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Pathogen Biology

Phytophthora infestans
...was named by Anton deBary. The name is derived from the Greek: Phyto = plant, phthora = destroyer. P. infestans is a member of the oomycetes, a group of organisms sometimes referred to as the "water molds" which are related to brown algae. The mycelium is hyaline and coenocytic (few septa), and the nuclei are diploid. The formal name for this group of organisms is Oomycota which have been assigned to the Kingdom Stramenopila of the eukaryotes. Oomycetes are no longer considered members of the Kingdom Fungi although they share many biological, ecological, and epidemiological characteristics with fungal plant pathogens.

Asexual reproduction
As is typical of this group, P. infestans produces sporangia on sporangiophores (Figure 15). The sporangiophores are indeterminate. (i.e., they grow and produce sporangia continuously). These stalk-like structures aid in air dispersal of the sporangia. P. infestans is one of the few species in the genus Phytophthora adapted to air dispersal. Sporangia may be dispersed to neighboring fields, but do not generally survive long-distance travel because of desiccation and exposure to solar radiation.

Figure 15

In cool, wet conditions, zoospores will form and emerge (Figure 16) from the sporangia after about two hours. In warmer conditions, sporangia may function as a single spore and germinate directly (Figure 17). Zoospores are biflagellate (have two flagella) (Figure 18), with one tinsel flagellum directed anteriorly and one whiplash flagellum directed posteriorly. After swimming on the surface of the host plant surface, zoospores encyst and infect the plant.

Figure 16	Figure 17
Figure 18	Figure 19

Sexual reproduction
If both mating types (A1 and A2) come into contact with one another, sexual reproduction may occur. A nucleus from the antheridium enters the oogonium. Following karyogamy (the fusion of two nuclei), a thick-walled, diploid oospore (Figure 19) is formed. Before the 1990s, only mating type A1 was present in potato-growing areas outside Mexico (see Historical Importance), so sexual reproduction did not play a significant role in the disease cycle. Now, mating type A2 has migrated to most potato and tomato growing regions of the world, and sexual reproduction is believed to occur in some areas. However, for the past 150 years, survival of P. infestans in most parts of the world has been in infected tuber tissues (see Disease Cycle).

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