Quantitative studies on downy mildew (<Emphasis Type="Italic">Peronospora destructor</Emphasis> Berk. Casp.) affecting onion seed production in southern Uruguay |
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Authors: | Pablo H González Paula Colnago Sebastián Peluffo Héctor González Idiarte Javier Zipitría Guillermo A Galván |
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Institution: | 1.Plant Protection Department, Facultad de Agronomía,Universidad de la República,Progreso,Uruguay;2.Plant Production Department, Centro Regional Sur (CRS), Facultad de Agronomía,Universidad de la República,Camino Folle km 36, Progreso,Uruguay |
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Abstract: | Onion downy mildew (Dm) symptoms and damage on seed production fields in southern Uruguay were quantified during two seasons
as the progress of incidence, severity, and as the effect of the level of seed-stalks infections on seed yield and quality.
In addition, the effects of two plantation dates and two plant densities on Dm were studied in a factorial experiment. Maximum
incidence along the season ranged from 15 to 65% in four commercial fields in 2005 and two fields in 2006. Maximum severity
ranged from 4.5 to 9.3% of leaf area affected in 2005, and 0.35 to 1.17% in 2006. Whereas Dm incidence varied among studied
fields, disease progress as Dm severity varied mainly between studied years. Crop rotation, crop vigour and plant density
were identified as major factors affecting Dm variation in the field. Intensive fungicide schedules did not control Dm when
other conditions favoured the disease. A high level of Dm severity defined as large necklace spots on seed-stalks significantly
reduced seed yield in comparison with healthy seed-stalks in 2005, as well as seed yield and germination in 2006. Weight of
1,000 seeds was not significantly reduced by Dm infections on seed-stalks. Six genera of fungi were recovered from seeds harvested
on highly infected seed-stalks (Stemphylium, Fusarium, Alternaria, Penicillium, Aspergillus and Botrytis), but not Peronospora. Late planting date and low plant density had significantly lower Dm incidence and severity. This finding questioned the
early planting dates and high densities previously recommended in order to achieve high yielding crops. The combination of
several practices to reduce initial inoculum, susceptibility status of the host, and environmental conditions promoting the
disease is discussed as forming a basis for effective disease control. |
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