Phytophthora Alni - Infection

Infection

Affected alder species include:

• Alnus cordata - Italian alder
• Alnus glutinosa - Common alder (most susceptible)
• Alnus incana - Grey alder (most resistant)
• Alnus viridis - Green alder

The only trees described as affected in the wild are alder trees; however, greenhouse inoculation trials have suggested that walnut (Juglans regia), chestnut (Castanea sativa) and wild cherry (Prunus avium) trees may also be susceptible. Risk factors for infection include: water temperature (disease risk is higher in warmer waters), soil type (disease risk is higher in fine textured soil, especially clay loams) and water course type (disease risk is higher in slow flowing water courses).

Infected trees have abnormally small, yellow and sparse leaves which frequently fall prematurely. The crowns of trees which have been infected for many years have many dead twigs and branches. 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. The course of the disease is varied, with many trees dying rapidly once symptoms appear, however, others may deteriorate slowly over many years.

P.alni is regarded as a serious threat to riparian woodland. In 2007 Oregon listed P.alni as one of its "100 most dangerous invasive species to keep out".