Despite the name, diamond scale is not an insect pest. Instead the fungus Phaeochoropsis neowashingtoniae (previously called Sphaerodothis neowashingtoniae) causes this common foliar disease, which derives its name from its characteristic diamond-shaped fruiting bodies.

Diamond scale attacks primarily the California fan palm (Washingtonia filifera) in coastal regions and the intermediate and interior valleys of California subject to marine influence; it rarely occurs in arid regions such as the Central Valley or the deserts of Southern California. Diamond scale can occur on hybrids of the Mexican fan palm (Washingtonia robusta), and the incidence and severity usually are proportionate to the amount of California fan palm in the hybrid. Diamond scale has not been observed on pure Mexican fan palm or any other species in California.

Initial infection sites are dark, water-soaked spots the size of a pinprick that eventually turn black and grow to shiny, diamond-shaped fruiting bodies 3/25 to 3/10 inch long by 3/50 to 3/25 inch wide. The fruiting bodies occur on the upper and lower surfaces of leaf blades and petioles, the stalk connecting the leaf base to the blade.

Diamond scale causes leaves to yellow and brown, then die prematurely, resulting in a reduced crown of leaves and an unattractive landscape subject. Older or lower leaves typically become the most infected, because the longer the leaf remains exposed, the greater the number of infections. Because of their more vigorous growth rate, young palms tend to have less disease and a fuller crown of leaves than older, less vigorously growing plants.

Disease severity often is cyclical. The dry, warm seasons of summer and fall favor California fan palm rather than diamond scale, so palms tend to grow quickly, producing leaves faster than the pathogen can colonize them. In contrast, the moist, cool seasons of winter and spring favor the pathogen over the host, so palms tend to grow more slowly, and the disease advances higher into the crown, resulting in a less-than-full set of leaves.

Heavily infected leaves have a black, sooty dust that rubs off easily when you brush against or handle them during removal, making the plant a nuisance to work around. Although not particularly lethal by itself, diamond scale reduces vigor and stresses the palm, leaving it vulnerable to other diseases such as pink rot.