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Striga and Orobanche parasitic weed controlAbstracts of papers presented in the parasitic weed session in the III International Weed Control Congress, Foz do Iguassu, Brazil, 6-11 June 2000, are given below. This session of the congress was sponsored by FAO. An overview of parasitic weed controlRICARDO LABRADA FAO Plant Protection Service, via delle Terme di Caracalla 00100 Rome, Italy Striga and Orobanche parasitic weeds constitute one of the most important biotic constraints to the production of food crops in Africa and the Middle East. Striga parasitic weeds may cause yield losses of up to 80% in the staple food and industrial crops of the region, including cereals, such as sorghum, pearl millet, finger millet, maize, rice, wheat, and sugar cane, and some broadleaf crops, such as cowpeas, sunflower, soybean, groundnut, tobacco, and various cultivars of beans. The subsistent resource-poor farmers in the marginal areas of rainfall and poor soil productivity are the most affected by Striga weeds. Serious losses of vegetables, legumes, sunflower and tobacco production are caused by Orobanche spp., which mainly occur in North Africa, the Near East and in a few Latin American countries. Permanent mono-cropping and lack of suitable crop rotation, impoverished soils and reduced rainfall, incorrect cultural practices (including the use of crop seeds infested by parasitic weed seeds) and the lack of other preventative measures are the major factors favouring continuous interference of parasitic weeds with several crops. There is no single effective method for the control of parasitic weeds. The most effective approach is the integration of different environmentally friendly control measures that are economically feasible to smallholders. Striga spp are best controlled by preventing their reproduction and build up of the seed bank in soil through the use of clean crop seeds, hand-weeding, direct application of 2,4-D in cereal crops for preventing seed-setting, rotation of affected crops with effective trap crops to induce suicidal Striga seed germination and increasing fertility of the soil through the use of organic manure and N- fertilisers. Some tolerant cultivars are available, but the attachment of haustoria to host crops can also be delayed using imazapyr for crop seed-dressing. Research has also been carried out for possible application of Fusarium pathogens for Striga control. For Orobanche spp the approach should be similar to the above with regards to improvement of soil fertility, but N-fertilisers are not always compatible in some affected crops. The use of tolerant cultivars is another option. Application of post-emergence glyphosate at reduced rates (0.02-0.04 kg a.i. ha-1) have been successfully used in some crops, such as faba beans and sunflower. Increased extension work is still required with farmers for them to adopt the available methods for the control of parasitic weeds.
Linking research, extension and farmers: Striga control strategies for western KenyaANDREAS OSWALD*1, JOSEPH AGUNDA2 and DENNIS FRIESEN3Institute of Crop Science, University of Kassel, Germany CARE-Kenya2 CIMMYT3 Striga hermonthica (Del) Benth., a parasitic weed that infests maize and other cereals, is a major constraint to food production in many sub-Saharan countries. However, small-scale farmers rarely adopt Striga control technologies. A project implemented in western Kenya in 1995 developed and tested agronomic methods for controlling Striga. In 1998, the project began to develop strategies and training materials to disseminate these technologies because surveys of farmers and extension agents showed that knowledge about Striga biology and control was very limited. Two organizations, the National Extension Service (NES) and a NGO (CARE-Kenya), each using different approaches to farmers' training and adaptive research, were identified as collaborators. The NGO uses a strictly participatory approach in which adaptive research farmers (ARFs) conduct on-farm trials while group resource persons train their farmers groups on new technologies. The major problems of this approach are sustaining farmer-managed-research in time and verifying whether farmer-to-farmer training has been successful. In contrast, the NES selects farmers to set-up on-farm trials and conducts training sessions and field days open to all farmers. Their major problem is the continuity of training since extension agents often lack incentives and the means to conduct their work. Extension agents of both organizations were trained on Striga biology and control. Manuals and picture-series on these topics were provided and scientific backup and assistance given for training sessions. During the first 9 months, the NES trained more than 2000 farmers and implemented 8 on-farm demonstrations. The NGO selected 64 ARFs and approximately 200 farmers' groups, implemented 16 on-farm trials and initiated the training of farmers. The NES will be encouraged to follow up farmers who participated in training sessions and assess the impact of these sessions on adoption. In 2001, a survey will be conducted to assess the impact of both approaches on knowledge dissemination and adoption of Striga control methods. Prospects for Orobanche control in the 21st CenturyDaniel M JoelDepartment of Weed Research, ARO, Newe-Ya'ar Research Center, P.O. Box 1021,Ramat-Yishay 30095, Israel Parasitic weeds of the genus Orobanche (broomrapes) are vicious pests of many agricultural crops. They have a tremendous impact