RESUMEN
The role of soilborne pathogens in flood damage on soybeans, Glycine max, was examined using six genotypes representing a reputed range of flood tolerances. Genotypes were planted in single-row plots from 1996 to 1998 with flood treatments of no flood, flood at emergence (3-day duration), or flood at the fourth leaf node growth stage (7-day duration). Three or four days after removing each flood treatment, plant stands were estimated and 15 plants were collected from each plot, weighed, and rated for root discoloration. Roots were assayed for the presence of fungi and other filamentous eukaryotic organisms. Plant stands were reduced by flooding at emergence compared with the nonflooded control. Flooding at both growth stages caused significant increases in root discoloration compared with nonflooded soybeans. Plant weights were reduced in 2 of 3 years for flooding at emergence. Pythium was the only genus of filamentous organisms whose isolation frequency increased with flooding. Of the 60 Pythium isolates evaluated representing the different cultural groups based on appearance and growth rate on potato dextrose agar, cornmeal agar, and V8 agar, 47% were moderately to highly virulent on soybean. Pythium spp. isolated from soybean included the pathogenic species P. ultimum, P. aphanidermatum, P. irregulare, and P. vexans and Group HS. Nonpathogenic P. oligandrum also was isolated from soybean.
RESUMEN
The effect of flooding and Pythium ultimum on soybean, Glycine max, was determined in a series of greenhouse experiments using the cultivars Hutcheson and Archer. Seeds were planted into pasteurized soil either not infested or infested with sand-cornmeal inoculum of P. ultimum and either flooded at emergence for 2 days or at the four leaf node stage (V4) for 5 days. A nonflooded control was included in each experiment. Seeds placed directly into infested soil resulted in little or no stand for Hutcheson regardless of flood treatment, whereas stand was reduced for Archer only in the flooded infested soil treatment. Additional experiments were conducted by placing seed onto a 2- to 5-mm layer of pathogen-free soil on top of the infested soil. Flooding at emergence reduced plant height, growth stage, and top dry weight for Hutcheson and root fresh weight for both cultivars. Greater reductions for Hutcheson in root weight, and top dry weight in P. ultimum-infested soil in the soil layer experiments, also indicated that Hutcheson was more susceptible than Archer. Flooding alone decreased root weights, and infestation with P. ultimum reduced weights further resulting in an additive effect. This also was the case for plant height, growth stage, and top dry weight for Hutcheson for flooding at emergence. Root discoloration was greatly increased for both cultivars in infested soil flooded at emergence. Similar results were found when plants were flooded at V4; however, the effect was not as great as with flooding at emergence. These studies indicate that Pythium damping-off and root rot may account for a portion of the negative response of soybean to flooding. The results also indicate that Archer has some resistance to P. ultimum.
