Diagnosis of Fusarium Wilt of Palms
Information extracted from http://edis.ifas.ufl
Field diagnosis can often be made based on the symptoms described above, especially if the symptoms are typical ones. But symptoms of Fusarium wilt look the same as petiole/rachis blight. The latter is not a true vascular wilt and usually does not kill the palm, but the resulting individual leaf symptoms are very similar on Phoenix species. See the fact sheet about this disease at http://edis.ifas.ufl.edu/pp145. It is also possible for a Canary Island date palm to have both diseases. Therefore, a laboratory diagnosis is required to confirm Fusarium wilt.
To confirm the field diagnosis, a laboratory must isolate a Fusarium oxysporum-like fungus from the affected leaf tissue and then conduct a molecular test on the isolated culture to confirm it is f. sp. canariensis. Contact your local county Extension office or the TREC Plant Diagnostic Clinic (786-217-9276) for complete details about correct sample submission procedures and cost of a laboratory diagnosis. The best tissue to send for a diagnosis is the petiole or rachis from a leaf exhibiting the typical symptoms of one-sided wilt. A dead leaf is not acceptable.
Molecular confirmation of the isolated fungus is critical. There are numerous Fusarium oxysporum isolates that do not cause disease but may be associated with palm material. Unfortunately, it is not possible to separate pathogenic isolates from nonpathogenic isolates based on their appearance in culture. Thus, isolating a fungus that looks like F. oxysporum does not confirm the field diagnosis. The fungus growing on the culture plate must be subjected to a molecular test that is specific for F. oxysporum f. sp. canariensis.
If the field symptoms appear typical for Fusarium wilt, but Fusarium is not isolated, then the symptoms may be an expression of petiole/rachis blight. Many of the petiole/rachis blight pathogens are not culturable from plant tissue. The next step would be to obtain new samples from the diseased palm or similarly diseased palm and resubmit to the clinic to confirm presence or absence of petiole/rachis blight pathogens. Be sure to discuss the problem with the clinic staff before resubmitting samples. If no symptomatic leaves are available, wait until new symptoms develop before resubmitting.