Leaf Spots and Leaf Blights
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Figure 2. Leaf spot exhibitng brown or black centers with yellow halos. Spots are randomly scattered on the leaf tissue. Photo by T. K. Broschat.
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Figure 1. The initial lesions of this leaf spot disease are pin point water-soaked appearing spots. As the spot expands, the center becomes gray with water-soaked edges As the lesions continue to expand, a yellow halo may be observed. Coalescing of expanding lesions are observed until large areas of blighted tissue result. Photo by M. L. Elliott.
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Figure 4. A leaf affected by both Graphiola leaf spot and Stigmina leaf spot. Signs of Graphiola phoenicis are the small back bodies (sori), many with filaments emerging from the sori. Stigmina palmivora symptoms are the large brown spots with dark edges and darker but flat centers. In some cases, a G. phoenicis sorus is superimposed upon the S. palmivora leaf spot. Photo by M. L. Elliott.
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Figure 3. Leaf spot that begins as a black spot with distinct yellow halo. As the lesion expands or lesions coalesce, the affected area becomes a leaf blight and the center becomes a gray color. Photo by T. K. Broschat.
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Figure 6. A leaf spot symptomatic leaf segment of the leaf in Figure 1 was placed under high humidity. The result was growth of the fungus from some of the spots. Note fungal growth is only associated with the leaf spot symptom and not with the healthy green leaf tissue. Photo by M. L. Elliott.
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Figure 5. Spots are initially black with no halos. As the spot expands, the center becomes tan and is outlined by a brown to brownish-black edge, but no yellow halo. The leaflet in the lower-left corner has large areas of necrotic tissue. The leaf spot is superimposed on iron deficiency symptoms. Photo by M. L. Elliott.
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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 baseleaf base:
the basal portion of a leaf petiole that is attached to the stem
. However, if the deficiency is severe, the oldest leaves may be necroticnecrotic:
see necrosis
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 chloroticchlorotic:
see chlorosis
and necroticnecrotic:
see necrosis
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 chloroticchlorotic:
see chlorosis
and necroticnecrotic:
see necrosis
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 necrosisnecrosis:
dead tissue, usually desiccated and brown to gray in color
, which may be confused with a leaf blight. Manganese deficiency occurs on the youngest leaves and is more severe at the leaf baseleaf base:
the basal portion of a leaf petiole that is attached to the stem
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 bladeleaf blade:
the broad, flattened distal portion of a leaf
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 rachisrachis:
the axis of a leaf beyond the petiole or an extension of the petiole into the leaf blade
blight and crowncrown:
see canopy
rot.
