Main features of the recent spreading of broomrape in French tobacco crop and efficiency of control methods
In France, the obligate root parasite Orobanche ramosa L. (broomrape) attacks tobacco and causes important yield losses in some areas. It is becoming a real agronomic threat. A national survey has been conducted by ANITTA (National Tobacco Technical Center) to evaluate infested surfaces and damages caused to the tobacco crop, and to determine the susceptibility of varieties. Eight out of the nine French tobacco co-operative societies are concerned by broomrape with 30 to 40% of the tobacco acreage being infested. A more accurate survey established that O. ramosa also attacks winter rape and hemp, showing ability of adaptation to various crops. The distribution of broomrape is spreading from South-West to North-East of France. Field observations were carried out during five months (May to September 2002) to define the kinetic of development of the different stages of O. ramosa and to propose a biological cycle of O. ramosa / tobacco pathosystem. This cycle is very important for farmers to detect the presence of broomrape in fields before its emergence. Laboratory research concerning preconditioning and germination of parasite seeds indicated that, besides the obligatory presence of susceptible crops, the development of O. ramosa did not require any specific conditions. Other abiotic and biotic factors are involved in broomrape expansion: susceptible weeds, monoculture, human activities, soil texture, soil seed bank … . Following treatment of the tobacco with maleic hydrazide, germination of broomrape seeds obtained from parasitized tobacco shoots was assessed in vitro . It was shown that 88,2 to 99,3% of seeds displayed an abnormal feature (transparent integuments and necrosed embryo) and did not germinate. The test of viability using Tetrazolium indicated that these seeds were dead. The research program on O. ramosa / tobacco pathosystem for the years 2003-2005 is presented and discussed. It focuses on host/parasite relationships, tobacco root exudates, Orobanche ramosa seed bank, tobacco weeds and control methods.