Phenology and growth of Orobanche crenata Forsk (broomrape) in four legume crops

Studies were conducted in the field in 2 years comparing the phenology and growth of Orobanche crenata (Forsk) (crenate broomrape) in lentils (Lens esculenta L.) cv. Castellana, peas (Pisum sativum L.) cv. Orix, vetch (Vicia sativa L.) cv. comun and broadbean (Vicia faba L. cv. Alameda. First attachment of O. crenata to these crops took place 9–14 weeks after mid-November planting and earlier after later plantings, Differences in the first O. crenata attachment dates and in the duration of the underground growth period of O. crenata were much greater between years for any given crop than between crops in a given year: both attributes were apparently affected more by seasonal climatic conditions than by crop species, Furthermore, there was no consistent relationship found between crop growth stages and time after first attachment of the parante. This occurred in lentils and vetch while they were vegetative, in peas at late vegetative-early flowering stages, and at widely varying growth stages in broadbean, depending on planting dates and years. The maximum number of O. crenata plants successfully attached to each individual crop plant decreased in the order: peas > broadbean > lentil > vetch, with 21, 14, 10 and 8 per plant, respectively, averaged over the two seasons. Similarly, plant parasites: host dry weight ratio were 1.0, 0.7, 0.3 and 0.2 For each of these crops, respectively.     

A competition index for Orobanche crenata Forsk effects on broad bean (Vicia faba L.)

The influence of broomrape (Orobanche crenata Forsk) infection levels on commercial broad bean (Vicia faba L.) crops was investigated and an equation relating yield losses to O. crenata density was derived. O. crenata growth was weakly and negatively related to the final height or shoot number of the broad bean plant. O. crenata decreased seed yield mainly by reducing the number of pods. The number of seeds per pod was also decreased by O. crenata infection when competition occurred at late-crop growth stages when most of the pods had already developed. Bean unit seed weight was only weakly affected by infection. Correlation coefficients between O. crenata number and broad-bean seed yield per plant were significant at P=0·05 and greater than 0·70 in most locations. On average, about four O. crenata per broad-bean plant decreased seed yield by half. An estimation of the percentage of losses in any field can be calculated from the equation: %loss = 100 × 0·124 × OcN. where OcN is the average final number of emerged O. crenata per plant.     

Effect of sowing date and host resistance on the establishment and development of Orobanche crenata in faba bean and common vetch

The effect of early and late sowing dates on the establishment of Orobanche crenata Forsk. (crenate broomrape) in resistant and susceptible cultivars of faba bean (Vicia faba L.) and vetch (Vicia sativa L.) were studied over four seasons in southern Spain. Differences in establishment, development and shoot emergence of the parasite were compared. With resistant faba bean and vetch cultivars, there was reduced attachment and shoot emergence of O. crenata with all sowing dates. Susceptible cultivars were more severely affected by the parasite with early sowing. Orobanche crenata development was also delayed in resistant cultivars. Crop yield, estimated by the number of pods per plant, decreased with late sowings. The combined use of resistant cultivars and early sowing is a useful tool as part of an integrated control strategy. Resistant cultivars allow early sowing (with low O. crenata attack), thus avoiding yield losses due to the short crop cycle with late sowing.     

Fenugreek root exudates show species-specific stimulation of Orobanche seed germination

Various Orobanche species are weedy and cause severe reduction in the yields of many important crops. The seeds of these parasitic weeds may remain dormant in the soil for many years until germination is stimulated by the release of a chemical signal from a host plant. In order to determine the effects of fenugreek root exudate on the induction of Orobanche crenata, Orobanche ramosa and Orobanche foetida seed germination, root exudate was collected from hydroponically grown fenugreek seedlings. Fractionation patterns obtained from column and thin layer chromatography of the fenugreek root exudate showed a set of metabolites differing in their polarity with stimulatory activity on Orobanche seed germination. The crude root exudate stimulated both O. ramosa and O. crenata seed germination to the same level caused by the synthetic germination stimulant GR24 at 10 mg L^?1. It also stimulated O. foetida seed germination which did not respond to GR24. Active fractions of root exudate stimulated the germination of Orobanche species differentially.     

Effect of the animal's digestive system on the infectivity of Orobanche seeds

Seeds of Orobanche aegyptica Pers., O. cernua Leofl. and O. crenata Forsk. were introduced into the rumen of three rumen-canulated rams. Infectious seeds of all Orobanche species were found in the faeces as indicated by infecting different host plants in pot experiments. There were indications that the seeds of O. crenata are the most susceptible to the detrimental effects of the animal's digestive processes on their infectivity, whereas the O. aegyptica Pers. seeds are the most resistant of the three species tested. The consequences of these findings in connection with the spread of the pest as a result of agricultural practices are discussed.     