on world agriculture. Unlike other weeds they are devoid of leaves and are totally dependent on a host plant for nourishment. In the 20th Century the means for Orobanche control included sanitation, weeding, soil fumigation and solarization, selection and breeding of resistant crops, trap and catch crops and biological agents. By the end of the century some herbicides proved relatively effective in certain crops. In addition, some new approaches developed only recently, like the use of herbicide resistant crops, and crop seed treatments. However, no method is thus far practical and economic in the majority of crops. Orobanche research in the last twenty years contributed to our understanding of some crucial steps in the development of the parasite and in host/parasite relations. Research has 1. Described seed metabolism during preconditioning and germination, and identified germination stimulants. 2. Discovered enzymatic processes during haustorium invasion. 3. Established host responses to Orobanche, resistance mechanisms, and host genes that are induced during penetration. 4. Revealed demographic data of the parasite and developed molecular markers for important species. These research achievements should be exploited for the development of novel control methods. Three new strategies for Orobanche control can be envisaged: A. Manipulation of known metabolic pathways, by developing specific herbicides that will only affect the parasite. B. Development of artificial resistances by genetic engineering, based on knowledge of crucial steps of infection and on the availability of suitable promoters. C. Maintaining an equilibrium between the parasite population and its hosts by constant supply of a biocontrol agent that will keep Orobanche seed production to a minimum. An integrated approach combining various control methods will allow a long-term solution of the Orobanche problem. Striga control by restoring soil fertilityGualbert Gbèhounou INRAB/Laboratoire de Défense des Cultures, B.P 128 Porto-Novo, République du Bénin Parasitic Striga species (Family Scrophulariaceae) thrive on degraded soils, the majority of soils in tropical Africa. They occur worldwide, in Africa (native continent of the genus), Asia, America and Australia. Literature review indicates S. asiatica (L.) Kuntze, S. gesnerioides (Willd.) Vatke and S. hermonthica (Del.) Benth. as the species currently causing agricultural problems. S. asiatica and S. hermonthica parasitize cereal crops in contrast with S. gesnerioides which parasitizes leguminous plants like cowpea (Vigna unguiculata (L.) Walp. and also plants of the families Agavaceae, Convolvulaceae and Euphorbiaceae. Parasitism by Striga may inflict up to 100% yield loss. Ammonium nitrogen impairs germination and attachment of S. hermonthica seedlings to roots of the host plant. It also reduces production of germination stimulant by the host. A more remarkable effect on Striga is expected from organic matter as compared to mineral fertilizers. In situ production of organic matter by growing a short fallow cover crop which improves soil fertility has been studied. Velvet bean (Mucuna utilis (L.) DC is adopted by farmers as cover crop in B‚nin. Leguminous trap crops like cowpea (Vigna unguiculata), soybean (Glycine max (L.) Merrill, groundnut (Arachis hypogaea L.), etc. are also available which induce suicidal germination of S. hermonthica seeds and may also contribute soil nitrogen. In contrast to mucuna they produce marketable seeds. Choice of a trap crop should be based on its specific and general effectiveness indices. Early sowing of a trap crop may be more effective for Striga control than late sowing. Establishment of a cover crop or a trap crop may require application of a limited amount of a mineral fertilizer. Rimsulfuron: a new tool for Orobanche ramosa control in potatoMustapha A. Haidar1 and Moatasim M. Sidahmad1 Faculty of Agricultural and Food Sciences, American University of Beirut, Beirut, Lebanon Field studies were conducted to evaluate the efficacy of rimsulfuron for Orobanche ramosa control in potato. Rimsulfuron was applied POST at 10, 20, 30, 40 and 50 g a.i. ha-1. Each rate was tested for single and sequential (twice or three times) application at 20, 40 and 60 days after potato emergence (DAPE). Results indicated that rimsulfuron at all tested rates significantly reduced Orobanche shoot number and dry weight compared to the control. Rimsulfuron at 10 g a.i. ha-1 reduced Orobanche infestation by 58% when applied once (20 DAPE) and 88% to 100 % when applied two (20 & 40 DAPE) or three times (20, 40 & 60 DAPE). Sequential application of 20-50 g a.i. ha-1 reduced Orobanche infestation and shoot dry weight by 100%. All tested rates except for the single application of rimsulfuron at 10 g a.i. ha-1 produced compact potato plants with small leaves. Phytotoxicity was mostly reflected in the tuber quality with a high incidence of malformed and small tubers. Effect of fodder legumes on stimulation, attachment and emergence of Striga hermonthica on maizeD. K. NDUNG’U1, A. OSWALD1, D. FRIESEN1, E. S. ARIGA1 and M. MBURU1P.O. Box 1221, KISUMU, Kenya A method to control Striga infestation is the use of trap-crops either in sole stands to decrease the Striga seed bank in the soil or as intercrops in maize to reduce Striga attachment to the host and suppress emerged Striga plants. Three pasture legumes, Mucuna gigantica, Stylosanthes guyanensis and Desmodium sp. were investigated for their ability to induce germination of conditioned Striga hermonthica seed, for