Fusarium oxysporum f. sp. palmarum: Kingdom Fungi, Phylum Ascomycota
The primary hosts of Fusarium oxysporum f. sp. palmarum are Syagrus romanzoffiana (queen palm) and Washingtonia robusta (Mexican fan palm). Phoenix canariensis (Canary Island date palm) has also been confirmed as a host.
USA (Florida, Texas)
Initial symptoms normally occur on the lowest (oldest) living leaves in the canopy. For Syagrus romanzoffiana, 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 (Figs. 1 and 2). 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. 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.
For Washingtonia robusta, there will be at least one older leaf with a mixture of healthy, chlorotic and necrotic leaflet segments in the leaf blade (Figs. 4 and 5). As with the above palm species, there will be a reddish-brown or dark-brown stripe on the petiole, with a corresponding internal discoloration observed in cross-section.
For both palm hosts, the disease symptoms normally begin on the lowest (oldest) leaves and then move up the canopy, progressively killing younger and younger leaves. In all situations, the spear leaf is the last leaf to die.
Fusarium wilt of Syagrus romanzoffiana and Washingtonia robusta develops very quickly. These palms often die within 2 to 3 months after initial symptom development. Due to the quick decline, a very characteristic symptom of this disease is the overall canopy appearance. The necrotic leaves do not droop or break and bend down around the trunk, but remain relatively rigid (Fig. 6).
Petiole (rachis) blight causes the same leaf symptoms in palms. 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.
Three other Fusarium oxysporum formae speciales occur on palms. Fusarium oxysporum f. sp. canariensis causes Fusarium wilt of Phoenix canariensis and occurs in Argentina, Australia, the Canary Islands, France, Greece, Italy, Japan, USA (California, Florida, Nevada, Texas). 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.