Leaf Spots and Leaf Blights

Other Common Names

No specific name can be assigned to a particular leaf spot or leaf blight until after the pathogen is determined.

Scientific Name of Pathogen

Acidovorax, Annellophora, Bipolaris, Botrytis, Cylindrocladium (=Calonectria), Cercospora, Colletotrichum, Exserohilum, Gliocladium, Pestalotia, Pestalotiopsis, Phaeotrichoconis, Phytophthora, Pseudocerspora, Stigmina

Acidovorax belongs to the Kingdom Bacteria. Phytophthora belongs to the Kingdom Stramenopila.  The remaining pathogens belong to the Kingdom Fungi.

More pathogens then the ones listed above may be associated with leaf spots and leaf blights.

Hosts

All palms are susceptible to at least one of the pathogens that cause leaf spots and leaf blights.

Distribution

Viewed as a group, these pathogens should be considered ubiquitous in their distribution.

Symptoms/Signs

Despite the many pathogens that cause leaf spots and leaf blights, the initial symptoms are very similar. Only those leaf spots that eventually produce easily recognizable signs of the disease have been separated out as an entity for this key. These diseases include diamond scale, Gliocladium blight, Graphiola leaf spot, and tar spot.

Initial leaf spots are usually round to oval in shape and vary in color from yellow to brown to black. The initial size may be as small as a pin point. Some leaf spots initially appear as water-soaked lesions (Fig. 1).  At some point during disease development, leaf spots will have a contrasting colored edge or halo - e.g., brown spot with a yellow halo, tan center with brown edge or gray center with black edge and a yellow halo (Figs. 2-4). All color combinations are possible. As the leaf spots expand in size, the shape and coloration may change (Fig. 5). As the disease progresses, leaf spots often coalesce (merge together) to form large areas of blighted tissue (Fig. 1), hence, the term leaf blight. If the disease continues to develop, leaflets or the entire leaf may die prematurely.

Any age leaf can be affected by leaf spots, and there usually is no distinct pattern to the spotting (see "may be confused with"). Leaf spot diseases may occur at any stage of palm growth, but are a more serious problem of seedling and juvenile palms because they have fewer number of leaves or the leaves are smaller in size than in a mature palm.

May Be Confused With

Potassium deficiency: This nutrient deficiency causes leaf spots that can look identical to leaf spots caused by plant pathogens. Leaf spots caused by potassium deficiency are normally confined to the oldest leaves and are more severe on the leaf tip than the leaf base. However, if the deficiency is severe, the oldest leaves may be necrotic or nearly so (and may have been removed) and the leaf spots due to the deficiency will then appear on the next youngest leaves. Furthermore, the chlorotic and necrotic tissue resulting from potassium deficiency is often colonized by leaf spot pathogens acting as saprobes or opportunistic pathogens. Once these fungi become established on the chlorotic and necrotic tissue, they may sporulate (under the proper environmental conditions) and the spores may then spread to healthy palm leaf tissue.

Manganese deficiency: This deficiency causes interveinal necrosis, which may be confused with a leaf blight. Manganese deficiency occurs on the youngest leaves and is more severe at the leaf base than the leaf tip.

Additional Comments

One should not guess as to the identity of the pathogen based on the leaf spot symptom observed. Identification is based on observation of the pathogen spores, either directly on the leaf tissue (Fig. 6) or in culture after isolating from the leaf tissue.

Leaf spot diseases that have blackened spore structures combined with blackened host tissue on the leaf blade are collectively referred to as "tar spot" diseases. They are very distinct from the leaf spots and leaf blights described in this fact sheet. With tar spot diseases, the pathogen is always an integral part of the "spot". However, the initial symptoms are usually small, water-soaked lesions - i.e., same as observed with the other leaf spot diseases described.

The fungus Pestalotiopsis can cause a leaf spot of Phoenix roebelenii that also develops into a rachis blight and crown rot.

December 1, 2010