Soybean rust (SBR) was reported on August 9 in two commercial soybean fields near Baxter and Jerome in Drew County in Southeast Arkansas. Soybeans were at R6 growth stage in both fields. Pustules were found on ~40% of the leaves inspected and disease severity was a little higher (1 to 100 pustules per leaflet) compared the first SBR report in Desha Co. Pustules were older indicating that infection occurred several weeks ago; however, several pustules were actively sporulating. The area has received ~2.5 in. of rain in the past month with temperatures averaging 92° and 72° F high and low, respectively. The fields from Desha to Drew are ~10 to 13 miles apart. It is likely other fields with soybeans at late R5 growth stages within these counties have low levels of SBR.
SBR (also called Asian soybean rust or Australasian soybean rust) is caused by the fungus Phakopsora pachyrhizi and requires a living host to survive. Typically, symptoms are observed first on leaves in the lower canopy at or after flowering (R1 to R3 growth stage); however, the positive samples observed this year have been at R5+ and R6 growth stages. Lesions appear as small, 2.0 to 5.0 mm tan or reddish brown angular spots on leaves. Lesions are often observed first at the base of the leaflet near the petiole. Volcano-shaped pustules (uredinia, Fig. 1&2) can be observed within the lesion on the underside of the leaf. When pustules are mature, they rupture and exude spores (urediniospores) that cause new infections. Pustules can be observed in the field with a 20x hand lens (Fig. 1 and 2), but may be misdiagnosed as bacterial pustule by untrained observers. Questionable samples should be sent the Plant Health Clinic in Fayetteville. Contact your local county agent about submitting samples to the clinic for SBR identificaiton. As disease progresses and secondary infection occurs, leaves begin to turn yellow and defoliate (Fig. 3). Severely diseased plants may completely defoliate resulting in fewer and smaller seeds.
Though SBR has been successfully managed with fungicides in the US (based on sentinel plots and early warning systems) in South America, 80% yield losses have occurred. These high yield losses have occurred when fungicides were not used when SBR infected R2 growth stage soybeans and conditions favored disease development (72° to 82° F with 12 hr leaf wetness per 24 hr period). Overall, SBR spread has probably been suppressed due to a high percentage of soybean acres in the South being sprayed with fungicides for other disease like frogeye leaf spot. Also, temperatures above 89° F retard disease development, thus hot conditions like we normally see in the state suppress SBR infection and sporulation. However, that does not seem to be the case this year as conditions are more favorable for SBR than last year. Infection can occur within 6 to 12 hours under optimum conditions, (long dew periods with leaf wetness of 10 to 12 hours) and new spores can be produced within 7 to 10 days after infection for as long as a 3 week period.
SBR is primarily managed with fungicides, and a list of fungicides and their efficacy for SBR management is available here. Since SBR is not the only disease that is active this year, producers will need to base their management on current threats rather than potential threats. For example, a field in the central or northern part of the state where strobilurin-resistant frogeye leaf spot (S-R FLS) is a greater threat than SBR, a fungicide should be applied for S-R FLS. It should be noted that premix fungicides with a strobilurin (protectant fungicide, FRAC group 11) and triazole (post-infection activity or curative fungicide, FRAC group 3) or a triazole alone are effective at managing both S-R FLS and SBR. So, if a field has been treated with a fungicide to manage FLS (scenario above), it is possible that it will be protected from SBR for 14 to 21 days. (Please refer to fungicide efficacy table in the hyperlink above.) Using a premix fungicide (or tank mix by combining a triazole-type fungicide with a protectant product) would be more favorable for SBR management. There is no threshold for how much SBR can be in the field at any one growth stage and not need a fungicide, but it is recommended to protect soybeans from SBR to R6 growth stage. SBR activity is too unpredictable, along with weather conditions, to determine these thresholds. It is recommended to scout fields treated for SBR (or after a fungicide has been sprayed) at least 10 days after application to determine the activity of SBR pustules. Collect several leaves with pustules and place in plastic bag with a moist paper towel and allow spores to incubate for 2 to 3 days near optimum temperatures for SBR sporulation then examine pustules for sporulation activity. A combination of spore activity and field observations can be helpful to determine when the first fungicide is wearing off. In situations where SBR is the only threat (i.e. rust in the field or nearby) and fungicides are applied before R3, it is likely another fungicide will be needed before R6.
Soybean producers in Southeast Arkansas with late planted beans should be prepared to spray a fungicide for SBR and/or FLS or other disease as they threaten their crop. Under severe rust situations, fungicides may need to be applied as early as R2 growth stage and continue 14 to 21 days after application, but this is dependent on environmental conditions for SBR. Producers in the central and northern parts of the state are at a lower risk but should be paying close attention to the row crops blog or IPM PIPE for updates and SBR activity. It is possible that late planted soybeans in the central and northern part of the state will need a fungicide application for SBR; however, FLS and aerial blight are currently active and many fields are being treated for those diseases.