Aggressiveness and host range of Phoma medicaginis isolated from Medicago species growing in Tunisia

Aggressiveness of 14 Phoma medicaginis isolates obtained from Medicago truncatula (barrel medic) and M. ciliaris (ciliate medic) growing in Tunisia was measured after inoculation on leaves and roots of M. truncatula. The ability of one isolate to cause disease on M. sativa (alfalfa), Cicer arietinum (chickpea), Pisum sativum (pea), Lens culinaris (lentil) and Phaseolus vulgaris (common bean) was also tested. The pathogen caused dark lesions that enlarged and coalesced causing yellowing and premature abscission of leaves, resulting in decreased shoot fresh weight in barrel medic plants. All P. medicaginis isolates infected barrel medic roots causing collar rot, brown root discoloration, yellowing of cotyledons and reduced shoot and root development. The pathogen colonized the cortex and the stele of plants and produced fertile pycnidia on infected roots. Symptoms on leaves allowed for greater discrimination in aggressiveness among isolates in comparison to symptoms on roots. No correlations were observed between the parameters measured on leaves and roots suggesting organ specialization in this pathogen. Phoma medicaginis infected leaves of alfalfa, pea, common bean and chickpea causing necrosis and tissue yellowing at 15 d post inoculation (dpi). Pycnidium production was observed on dead and dying foliar tissues of alfalfa, pea and common bean, but not on chickpea. The pathogen caused symptoms of collar rot and brown root discoloration on alfalfa, chickpea, pea and common bean, but did not cause symptoms on leaves or roots of lentil at 15 dpi. Phoma medicaginis was more pathogenic on barrel medic, the host of origin, in comparison to the other legumes, suggesting that these species are likely to be secondary hosts for this pathogen.