Temporal epidemiology of Botrytis leaf blight of onion under different fungicide spray regimens

ABSTRACT Botrytis leaf blight of onion is controlled by foliar fungicide application following a fixed schedule. However, the application regimen based on a forecast system can predict and minimize the disease progression. As the epidemiological behavior of the disease in southern Brazil is not known, this study aimed to analyze the temporal epidemiology of the disease under different fungicide application regimens. In a randomized block design with four replicates, the application regimens were evaluated based on the system of Marcuzzo & Haveroth (2016) with cumulative daily values of estimated severity (ES) of 0.20, 0.25, and 0.30, compared to the standard control (fungicide application at 5 and 7 days). The disease severity was weekly evaluated in ten plants previously demarcated at random for each replicate. The disease progress curve for each genotype was adjusted to Gompertz model, since the observed severity corresponded to the model and was confirmed by the consistency between the estimated points and by the residue. The 7-day regimen had the highest progression rate (0.80283), while ES=0.30 had the lowest rate (0.50526). The regimen showing ES=0.30 presented the maximum severity peak, 46.4%, and the 5-day regimen had the lowest cumulative value of the disease, 27.7%. Fungicide application regimens did not influence the temporal progression of Botrytis leaf blight in onion Empasc 352/Bola Precoce.