Meta-Analytic Modeling of the Decline in Performance of Fungicides for Managing Soybean Rust after a Decade of Use in Brazil

Abstract

An apparent decline of fungicide performance for the control of soybean rust in Brazil has been reported but the rate at which it has occurred has not been formally quantified. Control efficacy and yield response to three fungicides applied as single active ingredients (a.i.)—azoxystrobin (AZOX), cyproconazole (CYPR), and tebuconazole (TEBU)—and four applied as mixtures—AZOX+CYPR, picoxystrobin + CYPR, pyraclostrobin + epoxiconazole, and trifloxystrobin + prothioconazole (TRIF+PROT)—were summarized using network meta-analytic models fitted to mean severity and yield data from 250 trials (10-year period). The effect of year was tested on both variables in a meta-regression model. Overall control efficacy ranged from 56 to 84%; the three single-a.i. fungicides performed the poorest (56 to 62%). Yield increase for single-a.i. fungicides was as low as 30% but ranged from 47 to 65% for the premixes. Significant declines in both variables were detected for all fungicides except TRIF+PROT. For TEBU, control efficacy (yield response) declined the most: 78% (18%) to 54% (8%) from 2004–05 to 2013–14. The recent surge of resistant populations of Phakopsora pachyrhizi to both demethylation inhibitor and quinone outside inhibitor fungicides is likely the driving force behind a significant decline after 4 years of fungicide use.

A new standard area diagram set for assessment of severity of soybean rust improves accuracy of estimates and optimizes resource use

“…The Old SAD has been widely recommended and used in soybean rust field research, mainly to evaluate fungicide performance in field experimentation (Scherm et al , ; Dalla Lana et al , ). Our results show that its use should be discouraged, not only because of a clear lack of benefit, but also because of the detrimental effect on the estimates of raters who were inherently more accurate.…”

“…Soybean rust, caused by the fungus Phakopsora pachyrhizi , is one of the most damaging diseases of soybean ( Glycine max ; Hartman et al , ). In Brazil, since its discovery in 2002, the disease has caused tremendous losses due to yield reduction and increased control costs (Godoy et al , ), as well as indirect losses due to the development of resistance to fungicides in the fungal population (Dalla Lana et al , ). Regional to local management practices such as mandatory soybean‐free periods and early sowing of short‐cycle cultivars help to reduce inoculum levels, but an effective control is only achieved using sequential fungicide sprays during the season (Scherm et al , ; Dalla Lana et al , ).…”

“…The Old SAD has been used extensively to standardize ratings across raters of the national uniform fungicide trial (UFT) network. Visual estimates of severity are provided in a 10‐leaflet sample taken from each of three canopy heights per plot, which are averaged at the plot level (Dalla Lana et al , ).…”

“…The disease reduces healthy leaf area and, at high intensity, accelerates leaf senescence and defoliation. The proportion of diseased leaf area (severity) is the main variable used to compare treatments (e.g., fungicides and cultivars) for suppressing the disease, protecting yield (Scherm et al, 2009;Godoy et al, 2016;Dalla Lana et al, 2018), and to predict yield losses (Dalla Lana et al, 2015).…”

“…Visual estimates of severity are provided in a 10-leaflet sample taken from each of three canopy heights per plot, which are averaged at the plot level (Dalla Lana et al, 2018).…”