Recent Outbreak of Soybean Sudden Death Syndrome in Iowa.

Abstract

Sudden death syndrome (SDS) of soybean, caused by Fusarium solani f. sp. glycines, was first found in four Iowa counties in 1993 at low intensities. Following the first report of the disease, a statewide survey in 1994 and 1995 showed an overall low prevalence of the disease, mainly in eastern Iowa (2). In 1998, an epidemic of the disease occurred with drastic increases in prevalence and severity at regional, local, and field levels. The disease was found in 31 Iowa counties covering five out of the nine Iowa crop-reporting districts, with expansion in occurrence northward and westward. There were 12 SDS-affected counties in the North Central and Central districts, and 19 in the East Central, Southeast, and Northeast districts. To assess the extent of SDS increase at the local level, extensive surveys were conducted in 1998 in three areas where previous surveys were made in 1994 or 1995. In Washington County (Southeast District), the frequency of infestation has increased from 5% (2 of 40 fields surveyed) in 1995 to 30% in 1998. In Black Hawk County (East Central District), 26% of the fields examined had SDS, compared with 4.5% in 1995 (2 of 44 fields). In Story County (Central District), 35% of soybean fields were found with SDS in 1998, as opposed to 3% (2 of 62 fields) in 1994. In previous surveys (2), large disease patches (about 1 ha) were not found in central Iowa and were mainly limited to eastern counties along the Mississippi River. However, approximately 30% of the infested fields surveyed in 1998 had two to five patches with areas from 0.5 to 2.5 ha. The remaining 70% of the fields had several patches of diseased soybean plants with areas ranging from 3 m2 to 0.5 ha. The largest disease patch observed was about 10 ha, covering half of a field in Black Hawk County. Percentage of field area affected by SDS varied from 3 to 15% in 60% of the fields where the disease was found, and from about 20 to 60% in the remainder. In all fields surveyed, SDS was confirmed based on the presence of bluish sporulation or isolation from taproots of representative plants. The outbreak supports the 1996 risk assessment (1) that SDS would become a major production concern in Iowa.