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 Fungicides compared for the Control of Southern Rust and Yield Response in Late-Planted Irrigated Corn

June 12, 2017                                                                                   PP-775



A. K. Hagan

Extension Plant Pathology and Professor

Department of Entomology and Plant Pathology, Auburn University, AL 36849



Southern rust (Puccinia polysora) poses a significant threat to corn in the southern third of Alabama, particularly in Baldwin and Mobile Co.  Yield gains from fungicide inputs have exceeded 80 bu/A under severe rust pressure in late-planted irrigated corn in screening trials at the Gulf Coast Research and Extension Center (GCREC) in Fairhope, AL.   Destructive southern rust outbreaks occur every three to four years in Southwest AL, with disease onset typically occurring at tasseling or silking in outbreak years.  The risk of damaging rust outbreaks in corn declines with increasing distance from the Gulf of Mexico.  The later in the corn growth cycle that southern rust appears, particularly at the dough stage, the less likely that this disease will adversely impact yield.  Double crop corn sown behind early corn or wheat in South AL is particularly vulnerable to southern rust as inoculum pressure is often be very high in July and August.  Regardless of the planting date, rapid disease development and subsequent yield losses are also more likely in irrigated than dryland corn.  Frequent showers coupled with weather systems moving from Mexico or Florida over the Gulf of Mexico accelerates disease onset and development.


So far, low levels of southern rust have been found in corn at the GCREC but not yet at other Alabama locations.  Frequent showers over the past few weeks along with continued rain showers in the extended 10-day forecast should favor continued rust spread and intensification in SW AL.  In addition to southern rust, noticeable common rust development was noted in the lower and mid-canopy on corn at the GCREC and to a lesser extent at Brewton Agricultural Research Unit (BARU) in Brewton, AL, Wiregrass Research and Extension Center (WGREC)  in Headland, AL and Plant Breeding Unit (PBU), Tallassee, AL.  Also, Northern corn leaf blight and southern leaf spot disease along with gray leaf spot have been diagnosed on in corn variety trials at either BARU, GCREC, and PBU.  While common rust rarely causes sufficient damage to reduce corn yield, Northern and Southern corn leaf blight along with gray leaf spot can negatively impact corn yield. 


Should a southern rust outbreak occur, protective fungicides are key to slowing disease progress and protecting kernel yield.  Based on previous Alabama field trials, sizable yield gains from fungicide inputs are realized only when serious damage attributed to either southern rust or Northern corn leaf blight develops on the ear and ear-1 leaves.  When there is little or no disease activity, no yield gains from fungicide inputs will be realized.  All corn fields are not candidates for fungicide treatments. Name-brand fungicides at $20 to $30 per acre are fairly costly and producers are likely to see good returns on their fungicide investment in irrigated or dryland fields with yield potentials exceeding 175 to 200 bu/A, particularly with corn prices near the $4 per bu mark.  Ideally, fungicide treatments should be initiated for rust control based on a scouting report(s) and corn growth stage with a two-application program at growth stage VT-R1 and R2-R3 when disease pressure is high and weather favorable for further disease intensification.


The objective of this multi-year study at BARU was to assess the efficacy of registered fungicides for the control of southern rust and yield response of late-planted, irrigated corn.


Production Methods


The study site at the Brewton Agricultural Research Unit (BARU) was prepared for planting with a disk harrow and finished with a ripper bedder.  A broadcast pre-plant application of 25 to 51 lb actual nitrogen/A fertilizer supplemented with 10% sulfur was followed with two or three topdress applications of a total of 200 lb actual nitrogen/A.  Pioneer 2023YHR, Pioneer 1637YHR, and DeKalb 62-08 Smart Stax field corn was sown at a rate of 2 seed/row-ft (29,120 seed/A) during the first week of May in 2014, 2015, and 2016, respectively.  Weed control was provided by an at-plant application of 1.33 pt/A Dual Magnum II followed by a post emergent application of 0.5 gal/A Atrazine + 1 pt/A Dual Magnum II.  Plots were irrigated as needed to maintain optimum crop development with a lateral irrigation system. A factorial design with study year as the whole plot and fungicide program was used.  Individual experimental units consisted of four, 25-foot rows on 3-foot centers in four replications.  A non-fungicide treated control was included.  Fungicides were broadcast with a 'high-boy' sprayer with TX-12 nozzles spaced 19 inches apart mounted on a four row boom in 15 gal/A of spray volume at 40 psi at growth stage (GS) VT (tasseling) – R1 (silking) on July 8 and GS R2 (kernel blister) –  R3 (milk).  Induce non-ionic surfactant at 0.125% v/v was added to all fungicide tank mixtures.  Southern rust was assessed at GS R6 (black layer) on a scale of 1 to 11 where 1 = no disease, 2 = 1 to 10%, 3 = 11 to 20%, 4 = 21 to 30%, 5 = 31 to 40%, etc. of leaf area diseased on 5 ear leaves in each plot just prior to black layer.  Plots were combined on September 12.  Yields are reported at 15.5% moisture.  Statistical analyses for southern rust intensity were done on rank transformations of data.  For presentation, data are back transformed.  Means for all variables were separated using Fisher's least significant difference (LSD) test (P<0.05).




