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Significance

Phytophthora root rot is a problem worldwide on poorly drained soils (Costamilan et al., 1996; Jee et al., 1998; Wrather et al., 2001; Yanchun and Chongyao, 1993). The disease was identified in the US in Indiana in 1948 and again in Ohio in 1951 as soybean production was beginning to take off; however the causal agent was not described until 1958 (Kaufmann and Gerdemann, 1958).

In the US, this is probably the best managed soybean disease, as most cultivars have some level of resistance to P. sojae, either as an Rps gene alone or combined with partial resistance (Figure 14, Figure 15). Historically this was not always the case. Cultivars that were planted during the late 1970s had a single resistance gene, Rps1a. New races developed that killed plants with this gene. Ohio lost 300,000 acres in one year alone. As a result, single genes and partial resistance were widely deployed starting in the 1980s and still are relied upon today (Schmitthenner, 1985).

There has been much speculation about where P. sojae originated (Schmitthenner, 1988). It was only recently described from the Republic of South Korea or the People’s Republic of China, in1998 and 1993, respectively. However, the diversity of resistance, both in numbers of Rps genes and levels of partial resistance in soybeans, is much higher in soybean lines from the Republic of South Korea and some provinces in China than in plant material originating elsewhere.


Figure 14	Figure 15

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