Biology and management of the fast‐emerging threat of broomrape in rapeseed–mustard

Broomrapes (Phelipanche and Orobanche spp.) are obligate root parasites of the family Orobanchaceae. The natural variation in Orobanchaceae exists in plants involving Triphysaria versicolor (facultative hemiparasite), Striga hermonthica (obligate hemiparasite) and Phelipanche aegyptiaca, formerly Orobanche aegyptiaca (holoparasite). The family Orobanchaceae has 90 genera. The four major parasitic species of broomrape are Phelipanche ramosa in the Brassicas, especially in rapeseed, Phelipanche cernua in tobacco and sunflower, P. aegyptiaca in solanacious crops and in Brassica juncea and Phelipanche crenata in leguminous crops. They are prevalent, infesting nearly 1.2% of the world's arable land. In India, P. ramosa and P. aegyptiaca cause severe infestations of Brassica and have threatened its cultivation in major growing areas. In addition, a single broomrape plant can release >100 000 seeds that remain viable for decades in the soil. This provides the parasite with a great genetic adaptability to environmental changes, including host resistance, agronomical practices and herbicide treatments. Different control measures, such as manual weeding, herbicide applications, solarization, crop rotation and integrated broomrape management practices, have been proposed in order to reduce the broomrape menace and improve yields in farmers' fields. Breeding for broomrape resistance also could be an economic, feasible and environmental friendly control method. The present article reviews the current status of research on broomrape in India and abroad, as well as suggests strategies for its effective management.     

Germination response of Orobanche seeds subjected to conditioning temperature, water potential and growth regulator treatments

Broomrapes (Orobanche spp.) are parasitic weeds that cause significant losses of crop yield. Experiments were conducted to investigate the seed response to the artificial germination stimulant GR₂₄ in three species of Orobanche subjected to preconditioning under various temperatures, water potentials and with plant growth regulators. The highest germination percentages were observed in Orobanche ramosa, Orobanche aegyptiaca and Orobanche minor seeds conditioned at 18°C for 7 days followed by germination stimulation at 18°C. With the increase of the conditioning period (7, 14, 21 and 28 days), the germination percentage of O. ramosa and O. aegyptiaca progressively decreased. When conditioned at −2 MPa, the germination percentage was lower than at 0 and −1 MPa, especially at 13 and 28°C. Orobanche minor seeds could retain relatively high germination if conditioned at 18, 23 or 28°C, even after significantly extended conditioning periods (up to 84 days). GA₃ (30–100 mg L⁻¹), norflurazon and fluridone (10–100 mg L⁻¹), and brassinolide (0.5–1.0 mg L⁻¹) increased seed germination, while 0.01 mg L⁻¹ uniconazole significantly reduced germination rates of all three Orobanche spp. The promotional effects of GA₃ and norflurazon and the inhibitory effect of uniconazole were evident, even when they were treated for 3 days. Germination of Orobanche seeds was much lower when the unconditioned seeds were directly exposed to GR₂₄ at 10⁻⁶ m . This early GR₂₄-induced inhibition was however alleviated or even eliminated by the inclusion of GA₃ or norflurazon (10–50 mg L⁻¹) in the conditioning medium. On the contrary, the inclusion of uniconazole increased the inhibitory effect of GR₂₄, particularly in the case of O. ramosa.     

Granular formulation of Fusarium oxysporum for biological control of faba bean and tomato Orobanche

BACKGROUND: Orobanche spp. represent a serious threat to a wide range of crops. They are difficult targets for herbicides, and biological control could provide a possible solution. This work therefore aimed to formulate mycoherbicides of Fusarium with adequate shelf life and virulence against Orobanche but safe to faba bean and tomato. RESULTS: Only two isolates of Fusarium oxysporum Schlecht. (Foxy I and Foxy II) obtained from diseased Orobanche shoots were found to be pathogenic to Orobanche crenata Forsk. and Orobanche ramosa L. Conidial suspension of both isolates significantly decreased germination, attachments and tubercles of Orobanche. Microconidia and chlamydospores of both isolates were formulated as mycoherbicides encapsulated in a wheat flour–kaolin matrix (four different formulations). All formulations greatly diminished Orobanche emerged shoots, total shoot number, shoot height, attachment of emerged shoots, the germinated seeds that succeeded in emerging above the soil surface and dry weight. Meanwhile, disease incidence and disease severity of emerged shoots were enhanced. The shelf life was adequate, particularly for coarse, freshly prepared, low‐temperature‐stored, microconidia‐rich formulations. The induced growth reduction of Orobanche‐infected host plants seemed to be nullified by formulations, particularly at the highest dose. CONCLUSION: These formulations seemed to destroy Orobanche but appeared harmless to host plants. Hence, they could be efficiently used as mycoherbicides for biological control of Orobanche in faba bean and tomato. Copyright © 2008 Society of Chemical Industry     

