Diamond Scale

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 blade 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 blade and on the leaf petiole 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 canopy to younger leaves. Eventually, leaves turn chlorotic 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.

Last updated May 2015

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