Tar Spot
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Figure 1. The raised black structures on this leaf are stromata, a mixture of host tissue and mycelia of Phaeochoropsis neowashingtoniae. Photo by A. J. Downer, University of California.
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Scientific name of pathogen
Phaeochora, Phaeochoropsis, Brobdingnagia, Coccodiella, Malthomyces, Ophiodothella, Oxodeora, Phyllachora, and Sphaerodothis
All genera belong to the Kindom Fungi, Phylum Ascomycota. Other genera within the families Phaeochoraceae and Phyllachoraceae may be associated with this disease.
Hosts
The list of palm genera reported to be affected by these pathogens is relatively small, and it is probably incomplete: Aceolorrhaphe, Acrocomia, Allagoptera, Arenga, Astrocaryum, Attalea, Bactris, Butia, Calamus, Caryota, Chamaerops, Coccodiella, Coccothrinax, Cocos, Copernicia, Enterpe, Jubaea, Livistona, Phoenix, Roystonea, Sabal, Syagrus, Thrinax, Washingtonia.
Distribution
As a group, these pathogens are considered to be distributed worldwide.
Symptoms/signs
Disease symptoms and signs are usually most prominent on the oldest leaves in the canopycanopy:
the cluster of leaves borne at the tip of the stem
. These pathogens invade the leaf bladeleaf blade:
the broad, flattened distal portion of a leaf
, and not the leaf petiolepetiole:
leaf stem
or rachisrachis:
the axis of a leaf beyond the petiole or an extension of the petiole into the leaf blade
. Initially, spots or lesions will range from barely visible to obvious, depending on the pathogen invading the tissue. Discoloration and distribution on the leaf bladeleaf blade:
the broad, flattened distal portion of a leaf
is highly variable. Eventually, however, infected host tissue will become incorporated into a compact mass of mycelium and form a melanized stroma (plural = stromata), which contains the sexual fruiting bodies of the fungus. As with the initial symptoms, stromata vary in size and shape based on the pathogen invading the tissue. The stromata often erupt through the leaf epidermis, but size of the stromata will determine how visible these structures are on the leaf bladeleaf blade:
the broad, flattened distal portion of a leaf
surface. The pattern of stromata on the leaf vary from randomly scattered across the leaf bladeleaf blade:
the broad, flattened distal portion of a leaf
to occurring only in a straight line along the leaf vein. If disease severity is high, the entire leaf bladeleaf blade:
the broad, flattened distal portion of a leaf
may become necroticnecrotic:
see necrosis
.
May be confused with
Initial symptoms on the leaf tissue will look like any other leaf spot disease. Development of the stromata on the leaf bladeleaf blade:
the broad, flattened distal portion of a leaf
will separate this disease from other leaf spot diseases.
Additional comments
Tar spot is a disease name used specifically for diseases caused by leaf bladeleaf blade:
the broad, flattened distal portion of a leaf
invading pathogens that produce blackened stroma. Diamond scale is considered a tar spot disease, but the pathogen has a very restricted host range and geographic range, and so is listed separately. Petiolepetiole:
leaf stem
(rachis) blights are caused by the tar spot pathogens Cocoicola and Serenomyces, but these pathogens only invade the petiolepetiole:
leaf stem
or rachisrachis:
the axis of a leaf beyond the petiole or an extension of the petiole into the leaf blade
, never the leaf bladeleaf blade:
the broad, flattened distal portion of a leaf
. Most of the tar spot pathogens are not culturable.
