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SUMMARY Phoma macdonaldii, the causal agent of the sunflower black stem disease, is responsible for qualitative and quantitative damage which can result in up to 60% yield losses in France and worldwide. An early inoculation method has been developed in the laboratory to test sunflower genotype tolerance to Phoma. It was thus important to check whether the method was relevant by assessing the possible incidence of the growth stage of the plant on its reaction to Phoma. Six sunflower genotypes, which were known to be more or less susceptible to Phoma, were inoculated with an aggressive isolate at four different growth stages: cotyledon, unfolded leaves 3-6, budding and early flowering stage. Data analysis showed there is no interaction between the growth stage and the genotype: the test developed discriminates between genotypes early and the grading is representative of what occurs at a more advanced stage as it remains unchanged. Taking into account these data and the large genetic variability of Phoma macdonaldii, the strain-genotype interaction was investigated on isolates originating from France and other countries, in the search for possible pathotypes. For this purpose, ten isolates of Phoma, which were considered to be representative of the fungus variability, with differing aggressiveness, geographical origins and types of symptoms, were tested on ten different sunflower lines. Preliminary results suggest the existence of five pathotypes. Plant breeders could thus use these pathotypes for testing rapidly and reliably the tolerance of their genetic material to Phoma. |
