By Terry Spurlock, Extension Plant Pathologist
Soybean rust was confirmed in Ashley (33.249093°, -91.846713°) and Lincoln (34.077432°, -91.832217°) counties last week. Fields were R6 and R5.5, respectively. Only trace amounts were observed in the field in Lincoln County, but the field in Ashley County had quite a bit more rust. The occurrence of the disease at this time of year is not unusual. We typically find soybean rust in late August-early October as conditions become more favorable for disease development and spores move into our area from neighboring states.
In general, the fungus, Phakopsora pachyrhizi, reproduces and spreads during periods of cooler, wet weather (<84°F); much like the last two weeks. The combinations of cool fronts and the remnants of hurricanes Harvey and, presently, Irma have contributed to disease development. This weather, along with some later-planted soybeans just beginning or not yet completed pod fill may be cause for concern. Fields not yet to GS R5.5 should be scouted specifically for soybean rust and may require a fungicide application if rust is present and the current weather conditions continue. However, if the current forecast holds, sporulation and spread within fields should slow in the next week to ten days.
Evidence suggests triazole fungicides are most effective against soybean rust. See MP154 for product listings and rates specific for soybean rust. 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 reproduce and spread. Therefore, soybeans beginning to mature naturally slow disease development and are in no danger from the disease.
Proper identification of soybean rust is key to sound management decisions. 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 1). 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, somewhat concentrated down a leaflet vein (Figure 2), raised, often appearing as a volcano-like structure. More mature pustules will break open and be covered with reddish-orange spores (urediniospores) in masses (Figure 3). These are best seen under magnification (at least a 10x hand lens is recommended).
Additional information on soybean rust incidence and severity will be posted on this blog or Twitter at @SpurlockLab or @travisfaske.