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Root collar rot on Alder
The planting of contaminated alders from nurseries is responsible for the spread of Phytophthora disease of alder (root collar rot). This is confirmed by new cases, in which forest owners are confronted with the sudden dieback of Alder plantations.
The infection with the pathogen Phytophthora alni takes place in the root zone of alders via water, within which the free-moving flagellated spores (zoospores) of the pathogen are present. These zoospores can reach nurseries through contaminated water during flooding. Another possibility is the watering of the plants with zoospore infected river water. Furthermore, plant material is often purchased which is already infected in the root zone.
Infection happens in seedbeds or nurseries. Root infections through Phytophthora alni are only possible in artificial soils of seed beds and nurseries. In forest soils the spores are intercepted by competing fungi, which reduces the risk of infection. The alders do not show above-ground symptoms of infection until several years after planting. In the root zone, the phytophthora infection is not macroscopically visible. For this reason, it is not possible to check plants before purchase in order to find infected alders and reject them.

Black tar spots as diagnostic character

Infected alders thus grow powerfully and seemingly healthily on their assigned sites for several years before the root collar rot reaches the base of the stem. When only part of the root system is infected and the tree receives nutrients from a number of healthy roots, it will not show any above-ground symptoms at this time.
Symptoms only become apparent once the bark of the trunk begins to resist die off in the form of the noticeably black "tar spots", which mark the "battlefield" - the edge of the already dead areas of bark (pictured). Crown symptoms subsequently appear. Usually bark tissue in the lowest trunk sections have died off, and are thereby infectious.
When contact with water occurs (for example through floods), large numbers of zoospores develop and cause the spread of the pathogen downstream. When the number of planted and infected alders is large enough, enormous amounts of zoospores can mature simultaneously. These are capable of contaminating waterside alder stands along long distances within a short number of years, and of killing large numbers of alders of all age classes.

Solution: Uncontaminated water and certified seedlings.

Producers of alder plant material can avoid this domino effect: the simplest way is the raising of alder seedlings in beds, which are not irrigated with water from ponds, streams, or rivers. The irrigation of the beds should be achieved exclusively with water from well or tap water. The seedlings should not be raised in a mixture with alder plant material of unknown origin.


Figure: Above-ground symptoms of Phytophthora alni are only visible after a number of years

A further possibility is the production of container plants in controlled phytophthora free substrate, whereby uncontaminated irrigation should also be ensured. A safer way is also the purchase of controlled, zoospore-free alder plant material, which is offered by some nurseries as well as nurseries of the Austrian Research Centre for Forests (BFW). Further information: Dr. Heino Konrad, Department of Forest Genetics, Tel.+43 (1) 878 38-2112, E-Mail: heino.konrad@bfw.gv.at.

Forest owners should clear alders which are infected from the roots up, and reforest with other tree species. As the Phytophthora disease of alder is host-specific to the genus Alnus, there is no risk to other tree species.

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