Sensibilité aux Pythium et mauvaise germination en sol froid chez le haricot (Phaseolus vulgaris L)

French. Les manques à la levée observés sur haricot en semis précoce au printemps sont principalement dus aux attaques de Pythium. D’importantes différences de sensibilité peuvent être mises en évidence entre variétés par des méthodes de contamination les mettant en présence de souches de Pythium douées de divers niveaux d’agressivité. Les variétés à grain coloré sont, de façon générale, moins sensibles que celles à tégument incolore, et parmi celles-ci les flageolets verts sont les plus sensibles. L’étude de la descendance d’un croisement (noir résistant x flageolet vert) met en évidence l’effet sensibilisant de 2 gènes récessifs, l’un conditionnant l’absence de coloration, l’autre le caractère «flageolet». Cependant, il y a aussi des différences de sensibilité entre variétés à grains noirs. Les variétés plus sensibles exsudent plus de sucres et de composés azotés en cours de germination, leurs téguments sont moins riches en leuco-anthocyanes. L’aptitude physiologique à germer et à croître à basse température intervient également dans la réussite des semis précoces. Elle a fait, elle aussi, l’objet d’une étude variétale qui met en évidence les qualités de la variété Vernandon.

English. Bean growers and the canning industry would appreciate the improved success of early sowings of common beans, which are frequently damaged by Pythium spp. Twenty strains were collected and classified for agressivity and taxonomy: P ultimum (most of them very agressive), P sylvaticum (most of them fairly agressive), and palmatecoralloid strains, which are weakly agressive. Laboratory methods are described for inoculation of bean seeds with Pythium, analysis of bean seed exudates and estimation of leucoanthocyanin content of the seed coat. Significant differences in Pythium susceptibility could be demonstrated between bean cultivars. Black beans, and also some red, buff-colored and mottled beans were more resistant than white ones, amongst which differences in susceptibility could be demonstrated by the use of weakly aggressive Pythium strains. The highest susceptibility occured for the French flageolet vert cultivars. A study of the progeny of a cross between Pl226895 (the most resistant black bean) and Elsa (a "flageolet vert") demonstrate the increases in susceptibility caused by the 2 recessive genes inducing the absence of coloration and flageolet quality. High levels of resistance could be found in buff-colored as well as in black-seeded lines in the progeny of this cross. At the biochemical level susceptibility is associated wtih high level of soluble substances (sugars, nitrogen compounds) in seed exudates during imbibition, low content in leucoanthocyanins, and seed coat thinness. The pleiotropic influence of the gene (p) on all these characters could be demonstrated by the study of white mutants of the black-seeded cultivars PI226895 and Aiguillon. Some discrepancies could appear between ranking for Pythium susceptibilities in experiments with 10 d incubation at 11 °C and ranking for these biochemical characters. They could be linked to differences in physiological ability to germinate and grow at low temperatures. These differences were also studied: the best cultivar found for this character was Vernandon. Integration of control measures has been discussed. Even the low levels of resistance which can be bred for white-seeded lines can be improved by seed treatment with low amounts of fungicides. White-seeded cultivars are preferred by the snap bean canning industry. The water in the snap bean cans is blue-gray with black-seed cultivars. Light-buff colored resistant lines can be bred, resulting in a less unpleasant water coloration in the cans.