights from phytopathology and plant physiology in the broader sense to identify opportunities and knowledge gaps and to develop the way forward regarding research and development of combining genetics and plant nutrition for the durable control of Striga

Fertilisers differentially affect facultative and obligate parasitic weeds of rice and only occasionally improve yields in infested fields

Different fertilisers were field tested to investigate whether they (1) suppress the obligate parasitic weed Striga asiatica and the facultative parasitic weed Rhamphicarpa fistulosa, and (2) favour rainfed rice yields under parasitic weed infestation. Four years of experiments were conducted in southwest Tanzania, in a S. asiatica-infested rainfed upland and a R. fistulosa-infested rainfed lowland field. Treatments included sole mineral (NPK or Di-Ammonium Phosphate —DAP— plus urea), organic (cattle manure or rice husk), combinations of mineral and organic fertilisers and a no-fertiliser control. Fertilisers moderately suppressed the obligate parasite S. asiatica, but no correlations between infestation level and soil fertility status were observed. In the lowland field, fertilisers promoted the facultative R. fistulosa, in particular the ones that included organic components. Plant-available phosphorus, exchangeable potassium and soil organic matter content correlated with R. fistulosa infestation levels. Positive fertiliser effects on yields were found in both parasitic weed infested fields, except in years with high S. asiatica infestation levels. Rice husks alone and rice husks or manure combined with DAP and urea increased yields and soil fertility most. We conclude that fertilisation differentially affects the obligate and facultative parasitic weeds of rice systems; obligate parasites may be slightly suppressed, but facultative parasites may even be stimulated. Meanwhile, consistent rice yield increases under parasitic weed infested conditions cannot be obtained with fertiliser application when parasitic weed infestation levels are too high

Biochar improves fertility of a clay soil in the Brazilian Savannah: short term effects and impact on rice yield

The objective of this study was to report single season effects of wood biochar (char) application coupled with N fertilization on soil chemical properties, aerobic rice growth and grain yield in a clayey Rhodic Ferralsol in the Brazilian Savannah. Char application effected an increase in soil pH, K, Ca, Mg, CEC, Mn and nitrate while decreasing Al content and potential acidity of soils. No distinct effect of char application on grain yield of aerobic rice was observed. We believe that soil properties impacted by char application were inconsequential for rice yields because neither water, low pH, nor the availability of K or P were limiting factors for rice production. Rate of char above 16 Mg ha^(−1) reduced leaf area index and total shoot dry matter by 72 days after sowing. The number of panicles infected by rice blast decreased with increasing char rate. Increased dry matter beyond the remobilization capacity of the crop, and high number of panicles infected by rice blast were the likely cause of the lower grain yield observed when more than 60 kg N ha^(−1) was applied. The optimal rate of N was 46 kg ha^(−1) and resulted in a rice grain yield above 3 Mg ha^(−1)

Systems approaches to innovation in pest management: reflections and lessons learned from an integrated research program on parasitic weeds in rice

This paper provides a retrospective look at a systems-oriented research program, on the increasing occurrence of parasitic weeds in rainfed rice in sub-Saharan Africa, to qualitatively assess merits and identify challenges of such approach. We gained a broad contextual overview of the problem and different stakeholders' roles, which enabled identification of entry points for innovations in parasitic weed management. At the crop level parasitic weed infestation is associated with poor soil fertility and water management. Farmers' infrequent use of inputs to control them was caused by various factors, ranging from fears of undesired side effects (agronomic) to a lack of quality control of products (institutional). Furthermore, there may be enough extension agents, but they lack the required training on (parasitic) weed management to provide farmers with advice, while their organizations do not provide them with the necessary means for farm visits. At even higher organizational levels we observed a lack of coherent policies on parasitic weed control and implementation of them. Merits and challenges of an integrated multi-stakeholder and multi-level research project are discussed

Characterization of host tolerance to Striga hermonthica

One of the most promising control options against the parasitic weed Striga hermonthica is the use of crop varieties that combine resistance with high levels of tolerance. The aim of this study was to clarify the relation between Striga infestation level, Striga infection level and relative yield loss of sorghum and to use this insight for exploring the options for a proper screening procedure for tolerance. In three pot experiments, conducted in Mali (2003) and The Netherlands (2003, 2004), four sorghum genotypes were exposed to a range of Striga infestation levels, ranging from 0.0625 to 16 seeds cm−3. Observations included regular Striga emergence counts and sorghum grain yield at maturity. There were significant genotype, infestation and genotype × infestation effects on sorghum yield. The relation between infestation level and infection level was density dependent. Furthermore, the relation between Striga infection level and relative yield loss was non-linear, though for the most resistant genotype Framida only the linear part of the relation was obtained, as even at high infestation levels only moderate infection levels were achieved. The results suggest that for resistant genotypes, tolerance can best be quantified as a reduced relative yield loss per aboveground Striga plant, whereas for less resistant genotypes the maximum relative yield loss can best be used. Whether both expressions of tolerance are interrelated could not be resolved. Complications of screening for tolerance under field conditions are discussed