Pathogen Biology

Phytophthora sojae is an oomycete, a close relative of the golden brown algae, in the Kingdom Stramenopila (or Straminipila). This group of organisms produces swimming spores (zoospores) which have tubular flagella. While oomycetes have little taxonomic affinity to fungi, they are “fungus-like” in their growth since they also produce mycelium. The mycelium is coenocytic (aseptate – lacking cross walls) but as a laboratory culture ages, false septae will form. Lima bean agar (Figure 4) is one of the most common media used to culture P. sojae; on it, the mycelium grows close to the agar surface and has no color. Interestingly, P. sojae does not grow on full strength potato dextrose agar, a commonly used medium for many fungi and oomycetes. Reducing the concentration of potato dextrose agar and supplementing with water agar improves the growth (Figure 5). On V8 juice agar medium, the hyphae are white and branch mostly at right angles. The optimum temperature for growth of most isolates on any medium is 25 to 28°C (77 to 82°F).

Figure 4	Figure 5

On lima bean agar, oospores (thick-walled, sexual spores) form readily, often within 3 to 4 days. Phytophthora sojae is homothallic (self-fertile). Antheridia (male structures) are predominantly paragynous (attach to the side of the oogonial stalk), but some will be amphigynous (encircle the oogonial stalk). Oospore walls are smooth and the oogonia are 40µm in diameter, sometimes as large as 45µm (Figure 6a, Figure 6b)

Figure 6a	Figure 6b

Sporangia (thin-walled lemon-shaped spores) do not form in agar culture but can be induced with repeated flooding and washing of cultures (Qutob et al., 2000; Schmitthenner and Bhat, 1994). Once formed, a sporangium has an inconspicuous papilla (a small rounded process on the tip) so it appears nonpapillate. Sporangia have long pedicels and are non-caducous (do not shed or break off from main mycelium) (Figure 7). Sporangia are approximately 40 µm long by 28 µm wide. Germination is either direct by production of hyphae or indirect by the production of large numbers of zoospores (>20). From the empty sporangium, another sporangium will develop in an extended and nested pattern.

Figure 7

Zoospores swim toward the root and encyst on the root surface. A single germ tube emerges and penetrates the root cell wall.

Nomenclature

Phytophthora sojae can readily be mistaken for Phytophthora megasperma. In fact, at one time, P. sojae and P. medicaginis were known as Phytophthora megasperma f. sp. glycines and P. megasperma f. sp. medicaginis, respectively. Molecular analysis of the mitochondrial region (Forster et al., 1989) and isozyme analysis (Nygaard et al., 1989) provided evidence that these were in fact distinct species.

P. sojae has a gene-for-gene relationship with its host, soybean. Fourteen different single resistance genes, designated Rps genes, have been described (Grau et al., 2004). A number of sets of differential cultivars are available to characterize pathotypes (races) of P. sojae. These sets were developed by backcrossing different sources of resistance into common genetic backgrounds (Williams, Bedford, Harosoy and Corsoy) to develop isolines, i.e., varieties that are almost identical to each other, except that they differ in one resistance gene. There are 55 coded races, but many more virulence combinations have been described in the US. Many publications today refer to the pathotype, the exact listing of virulence reactions that each isolate has on the isolines of the differential set rather than a race code number, as the number of races and diversity has become too great. A pathotype is determined by inoculating an isolate on the series of differentials. If a spreading brown lesion forms following an inoculation (compatible or susceptible interaction) on a differential carrying a specific R-gene, then that response becomes part of the pathotype. For example, isolates that are designated Race 30 cause susceptible responses on differentials with Rps1a, Rps1b, Rps1k, Rps3a, Rps6 and Rps7. A brief listing of races and their corresponding pathotype is shown in Table 1.

Table 1. Race designations^a, corresponding pathotypes, and reactions following inoculation of R-gene specific differentials used to characterize Phytophthora sojae.

^aDesignations for race were based on the response of differentials, in which R indicates a resistant response (no lesion develops) and S indicates a susceptible response (spreading brown lesion).

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by The American Phytopathological Society