Why is my peach tree covered in brown scales?

Hard or waxy brown bumps encrusting the bark of a peach or nectarine tree are scale insects — sedentary, sap-sucking pests that protect themselves under a tough shield and can be easy to overlook until a severe infestation has already established. Several species affect peach in temperate gardens, including San Jose scale and mussel scale.

Identifying scale insects on peach

Scale insects appear as flat or slightly domed brown or grey-brown shells, 1–4 mm across, firmly attached to bark, branches, and sometimes young shoots. Unlike woolly aphid, there is no visible soft body — the scale is a hardened protective covering. In heavy infestations entire branches may be encrusted, giving the bark a rough, scurfy appearance. Scraping a scale with a fingernail reveals a yellowish or pinkish soft body underneath. Honeydew excreted by large colonies causes sticky surfaces and black sooty mould.

Winter dormant oil spray

The most effective control is a winter wash of plant-based horticultural oil applied to dormant wood during January and February. The oil penetrates under the scale and smothers the overwintering females and eggs. Coat every part of the bark thoroughly — trunk, main branches, and smaller shoots — with a pressure sprayer. Two applications, ten days apart, are more effective than one. This also helps control woolly aphid and mite eggs at the same time.

Targeting crawlers in June

In early summer, young scale insects ("crawlers") hatch and move briefly across the bark before settling and producing their protective shield. This is the only stage at which a contact spray is truly effective. Apply an insecticidal soap spray in early June and again two weeks later. Time the application carefully — crawlers are active for only two to three weeks and can be difficult to see without a hand lens.

Physical scrubbing of light infestations

On lightly infested trees or newly introduced plants, scrubbing the bark with a stiff brush dipped in diluted insecticidal soap will physically remove scales. This works well for isolated patches on young shoots. Follow up with a dormant oil application the following winter to clear any missed individuals and reduce the breeding population before the next generation hatches.

Keeping trees vigorous

Scale insects are opportunist pests that tend to build up on stressed or weakly growing trees. A peach tree that receives appropriate nutrition, adequate water, and proper summer pruning to maintain an open, healthy canopy is far less susceptible than a neglected specimen. Encouraging natural predators — particularly parasitic wasps — by avoiding broad-spectrum insecticides also helps keep scale populations in check.