First record of Bremia lactucae infecting Sonchus oleraceus and Sonchus asper in Brazil and its infectivity to lettuce

Abstract

Bremia lactucae is recorded for the first time causing downy mildew on common sowthistle (Sonchus oleraceus) and spiny sowthistle (Sonchus asper) in Brazil. The disease and etiological agent are described. Pathogencity of sporangia obtained from S. oleraceus was tested on 12 species belonging to the Asteraceae, already recorded in the literature as hosts of B. lactucae, and four commonly cultivated varieties of lettuce. All four cultivars of lettuce, S. oleraceus and S. asper showed symptoms of the disease and sporulation of the pathogen 5 days after inoculation. It has been observed that the disease occurs on Sonchus spp. throughout the year in Vic ̧osa (state of Minas Gerais), being more common on these two hosts than on lettuce. Both weeds are very common invaders of vegetable growing areas in Brazil. This indicates that those two hosts for the fungus may be important inoculum reservoirs for disease occurring in lettuce, highlighting the need for control of these weeds as part of the pro- cedures aimed at controlling this disease. These results are also in agreement with the views that B. lactucae is split into host-specific infraspecific taxa. Lettuce and sowthistles are regarded as closely related, belonging to the same subfamily and tribe in the Asteraceae (Subfamily Cichorioideae: Tribe Lactuceae).