Discovering white mold on your palm can be alarming. Palms and palm-like trees span many diverse plant species that aren't bothered by mildew or mold diseases common in nontropical trees. Closer examination of the apparent fungus reveals something more disconcerting. Two non-native insects have invaded the United States, leaving trails that only look like mold. Treatment varies depending on the extent of your infestation.
A tiny insect known as cycad or aulacaspis scale (Aulacaspis yasumatsui) wreaks havoc on King sago palms (Cycas revoluta) and related plants. Hardy from U.S. Department of Agriculture plant hardiness zones 8b through 11, King sagos are palms in name only. Cycad scale does not affect true palms, but it will kill sagos if left untreated. By the time you see snowy infestations, large colonies of scale are present. It infests undersides of fronds first, then moves to the upper surfaces. Not content, it attacks the stems and goes more than 1 foot down into soil. The white moldy substance is waxy, white, female scales. Infestations amass thousands of insects per inch.
Rugose Spiraling Whitefly
True palms such as the Coconut palm (Cocos nucifera), hardy in USDA zones 10b through 11, battle an insect called the rugose spiraling whitefly (Aleurodicus rugioperculatus). A white, waxy substance left in distinctive spiraling patterns covers the leaf undersides of diverse trees and shrubs affected by this pest. The insects feed on tree juices and then excrete honeydew on the plant. The sugary liquid attracts sooty black mold that accumulates on the leaves and drips onto anything beneath. Rugose spiraling whitefly doesn't kill plants, but can leave them stressed. Infested plants are flocked in white mold-like patterns and sooty mold.
Manual and Biological Controls
The first course of treatment for cycad scale and rugose spiraling whitefly is to inspect susceptible trees regularly. Prune heavily infested fronds or leaves from infected plants using sharp pruning implements. Sterilize the blades before and after each cut to avoid spreading the invasive pests. Seal cut fronds in a plastic bag and dispose of them. Spray remaining fronds with strong blasts of water to remove dead scales, honeydew and sooty mold. Both these pests are non-native species inadvertently introduced to the United States. No commercially available natural predators or parasites exist, so releasing beneficial insects is not an option. Keep trees as healthy as possible to reduce stress and susceptibility.
If infestations increase, you can treat both these insects with ultra-fine horticultural oil. Mix up to 3 tablespoons of paraffin-based horticultural oil concentrate with 1 gallon of water in a sprayer, or follow label instructions on your product. Reserve the highest rate for severe or rapidly increasing infestations. Spray to cover foliage and stems completely on both sides; the oil must contact the pests. Repeat this weekly for five weeks, or the recommended label rate, to kill insects present on the fronds and those migrating from trunks and roots. Chemicals don't discriminate between harmful invasive insects and beneficial populations that may combat these pests. Use chemicals as a last resort so beneficial populations aren't harmed.