GENETIC ASPECTS OF BLACKLEG (LEPTOSPHAERIA MACULANS) RESISTANCE IN RAPESEED (BRASSICA NAPUS)

A method of inoculating and evaluating the cotyledons of rapeseed (Brassica napus L.) seedlings with the blackleg pathogen, Leptosphaeria maculans (Desm.) Ces. et de Not.(Phoma lingam (Fr.)Tode) was developed which allows single plants to be tested with several isolates of the fungus simultaneously, and which allows tested plants to survive and to be used in genetic studies. This seedling screening methods was used: (1) to determine the pathogenicity of isolates from a large collection of European isolates of the blackleg organism; (2) to conduct genetic studies on the inheritance of blackleg resistance; and (3) to compare the response of several cultivars or breeding lines to isolates of the pathogen representing a broad geographic range from the European rapeseed crop.; On the basis of seedling response, approximately half of the 70 European isolates tested were of the strongly pathogenic strain, and half were of the weakly pathogenic strain. Strongly pathogenic isolates were found among isolates from seven of the eight sites sampled. Seedling resistance in the French breeding line R39 to several strongly pathogenic isolates of P. lingam is controlled by one dominant gene, Lm1. Resistance in the cultivar Girita to two strongly pathogenic isolates is also controlled by a single dominant gene, Lm2. Recombination frequency between Lm1 and Lm2 is estimated at 22.9 (+OR-) 2.3%. Seedling response of S(,1) lines derived from six cultivars or breeding lines to 28 P. lingam isolates demonstrated variation in virulence among populations of the pathogen, as well as variation among and within host cultivars in their resistance to specific isolates. The 97 S(,1) lines tested were grouped into seven pathogenicity groups (pathotypes). All S(,1) lines from the cultivar Lesira were susceptible to all 28 isolates. Some S(,1) lines from Primor and Girita were also susceptible to all 28 isolates, but both Primor and Girita also contained individuals resistant to some isolates and susceptible to some. The French breeding lines R39 and R47 were differentiated from the susceptible cultivar Primor in their intermediate level of resistance to a group of French isolates, whereas Primor was susceptible to this group of isolates.; Cultural characteristics of the strongly and weakly pathogenic strains of the fungus were compared. Weakly pathogenic isolates grew more rapidly on V-8 juice agar and on malt extract agar (MEA) than did strongly pathogenic isolates, and produced a brownish-yellow, water-soluble pigment on Czapek-Dox broth + yeast extract (CDY) whereas strongly pathogenic isolates did not produce the pigment. Growth of weakly pathogenic isolates was only partially inhibited on PDA + 0.02% lactic acid (PDA + LA), while strongly pathogenic isolates were completely inhibited. The weakly pathogenic strain had an appressed colony morphology on MEA; strongly pathogenic isolates were somewhat more floccose. Radial growth on MEA, pigment production on CDY and growth inhibition on PDA + LA appear to be the most useful cultural characteristics to differentiate the two strains.