Fusarium oxysporum f. sp. canariensis: Kingdom Fungi, Phylum Ascomycota
The primary host of Fusarium oxysporum f. sp. canariensis is Phoenix canariensis (Canary Island date palm). Phoenix reclinata (Senegal date palm) has been confirmed as a host.
Argentina, Australia, the Canary Islands, France, Greece, Italy, Japan, USA (California, Florida, Louisiana, Nevada, South Carolina, Texas)
Initial symptoms normally occur on the lowest (oldest) living leaves in the canopy. For Phoenix canariensis, there will be at least one leaf with leaflets, either all of them or only a portion, on only one side of the rachis that will be discolored, usually a shade of brown due to desiccation or death. The leaflets on the opposite side of the rachis will be a healthy green color (Fig. 1). This very distinctive symptom is often referred to as a “one-sided wilt” or “one-sided death”. A reddish-brown or dark-brown stripe will be observed on the petiole and rachis of the affected frond, on the same side where the first dead leaflets appear (Fig. 2). This streak may run the full length of the petiole and rachis, or just a portion of it. Internal discoloration will be observed in cross-sections of the discolored petiole and rachis (Fig. 3). Eventually the leaflets on the other side of the rachis will turn brown also, and the entire leaf dies.
The disease symptoms normally begin on the lowest (oldest) leaves and then move up the canopy, progressively killing younger and younger leaves. For some leaves, the leaflets may begin turning brown at the leaf tip and on both sides at once, rather than just on one side. In all situations, the spear leaf is the last leaf to die.
Fusarium wilt of Canary Island date palm often progresses at a relatively slow rate, and the palm can survive for as long as two years after initial symptom development. Since the primary method of pathogen transmission between Phoenix canariensis is pruning tools, it is not uncommon to see a landscape of Phoenix canariensis in various stages of disease development (Fig. 4).
Petiole (rachis) blight causes the same leaf symptoms in Phoenix canariensis. Laboratory diagnosis is necessary to confirm which pathogen is responsible for the symptoms. It is possible for a Fusarium wilt pathogen and a petiole (rachis) blight pathogen to be present in the same tissue.
Fusarium wilt caused by Fusarium oxysporum f. sp. canariensis has been observed naturally occurring on Washingtonia filifera in limited areas of California and Australia. While this disease has been artificially induced on Phoenix dactylifera, it has not been observed to occur naturally.
Three other Fusarium oxysporum formae speciales occur on palms. Fusarium oxysporum f. sp. albedinis causes Fusarium wilt of Phoenix dactylifera, but is limited to Morocco, Algeria and Mauritiana. Fusarium oxysporum f. sp. elaeidis causes Fusarium wilt of Elaeis guineensis, but is limited to western African and very limited areas within Ecuador and Brazil. Fusarium oxysporum f. sp. palmarum causes Fusarium wilt of Syagrus romanzoffiana and Washingtonia robusta, but is limited to Florida (USA).