Farm productivity gaps and environmental impact on vegetable-beef cattle family farming systems in the south of Uruguay

Abstract

Downy mildew caused by Peronospora destructor is an important disease worldwide. The development of resistant cultivars is a strategy to control this disease which would reduce the use of chemicals, saving the environment and people´s health. In Uruguay, landraces and cultivars presented differences in the response to downy mildew in field conditions. This could be the result of difference in infection rate or latency. This research aimed to identify sources of resistance to downy mildew in the local onion germoplasm. Field trails were conducted in 2007 and 2008, in which 34 landraces, five cultivars and one advanced population were assessed. The proportion of plants with sporulation or typical downy mildew spots was evaluated once a week and data was integrated in a logistic model to calculate the Area Under Disease Progress Curve (AUDPC). Disease severity was assessed once in 2007 and three times in 2008. In 2007, local germoplasm differed in the disease incidence (3 to 99%), severity (7 to 40% of infected leaf area), and AUDPC. Accessions assessed in 2008 showed differences as well, but smaller than in 2007. Resistance and growing cycle were related. Long-day accessions were more resistant than intermediate and short-day accessions. This behavior could involve either specific resistance factors or could be the result of physiological differences related to developmental stage for each accession when infections with P. destructor took place. Evolution of disease was very variable in 2008. The disease incidence decreased in one evaluation date, and increased markedly the week after up to achieve 100% of infected plants for most accessions. Inoculations under controlled environmental conditions were carried out to maintain P. destructor and evaluate components of resistance in different accessions. Different ways of collecting inoculum, preparing the inoculum, inoculation techniques, whole plant versus detached leaves, and different environmental conditions were evaluated. Downy mildew infection was observed using whole plants and dry sporulation distributed as inoculum on the leaf surface previously wet. Large diversity in the response to P. destructor confirms the value of onion local germoplasm to be used as resistance source in onion breeding.