Palm Tree Diseases & Treatments

Diamond Scale

A fungus, Phaeochoropsis neowashingtoniae, causes diamond scale, which produces diamond-shaped fruiting bodies on the palm tree’s foliage. The California fan palm and hybrids of Mexican fan palm (Washingtonia robusta) are the primary hosts for the disease. Diamond scale is more prevalent in coastal areas and some inland valleys. Tiny, watery-looking dark spots form first, growing to diamond-shaped, black, shiny fruiting bodies on the upper and lower surfaces of the leaves and leafstalks. Leaves turn yellow, then brown. Consider replacing badly infected trees with resistant species. Keep existing trees healthy and give supplemental water as necessary. Improve drainage, if needed, and fertilize with a product formulated for palm trees. Keep other vegetation at least 2 feet away from the trunk.

Fusarium Wilt

Another fungus, Fusarium oxysporum f. sp. canariensis, can infect Canary Island date palms (Phoenix canariensis). Leaves in the lower part of the canopy behave much like those infected with diamond scale, turning brown and hanging onto the plant. The disease may affect only one side of the leaves. Leafstalks may become discolored and brown to black streaking may be noticeable. The disease will kill the tree sooner or later. Fusarium wilt also stresses the palm tree, leaving it susceptible to a secondary infection, pink rot. The fungus can enter the palm tree through wounds, so disinfect pruning tools and avoid injuring the tree with lawn mowers, chainsaws or other tools. Fusarium wilt may also attack at the roots during periods of heavy rainfall or excessive watering. There is no cure.

Pink Rot

Although it can attack any part of the palm tree, pink rot is considered a secondary disease. Pink rot causes leaf spots, rot and stunted or deformed growth. Visible pink spore masses are a telltale symptom. Pink rot may also produce a brown, syrup-like ooze. Keep palms as healthy as possible to avoid this disease. Make sure the trees are not planted too deeply. Do not over-water or under-water, and use a fertilizer appropriate for the tree. Avoid injuring the palm with gardening equipment or pruning tools. Fungicides may help protect the tree after pruning. Follow label instructions carefully for proper application.

Other Types of Rot

Collar, foot, root and crown rots caused by species of the fungus Phytophthora can infect sago palms as well as standard palm trees. The fungus attacks the roots or the root crown of the plant, causing distorted, discolored leaves. The fronds fall prematurely and the entire plant may wilt. These infections may also cause cankers on the trunk and oozing sap. Improve drainage to keep the soil from staying wet too long. Make sure the tree is not planted too deeply. Avoid over-watering and water only at the base to avoid getting the trunk wet. If the tree has crown rot, try removing soil from around the root crown to let it dry out. Fungicides may help ward off infection or keep it from spreading. Consult with a cooperative extension or university horticulture department for information on fungicides appropriate to your area and the specific palm species infected.

Ganoderma Butt Rot

Queen palms infested with Ganoderma butt rot (Ganoderma zonatum) fungus have received their death sentence, although they seldom show more than a slow decline with stunted growth and discolored, wilting leaves until the disease reaches its final stages. This soil-borne, white-rot fungus destroys a palm’s lignins and cellulose, eventually disrupting the nutrient-transporting xylem tissue. It spreads from spores released by the conks — flat, mushroomlike growths — that sprout from a dying palm’s trunk. By the time the conks emerge, the lowest 4 or 5 feet of the trunk have already rotted from the inside. Some palms, however, never develop conks. The only way to diagnose Ganoderma accurately is to cut down a declining palm and examine the trunk.

Sudden Crown Drop

Canary Island and other types of date palms may fall victim to sudden crown drop, a mysterious disease that causes the entire crown to suddenly fall off the palm tree. As the crown can weigh several tons, this phenomenon can be dangerous to anyone near the tree when it happens. According to the University of California, Davis Integrated Pest Management website, the fungus Thielaviopsis paradoxa may be to blame. The disease rots the trunk on the inside, with no outward symptoms. Pounding on the trunk to listen for internal decay in the upper area can help uncover the problem before the crown falls. Keep trees healthy and keep pruning to a minimum, preferably without chainsaws, to reduce wounds where the pathogen may enter.