Fruit Tree Insects & Diseases

Insects and diseases can damage trees and decrease yields.

Insects and diseases can damage trees and decrease yields.

From apples and apricots to olives and plums, hungry insects and malevolent fungi and viruses target your fruit trees and their produce. Hundreds of insects and dozens of diseases may find your home orchard full of tasty treats and comfortable residences. Some of these visiting critters and pathogens are just nuisances, some can damage trees and fruit, and some pose serious risks because there are no effective management tools for them.


Aphids, mealy bugs, whiteflies and many hard scales are nuisance insects that may disfigure or discolor leaves and branches, but seldom do real damage unless they descend in a plague. Many are controlled using natural enemies such as lady beetles, green lacewings, tachinid flies and parasitic wasps. Nuisance pathogens such as powdery mildew (Podosphaera spp. and Sphaerotheca spp.), which results from cool, moist weather, can be controlled with fungicides, such as copper sulfate, neem or jojoba oil, and good cultural practices, such as careful watering, pruning to improve air circulation and the use of slow-release fertilizer.


Some insects pose serious threats to fruit trees by killing young branch and leaf growth. Armored or hard scales include San Jose scale (Quadraspidiotus perniciosus) feeding on stone fruits such as apricots and plums and California red scale (Aonidiella aurantii) feeding on apples, peaches and olives. Soft scales, such as calico scale (Eulecanium cerasorum) and Kuno scale (Eulecanium kunoense) feed on stone fruits, including cherries, while brown soft scale (Coccus hesperidum) and black scale (Saissetia oleae) feed on citrus, causing more damage due to their mobility. Scale infestations require timely pruning and targeted insecticides.

Borers and More

Borers and some other insects use fruit as a nursery for their young, laying eggs inside the fruit, giving hungry young larvae a ready source of food. Borers include American plum borer (Euzophera semifuneralis), Peachtree borer (Synanthedon exitiosa) and Shothole borer (Scolytus rugulosus) feeding on stone fruits, while Shothole borer (Scolytus rugulosus) and Pacific flatheaded borer (Chrysobothris malis) also feed on apples and pears. Other insect pests include carpenterworm (Prionoxystus robiniae), spotted wing drosophila (Drosophila suzukii) and olive fruit fly (Bactrocera oleae). Borers and other fruit eaters can ruin part or much of a crop and must be caught early in the home orchard with targeted insecticides.


Mosaic viruses target many fruits, spreading necrotic, tough patches on green wood and fruit surfaces. Although apples are particularly susceptible to mosaic virus, its effect is to reduce the harvest rather than kill the tree. Fire blight, caused by the bacterium Erwinia amylovora, is another more destructive pathogen which attacks apples and peaches. Apple and pear scabs, caused by Venturia inaequalis and V. pirina fungi are most severe in damp, coastal areas. Sphaerotheca species fungi attack stone fruits. Pathogens are managed by preventive fungicides, good cultural practices, prompt removal of fallen leaves and use of resistant varieties. Mosaic-infected trees cannot be cured.

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