Seed bank and other demographic parameters of broomrape (Orobanche crenata Forsk.) populations in faba bean (Vicia faba L.)

A study on the population dynamics of broom-rape (Orobanche crenata Forskal) in faba bean (Viciafaba L.) was conducted in two locations for 8 years. The O. crenata seed bank increased every year in the 20-cm depth arable layer until it reached approximately 4 million (M) seeds m^?2. Seed viability ranged between 53% and 68%, approximately half of the seeds remaining dormant. About 3 × 10^?3% of the seed bank became attached to the root system of the faba beans. Only 9% of the attached broomrapes developed and emerged from the soil, possibly reflecting high levels of intraspecific competition. Maximum broomrape seed production for a population of 53 emerged broomrapes per m² was approximately 4 M seeds m^?2. About 43% of the seeds produced were not incorporated into the soil, most probably as a result of their degradation, decomposition or dispersion. Stock semencier et autres paramètres démographiques de I'orobanche crénelée (Orobanche crenata Forsk.) dans des cultures de féverolle (Vicia faba L.) Une étude de la dynamique de populations d'orobanche crénelée (Orobanche crenata Forsk.) dans des cultures de féverolle (Viciafaba L.) a été menée pendant 8 années sur deux sites. Le stock semencier de O. crenata a augmenteéchaque année dans la couche (20 cm) arable du sol jusqu'à atteindre 4 millions (M) de graines m^?2. La viabilité des graines était comprise entre 53 et 68%, approximativement la moitié d'entre elles présentaient une dormance. Environ 3 × 10^?3% du stock semencier adhérait au système racinaire de la féverolle. Seulement 9% des orobanches adhérantes se développaient et levaient, ce qui pourrait refléter une compétition intraspécifique importante. La production maxi-male de graines par une population de 53 orobanches levées au m²était d'environ 4 M graines m^?2. Environ 43% des graines produites n'étaient pas incorporées au sol, très prob-ablement en raison de leur dégradation, de leur décomposition ou de leur dispersion. Bodensamenbank und andere populationsde-mographische Parameter der Sommerwurz Orobanche crenata Forsk. in Bestanden der Acker-Bohne (Vicia faba L.) Bei einer Untersuchung der Populationsdy-namik der Sommerwurz Orobanche crenata Forsk. in Beständen der Acker-Bohne (Vicia faba L.) an 2 Orten über 8 Jahre hin nahm die Bodensamenbank in der Pflugzone von 20 cm Tiefe jedes Jahr zu, bis sie rund 4 Millionen Samen m^?2 erreichte. Die Lebensfähigkeit der Samen lag bei 53 bis 68%, etwa die Hälfte war dormant. Ungefähr 3 × 10^?3% kamen mit den Bohnenwurzeln in Berührung. Nur 9% davon entwickelten sich und wuchsen heran, worin vielleicht der hohe Grad intraspezifischer Konkurrenz zum Ausdruck kommt. Die höchste Zahl gebildeter Samen eines Sommer-wurzbestands von 53 Pflanzen m^?2 war rund 4 Millionen. Etwa 43% der gebildeten Samen gelangten nicht in den Boden, wahrscheinlich weil sie zerstört oder verweht wurden.     

Characterization of the Orobanche–Medicago truncatula association for studying early stages of the parasite–host interaction

There is an increasing interest in the legume species Medicago truncatula as a model plant for structural and functional genomic studies that can be used to identify agronomically important genes in legumes. Field screening has shown very high levels of resistance to Orobanche crenata in M. truncatula. However, in vitro studies with O. crenata, Orobanche foetida, Orobanche ramosa and Orobanche minor showed useful variation among accessions at early stages of the parasite–host interaction. Significant differences were observed in the levels of germination of O. crenata and O. foetida seeds induced by different accessions of M. truncatula. Only limited germination was observed on accession SA‐4327. All accessions induced little O. ramosa and O. minor germination. Accessions also varied in the number of O. crenata and O. foetida attachments supported, with few developing on accession SA‐27774. The variation observed for induction of germination and of subsequent attachment will be useful to isolating and characterizing genes involved in the early stages of Orobanche–host plant interaction and for the study of the biosynthetic pathways of production for germination stimulants.     

