Tar Spot

Scientific Name of Pathogen

Phaeochora, Phaeochoropis, Brobdingnagia, Camarotella, Coccostromopsis, 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, Chamaedorea, Chamaerops, 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 canopy. These pathogens invade the leaf blade, and not the leaf petiole or rachis. Initially, spots or lesions will range from barely visible to obvious, depending on the pathogen invading the tissue. Discoloration and distribution on the leaf blade 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 blade surface. The pattern of stromata on the leaf vary from randomly scattered across the leaf blade to occurring only in a straight line along the leaf vein. If disease severity is high, the entire leaf blade may become necrotic.

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 blade will separate this disease from other leaf spot diseases.

Additional Comments

Tar spot is a disease name used specifically for diseases caused by leaf blade 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. Petiole (rachis) blights are caused by the tar spot pathogens Cocoicola and Serenomyces, but these pathogens only invade the petiole or rachis, never the leaf blade. Most of the tar spot pathogens are not culturable.

December 1, 2010