Assessment of sunflower genotype tolerance to Phoma macdonaldii

ENSAT, UMR AGIR, BP 52627, F-31326 Castanet-Tolosan cedex, France

^* dechamp@ensat.fr

Abstract

Sunflower genotypes with differing susceptibility to Phoma macdonaldii, the causal agent of sunflower black stem disease, were inoculated artificially with an aggressive fungal strain according to various modalities. A first series of experiments carried out in a greenhouse on plants at various phenological stages (2-leaf pairs, 5-6 leaf pairs, budding and early flowering) showed that the behaviour of sunflower cultivars, distributed into different susceptibility groups, remained unchanged irrespective of the phenological stage reached at the time of the inoculation. The development of the symptoms varied with genotype susceptibility to the disease: it was fast with coalescing necrotic spots on the stems for the most susceptible cultivars. The intensity and the development rate of the symptoms were also increased when the inoculation had been carried out at later stages. Two additional series of experiments were carried out in the field. In each series, a first block accounted for natural contaminations and a second block was inoculated artificially. The development of the symptoms and the susceptibility groups within the two blocks were identical to those observed in the greenhouse. These results led to the validation of an early inoculation test, at the cotyledon stage, which accounts satisfactorily for the behaviour of sunflower cultivars under natural conditions and is thus likely to be used in breeding programs.