Where is the disease found?
All species of alder that occur commonly in Britain are susceptible to the disease to varying degree. This includes:
- Common alder (A. glutinosa)
- Grey alder (A. incana)
- Italian alder (A. cordata).
Although the disease is usually seen along river systems it has been found in sites well away from riverbanks or other water courses, for example in orchard shelter belts and in new woodland plantings. This suggests that the alder plants were already infected prior to planting.
Dissemination from nursery stock?
There is evidence that the pathogen can be disseminated on alder plants which have become infected in the nursery, a common occurrence with other species of Phytophthora that affect ornamental nursery stock. Alders could become infected in the nursery either by watering with contaminated river water or through contact with infected material.
In Germany, the alder pathogen has been found in rootstocks of alders from three out of four commercial nurseries that were screened.
In the UK the evidence of nursery involvement is indirect. In one instance, disease in two young woodland plantations had the common feature that both had been planted up with alders supplied by the same nursery: the plants had been imported from elsewhere in Europe. Many nurseries do not grow their own alder from seed but buy them in as young plants from other countries and grow them on before resale. If infected plants are planted out, new infection foci may be established that may only become apparent after several years.
If you are concerned about the risk of buying infected plants
Evaluate potential suppliers carefully and, if possible, see the growing stock in the nursery prior to purchase to determine that there are no apparent health problems. Partners in the European Union Concerted Action project (FAIR5 CT97 3615) working on disease of alder suggested that good practice in nurseries should include:
- No irrigation with river water in the nursery.
- At least one growing season inspection of plants to look for symptoms of Phytophthora infection.
- Routine disinfection of the nursery before new alder plants are introduced into an area where alder plants have been previously.
- Replanting of alders in ground where diseased alder plants have been growing should not be attempted for three years.
- Limiting use of Phytophthora controlling fungicides applied to plants prior to sale, because these can suppress symptom development in infected plants.
Planting alders
The planting of alder on sites liable to flooding by rivers, on the banks of rivers where diseased alders are known to occur, presents a high risk.
Although alder is often the most suitable genus for a variety of reasons, take account of the threat of disease and consider other flood-tolerant species, such as willow, as replacements or in mixture.
Special care should be taken not to introduce the fungus to remote riparian sites. Instead, natural regeneration of alder from seed can be encouraged as it occurs readily although the young plants may need to be protected with a stock-proof enclosure.
Coppicing?
Trials in Britain have shown that coppicing can regenerate diseased trees. It also reduces the likelihood of affected trees becoming unstable and causing erosion to riverbanks if they fall.
