Soybean Rust Epidemics as Affected by Weather Conditions in Brazil

Gustavo C Beruski,Gil M S Câmara,André B Pereira,Paulo C Sentelhas,Ivan P A Junior,Luis M Schiebelbein

doi:10.5539/jas.v12n1p213

Abstract

The soybean rust (SBR) epidemics are often triggered by weather conditions, which interfere actively on the disease progress. Therefore, weather variables can be used to estimate the risk of occurrence and severity of SBR outbreaks. This research aimed to determine the influence of weather variables on SBR progress in different field trials in Brazil. Field experiments were conducted during 2014-15 and 2015-16 soybean growing seasons in Piracicaba (SP), Ponta Grossa (PR), Campo Verde (MT) and Pedra Preta (MT). For all sites and seasons, a susceptible soybean cultivar was drilled with 0.45 m row spacing and 12 plants per linear meter. No fungicide sprays were applied to ensure natural disease occurrence. In order to create different environmental conditions, sequential sowing dates, of roughly 30-day intervals were carried out. In Piracicaba, Ponta Grossa, Campo Verde, and Pedra Preta the main weather variables influencing SBR were leaf wetness duration - LWD (R = 0.340), air temperature during LWD (R = 0.313), and cumulative rainfall (R = 0.304). The final severity was assessed only at Piracicaba and Ponta Grossa, and it was mainly influenced by LWD (R = 0.643). It is possible to conclude that epidemics of SBR were mainly influenced by leaf wetness duration, accumulated rainfall and air temperature during the LWD. Therefore, future researches aiming to develop a disease warning system for SBR should include the cumulative rainfall, LWD and the air temperature during LWD, together or individually, as inputs.

Highlights

Many factors contribute to reduce soybean production and among them, diseases are one of the most important (Strange & Scott, 2005)
3.1 Weather Characterization In Piracicaba, the mean air temperature for each sowing date remained between 22.9 and 24.7 °C, which are within the optimum range for soybean rust (SBR) development, which is considered to be in the range between 20 and 25 °C (Melching et al, 1989; Alves et al, 2007; Del Ponte & Esker, 2008)
For the fifth sowing date in 2015-16, SBR was not observed, probably due to the occurrence of low temperatures combined with the absence of moisture, which was caused by the intense drought that occurred in this year

Summary

Introduction

Many factors contribute to reduce soybean production and among them, diseases are one of the most important (Strange & Scott, 2005). For the establishment of disease, the existence of a susceptible host and favorable environmental conditions, which characterize the disease triangle, are necessary. Environmental conditions should be seen as the main triggering factor to promote disease occurrence in conjunction with the pathogen and host populations (Kranz & Rotem, 2012). Relative humidity, rainfall, leaf wetness duration (LWD), solar radiation, and wind are the main weather variables that affect the infection process and disease spread in soybean fields (Cao et al, 2014). Weather regimes characterize the local climate, which can create favorable or unfavorable environmental conditions for disease occurrence. Weather variables are directly linked with fungicide spray effectiveness, affording a better or lower level of disease control (Stefanello et al, 2016)

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