Ganoderma zonatum: Kingdom Fungi, Phylum Basidiomycota
All palm species are considered hosts of this pathogen.
Southeastern USA, primarily Florida, but also known to occur in Georgia and South Carolina. While the fungus may be present in Mexico or the Caribbean basin, it has not been documented thus far.
The primary symptoms that may be observed include: wilting, mild to severe, of either all leaves or just the lowest leaves in the canopy (Fig. 1); premature death of the oldest leaves (Figs. 1 and 2); or a general decline of the palm (slower growth and off-color foliage, despite good fertilization).
The basidiocarp or conk is the most easily identifiable structure associated with the fungus. The basidiocarp originates from fungal growth inside the palm trunk. When the basidiocarp first starts to form on the side of a palm trunk or palm stump, it is a solid white mass that is relatively soft when touched (Fig. 3). It will have an irregular to circular shape and is relatively flat on the trunk or stump. As the basidiocarp matures, a small shelf or bracket will start to form as the basidiocarp begins to extend or protrude from the trunk (Fig. 3). It will still be white, both on the top and bottom surfaces. Eventually, the fungus will form a very distinct shelf-like structure that is quite hard with a glazed reddish-brown top surface and a white undersurface (Fig. 4).
A mature basidiocarp will have distinct zones, hence the name Ganoderma zonatum (Fig. 4). The basidiocarp will have a half-moon shape that is directly attached to the trunk. There is no stalk that attaches the basidiocarp to the trunk. Once the basidiocarp is mature, it will release rust-colored spores that will cover the basidiocarp and the surrounding area. Basidiocarps of Ganoderma zonatum can be up to 8 inches at their widest point and 2 inches thick. However, basidiocarps will take on the shape and size of the area in which they are forced to grow.
If a basidiocarp is present on the trunk at the same time the wilt or decline symptoms appear, then it is safe to diagnose Ganoderma butt rot. However, it is quite common for basidiocarps not to appear prior to severe decline and death of a palm. In that situation, the only way to determine if Ganoderma butt rot is the cause is to cut cross-sections through the lower four feet of trunk after the palm is felled and examine the cross-sections for the typical pattern of rot - greatest near the soil line, decreasing in sections further from the soil line (Fig. 5). Basidiocarps may form on the palm stump after the diseased trunk is felled.
The tissue degradation (rot) that occurs does not result in a soft, mushy trunk, nor does it smell "rotten". Instead, it is a dry rot of lignified tissue found in the vascular bundles (Figs. 6 and 7).
Ganoderma butt rot may be confused with the initial symptoms caused by the phytoplasma diseases, lethal yellowing or Texas Phoenix palm decline - i.e., a wilt or premature death of the lowest leaves in the canopy. Thielaviopsis trunk rot normally causes a trunk rot in the upper half of the trunk, but Thielaviopsis paradoxa rots the trunk from the outside to the inside of the trunk, beginning at a precise infection point. Drought stress and deep planting would also cause wilt symptoms.
The fungus is spread primarily by the spores produced by the basidiocarp (conk). It is believed that the fungus uses the roots as a means of moving from the soil to the woody trunk tissue at or below the soil line. Once a palm is infected with Ganoderma zonatum, it may remain symptomless for months or years. However, the fungus will move with that infected palm to the location in which it is transplanted. It is also possible that soil associated with transplanted palms is infested with the fungus. Another Ganoderma species, G. boninense, is the cause of a similar disease of Elaeis guineensis in southeast Asia.