By Terry Spurlock, Extension Plant Pathologist
Soybean rust was confirmed in Ashley, Chicot, Lincoln and Phillips counties the week of August 30th. Fields were all R6-R7 and rust was not difficult to find. The occurrence of the disease at this time of year is not unusual. Soybean rust is typically found in Arkansas beginning late August through early October as conditions become more favorable for disease development and spores move into our area from neighboring states. This year, rust was identified in Concordia Parish in Louisiana mid-July and in Alabama and Mississippi even earlier. The combinations of southernly winds and tropical storms moving from the south have spread spores to our area. The progression of the disease has been monitored through ipmPIPE. (https://www.ipmpipe.org/).
The fungus that causes soybean rust, Phakopsora pachyrhizi, infects and spreads during periods of cooler, wet weather (optimally between 61°F – 83°F). Lesions on the upper leaflet surface appear dark reddish-brown and angular, often confined by the leaflet veins. Leaves tend to yellow as disease progresses (Figure 2). Unfortunately, soybean rust is sometimes confused with Septoria brown spot or bacterial pustule. To avoid this, make sure to observe rust pustules (uredinia) on the underside of leaves (Figure 3). They should be raised, often appearing as a volcano-like structure (Figure 4A). More mature pustules will break open and be covered with reddish-orange spores (urediniospores) in masses (Figure 4B). These can be seen under magnification with a 20x hand lens.
While it has been extremely rare for fungicide application to be necessary to control soybean rust in Arkansas, the combination of mild temperatures along with some later-planted soybeans just beginning or not yet completing pod fill may be cause for concern. Fields not yet to R5.5 should be scouted for soybean rust. See MP154 for product listings and rates specific for control of soybean rust as well as a chart detailing product efficacy specific to soybean rust (pp 25-26). If a field is R5.5 or later, a fungicide application will likely not be beneficial. It is worth noting that P. pachyrhizi is an obligate parasite. As such, it requires living plant tissue to continue to infect and spread. Therefore, fields beginning to mature naturally slow disease development and are in no danger from the disease.
Below are key factors to determine if a fungicide application is necessary.
- Soybean rust was correctly identified and found in many locations across the field.
- Soybean rust has moved into the top 1/3 of the canopy.
- The field has not yet progressed to R5.5 (beans filling half the space in the pod on the upper four nodes).
- The yield potential suggests the field will still be profitable after including the cost of the application.
Additional information on soybean rust incidence and severity will be posted on this blog or Twitter at @SpurlockLab.