Bull. Spec. CORESTA Symposium, Kallithea, 1990, p. 166, P09, ISSN.0525-6240

Comparison of two new methods for measuring the resistance of tobacco to Phytophthora parasitica var. nicotianae usable in selection

INRA, Station de Botanique et de Pathologie Végétale, Antibes, France
Two methods for inoculating tobacco with the black shank agent Phytophthora parasitica var. nicotianae (Ppn) : the inoculation of a population of young plants in the cotyledon stage ("cotyledons test") and the inoculation of young plants at the two leaf stage ("miniplant test"). The ability of these methods to express the varietal resistance to Ppn was evaluated by applying them to a series of varieties with varied but known behaviors. The "cotyledons test" involved groups of 500 seedlings. The culture medium and the inoculation conditions (by spraying with zoospores) were optimized. Under these conditions, the test was correctly able to distinguish between varieties presenting clearly different resistance. This simple and rapid method can, therefore, be used to obtain an early selection of line leaders in the selection process. In the"miniplant test", the inoculation was carried out individually by adding the mycelium around the roots. With this method, the previous estimation can be made more accurate and by comparing the effect of different strains of Ppn (of varied species and aggressivity) it was possible to distinguish the"vertical"resistance (the R1 gene) from the degrees of"horizontal"resistance, even though this is only partially expressed a this stage. The two tests are therefore complementary and should assist in the selection of new varieties of tobacco with increased resistance to black shank.