Goss's bacterial wilt and blight, also known as leaf freckles and wilt was first identified in Dawson Co. in south central Nebraska in 1969. The disease was soon identified in 54 Nebraska counties and six bordering states. Widespread development of symptoms in NE, WY, and CO early in the 2006 growing season led to submission of more than 50 samples to the UNL Panhandle Research and Extension Center's Plant Disease Diagnostic Clinic in Scottsbluff, NE for analysis. Pathogen identification was made based on a combination of test results. Accepted for publication 5 April 2007. Published 19 September 2007.

Soybean [Glycine max (L.) Merrill] yield losses as a result of plant diseases were estimated by university and government plant pathologists in 29 soybean-producing states in the United States and in Ontario, Canada, from 2015 through 2019. In general, the estimated losses that resulted from each of 28 plant diseases or pathogens varied by state or province as well as year. Soybean cyst nematode (SCN) (Heterodera glycines Ichinohe) caused more than twice as much loss than any other disease during the survey period. Seedling diseases (caused by various pathogens), Sclerotinia stem rot (white mold) (caused by Sclerotinia sclerotiorum [Lib.] de Bary), and sudden death syndrome (caused by Fusarium virguliforme O'Donnell & T. Aoki) caused the next greatest yield losses, in descending order. Following SCN, the most damaging diseases in the northern U.S. and Ontario differed from those in the southern U.S. The estimated mean economic loss from all soybean diseases, averaged across the U.S. and Ontario, Canada was $45 U.S. dollars per acre ($111 per hectare). The outcome from the current survey will provide pertinent information regarding the important soybean diseases and their overall severity in the soybean crop and help guide future research and Extension efforts on managing soybean diseases.

Fusarium solani f. sp. glycines is the causal organism of soybean sudden death syndrome (SDS). This organism is difficult to detect and quantify because it is a slow-growing fungus with variable phenotypic characteristics. Reliable and fast procedures are important for detection of this soybean pathogen. Protocols were optimized for extraction of DNA from pure fungal cultures and fresh or dry roots. A new procedure to test polymerase chain reaction (PCR) inhibitors in DNA extracts was developed. Novel real-time quantitative PCR (QPCR) assays were developed for both absolute and relative quantification of F. solani f. sp. glycines. The fungus was quantified based on detection of the mitochondrial small-subunit rRNA gene, and the host plant based on detection of the cyclophilin gene of the host plant. DNA of F. solani f. sp. glycines was detected in soybean plants both with and without SDS foliar symptoms to contents as low as 9.0 × 10-5 ng in the absolute QPCR assays. This is the first report of relative QPCR using the comparative threshold cycle (Ct) method to quantify the DNA of a plant pathogen relative to its host DNA. The relative QPCR assay is reliable if care is taken to avoid reaction inhibition and it may be used to further elucidate the fungus-host interaction in the development of SDS or screen for resistance to the fungus.

Isolates of Phytophthora sojae were collected during 1995 to 1998 from soil samples collected in 23 Arkansas soybean fields in 14 counties, and characterized by race. A total of seven races (races 2, 10, 14, 15, 24, 26, and 38) were found. Races 10, 24, and 15 were the most common and comprised 47, 22, and 9% of the 32 isolates, respectively. A single isolate each of races 2, 14, 26, and 38 also was found. Three of the isolates collected could not be characterized to race due to inconsistent results. In 1997 and 1998, a portion of a single soybean field at the University of Arkansas Southeast Research and Experiment Center near Rohwer, AR was surveyed intensively for P. sojae. The area was planted each year to the P. sojae-susceptible cv. Williams and both plants and soil were collected to assay for P. sojae from 16 and 28 plots (4.9 by 7.6 m) in 1997 and 1998, respectively. A total of 83 isolates were collected (11 from plants and 72 from soil), and found to represent 13 pathotypes, including 6 with virulence formulae that have not been described previously. Nine commercial soybean cultivars representing a range of reported resistance and tolerance to Phytophthora root and stem rot were screened for resistance to races 10, 15, and 26 of P. sojae using both hypocotyl injection and inoculum layer techniques. Cvs. Manokin, Hartz Variety 5545, and Riverside 499 were consistently resistant to all of the races using both inoculation methods. These results indicate that, although considerable pathogenic variability in P. sojae exists in soybean fields in Arkansas, cultivars with effective resistance are available to help growers manage Phytophthora root and stem rot.

The soybean cyst nematode, Heterodera glycines, and the fungus that causes sudden death syndrome (SDS) of soybean, Fusarium solani f. sp. glycines, frequently co-infest soybean (Glycine max) fields. The interactions between H. glycines and F. solani f. sp. glycines were investigated in factorial greenhouse experiments with different inoculum levels of both organisms on a soybean cultivar susceptible to both pathogens. Measured responses included root and shoot dry weights, H. glycines reproduction, area under the SDS disease progress curve, and fungal colonization of roots. Both H. glycines and F. solani f. sp. glycines reduced the growth of soybeans. Reproduction of H. glycines was suppressed by high inoculum levels but not by low levels of F. solani f. sp. glycines. The infection of soybean roots by H. glycines did not affect root colonization by the fungus, as determined by real-time polymerase chain reaction. Although both pathogens reduced the growth of soybeans, H. glycines did not increase SDS foliar symptoms, and statistical interactions between the two pathogens were seldom significant.