Phytophthora

Several species of Phytophthora attack Pacific madrone.

• Phytophthora cactorum is commonly recovered in Washington. It infects a variety of hosts, including Acer and Arctostaphylos species. It can also infect apple and other fruit trees, and is occasionally found in eastern Washington.

• Phytophthora cinnamomi also causes major issues in warmer temperatures such as California. This species is known to infect more than 5000 plant species and is considered one of the worst invasive species in the world by the IUCN.

Phytophthora species produce swimming spores that seek out and infect plant roots during periods of extended soil moisture. If they are successful in finding a host, they invade fine feeder roots and begin to kill parts of the root system. The pathogen can then spread into the woody roots and up into the root collar, where it forms a dark “cat ears” type of lesion on Pacific madrone trees.

Phytophthora infection will eventually girdle the stem and choke off the water supply from the roots. Damaged roots do not conduct water and the tree will show symptoms of drought stress. These symptoms include wilting top dieback, or yellowing foliage. The added water stress can also make the plant more susceptible to canker and dieback fungi.

In chronic cases, the size of the foliage is reduced and the tree canopy appears thin. The symptoms are usually apparent in the summer when water demand is higher.