Evaluation of the Response to Peronospora Destructor (Berk.) Casp. in Local Onion Germplasm in Uruguay

Abstract

Downy Mildew (DM) caused by Peronospora destructor is one of the most economically important diseases in onion production. This research aimed to identify sources of resistance to DM in local onion germplasm. Field trials were conducted in 2007 and 2008 in order to evaluate a total of 40 genetic materials. The incidence of P. destructor was assessed weekly, a logistic equation was adjusted to the data, and the Area Under Disease Progress Curve (AUDPC) was calculated. In 2007, local germplasm showed differences in the proportion of diseased plants (3 to 99%), in severity (7 to 40% of total leaf area was affected) and AUDPC. In 2008, the materials tested showed differences as well, but smaller than in 2007. Resistance and phenological cycle were related. The group of long day accessions showed higher resistance than the groups with intermediate and short-day accessions. This behavior could involve specific resistance factors present in the germplasm of long day, or could be the result of physiological differences related to the stage of development that determine susceptibility to the pathogen. In 2008, evolution of disease was very variable. The incidence of P. destructor fell sharply on an assessment, and the following week rose to 100% of the plants for several accessions. Diversity observed in the response to P. destructor supports the potential value of local germplasm to be used as source of resistance in onion improvement.