Biological Control of Root Parasitic Weeds with Plant Pathogens

Abstract

Root parasitic weeds are nutrition specialists. Using special organs, the haustoria, they penetrate into the vessels of roots of host plants in order to supply themselves with water and nutrients. They can be either facultative or obligate, obligate parasitic weeds being considered as one of the most serious constraints to food production in many parts of the world. Obligate parasites are either hemi-parasites with chlorophyll, like species of the genus Striga (Scrophulariaceae), witchweeds, or bolo-parasites without chlorophyll such as species of the genus Orobanche (Orobanchaceae), broomrapes. Forty Striga species are reported worldwide: 33 in Africa, 7 in Asia and 4 endemic in Australia. Eleven species are parasites on agricultural crops. The genus Orobanche has more than 100 species but only 7 are considered as economically significant parasitic weeds (Parker and Riches, 1993; Raynal-Roques, 1996). ECONOMIC IMPORTANT STRIGA AND OROBANCHE SPECIES AND THEIR MAIN HOST PLANTS Striga spp. hamper cereal and legume production in Sub-Saharan Africa. Striga hermonthica (Del.) Benth. and S. asiatica (L.) Kuntze are specific to grasses and cause severe production losses in sorghum (Sorghum bicolor (L.) Moench), millet (Pennisetum americanum L.), maize (Zea mays L.) and rice (Oryza sativa L.) whileS. gesnerioides (Willd.) Vatke prefers dicot hosts and affects mainly cowpea (Vigna unguiculata (L.) Walp.). Orobanche spp. occur primarily in the Mediterranean region, West Asia and North Africa. Orobanche crenata Forsk., 0. ramosa L., 0. aegyptiaca Pers., 0. cernua Loeffl., 0. cumana Wallr., 0. minor Sm. and 0. foetida Poir. are major biotic limiting factors to the production oflegumes like faba bean (Viciafaba L.), chickpea (Cicer arietinum L.), lentil (Lens culinaris Medick.), and to crops of the family Solanaceae (tomato, Lycopersicon esculentum Mill.; potato, (Solanum tuberosum L.; tobacco, Nicotiana tabacum L.) and Asteraceae (such as sunflower, Helianthus annuus L.) (Parker and Riches, 1993).