Competition between annual weeds and vining peas grown at a range of population densities: effects on the crop

Competition between annual weeds and vining peas (Pisum sativum L.) at five target population densities between 11 and 194 plants/m² was examined by means of periodic destructive sampling of weedy and weed-free plots. A further area of each plot was cut and vined to assess yields. Weeds impaired vegetative development, particularly by reducing tillering in low density crops. This resulted in weedy plots having fewer pods per plant at harvest but a lower proportion of flat pods than weed-free plots. Weeds had no effect on numbers or weights of peas in full pods nor on tenderometer values of samples of vined peas. Adverse effects of weeds on the growth of individual crop plants decreased with increasing crop density. However, at lower crop densities many of the additional pods on weed-free plots contributed little to vined yield, while at higher densities, direct or indirect effects of weeds increased the problem of maintaining sufficient photosynthetic area during pod swelling to prevent pod abscission and poor ovule development. Regression analysis of yield on crop density and of yield on numbers of pods per plant showed that vined yield per hectare was reduced by weeds by a constant amount across the range of densities and numbers of pods examined. Vining throughput was also reduced in weedy as compared with weed-free crops, even on high density plots where little or no weed vegetation remained at harvest. In general, weed presence had effects similar to those caused by increasing crop density, but without the additional contribution to yield made by extra plants. Higher density crops suppressed weeds very effectively but were no less vulnerable to yield loss than those of lower density; they therefore merit just as much attention to effective weed control as crops suffering more visibly from competition by weeds.     

Detecting<Emphasis Type="Italic">Orobanche</Emphasis> species by using cpDNA diagnostic markers

Some species of the genusOrobanche are among the most devastating parasitic weeds, causing extensive damage in agricultural fields. Considering the difficult control due to seed longevity in the soil, small seed size, high fecundity and a subterranean phase that allows them to parasitize the host before they emerge and become evident, the development of diagnostic markers is highly recommended. In our study we identified potential molecular diagnostic markers from the plastid genome in order to distinguish among the most importantOrobanche species attacking crops in Andalusia, the southern region of the Iberian Peninsula. The study has consideredO. crenata, O ramosa andO. cumana causing serious losses in legumes, solanaceous crops and sunflower fields, respectively, andO. minor that, although abundant in Andalusia, has to our knowledge not yet been found parasitizing agricultural hosts. We amplified a non-coding region from the plastid genome, studied sequence differences among the amplified fragments and digested those of the same length with selected restriction enzymes. Here, we propose a molecular protocol to distinguish the main parasitic plants in crop fields of southern Spain. Different applications such as identification ofOrobanche seeds in soil or crop seed lots are discussed in order to offer right crop recommendations or to prevent new infestation of parasite-free fields. Recommendations for further development of these diagnostic markers are also considered. http://www.phytoparasitica.org posting Jan. 15, 2007.     

Influence of tillage, straw disposal system and seed return on the population dynamics of Alopecurus myosuroides Huds. in winter wheat

Alopecurus myosuroides Huds. was studied over a 2-year period in winter wheat established after tine cultivations or direct drilling. Straw was removed by baling or spread and burnt. Seed production was either allowed or prevented by cutting and removing all vegetation at the end of the first year. Cultivation differences had no consistent effect on plant or seed populations. Straw burning destroyed about 50% of seeds and encouraged the germination of surviving seeds. Weed populations in the crop were lower on burnt than on baled areas. Where seed shedding was allowed, populations of seeds in soil and plants increased by up to nine-fold per year. Straw burning resulted in smaller population increases. Seed decline in the soil averaged about 80% per year, so that less than 6% of the weed seeds sown were still viable after 2 years’burial in the soil. Most of the seed decline occurred between July and October and was slightly greater on burnt than on baled areas. Only part of this seed loss was accounted for by germination and emergence of seedlings during summer and autumn. Plants emerging in the crop represented less than 26% of viable seeds present in the soil at time of drilling the crop. Few seedlings emerged in spring. The viability of shed seeds varied with year and with weed density. High infestations were associated with lower seed viability and also fewer heads per plant.     

