NADJIA ZERMANE1, JÜRGEN KROSCHEL2, THOURAYA SOUISSI3
1University of Kassel, Institute of Crop Science (INK), Germany
2University of Hohenheim, Institute of Plant Production and Agroecology in the Tropics and Subtropics, Germany
3National Institute of Agronomy of Tunisia (INAT), Laboratory of Botany, Tunisia
Broomrapes (Orobanche spp.) are the most economically important group of root parasitic weeds causing high yield losses in many essential crops mostly in the Mediterranean area, the Middle East and Eastern Europe. In North Africa, Orobanche constitutes a major constraint to food legume crops production and complete crop failure can be expected in case of heavy infestations. Application of natural enemies of Orobanche could be an effective control method to be combined to other control measures in an integrated weed management program. Consequently, surveys for natural antagonists (insects, fungi and rhizobacteria) of the parasitic weed Orobanche was carried out during the growing season of 2000 and 2001 in the major legume-cropping areas in the North African country Tunisia. Insects herbivores of Orobanche were investigated at 21 locations in northern Tunisia. Stems and fruit capsules from 315 Orobanche plants, sampled in faba bean (Vicia faba L.) fields, were examined separately for the presence of insects. Phytomyza orobanchia (Diptera: Agromyzidae) and Smicronyx cyaneus (Coleoptera: Curculionidae) were found to attack O. crenata Forsk. and O. foetida Poiret in almost all surveyed areas. Both insects inflict significant damage to the parasitic weed under natural conditions. One hundred and five fungal isolates were obtained from infected Orobanche underground stages with Fusarium spp. being the most prevalent. Fusarium oxysporum and F. culmorum showed promising control of Orobanche in screening experiments. Bacteria were isolated from the Rhizosphere of faba bean as well as from diseased Orobanche underground stages or suppressive soil samples. Of 351 bacterial isolates obtained, 337 were screened for antagonistic activity against both Orobanche species. Both, Orobanche seed germination stimulating bacteria and Orobanche growth inhibiting bacteria have been selected and had a significant antagonistic activity against the parasitic weed. Most promising isolates were Pseudomonas fluorescens, Ps. marginalis and Ralstonia pickettii. The diversity of natural antagonists of Orobanche in Tunisia provides promising prospects for the biocontrol of the parasitic weed.
Keywords: Biocontrol, Fusarium spp., Herbivores, orobanche spp., Pseudomonas spp., Rhizobacteria, Survey, Tunisia