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Title:Soybean disease research on green stem disorder, effects of seed treatments on Sclerotinia stem rot, and resistance to Cercospora leaf blight
Author(s):Harbach, Chelsea Jean
Advisor(s):Hartman, Glen L.
Department / Program:Crop Sciences
Discipline:Crop Sciences
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):green stem disorder
Sclerotinia stem rot
Sclerotinia sclerotiorum
seed treatment
Cercospora leaf blight
Cercospora kikuchii
Abstract:Soybean plants often exhibit varying delayed maturity symptoms at the end of the growing season. One delayed maturity malady known as green stem disorder (GSD) is the occurrence of non-senescent, fleshy green stems at harvest maturity with normal, fully mature pods and seeds. Data on GSD incidence were collected for 1090 soybean cultivars from 2009 to 2012 at seven locations throughout Illinois. Data on six agronomic traits were also collected for every GSD observation, including height, lodging, moisture, protein, oil, and yield. Correlations of GSD incidence with agronomic traits were estimated for every year x location x trial (maturity group and herbicide tolerance type) combination to account for the change in cultivars from year to year and the effect of year x location on GSD incidence. To study the effects of location on GSD incidence, pair-wise comparison best linear unbiased predictors (BLUPs) were estimated between all locations within years for all trials. Correlations showed no consistent significant relationship with yield and GSD, but GSD incidence was positively correlated to height, lodging, moisture, and protein, while negatively correlated to oil. Pairwise comparison BLUPs showed an overall trend of locations in northern regions of Illinois having significantly more GSD than regions in the south. There were locations that had significantly more GSD compared to all other locations consistently throughout the analysis. The general significant effect of locations on GSD incidence provides an important consideration when designing future research with GSD in soybean. The significant correlations with agronomic data in this study support the findings that GSD incidence is a quantitatively controlled genetic trait that can be influenced by the environment. Sclerotinia sclerotiorum is one of the most important pathogens infecting soybean plants. When the fungus is seedborne as mycelia or when it is infested with seeds as sclerotia, the fungus can kill germinating seedlings. The use of fungicide seed treatments may help to manage this phase of the disease and may provide residual protection to infection beyond the seedling stage. The objectives of my study were to use the data from the University of Illinois Soybean Variety Testing Program (UISVT) to document the increase in deployment of fungicide seed treatments from 2005 to 2014 and to determine if seed treated fungicides would provide control of S. sclerotiorum on inoculated germinating seedlings and on plants beyond the seeding stage inoculated with the fungus. The data from the UISVT showed that the deployment of seed treatments with fungicides on cultivars entered into the program increased from 66% in 2005 to 92% in 2014. To test the efficacy of fungicide seed treatments, four fungicide seed treatments, fludioxonil, trifloxystrobin, trifloxystrobin + saponins, and penflufen + prothioconazole, and an untreated control were applied to seeds of four soybean cultivars. The plants were inoculated at various growth stages from seedling germination to flowering. In the seed germination stage, fludioxonil provided complete control, penflufen + prothioconazole provided moderate control while the trifloxystrobin and trifloxystrobin + saponins provided no control. There was little residual activity detected when plants beyond the seed germination stage were inoculated. Although seed treatments included in this study do not offer residual protection against S. sclerotiorum infection beyond germination, there is hope that future seed treatment fungicides will have longer residual activity that could provide protection to plants at later growth stages. Cercospora leaf blight (CLB) and purple seed stain (PSS) caused by Cercospora kikuchii are important diseases of soybean worldwide. While there are no commercially available cultivars advertised for resistance to PSS or CLB, sources of resistance have been reported in plant introductions or older public soybean cultivars. In this study, nine public soybean cultivars with varying resistance or susceptibility to CLB, PSS, and frogeye leaf spot (Cercospora sojina) were screened for differences in CLB disease severity. Soybean plants were inoculated with two different Cercospora isolates that were isolated from soybean seeds symptomatic of PSS and leaves symptomatic of CLB in Illinois. The intergenic spacer (IGS) region of these isolates were sequenced and compared to previously published IGS sequences of C. kikuchii isolates. Bioassays for differences in disease severity showed no significant differences between leaf and seed C. kikuchii isolates though significant differences were observed between cultivars included in the study. Soybean cultivar Mejiro (PI80837) had the highest disease rating overall. Comparison of the IGS sequences from Cercospora isolates showed differences in the sequence that followed the previously published differences defining three haplotypes of the fungus. This indicates that C. kikuchii isolates in Illinois vary genetically from those collected in the southern United States.
Issue Date:2015-07-23
Rights Information:Copyright 2015 Chelsea Harbach
Date Available in IDEALS:2015-09-29
Date Deposited:August 201

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