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Soybean rust moves north toward Arkansas
Author: Travis Faske, Extension Plant Pathologist

Soybean rust (SBR) was detected on 7/26 in Jefferson and Issaquena counties, Mississippi (Fig. 1. Note Issaquena Co. is not in red).  This is the closest threat this season to the Arkansas soybean industry (Fig. 1).  In Issaquena Co., rust was found on 5 to 6 leaves on MG 5 in the lower canopy with a relatively high disease severity (25 to 30%).   Conditions are favorable for rust development along the Gulf Coast where rainfall has been more frequent than most of Arkansas.  Though there has been no report of SBR in Arkansas, it is likely SBR will continue to spread along the Mississippi River into Southeastern Arkansas.  There is a low risk of disease development further northward due to drought conditions that cover much of central and northern part of the state, and hot, dry weather is in the extended forecast.  Producers and consultants in the southeastern part of the state are encouraged to monitor for SBR, and we will continue to monitor sentinel plots and commercial fields.

Soybean rust is caused by a fungus (Phakopsora packyrhizi), which needs a living host to survive. Spores are typically blown in from the Caribbean or Mexico, but mild winters in Texas and Florida would be suitable for the pathogen to overwinter.  Typically, symptoms are first observed on the leaves in the lower canopy.  Lesions appear as small, yellow, irregular shaped spots and progress to 1.5 to 2.0 mm in diameter.  A few volcano-shaped pustules (uridinia) can be observed in these lesions on the underside of the leaf.  As the disease progresses, these lesions turn tan to brown or reddish, and plants prematurely lose their leaves. Conditions that favor disease are extended periods of leaf wetness over a wide range of temperatures (59 to 82 F).  Temperatures above 86 F retard disease development.

SBR Distribution 7/29/2012

Figure 1. Distribution of soybean rust in the Southern United States, July 29, 2012. Image provided by the IPM-PIPE website, an excellent resource for latest information on soybean rust movement in the US.

Managing SBR primarily consists of applying foliar fungicides during the susceptible stage of soybean rust, which is from flowering (R1) to beginning seed (R5).  Though it is possible for SBR to develop on late vegetative stages of soybean, it is unknown if there would be a beneficial yield response from fungicide application.  Certainly identification of rust in late vegetative stages would strongly warrant a fungicide application when flowering begins (R1). The first application should be made from R1 to R3 for high-risk fields where rust threatens and at R1 if rust has been observed in the field.  A second application may be needed 14 to 21 days after the first depending on environmental conditions and disease development.  To manage the rapid development of fungicide-resistant SBR to strobilurin fungicides, repeated applications of strobilurin fungicides alone should be avoided.  Spraying at late growth stages (R6 or later) is not recommended due to lack of yield response.  See MP 154 for numerous fungicides labeled for SBR.  Though no fungicide is recommended at this time, future updates will be posted on this site and IPM-PIPE.

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