their effect on Striga attachment and on Striga shoot emergence. Laboratory experiments showed that the root exudates of the legumes stimulated up to 70% more Striga seeds to germinate than exudates of maize.Legumes and maize were also grown in pots to observe Striga attachments 90 days after planting. The maize-Mucuna combination had the highest number of attachments while all other combinations and maize planted in pure stand had lower numbers of attached Striga.The experiment showed also that Striga attached in considerable numbers to Mucuna roots but seedlings did not develop further. The other legumes had none or only very few Striga seedlings attached to their roots. In a field trial the number of emerged Striga shoots 12 weeks after planting were counted. Maize intercropped with legumes had a clearly lower Striga infestation than maize in pure stand. Grain yields of maize were highest in the maize-Mucuna combination followed by maize-Stylosanthes, maize in pure stand and maize-Desmodium respectively. These experiments show that the legumes tested are better stimulant producers than maize and therefore have a potential as Striga trap crops However, they do not prevent or reduce attachment of Striga to the maize roots if planted as intercrops. The lower number of emerged Striga shoots in maize intercropped with legumes can be attributed to the shading effect of the legumes or a change in humidity and temperature conditions due to their dense canopy. Fodder legumes can reduce Striga infestation as trap crops or due to suppression of emerged Striga. They can form a valuable part of an integrated Striga control strategy for western Kenya. Progress on Striga hermonthica control in East AfricaGEORGE D. ODHIAMBO1Kenyan Agricultural Research Institute Striga is a parasitic weed which affect cereal productivity in East Africa with losses of between 30-90% very common. Major progress in the Striga witchweed control has been in the identification of resistance in major cereal crops especially maize. Striga resistance genes have been identified in Zea diploperenis, a wild relative of maize which is being incorporated into the adapted local maize genotypes through backcrossing. The genes would then be mapped through marker assisted selection. Parallel to this effort, a large collection of transposon-induced mutations in the genome of maize adapted to African conditions are being constructed using the mutator type transposable elements. After the resistance is confirmed, genes for resistance will be tagged then isolated for clonning. Seed dressing acetolactase (ALS) target-site resistance maize mutants (PH 3245-IR) seed with Mg-imazapyr or pyrithiobac-Na at 30 g a.i ha-1 has been observed to provide up to 12 weeks of Striga control at minimal cost. In addition to the use of these new genetic tools to identify resistance, cereal germplasm with wider genetic variability continue to be evaluated for durable resistance to Striga. As part of the integrated control strategy, new cultural practices or old ones are being refined to manage the weed. Improved short term land fallows for 6 months using either of Crotalaria, Sesbania, Tephrosia and Acacia species among others cause suicidal germination of Striga, and thus help reduce the seed bank. Catch cropping with susceptible hosts (Sudan grass or sorghum), transplanting maize or sorghum to escape the most vulnerable stage of Striga attack; inter-cropping susceptible host with Desmodium, a fodder legume, to suppress Striga growth while at the same time repelling stem borers from attacking the cereal can each or all be integrated with resistant genotypes to help reduce the harmfull effect of the weed. New possibilities for Cuscuta management in some vegetable cropsABDUR-RAHMAN SAGHIRP.O. Box 11-8281, Beirut, Lebanon e-mail: absaghir@inco.com.lb Cuscuta is a parasitic flowering plant which twines its tendrils on the foliage of several vegetable crops. It causes serious losses on chard, mallow, mint, onions, radish, red beet, tomato, turnip and other vegetables grown in the Middle East. Several control measures have been used to alleviate crop losses. These include prevention methods such as planting of uncontaminated crop seeds, crop rotation, various cultural practices, biological and chemical control methods.Soil-applied, contact and post attachment application of herbicides are used with varying degrees of success. Pot experiments were conducted to study the efficacy of glyphosate and glufosinate ammonium for the control of Cuscuta campestris L. on some vegetable crops. It was found that post-attachment application of glufosinate at 25 ppm (a.i.) , sprayed on mint at a volume of 100 cc m-2, gave good selective control of Cuscuta. In the same trial , glufosinate at 50 ppm and above was toxic to mint as was glyphosate at 100 ppm and above. Chard, red beet and radish were tolerant to 25-50 ppm glufosinate, and to 75-100 ppm glyphosate. In the case of tomato, glyphosate at 25-75 ppm controlled Cuscuta, but caused some phytotoxicity to the crop. The application of glyphosate at 25 ppm controlled the parasite without apparent phytotoxicity to mallow. It is recommended that lower concentrations of the herbicides tested as well as others such as chlorsulfuron and imidazolinones be evaluated in the future for selective control of Cuscuta in vegetable crops. With the release of genetically modified cultivars of herbicide resistant crops, post-attachment application of herbicides becomes more feasible.
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