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Soybean rust activity increases in Southeastern Arkansas
Author: Travis Faske, Extension Plant Pathologist

Soybean rust (SBR) has recently been confirmed in Chicot and Desha Co.  Rust was identified (9/13) on soybean near Bellaire in Chicot Co. with actively sporulating pustules on ~6% of the soybean leaflets.  The soybean growth stage was R7.  In Desha Co., SBR was confirmed on 9/17 from leaves collected in a soybean (R5) field near Gould.   Pustules were actively sproulating on ~25% of leaflets and some leaflets had 10+ pustules.  In late July, SBR was confirmed in Desha Co. but was harvested one week after that report.  Current conditions (mild temperature and heavy dew/light rain) are more favorable for SBR than when it was first confirmed in July.  Tropical storm Isaac contributed to the spread of rust throughout the Mid-South (Fig. 1).  Thus, it is likely SBR will be found in other counties, especially those neighboring Chicot and Desha Co.  Though most of the soybeans are past the recommended growth stage for treatment (R6), rust is a threat to late-planted soybeans that are in the mid-stages of reproduction.   Producers and consultants with young soybeans in the area where rust has been confirmed should consider management options and continue to monitor fields.  We will continue to monitor fields for SBR movement in throughout AR and post positive observations on this blog and IPM-PIPE.

Soybean rust distribution as of 9-17-2012

Figure 1. Distribution of soybean rust in the Southern United States, September 17, 2012. Map was from IPM-PIPE website. Click picture to visit the site.

Soybean rust is caused by the fungus Phakopsora packyrhizi, which requires a living host to survive. Symptoms are first observed on the leaves in the lower canopy.  Lesions appears as small, yellow, irregularly-shaped spots and progress to 1.5 to 2.0 mm in diameter (Fig. 2).  Volcano-shaped pustules (uridinia) can be observed on the underside of the leaf that rupture and exude spores (Fig. 2).  These pustules can be observed in the field with a 20x lens, but can be confused with bacterial pustule.  Contact your local county extension agent about submitting suspect samples for diagnosis.

Soybean rust stages

Figure 2. Soybean rust pustule (yellow arrow) on leaflet with no magnification (right), 10 x magnification, and 50 x magnification.

Fungicides are effective tools for managing this disease, though no fungicide is recommended in areas where rust has not been detected.  Triazole and strobilurin fungicides are effective in managing SBR.  Strobilurins are protective and should be applied prior to disease, whereas triazoles can be effective after disease has been observed in the field but are most effective when applied prior to disease development.  Take into account growth stage, time since last fungicide application (if applicable) and nearest confirmed SBR location when considering a fungicide application for SBR.  Fungicide applications should be made from R1 to R3 for high-risk fields where rust threatens and at R1 if rust has been observed in the local area. The second application should be made 14 to 21 days after the first application.  It is recommended to apply a fungicide until growth stage R6.  Applying a fungicide in late reproductive stages (R6) my not provide a significant economical return; however, untreated fields may supply spores to later-planted soybeans in the area.

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