Common causes of plant galls
||Plant parts affected
||Leafy, green, parasitic plant causes bark and wood to swell where mistletoe attaches to its host.
||basal stem, root crown; uncommon on more aerial parts
||Surface of swelling same color and as firm as surrounding, ungalled tissue. On woody hosts, when gall is cut with a knife, internal tissue is softer than normal wood and lacks the typical pattern of annual growth rings.
||Roots may appear fuzzy or excessively forked or swollen, so they may appear to be diseased. However, there is no root decay associated with mycorrhizae. Roots must be specially stained and examined under a microscope to confirm that these beneficial fungi are present.
|eriophyid or tarsonemid mites
||buds, stems, foliage
||Tiny mites, cast skins, or excrement occur in and around distorted tissue. A microscope is needed to clearly discern these mites.
|fasciation, bacterial or other causes
||crown, stems, foliage, buds
||Plant often becomes extensively distorted, and secondary decay may be present. Bacterial fasciation often develops under wet conditions.
|fungal leaf galls
||Fungi, including Exobasidium and Taphrina spp., hosts include azalea, California buckeye, certain stone fruits, and oaks.
|insects that bore, mine, or chew inside plant tissue
||roots, stems, foliage
||Cast skins, emergence holes, frass, tunnels, or the insects themselves (often maggotlike) may be observed in and around galls. Common groups are larvae of beetles, cynipid wasps, gall midges, moths, and sawflies. Certain insects are secondary, attracted to galls originally formed due to other causes.
|nitrogen-fixing beneficial bacteria
||Galls easily rub off of roots; a thumbnail can easily be pressed into galls. They occur only on plants in certain groups, especially legumes.
|root knot nematodes
||Surface of galls is as firm as surrounding ungalled tissue; swellings cannot be rubbed off; cutting into gall may reveal pinhead-sized, shiny, white female nematodes inside, which look like tiny, pear-shaped pearls that are visible if galls are inspected through a hand lens.
||buds, stems, terminals
||Aphids, thrips, true bugs, or other insects or their cast skins or excrement are present. Sucking insects commonly feed on new growth, which later distorts.
||roots, bark, foliage
||Whitish flocculent material around galls during certain times of the year; aphids or cast skins commonly visible.
Eulophid gall wasp damage to blue gum eucalyptus