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ISHS Acta Horticulturae 1143: VII International Symposium on Edible Alliaceae

Selection for resistance and histopathological relationships in the onion 'Regia' against downy mildew (Peronospora destructor)

Authors:   G.A. Galván, M. Arias, N. Curbelo, S. Peluffo, P.H. González
Keywords:   disease severity, infection process, partial resistance
DOI:   10.17660/ActaHortic.2016.1143.3
Quantitative differences among onion accessions in the reaction against downy mildew can be exploited in breeding. This research describes the use of 'Regia' as a resistance source and a study of histopathological relationships. Field trials were performed to evaluate downy mildew incidence and severity. The widely grown cultivar 'Pantanoso del Sauce' (PdS) was among the susceptible accessions, whereas 'Regia' presented low severity, low proportion of plants with sporulating spots and delayed progress of the disease. Plants from a cross 'PdS' × 'Regia' were selected for resistance and quality traits, and thereafter selfed to obtain F1S1 lines. Fifty-nine lines were evaluated in 2013 and 2014, and only 9 and 7 F1S1 lines did not differ from 'Regia' each season. Histopathological observations of sporulating spots collected from field trials were done for some accessions. 'Regia' had a larger proportion of healthy stomata than 'PdS' (62 vs. 33%) but lower proportions of sub-stomatal colonization (14 vs. 31%) and stomata with emerging sporangiophores (28 vs. 46%). Randomly sampled 'PdS' × 'Regia' F1 plants showed intermediate values between the parents, but a selected F1S1 line did not differ from 'Regia' in sub-stomatal colonization. The infection process was studied by inoculation of detached leaves. At 48 hpi, 5-9% of the sporangia were germinated and/or penetrated the stomata in 'PdS' and other susceptible accessions, whereas for 'Regia' colonized stomata were found after 72 hpi, and the proportion of germinated sporangia was significantly lower (2-3%). The lower rate of infection and slower colonization within the leaf parenchyma could be components of resistance in 'Regia'.

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