Phytophthora alni is an oomycete plant pathogen that causes lethal root and collar rot in alders.

[2] It has subsequently been reported in many European countries including the Netherlands, Germany, France, Sweden, Belgium, Austria and Hungary[3][4] and has recently been found in Alaska.

P. alni is homothallic meaning that both structures for sexual reproduction (antheridia and oogonia) appear in a single culture.

[3] As a result, it is believed to spread predominantly via asexual means, namely zoospores which are produced in a specialised structure known as the sporangium.

Tarry or rusty spots may appear at the base of the trunk which are indicative of the death of the phloem caused by the P. alni invasion.