Excess rain and over-irrigation on poorly drained fields or low-lying areas can result in flooding and soil waterlogging. Soil waterlogging for as little as two days can reduce yields by 25%. Soybean may never fully recover from flooding injury. Genetic variability for flooding tolerance exists among soybean varieties.

In a field screening of soybean cultivars for tolerance to severe soil water-logging, most tolerant cultivars had yield reductions of 40% versus an 80% reduction for most susceptible cultivars. Still the 40 % yield loss to severe flooding for tolerant cultivars would severely impact a farmer's profits. Thus, varieties that limit yield losses by 10% or less are needed to protect soybean yields under severe field flooding.

Soybean plant introductions (PIs) from the USDA collection could offer higher levels of soil water logging tolerance than in current soybean cultivars. In 2005, 262 maturity group III PIs collected from both wet and dry areas of the world were planted in three replicate hills near New Franklin, MO in specially constructed channels where the duration and amount of flooding can be precisely controlled by pumping water on and off of plots as necessary. Plots were flooded about 5cm deep at flowering until plants began to yellow, wilt and die (about 14 d). Plots were then drained, soil allowed to dry and PIs were rated for injury on a 1 (no injury) to 5 (all plants dead) scale after a two week recovery period. Twenty (20) soybean PIs had flood tolerance scores of 1.0 to 2.0 and showed little injury to severe flooding. Seventy-eight (78) PI lines had scores of 2.3 to 3.0 and were moderately tolerant - moderately sensitive to excess water; and 164 PIs were severely injured from severe soil water-logging with scores of 3.3 - 5.0. Data combined from New Franklin in 2005 and in 2006 from New Franklin and Portageville, MO will identify PIs that show the best flooding tolerance over years and locations. The range in levels of tolerance of the 300 PIs is shown in Figure 1.

Figure 1 - Number of Flood Tolerant, Moderately Tolerant, Intolerant and very Intolerant Based on Scores of 1=No injury to 5 all plants dead for 300 Group III and IV Soybean Plant Introductions (PIs), New Franklin, MO, 2005.

Phytophthora root rot (PRR) and flooding in soybeans are often a problem on heavy clays or poorly drained soils. The question then arises do phytophthora root rot (PRR) resistance genes protect soybeans from flooding? There are genes for PRR resistance at eight or more loci. Yields of Williams rps (susceptible) and 13 Williams isolines with different PRR genes for resistance were grown on a gumbo soil at Portageville under severe flooding and normal irrigation. Yield of Williams rps, Rps 1a, rps1b, rps1k and AG 3906 is shown in Figure 2. Yield of Williams without a PRR resistance gene was reduced 10 Bu/A. Yields were also reduced for Rps1a, rps2, Rps3a, Rps4, Rps5, and Rps 6. On the other hand Williams with Rps1k, and Rps3b yielded the same or more under flooding compared to non-flood treatments showing that PRR resistance genes can protect soybeans from yield losses under excess moisture. Combinations of PRR resistance genes at different loci could be of even greater benefit for increased protection under wet soils where damage from this disease is likely. More than 50 races of Phytophthora sojae cause PRR and different resistance genes protect against specific races. Thus, the effectiveness of PRR resistance genes under conditions of excess water will depend on races of the pathogen present in the field. However, because other factors are obviously involved, PRR resistance will not entirely eliminate losses due to flooding.

Figure 2 - Yield Bu/A comparison of Williams isolines with different phytophthora root rot resistance genes rps (susceptible), Rps1a, Rps1k, Rps3a, Rps3b and variety AG 3906 under flood (F) and non-flood (NF) conditions, Portageville, MO, 2005.

2006 Field Day Report