Southern rust severity significantly differed by study year and fungicide program (Table 1).  For all fungicide programs, southern rust intensity ratings were higher in 2014 than either of the following study years (Fig. 1).  With the exception of the non-fungicide treated control, Quilt Xcel and Aproach fb Aproach Prima programs, lower rust ratings were observed in 2016 than 2015.  Noticeable better southern rust control was provided by Stratego YLD and Fortix in 2015 and 2016 as compared with 2014 when both products proved less efficacious against this disease than Aproach fb Aproach Prima, Priaxor, and Quilt Xcel.  The latter three fungicide programs consistently gave effective disease control, particularly Quilt Xcel under severe disease pressure in 2014.  The two least effective fungicides for controlling southern rust in corn were Tilt 3.6E and Muscle 3.6F.  While the Muscle 3.6F-treated corn reduced rust severity compared with the non-fungicide treated control in two of three years, this fungicide failed to match the efficacy of Aproach fb Aproach Prima, Priaxor, and Quilt Xcel.  When compared with the non-fungicide treated control, Tilt 3.6F failed to reduce southern rust severity in two of three study years and was the least efficacious fungicide program in 2015. 


Table 1. P values for generalized linear mixed models for effects of year and fungicide treatment on southern rust severity, test weights, and yield of corn. 


Source of VariationSouthern rustTest weightYield
Year80.51*** Z220.24***107.13***
Fungicide program76.23***  11.11***  53.47***
Year × Fungicide program  6.20***    5.67***    6.75***

Z Significance of F values at the 0.05, 0.01, and 0.001 levels is indicated by *, **, or ***, respectively.


Figure 1. Southern rust severity as influenced by fungicide program. Means followed by the same letter are not significantly difference according to Fisher's protected least significance (LSD) test (P<0.05).   

 Pooled Corn Fungicide Summary Slide 1.jpg

Yield significantly differed for all fungicide programs by study year (Table 1).  The greatest yield differentials between the non-treated control and the superior fungicide program(s) was noted in 2014 and 2015 when a 50 and 65 bu/A yield gain, respectively, was obtained with Quilt Xcel (Fig. 2).  Yield gains from fungicide inputs was lower in 2016 due to reduced disease severity compared with the previous two study years.  When compared with the non-fungicide treated control, significant yield gains were obtained in all study years with Aproach/Aproach Prima, Fortix, Priaxor, and Quilt Xcel, as compared with two of three years with Stratego YLD and one of three years with Tilt 3.6E and Muscle 3.6E.  In all study years, greater yields were recorded for Quilt Xcel than for Muscle 3.6E, Stratego YLD, and Tilt 3.6E.  Priaxor-treated corn also produced greater yields than Muscle 3.6E and Tilt 3.6E in all study years and Stratego YLD in one of three study years.  Aproach/Aproach Prima and Fortix programs matched the yields obtained with Quilt Xcel in 2016 but not 2014 or 2015 under elevated disease pressure.  Overall, the poorest yield response was recorded for Tilt 3.6E and Muscle 3.6E as neither of these fungicides were able to match the performance of the above fungicides in at least two of three study years.    


Figure 2. Yield response to fungicide inputs by study year. Means followed by the same letter are not significantly difference according to Fisher's protected least significance (LSD) test (P<0.05).  

 Pooled Corn Fungicide Summary Slide 2.jpg



As the results of this study demonstrate, southern rust, particularly under favor weather patterns for disease development, can drastically reduce corn yields.  When compared with Quilt Xcel, yield for the non-fungicide treated control was reduced up to 60 bu/A for a farm gate income loss of $240 per acre at current contract prices.  Yield losses up to 80 bu/A have previously been recorded in previous fungicide screening trials in Alabama.  


The fungicides screened are among many but not all of the products currently recommended for the control of Southern rust and other foliar diseases in corn.  Of the fungicides screened, Quilt Xcel followed by Priaxor, Fortix, and Aproach/Aproach Prime provided the most consistently effective southern rust control.  That high level of disease control was reflected in the sizable yield gains obtained with the two former and to a lesser extent the latter two fungicides in all study years.  The generic fungicides Tilt 3.6E (propiconazole) and Muscle 3.6E (tebuconazole) are considerably less expensive than the above name- brand fungicides but proved to have limited activity against southern rust and often failed to boost yields as compared with Quilt Xcel, Priaxor, Fortix, and Aproach/Aproach Prima. As a result of this and previous Alabama fungicide screening trials, the use of Tilt 3.6E and Muscle 3.6F for southern control in corn should be avoided.  In contrast, the name-brand fungicides in general and Quilt Xcel and Priaxor in particular gave effective rust control over the three-year study period that was backed up with consistently high yields.    

 2017 Corn Fungicide TI.rev.pdf


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