Impaired internal drainage and Aphanomyces euteiches root rot of pea caused by soil compaction in a fine-textured soil

The Aphanomyces euteiches Drechs. root rot in pea (Pisum sativum L.) causes serious crop loss. Genetic resistance and fungicidal control are not available commercially. Moreover, the pathogen may remain viable in soil for many years. The disease is favoured in poorly drained soils, especially when soil compaction with heavy axle loading may impair drainage. Precrop species (oat (Avena sativa L.), pea, and several others) and two different tillage tools (moldboard vs chisel) for late fall incorporation of the precrop residue were tested for disease control in two farm fields (fields 1 and 2) abandoned from processing pea production because of high inoculum concentrations of A. euteiches and poor drainage below the tilled layer. Disease severity, growth staging of the pea, and soil-water potential (SWP) were observed nearly every other day during the pea test-year. All pea plants were dead before the node-8 stage in 1991 and 1993 in field 1 and in 1993 in field 2. Pea in field 2 in 1994 produced a harvestable yield after a pea precrop, but the oat precrop increased yield at least 270%. Tensiometers repeatedly measured SWP > -5kPa at the 15 cm depth when pea plants did not survive the disease in 1991 and 1993 in both fields. Periods of SWP > -5kPa did not occur in 1994 at depths < 20 cm in field 2, when the pea plants survived to harvest and the oat precrop was successful. Bulk density profiles differed between tillage implements for precrop incorporation, but these profiles were poor predictors of saturated hydraulic conductivity (K_sat). K_sat < 1 mm per day indicated impaired drainage in the upper 50 cm of the abandoned fields. Soil-water retention and field observed SWP indicated < 10% air-filled pores frequently in the 15-30 cm layer, yet there were no symptoms of anoxia. Where there was no heavy axle loading in an adjacent pea nursery, traffic compaction decreased K_sat in the tilled layer only and suppressed disease relief from use of an oat precrop. Pathogenic responses to poor internal soil drainage are likely explained by measured SWP near the soil surface, because SWP influences the mode of infection. When an oat precrop is grown the previous season as a cultural control for root rot, internal drainage must not be impaired by excessive compaction related to tillage and traffic management.