Bioassay-led isolation of Myrothecium verrucaria and verrucarin A as germination inhibitors of Orobanche crenata

A total of 188 fungal isolates was obtained from the rhizosphere of Vicia faba grown in an Egyptian soil heavily infested with Orobanche species. Agar cultures of 58 isolates inhibited the germination of conditioned seed of Orobanche crenata exposed to the germination stimulant, GR24. Filtrates of inhibitory fungi grown in liquid medium for 9–15 days were also assayed and those of five isolates, which were morphologically similar, inhibited germination even when diluted 16-fold. The fungus was identified as Myrothecium verrucaria (Alb. & Schwein.) Ditmar by its morphology and the nucleotide sequence of the ITS1 and ITS2 regions of the ribosomal repeat unit. Purification of the inhibitor to homogeneity was accomplished by solvent partitioning, flash chromatography on silica gel, semi-preparative HPLC on a reversed phase C18 column, solid phase extraction and tlc on silica gel. The inhibitor was identified as verrucarin A by nuclear magnetic resonance spectroscopy and comparison of the spectra with those of an authentic sample of the compound. A preliminary experiment demonstrated that infection of V. faba by O. crenata could be prevented by addition of spores of the fungus to soil infested by the parasite.     

Use of ITS nuclear sequences from Phelipanche aegyptiaca as a direct tool to detect single seeds of broomrape species in the soil

Broomrape (Phelipanche and Orobanche spp.) are obligate holoparasites that attack roots of almost all economically-important crops in semiarid regions of the world. Broomrape seeds are extremely small (dust-like seeds), averaging 200 to 300?μm in size and because of the miniscule seed size it is difficult to detect and confirm via conventional methods. In this study our aim was to develop a PCR-based assay specific for broomrape soil-borne seeds and sensitive enough to detect a single or few broomrape seeds in a soil sample. For this purpose, we used complementary polymerase chain reaction (PCR) primers based upon unique sequences in the internal transcribed spacer (ITS) regions of the nuclear ribosomal DNA of Phelipanche aegyptiaca. Genomic DNA was extracted from soil samples artificially infested with broomrape seeds or tissue of Phelipanche aegyptiaca Pers., Orobanche cumana Wallr. and Phelipanche crenata Forsk. and subjected to PCR analysis. Using ITS-350 primers, a specific PCR product (350?bp) was amplified and detected in all samples containing broomrape species, but was not detected in soil sample free of broomrape seeds or tissues. Additionally, the PCR-based assay was sensitive enough to detect even a single broomrape seed in the soil. As expected the universal internal control primers amplified a PCR product (555?bp) of genomic DNA extracted from soil samples with or without broomrape tissues or seeds. This diagnostic method is simple, reliable and rapid and could help for assessment of broomrape seed contamination in a crop field.     

Influence of Root Exudates on Orobanche Seed Germination*

Broomrape (Orobanche ramosa L.) is a common root parasite of solanaceous, leguminous and other crops grown in the semi-arid regions of the world. The seeds germinate when root exudates from host plants are released in their immediate vicinity (Lindley, 1853; Koch, 1887; Chabrolin, 1934). Brown et al. (1951a) reported that non-host plants, such as flax (Linum usitatissimum L.) may stimulate Orobanche seed germination without being parasitized. The stimulating properties of flax exudate were studied by Brown et al. (1951b). They reported that the stimulant was unstable in alkaline solutions, but moderately stable in weakly acidic media, which may indicate the presence of an acidic (lactone) grouping. Nash & Wilhelm (1960) reported that gibberellic acid in agar media stimulated O. ramosa seed germination. Abu- Shakra, Miah & Saghir (1970) found that pre-treatment of 0. ramosa seeds with 100 ppm of gibberellic acid followed by incubation on a flax-root diffusate agar medium gave a high (81·7%) germination. The purpose of this study was to collect root exudates from three species of plants cultured under three experimental systems, namely (a) germ-free, (b) glasshouse (non-sterile), and (c) growth chamber (hydroponic, initially aseptic), and to evaluate their biological activity as germination stimulants for O. ramosa seeds. The plants used were tomato (Lycopersicon esculentum Mill.), sorghum (Sorghum vulgare Pers.) and flax. Exudate from marigold (Tagetes erecta L.) also was collected from germ-free culture.     

