Weather conditions have been unfavorable for many foliar fungal disease of soybean, especially soybean rust in Arkansas. Soybean sentinel plots and commercial field sites have been designated and monitored to detect soybean rust. Twenty-nine county agents are participating in this IPM program by collecting soybean or kudzu leaves to be assayed for soybean rust in Arkansas. At this time, there has been no report of soybean rust nor does the long-range forecast favor soybean rust in Arkansas.
Soybean rust was confirmed on 31 May in Hildago, Co., TX, which was the first report this year on soybean. It has been reported in Alabama, Florida, Georgia, and Louisiana on soybean or kudzu (Fig. 1). The IPM-PIPE website (http://sbr.ipmpipe.org/cgi-bin/sbr/public.cgi) is an excellent resource for latest information on soybean rust movement in the US.
Soybean rust is caused by a fungus (Phakopsora packyrhizi), which needs a living host to survive. Spores are typically are 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 appears as small yellow irregular shaped lesions 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 lesion 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). However, temperatures above 86 F retard disease development.
Management of soybean rust has primarily relied on foliar fungicides during the susceptible stage of soybean crop, which is from flowering (R1) to full seed (R6). See MP 154 for fungicides labeled for soybean rust, though no fungicide use is necessary at this time. The monitoring of movement of soybean rust is critical for plant protection since fungicide effectiveness is reduced when applied after 10% disease severity. Soybean rust will continue to be monitored in sentinel plots and commercial field plots throughout this season, though there is a very low possibility this disease will threaten the 2012 soybean crop.