1a.Objectives (from AD-416):
To idenify new sources of charcoal resistance in adapted and exotic germplasm.
1b.Approach (from AD-416):
A uniform cultivar screening protocol will be developed and implemented. A set of standard susceptible cultivars will be used. These standard susceptible cultivars will represent early and late cultivars with in maturity groups 3 through 5. Tests will be established in naturally infested fields supplemented with adding inoculum at planting. Disease will be evaluated by rating the development of foliar symptoms of charcoal rot, rate of premature plant death, and by splitting the stems at harvest, visually determining the extent of stem colonization and quantifying colonization by determining the colony forming unit of M. phaseolina. Yields will be taken. Test cultivars will be selected based on previous observations and from other charcoal rot screening tests in the field and in greenhouse and laboratory assays. Soil temperature and moisture and rainfall data will be taken and related to disease development.
There are no commercial cultivars marketed as resistant to charcoal rot of soybean. The main objective for this study was to evaluate and confirm cultivars thought to be resistant to charcoal rot. Seventeen accessions consisting of maturity group (MG) III, MG IV, and MG V were evaluated for resistance against the charcoal rot fungus for three years: 2008, 2009 and 2010 in Tennessee. These genotypes were grown and disease assessments were made under no-till and non-irrigated environments. The results show that there is no genotype that was immune to Macrophomina (M.) phaseolina, but there were genotypes with moderate levels resistance. Five of the genotypes namely, DT97-4290, DT991755, DT991686, DT987553 and DT991748 had the lowest colony forming unit (CFU) mean values. The mean CFU value for all genotypes in 2010 was far less than the CFU for 2008 and 2009 and this may be due to the high amount of precipitation in 2010 (973 mm) than in 2008 (570mm) and 2009 (853 mm). High soil moisture has been reported to reduce the severity level of charcoal rot. Using the stem and root severity values and the height of discoloration, all the DT genotypes showed the lowest value and DT97-4290 had a higher yield with 49 Bu/A among these resistant lines. This test further confirmed the earlier findings on the resistance of these DT lines. The genotypes identified as having moderate resistance to M. phaseolina across three years could be useful as a source of resistance in soybean breeding programs. The regional cultivar test consisting of 18 soybean lines were planted in Tennessee on May 18th. These lines represent three genotypes from MG III, five from MG IV and seven from MG VII all planted in artificially infested and in non-irrigated soil. Ten samples from each plot were taken for measuring stem severity rating and CFU assessments. Five of the breeding lines developed under the Crop Genetics and Research unit showed resistance across multiple environments. In addition, among the 649 plant introductions evaluated in 2010, four lines showed a consistent resistance. Seeds for three of these lines will be increased in Costa Rica and may be included for testing in multiple environments by 2013. The test plots in Tennessee are inoculated each year with infested millets to reduce plot to plot variability. Soil, air temperature, soil moisture relative humidity, rainfall, and solar radiation were all recorded.