Cocoicola californica and Serenomyces spp. are considered the primary pathogens of petiole (rachis) blight. Diplodia, Dothiorella, Fusicoccum, Macrophoma, Phoma and Phomopsis have also been associated with symptoms of this disease. All of the pathogens belong to the Kingdom Fungi.
Because there are a number of pathogens associated with this disease, all palms may be susceptible to at least one of the pathogens. Palms are the only known plant hosts for Cocoicola californica and Serenomyces spp., whereas the other pathogen species have multiple plant family hosts.
In general, the pathogens are distributed worldwide, although individual species may have a limited geographic range.
Cocoicola californica has only been found in association with Washingtonia spp. in the USA (California, Florida).
The term "petiole" refers to the portion of the leaf between the leaf base and the leaf blade. A palmate leaf palm has only a petiole. The term "rachis" refers to the extension of the petiole into the leaf blade that the leaflets are attached to in a pinnate leaf palm. The leaves of pinnate leaf palms have both a petiole and a rachis.
Initial symptoms normally occur on the lowest (oldest) living leaves in the canopy. For palms with pinnate leaves, 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). 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. 2). Eventually, the leaflets on the other side of the rachis will turn brown also, and the entire leaf dies.
For palms with palmate or costapalmate leaves, there will be at least one leaf with a mixture of healthy, chlorotic and necrotic leaflet segments (Fig. 3). As with palms with pinnate leaves, there will be a reddish-brown or dark-brown stripe on the petiole (Figs. 4 and 5), with a corresponding internal discoloration observed in cross-section.
The pathogens are found in the petiole or rachis tissue, and not the leaflet or leaf segment tissue. Under high humidity situations, the pathogens will erupt through the epidermal tissue to expose fungal fruiting structures. These eruptions may look like blisters on the petiole (Fig. 6), or be very distinct, individual structures with spore masses emerging (Fig. 7).
For Phoenix canariensis, Syagrus romanzoffiana and Washingtonia robusta, Fusarium wilt causes the same symptoms as petiole (rachis) blight. 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 of these palm species.