Fusarium Wilt in Canary Island Date Palms (Phoenix canariensis)
 
Over the past few years numerous customers have asked about the ongoing problems with Canary Island Date palms (Phoenix canariensis). This variety of palm tree has become very popular and is transplanted regularly. Unfortunately, this tree is prone to infection with a fungal disease known as fusarium wilt. 
 
The pathogen is Fusarium oxysporum f. sp. Canariensis. Caution must be taken in diagnosis of this disease as another fungus, Diothorella, produces similar one sided die back along the length of infected fronds. I recommend a lab diagnosis using a DNA identification method.
 
One laboratory in California is recommended by Dr. Henry Donselman, which is the CA Seed & Plant Lab, 7877 Pleasant Grove Rd., Elverta, CA 95626, tele. (916) 655-1581. Dr. Parm Randhawa.
 
Pruning
All of the modes of transmission of the disease are not known. It is believed that the most common method of disease transmission is through pruning. Therefore it should be common practice to only prune with clean, sterilized implements. 
 
However, in some instances trees that have only been pruned in a very clean sterile manner have become infected. Plant-Tek, Inc. has concluded, along with The University of Florida (Elliot, Jan. 2006), that only old, dead, brown fronds that no longer have viable vascular tissue should be removed from the trees. 
 
A fresh wound is created by pruning off a green frond. This allows spores of the Fusarium pathogen the ability to gain access to the plant through the open wound. As this may be a pathway to infection, it appears that many well cared for trees may have been innocently infected by knowledgeable tree care professionals.
 
Convention
For the Canary Island Date palms to flourish they should be allowed to maintain or re-grow a full head of fronds. Palms transplant companies typically remove many fronds in order to lighten the weight of the tree. 
 
These companies should be urged to only remove what is absolutely necessary as this trimming is a prime suspect for the introduction of the Fusarium pathogen. It is possible for Canary Island Date palms to be transplanted very successfully with intact, full heads of fronds. 
 
Many palm transplant companies remove too many fronds and shape the area beneath the head into a “pineapple”. This appears to be done due to convention within their industry. Companies emulate what other companies are doing for fear that they will be different, when in fact they are following each others lead without any scientific or otherwise good reason for this practice. 
 
The latency of the Fusarium disease does not cause the trees to show symptoms after inoculation for up to or more than 18 months. The tree transplanting industry might not realize that they are responsible for the infection of the trees that they commonly heavily prune (most commonly with chainsaws). This is an excellent reason to request a two year warrantee on Canary Island Date palms.
 
Placement
The rampant spread of fusarium wilt disease in the landscape is promoted by improper placement of these very large plants. Whether the Canary Island Date palm is very tall or a young tree with a trunk height of only 2 feet, these trees are being placed in locations that almost guarantee that they will be pruned heavily. 
 
For aesthetic reasons, Canary Island Date palms are placed too close to driveways, walkways, spas, swimming pools, home walls & fencing, or in areas that eventually block views or limit access through walkways, driveways, etc. After these expensive trees are planted in these areas there is little recourse but pruning to alleviate the situations. 
 
From the homeowner or property manager’s viewpoint trimming is the only way to live with the trees in their crowded location. By being forced to trim the trees in order to live with them in the wrong locations, unsanitary pruning may occur which is detrimental to the long term health and survival of the trees.
 
Summary
Guidelines for transplanting and maintaining a healthy Canary Island Date palm are:
  • Do not use chainsaws for pruning
  • Do not prune green fronds (only remove old, dead, brown fronds)
  • If the trees are placed in unlivable situations which require pruning, treat the wound immediately with a copper hydroxide based fungicide (and hope for the best) or optimally move the tree to a location that will not require pruning.
  • Sterilize pruning tools (Do not place on ground between cuts!)
  • Ask for a two year warranty when purchasing a tree with unknown history.
  • Test suspected infested trees to be sure it is fusarium and not dothiorella
 
Preventative Treatment for Canary Island Date Palms
Immediately after transplant (within 7 days), foliar and soil injection treatments of fungicides and nutrients is recommended. A follow-up treatment should be scheduled 45-60 days later. Thereafter, treatments should be made every six months until the tree is established (1-2 years).