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Pathogen Biology

The division Oomycota includes important plant pathogens, such as the downy mildews and the water molds, Phytophthora and Pythium. Though traditionally taught along with fungi in plant pathology courses, recent studies indicate these organisms are closely related to golden brown algae and are now placed in a different kingdom.

Important differences exist between the oomycetes and true fungi. For example, oomycetes have cell walls composed of β-glucans and cellulose rather than chitin. Oomycetes also are unable to produce sterols. These differences can affect isolation, culture, and management of oomycete pathogens. The mycelium of Phytophthora species is composed of tube-like hyphae with few or no cross-walls (coenocytic), in contrast to the septate hyphae of fungi. Their nuclei are diploid (2n), compared to haploid (n), or dikaryotic (n + n) nuclei in the mycelia of most fungi.

Asexual reproduction by Phytophthora colocasiae, an aerial species similar to P. infestans, occurs during wet weather. Sporangia are formed at the end of short, unbranched or sparingly branched sporangiophores at the edge of lesions. They are ovoid to ellipsoid with a distinct narrow apical plug (semi-papillate), average 40-50 x 23 µm, and have a length-to-width ratio of 1.6:1 (Figure 10). Sporangia are usually separated from sporangiophores by rain (caducous), leaving a stalk (pedicel) 3-10 µm in length attached to their base (Figure 11). During wet weather, sporangia germinate on the upper surface of leaves. When temperatures are near 20°C (68°F) and humidity is high (90-100%), most germination is indirect (Figure 12), producing zoospores that swim for a few minutes, encyst, and form germ tubes (Figure 13). This process can occur in two hours or less. Sporangia germinate directly (Figure 14) between 20-28° C (68-82°F), but may account for only a small percentage of total germination. The incubation period (time from germ tube penetration to development of symptoms) is 2-4 days at optimal temperatures of 24-27°C (75-80°F).

Figure 10	Figure 11
Figure 12	Figure 13
Figure 14

Sexual reproduction of this heterothallic species depends on the presence of both A1 and A2 mating types. Hormones produced by one mating type stimulate production of antheridia (male) and oogonia (female) in the opposite mating type. Each antheridium attaches to the base of an oogonium, surrounding its stalk-like attachment (amphigynous) (Figure 15). The nuclei in these organs undergo meiosis and the haploid nucleus from the antheridium unites with a haploid nucleus in the oogonium, forming a diploid oospore 18-30 µm (average 23 µm) in diameter (Figure 15).

Figure 15

Phytophthora colocasiae is a warm-weather pathogen, growing most rapidly at temperatures between 27-30°C (80-86°F). Minimum and maximum temperatures for growth are 10°C (50°F) and 35°C (95°F), respectively. Chlamydospores (thick-walled asexual spores) are sometimes produced in culture and develop either at the end of (terminal), or along (intercalary) hyphae. They measure 17-38 µm (average 27 µm) in diameter, with walls 2-3 µm thick.