Quantitative studies on downy mildew (Peronospora destructor Berk. Casp.) affecting onion seed production in southern Uruguay

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.

Characteristics of Meteorological Conditions during a Severe Outbreak of Onion Downy Mildew and Metalaxyl Sensitivity of Peronospora destructor in Saga, Japan, in 2016

“…In the disease survey fields, symptoms of downy mildew caused by secondary infections appeared earlier in early maturing onion varieties than in the medium-latematuring onion varieties in 2016, 2017, and 2018 (Figures 5-7). Several studies have reported that host-plant age and size can affect the incidence of onion downy mildew, as shown below; thus, even in favourable environments for disease development, it has been observed that onset occurred at 9 weeks after transplanting [39] or that disease development only occurred late in the growing season [40]. Furthermore, in a study examining the effects of planting date on onion plants, the size of plants when the first symptoms of downy mildew were observed was approximately 25 cm in length [41].…”

Diversity of vegetable landraces in the Pampa biome of Brazil and Uruguay: utilization and conservation strategies

“…The seed certification program created opportunities for research in seed production technology, encompassing various aspects of crop protection and management (Gonzaĺez et al, 2011;Peluffo et al, 2016b). Thanks to the genetic richness of onion landraces, improved cultivars enabled their competitiveness in major vegetable markets.…”