Phytophthora species are well adapted to the diverse environments that they encounter in different seasons. They produce several types of structures. Some ensure survival in the absence of a host plant. Others are well suited for infecting plants or dispersing to other plants. All the structures are so small that a microscope is necessary to see them.
Chlamydospores and Oospores
Phytophthora produces two kinds of spores designed for long-term survival: chlamydospores (clam-EE-doe-spores) and oospores (OH-oh-spores).
Both types of spores are round and have thick walls that protect them.
The important difference is that chlamydospores are asexual (clonal) spores, whereas oospores result from sexual recombination.
Sexual recombination can be very important in increasing the genetic variability within a species. For example, it could lead to faster development of resistance to fungicides.
In the presence of water, such as during wet weather, Phytophthora chlamydospores or oospores germinate to form sporangia (spor-ANJ-ee-uh; the singular is sporangium). It is important to manage water in the nursery to minimize conditions favorable for sporangia production (this will be covered in detail in Module 2).
Phytophthora species produce sporangia on plant surfaces, including leaves and roots. In some Phytophthora species, sporangia can detach and be blown or splashed with water to healthy plants. Sporangia release small, one-celled, swimming spores called zoospores (ZO-oh-spores).
Zoospores can swim through water on leaf surfaces and in water-logged potting media and soil. They are very fragile and susceptible to drying.
"Zoospore release from sporangium" by M. Putnam
Cysts and Hyphae
Zoospores are attracted to plant roots when in the soil. When they reach a susceptible plant, they stop swimming and form a cyst. A cyst is a short-lived resting structure.
Cysts germinate (much as a seed germinates) to form microscopic, thread-like structures called hyphae (HY-fee). Hyphae allow the pathogen to grow into plant cells to get food.
Once Phytophthora infects the plant, it produces more chlamydospores, oospores, and/or sporangia, thus completing its life cycle.