Survival
Germination & Infection
Early Root Disease
Late Root Disease
Spread
Foliar Disease

General Cycle of Phytophthora Diseases On Woody Ornamentals

There are many species of Phytophthora that infect nursery plants. This discussion focuses on Rhododendron and Phytophthora without regard to specific species of either host or pathogen.

The phases of the disease cycle are:

• survival
• spore germination and infection
• early root disease development
• late root disease development
• spread
• foliar disease development
• production of survival structures

Survival
Oospores and chlamydospores form in infected leaves, roots, or stems. When these plant parts die, the spores stay with the organic debris but may wash down into the soil or potting medium. They may survive a long time in soil, even after the plant tissue has decomposed.

Spore Germination and Infection
The spores lie dormant until environmental conditions and the presence of a host plant stimulate them to germinate. Roots grow near an oospore or chlamydospore. Soil may become saturated with water from rain or irrigation. Chlamydospores and oospores can then germinate to form hyphae or sporangia. Under these wet conditions, zoospores form inside the sporangia.

Zoospores are expelled from the sporangia and begin to swim around. Normally, they swim toward a growing host root, attracted to chemicals from the root. The zoospores congregate either just behind the root tip, in wounds, or in areas where root branches are produced.

The zoospore becomes a cyst just before the organism infects the root. Now, the thread-like hyphae begin to grow inside the root.

Infection also can result from hyphae coming directly from oospores or chlamydospores.

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