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Disease Management

Phytophthora sojae is prevalent in many soybean production regions of the world and is managed primarily with host resistance. P. sojae consists of many races but the occurrence of these races or virulence pathotypes is quite variable across the north central region of the United States both within fields and between fields. This diversity of pathotypes can make soybean cultivar selection challenging for producers.

Host resistance

Several types of resistance have been described in the P. sojae-soybean pathosystem: R-gene mediated resistance, root resistance, and partial resistance. R-gene mediated resistance has been described for 14 Rps genes and most have been mapped on the soybean genome. Commercially, only six genes, Rps1a, Rps1b, Rps1c, Rps1k, Rps3a and Rps6, have been deployed and one more, Rps8, is in commercial development. This type of resistance is expressed as a hypersensitive response and is most often measured in laboratory and greenhouse assays through inoculation of the hypocotyl region on young seedlings. However, it is only effective against some races (see Pathogen Biology page).

Figure 10

Root resistance and partial resistance are both expressed in the roots. Root resistance is almost a complete resistance that is expressed in the roots but is quantitatively (several genes that each contribute to the level of resistance) inherited. Partial resistance is also quantitatively inherited and is primarily expressed as reduced colonization of the roots.

Figure 11

Partial resistance is expressed in plants after the cotyledons and first true leaves are visible, but Rps gene resistance is expressed in the seed and therefore is effective from germination onwards. Partial resistance is expressed as reduced colonization and slower lesion expansion, and is inherited as a multi-genic trait. Since more genes are involved in the expression of this resistance and the assays take longer, partially resistant cultivars are more difficult to breed. A combination of Rps gene and partial resistance has provided the best protection, especially in regions where individual fields may harbor a large number of races.

Chemical control

Fungicide seed treatments are the only chemical control practiced to manage P. sojae and they are used for managing early season seed decay and damping-off. Mefenoxam and metalaxyl, two very similar compounds, can both be used as seed treatments. For adequate management of P. sojae, higher rates of seed treatment product have higher efficacy than low rates. Many oomycete species have been identified that have adapted to these fungicides and are no longer sensitive. However, the north central region of the United States was surveyed from 2001 to 2005 for the presence of metalaxyl/mefenoxam-insensitive isolates of P. sojae, but none were found.

Figure 12

Cultural practices

Phytophthora root rot is more severe in poorly drained or flooded areas of the field. Any cultural practice that improves soil drainage ultimately reduces the time that soils are saturated, and thus reduces the infection period. Soil tillage and placing drain tiles in fields either in combination or alone may reduce the incidence and severity of Phytophthora root and stem rot (Figure 13). Rotation and tillage are not effective management options since the oospores of P. sojae are capable of surviving for long periods of time in soil.

Figure 13

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