Date of Graduation


Document Type


Degree Name

Doctor of Philosophy in Plant Science (PhD)

Degree Level



Plant Pathology


John Rupe

Committee Member

Burt Bluhm

Second Committee Member

Pengying Chen

Third Committee Member

Edward Gbur

Fourth Committee Member

Douglas Karcher

Fifth Committee Member

Larry Purcell


Drought, HAP3, Machrophomina phaseolina, Phomopsis longicolla, seeds, Soybean


Soybean (Glycine max), a legume, is an economically important crop in many parts of the world, including the USA, Brazil, Argentina, China, and India, currently the top five producing countries. Soybean is primarily used as feed, with incising markets for food and biodiesel. Similar to most crops, soybean yield and quality are affected by a diverse group of plant pathogens. In particular, several species of filamentous fungi have been the cause of severe yield losses in most growing regions world-wide. The soil born fungus Macrophomina phaseolina, causal agent of charcoal rot, has been found to be endemic to several soybean production areas. Charcoal rot has been historically associated with severe yield reduction, particularly when coupled with drought stress. However, little is known about how irrigation-regimes change affect the relationship between disease severity and yield. A different soil and seed born fungus Phomopsis longicolla, causes severe seed decay under conducive environmental conditions, warm and wet weather during soybean senescence. Although both fungi co-inhabit soybean roots and stems for extended periods of times, little is known about their interactions. Recently, P. longicolla has been shown to be amenable to genetic manipulation via Agrobacterium tumefaciens. However, no study has yet taken advantage of this amenability to dissect potential mechanisms associated to Phomopsis seed decay. This study was designed to understand how different irrigation regimes may impact severity of charcoal rot and its relationship to soybean yield. Additionally, the interplay between M. phaseolina and P. longicolla in soybean tap roots via the formation of zone lines was investigated. Lastly, taking advantage of available genetic transformation methodology, experiments were implemented to elucidate putative molecular mechanisms of Phomopsis seed decay pathogenesis. Results indicated that drought stress significantly increases charcoal rot severity and decreased yield. However, no consistent relationship was observed between these variables. Observations also showed that P. longicolla forms zone lines in soybean taproots thereby precluding M. phaseolina from colonizing these tissues. In this study, for the first time, forward and reverse genetic approaches and transcriptomics were employed on P. longicolla successfully. Multiple genes linked to decreased pathogenicity were identified in P. longicolla.