Date of Graduation

8-2012

Document Type

Thesis

Degree Name

Master of Science in Plant Pathology (MS)

Degree Level

Graduate

Department

Plant Pathology

Advisor

John C. Rupe

Committee Member

Sreekala G. Bajwa

Second Committee Member

W S. Monfort

Third Committee Member

Craig Rothrock

Abstract

Charcoal rot of soybean, caused by the soilborne fungus Macrophomina phaseolina (Tassi) Goid, is a disease associated with high soil temperature and low soil moisture. Above-ground symptoms, which can be difficult to distinguish from drought symptoms, include low vigor, dead leaves that remain attached to the plant, early senescence and yield loss. Irrigation limits damage, but does not prevent colonization of the tissue by the pathogen. No soybean line is immune to M. phaseolina, but a few lines may have moderate resistance. The objectives of this research were to 1) determine the effects of genotype and drought on the development of charcoal rot and 2) determine the efficacy of non-destructive techniques to measure disease development during the season. In 2008 and 2009, soybean genotypes, DT97-4290 and Delta Pineland 4546 (moderately resistant), LS980358 (susceptible), and R01-581F (drought tolerant) were grown in microplots. Treatments included presence or absence of M. phaseolina and water-stressed and non-stressed in a factorial randomized complete block design experiment with five replications. Soil moisture and temperature, and rainfall were monitored. Root colonization and plant growth were assessed at growth stages V2/V3 and V4/V5. Stomatal conductance was measured with a porometer. Canopy temperature was measured with an infrared thermometer and used to calculate Crop Water Stress Index (CWSI). Canopy reflectance was measured with a spectroradiometer and used to calculate ten vegetative indices. Yield, root/stem disease severity, plant height, stem discoloration and microsclerotial densities of M. phaseolina were determined at harvest. When differences between treatments occurred, plants in infested plots had lower stomatal conductance and higher CWSI than those in non-infested plots. Differences between treatments in canopy reflectance were not consistent. In 2008, there was only a genotype effect on yield. In 2009, LS980358 and R01-581F plants had 38 % and 49 % lower yields, respectively, in infested than non-infested plots, whereas yields for DT97-4290 and Delta Pineland 4546 did not differ between infestation treatments. The results from this study show that M. phaseolina affects soybeans at all growth stages and that some genotypes are very sensitive to colonization.

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