Diamond Scale
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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
Phaeochoropsis neowashingtoniae: Kingdom Fungi, Phylum Ascomycota
Hosts
Washingtonia filifera, Washingtonia robusta
Washingtonia filifera is more susceptible than W. robusta, and hybrids of these two palm species appear to be intermediate in susceptibility.
Distribution
Mexico (northern), USA (Arizona, California, Nevada)
Symptoms/signs
The initial symptom will appear as small, water-soaked lesions on the leaf bladeleaf blade:
the broad, flattened distal portion of a leaf
of the oldest leaves. The fungus quickly forms stromata (singular = stroma) that are black in color and distinctly diamond-shaped (paralleling the leaf veins) (Fig. 1). A stroma is a compact mass of mycelium and host tissue in which the sexual fruiting bodies are embedded. Stromata can be as long as 3/16 inch (about 1 centimeter). They form on both sides of the leaf bladeleaf blade:
the broad, flattened distal portion of a leaf
and on the leaf petiolepetiole:
leaf stem
and may number in the thousands on a single leaf. While the disease begins on the oldest leaves, the sexual spores produced within the stroma move the disease upwards through the canopycanopy:
the cluster of leaves borne at the tip of the stem
to younger leaves. Eventually, leaves turn chloroticchlorotic:
see chlorosis
and senesce prematurely. Extensive loss of leaves can occur.
May be confused with
Initial symptoms on the leaf tissue will look like any other leaf spot disease. Development of the stroma will separate this disease from other leaf spot diseases.
Additional comments
Diamond scale is included within a group of diseases referred to as "tar spot" diseases. Diamond scale is separated out from the other tar spot diseases because it has a restricted geographic range and only two palm hosts.
