Phytophthora is a type of water mold, also known as an oomycete.

Just like pythium, phytophthora is an exceptionally aggressive fungus that causes both economic and environmental damage. When a plant is infected, it is unable to absorb nutrients or water because of dead root tissue.

The word phytophthora means "plant-destroyer." There are numerous subspecies of the fungus, each responsible for infecting a different crop.

One strain of phytophthora, known as Phytophthora Infestans, was the fungal agent responsible for the Great Irish Potato Famine. It is still a major obstacle for potato farmers worldwide.

Preventing phytophthora is difficult, but possible. Since the mold thrives in water, the most important aspect of prevent is correct water management: 

• Water should be uniformly distributed to each plant, without causing the plant to stay wet for unnecessary amounts of time.

• An irrigation system must be set up that moves the water away from roots.

• A fungicide can help with initial avoidance of the mold, but is not effective for long term use.

Unfortunately, once phytophthora is already found in a certain area, the best thing to do is get rid of the infected plants and exterminate the area. However, once the area is cleared of the fungus, the space can be reused, especially for phytophthora-resistant crops.

If no resistant crops can be planted, make sure the new crops have at least 25 percent air filling space, good drainage, and a well-kept surrounding area.

More resources:

Phytophthora on Wikipedia

Phytophthora on Down Garden Services

See also:

What is pythium?

Damping off: What it is and how to prevent it

Tomato diseases