Soybean is the fourth most cultivated crop worldwide and provides one of the most
important sources of oil and protein. Drought stress is one of the major constraints to soybean
yield in the United States. Charcoal rot of soybean, caused by Macrophomina phaseolina, is a
soil-borne fungal pathogen that infects over 500 plant species worldwide and also has been
known to negatively impact soybean yields. Environmental conditions, such as severe drought,
have been associated with increased charcoal rot severity. Both drought tolerance and charcoal
rot resistance are complex traits, and their interaction is not well understood.
In Chapter 2, a collection of 41 SoyNAM parents were evaluated for their response to
drought tolerance. Greenhouse gravimetric tests were conducted on the 41 SoyNAM parents and
on a selection of the 41 SoyNAM parents representing the most and least drought tolerant
genotypes. Another experiment tested the 41 SoyNAM parents in a growth chamber based on
sudden water depletion drought stress assessed by image-based ratings. Soybean genotypes for
both the greenhouse and growth chamber tests were significant (P < 0.01), and there was a
positive correlation (r = 0.37, P = 0.02) between the tests. Soybean genotypes U03-100612,
LG94-1906 and Skylla, demonstrated drought tolerance in both tests, and PI 404188A was
drought sensitive in both tests.
In Chapter 3, charcoal rot resistance of SoyNAM parents were evaluated by a cut-stem
inoculation method in controlled conditions in a growth chamber. Soybean genotypes were
significant (P < 0.001) for the percentage of dead plants. The parents were then tested with cutstem
inoculation incorporated with and without a drought treatment. The drought treatment
increased (P < 0.05) disease severity of charcoal rot for all variables measured, which indicted that drought enhances charcoal rot severity. Genotype had significant (P < 0.05) effect on disease severity in all but one variable. The evaluation at 15 days after inoculation of the two tests were positively correlated (r = 0.36, P = 0.02). Soybean genotypes Skylla had the low disease ratings among the SoyNAM parents in both tests.
In Chapter 4, a genome-wide association mapping study was conducted by phenotyping a diverse soybean collection of 350 genotypes for their response to M. phaseolina inoculation. Data from the cut-stem inoculation method were associated with the SoySNP50K marker dataset to detect and map significant (P < 0.001) SNPs. Eight SNPs were associated with charcoal rot resistance located on chromosomes 3, 11, 13, 14, 18 and 20. The functions of the candidate genes located near these SNPs involve plant defense, metabolite transportation and protein synthesis
