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Answers to Exercise Questions

Virginia Heffer1, Mary L. Powelson2, and Kenneth B. Johnson2 
Former graduate student, Oregon State University; currently Clinical Microbiology Institute, Wilsonville, OR and 2Department of Botany and Plant Pathology, Oregon State University


1. a. How may environment play a role in the type of germination of sporangia?

In some species of Oomycetes, cool temperatures and high moisture conditions tend to favor the production of zoospores (indirect germination), whereas warm temperatures and low moisture conditions favor direct germination by germ tube.

1. b. How does the type of sporangial germination relate to the mechanism by which spores are dispersed?

Sporangia that germinate directly are often dispersed by wind. Sporangia that germinate indirectly produce zoospores. Zoospores can move actively or passively in water.

2. What fungal structure differentiates species of Pythium from those of Phytophthora? What is the location and role of this structure in the Pythium life cycle?


The vesicle is connected to the sporangium by a short tube. In asexual reproduction, zoospores are differentiated in and released from the vesicle.

3. How can wet, poorly drained soil contribute to root rots caused by Pythium and Phytophthora?

Germination of sporangia and oospores is more likely to be indirect (i.e., production of zoospores) in wet soil. Zoospores can move in soil water in response to exudates released by plant roots. Zoospores can then encyst and germinate on roots of susceptible plants.

4. In the U.S. and Europe, sexual reproduction in Phytophthora infestans, the cause of late blight of potato and tomato, was for many years absent because only one mating type was present in the pathogen population. Recently, however, both mating types have been found. If you suspected that both mating types were present in your potato field:

a. Where would you look to determine if sexual reproduction was occurring?

On leaf tissue with two or more lestions.

b. What fungal structure would be evidence of sexual reproduction?