Quantitative determination of broomrape (Orobanche spp.) seeds in vetch seeds

A method was developed by which the presence of broomrape (Orobanche spp.) seeds in vetch seeds was determined quantitatively. The broomrape seeds are washed out of the vetch seeds by a combination of two sieves. The vetch seeds remain on the top sieve and the broomrape seeds are collected on the lower sieve with square openings of 100 μm, which do not permit passage of the broomrape seeds. An appropriate statistical method for estimating the true mean number of broomrape seeds per sample in a large vetch seeds lot is presented and discussed. Détermination quantitative des graines d' orobanche dans des graines de vesce. Une méthode permettant de dénombrer la présence de graines d'orobanche dans un lot de graines de vesce a été mise au point. Les graines d'orobanche sont séparées de celles de vesce par une succession de deux tamis. Les graines de vesce sont maintenues sur le tamis supérieur et les graines d'orobanche sont collectées sur le tamis le plus bas avec une maille de 100 μm qui ne permet pas le passage des graines d'orobanche. Une méthode statistique appropriée pour estimer la quantité réelle d'orobanches dans un grand lot de semences de vesce est présentée puis discutée. Quantitative Bestimmung von Orobanche-Samen in Wicken-Saatgut Es wurde eine Methode entwickelt, mit der Orobanche-Samen in Wicken-Saatgut quantitativ erfasst werden können. Zum Trennen der Samen wurden 2 Siebe verwendet, wobei die Wicken-Samen im oberen, die Orobanche-Samen im unteren mit einer Maschenweite von 100 μm gesammelt wurden. Eine statistische Methode für die Schätzung der Orobanche-Samenmenge in grossen Wicken-Saatgutpartien wurde vorgestellt und diskutiert.     

Insect pests

Abstract

Investigation on seed production potentialities of weeds of Dharwar was carried out at the Agricultural College Farm, during January 1969. Both the parasitic weeds Striga and Orobanche showed high seed production, Striga producing as many as 380,000 seeds per plant followed by Orobanche with 230,000 seeds per plant. Among the weeds studied, Desmodium diffusum produced the least (425) seeds per plant.     

Soyasapogenol B and trans-22-dehydrocam- pesterol from common vetch (Vicia sativa L.) root exudates stimulate broomrape seed germination

BACKGROUND: Orobanche and Phelipanche species (the broomrapes) are root parasitic plants, some of which represent serious weed problems causing severe yield losses on important crops. Control strategies have largely focused on agronomic practices, resistant crop varieties and herbicides, albeit with marginal success. An alternative control method is the induction of suicidal seed germination with natural substances isolated from root exudates of host and non‐host plants. RESULTS: Soyasapogenol B [olean‐12‐ene‐3,22,24‐triol(3β,4β,22β)] and trans‐22‐dehydrocampesterol [(ergosta‐5,22‐dien‐3‐ol, (3β,22E,24S)] were isolated from Vicia sativa root exudates. They were identified by comparing their spectroscopic and optical properties with those reported in the literature. Soyasapogenol B was very specific, stimulating the germination of O. minor seeds only, whereas trans‐22‐dehydrocampesterol stimulated P. aegyptiaca, O. crenata, O. foetida and O. minor. CONCLUSION: Soyasapogenol B and trans‐22‐deydrocampesterol were isolated for the first time from Vicia sativa root exudates, and their biological activity as stimulants of Orobanche or Phelipanche sp. seed germination was reported. Copyright © 2011 Society of Chemical Industry     

Effect of residue management and fallow length on weeds and rice yield

Reduced fallow length in slash-and-burn rice (Oryza sativa L.) production systems of northern Laos increased weed pressure, labour requirement and the need for soil and moisture conservation. On-farm and on-station studies and on-farm surveys were used to evaluate the effect of residue management and cropping intensity on weed population, rice yield and nematode density. Residue loads were 2.3–4.4 t ha^?1 after a rice crop and 9.5 t ha^?1 after 1 year of fallow. Compared with farmers' traditional burning of crop and weed residues, mulching reduced rice yield by 43% in one out of four comparisons and increased weed biomass by 19–100%. Compared with continuous rice treatments (averaged over burning and mulching treatments), treatments with fallow or cowpea [Vigna unguiculata (L.) Walp.] in the previous year had 32% less herbaceous weed biomass, 90% fewer Ageratum conyzoides L. and over 99% fewer Meloidogyne graminicola Golden & Birchfield. Rice yield was negatively associated with A. conyzoides density (?0.62, P < 0.01) and M. graminicola number (?0.42, P < 0.05). Less striking effects of fallow period on A. conyzoides and M. graminicola, observed from on-farm surveys, demonstrate the limitations of on-farm studies because of undocumented effects of farmers' management decisions.