Fusarium wilt is a devastating disease of certain species of palms that was first observed in Australia in the early 1980s when palms began to die at Centennial Park in Sydney. Most of these palms have since died, and the disease has been observed at other locations in Sydney. It is likely that more palms will die and that the disease will limit the use of certain palm species used for landscaping.
Palms affected by this disease are characterised by an unusual type of frond death - fronds may die more rapidly on one side of the tree, or from the base or from the centre of the tree. Most characteristically the pinnae and spines on one side of an individual frond die first and the lower fronds die rapidly so that eventually only a few surviving fronds form a spike at the top of the tree.
Eventually the whole palm will die. Affected fronds when removed from the plant will often show discolouration of the vascular bundles. This is best seen if the cut surface is wet and cleaned. Small blackened areas should be visible on the cut surface.
The disease is caused by a fungus, Fusarium oxysporum. This fungus has many different strains that cause wilt diseases in a range of plants, although each strain is restricted to a very limited host range - the strain that affects palms is restricted to palms.
Host range: the fungus was thought to only be a significant problem in certain species of Phoenix, viz., P. canariensis and P. dactylifera. Glasshouse tests have shown that P. reclinata can be infected by the fungus, but symptoms have been rarely observed in the field. Recently similar symptoms were observed in Washingtonia filifera planted at Centennial Park and Fusarium oxysporum was isolated from them. Similar results have been recorded for this species in California, so it seems that this species of palm is also a susceptible host of the fungus.
Although the symptoms of the disease are distinctive, in many cases the disease can only be accurately diagnosed by laboratory tests. Identification of Fusarium species is difficult and requires specialised training. It is not possible to test soils for the presence of Fusarium oxysporum because of the presence of non-pathogenic strains that are indistinguishable from the pathogen.
Factors affecting the spread of the disease: there is very little information on the factors that predispose palms to this infection. The fungus may have spread via the common water table below eastern Sydney. It is more common on sandy soils, however we do not know whether clay soils are unfavourable to the disease or whether it has not yet been introduced to palms on these type of soils. The two most common methods of the disease being introduced into new areas are by transplantation of infected palms or by the use of cutting implements that have not been sterilised between palms.
Once a tree is infected it will eventually die and this may take as little as two months or up to several years. There are no effective fungicides and control is dependent on avoidance of
the disease - several steps must be taken:
- Assume that every susceptible palm is a source of the fungus and always sterilise chain saws and cutting implements between use.
- Do not move palms from suspect areas (e.g. eastern Sydney) to areas where the disease does not occur. Obtain transplant palms from areas thought to be clean of the disease and have them tested.
- Use high levels of hygiene in all horticultural practices relating to palms. Clean equipment after each job and do not transport soil.
- Ensuring adequate water and a good supply of nutrients to palms. Potassium is often important in their nutrition and disease resistance.
- Do not plant Phoenix or Washingtonia species of palms in areas where the disease is known to occur.
We are conducting research to determine what species of palms are resistant to the disease so that we can more confidently recommend palm species that are suitable for planting in Fusarium affected areas.
What to do if the palm is diseased: once the palm is infected all the evidence to date suggests that it will die. Obviously the best option is to remove the tree and as much of the root ball as possible once the disease has been diagnosed. This is especially important if the palm is located close to other susceptible palms. The infected remains of the palm should be disposed of at a tip.