Another oomycete of concern is Phytophthora alni, cause of alder root and collar rot. This disease was first discovered in 1993 killing alders in Great Britain, and identified as a new species in 2004. Currently, P. alni is considered a species complex of 3 subspecies: alni, multiformis, and uniformis. P. alni subsp. alni (Paa) is considered more aggressive than P. alni subsp. multiformis (Pam) or P. alni subsp. uniformis (Pau). Paa is frost sensitive, while Pau is cold tolerant. Overall, P. alni is favored by mild winters and warm summers (70- 86°F).

P. alni has caused alder dieback and mortality across Europe. It was probably spread by infected nursery stock and infested irrigation water. Generally, there has been higher incidence (rate of occurrence) in riparian zones (next to rivers and streams), but it has been found away from waterways. Stresses caused by flooding can increase damage.

Alders are important pioneer species, capable of fixing atmospheric nitrogen. They tolerate wet soils, provide shade and cooler water temperatures, provide habitat for aquatic and terrestrial organisms, contribute significant amounts of nitrogen to stream food webs, and stabilize streambanks. Changes in riparian tree species can increase soil erosion, alter currents, and affect fish habitat (including salmonids).

Widespread dieback and mortality of thinleaf alder in Alaska led to surveys in 2007. Phytophthora alni subsp. uniformis (Pau) was discovered near Cooper Landing and Denali National Park, in remote, unmanaged stands. Further surveys found populations near Fairbanks and Tok. However, root and collar rot symptoms were rarely found, suggesting dieback due to other factors. Genetic diversity studies indicate the Alaska Pau isolates are probably indigenous to North America. The risk of introduction of the more aggressive forms still exists.

Hosts and Symptoms

Native and domesticated P. alni hosts include: European or black alder (Alnus glutinosa), gray alder (A. incana), thinleaf alder (A. incana subsp. tenuifolia), green alder (A. viridis), mountain alder (A. viridis subsp. crispa), Sitka alder (A. viridis subsp. sinuata), Italian alder (A. cordata), red alder (A. rubra), and sweet cherry (Prunus avium, experimentally).

P. alni doesn’t appear to produce chlamydospores (tough, asexual spores), but does produce oospores (tough, sexual spores). Sexual recombination allows for greater genetic variability. Swimming zoospores are produced, which can infect fine roots, as well as bark near the root collar.

microscopic ball like spore

Phytophthora alni, oospore, Thomas Jung, Bugwood.org

microscopic sprores

Phytophthora alni, sporangia, Thomas Jung, Bugwood.org

Injuries allow pathogen entry, but P. alni does not require injury to initiate infection. Root collar tissue is killed, resulting in stem girdling. When most of the root collar has been girdled, foliar and crown symptoms start to appear. This may take up to 10 years. Leaves in mid to late summer may be small, yellow and sparse; they may drop prematurely. Crown dieback can occur, with twigs dying in the upper or outer crown.

a deciduous tree in a forest — *Phytophthora alni, dieback, E. black alder, Thomas Jung, Bugwood.org*

Tarry spots or rusty exudates may form on the lower portions of the stem. Underlying bark is dead.

large black blotches on tree trunk — *Phytophthora alni, tarry spots, European black alder, Thomas Jung, Bugwood.org*

Peeling away the bark may reveal orange-brown necrotic (dead) inner bark in a tongue pattern. Older inactive lesions are dark brown.

Phytophthora alni, alder collar rot showing orange inner bark, Andrej Kunca, National Forest Centre- Slovakia, Bugwood.org

Phytophthora alni, collar rot with tarry spots below, and orange tongue-shaped necrosis of the inner bark, gray alder, Thomas Jung, Bugwood.org

In advanced stages, live strips of bark may produce new shoots. Adventitious roots may form above bark lesions on the stem, as the vascular system is damaged. Early or excessive cone production may occur, cones are unusually small. Many symptoms are similar to those caused by other root and collar infecting Phytophthora spp., Pythium spp., and several fungi. As with P. ramorum, identification to genus-level is possible with “dipstick” field kits, identification to species level requires submission to a laboratory.

Spores are spread by movement of plants, rain splash, infested soil, flood and irrigation water. Vehicles and footwear used in high-risk areas (streamside or near diseased alders) should be cleaned and disinfected to reduce movement of infested soil. Outside a living host, spores in soil probably survive less than one year. Recently dead trees (1-3 years) produce little inoculum. Nurseries should use pasteurized or sterilized soil and clean pots. Careful inspection of incoming plant materials helps to exclude diseased material. Periodic scouting allows for early detection and eradication. A 2009 Federal Order (DA-2009-27) prohibits import of Alnus spp. for planting from all countries (excluding seed).

Although a less aggressive form of P. alni is found in Alaska, we can reduce its spread. The more aggressive form found in Europe is causing disruptions to stream and river ecosystems. Vigilance on our part can reduce the chances of similar damage in Alaska.