Introduction to Plant Galls Talk by BRIAN SPOONER, 12th January 2005

Most of us are aware of Brian's passion for fungi through his many talks on the subject to the Society, and leadership of (mainly Autumn) fungal walks. However, another of his favourite study subjects is plant galls and, through the years, he has kept a collection of these 'structures' among the collections at the Royal Botanic Gardens Kew, mainly as a personal reference tool. He offered to talk about them and, although he apologised for the fact that he is not a professional in the field, his lecture and many slides kept us busy for nearly two hours. Below is a brief summary of what he told us:

Galls are structures formed by abnormal growth or multiplication of cells, mainly of plant tissues. The growth is a reaction to the attack of another organism (from a surprisingly wide range of groups) on or in the plant cells. The scientific name of the gall is that of the organism that causes it, although this is usually not the case when using common names. Galls are abundant and widespread, to be found in woods, hedgerows and meadows, and most of the best known ones are caused by insects such as gall wasps (Hymenoptera, Cynipidae). Oak apples (caused by Biorrhiza pallida) are a familiar example, being one of many galls of the buds of oaks, whereas the introduced but well-established Knopper gall (caused by Andricus quercus-calicis) develops on the acorn cupules. In fact we learned that oak trees make a particularly interesting subject for the study of galls, being host to more species than almost any other tree. Each gall usually consists of a single larva, but is commonly inhabited by larvae of other species that take advantage of the extra nutrients and growth. These inquilines should not be confused with the gall causer itself. Other gall-causing Hymenoptera include some of the sawflies (Tenthredinidae) exemplified by the Common bean gall, due to Pontania proxima, on willow leaves. Gall midges (Diptera, Cecidomyiidae) are also important gall-causing insects, with many species in Britain and causing galls, usually of the leaves, on a wide range of trees and herbs. Various other groups of Diptera, notably Tephritidae, are also gall causers, as are many aphids and other Homoptera. The Spiral petiole gall due to Pemphigus spirothecae, often common on Black poplars and hybrids, is a familiar example.

Another important group of gall-causing organisms are the gall mites (Acarina, Eriophyidae). Sometimes mite galls are not as easily recognized as galls because they tend to form leaf rolls, and various felty and hairy pustules where ants also move around and feed. These mites are also curious since they have only two pairs of legs. There are many species, though generally they are not well known and most require further study. Many nematodes, notoriously abundant in plant tissues, also cause plant galls. These are commonly found in the roots of herbs, including crop plants, but also sometimes in the leaves or flowers as in some grasses.

A wide range of fungi also induces plant galls, and among these the peach leaf curl due to Taphrina deformans is well known. This ascomycete causes bright red blisters and swellings on the leaves of some species of Prunus, and its effects on ornamental trees can be devastating. Other gall-causing fungi include many of the rusts and smuts (basidiomycetes) that can cause swelling, deformation and decoloration of leaf and stem tissues. Finally, two other groups of gall causers found in our borough should be mentioned, these being flowering plants, notably the parasitic mistletoe (Viscum album, family Viscaceae) which causes swelling of the host branches at the area of attachment, and bacteria. The latter may infect roots, shoots or other plant parts; sometimes gall formation is induced by the presence of plasmids on the bacteria. An example of this is the Crown gall found on many plants. Other galls are induced by the bacteria-like phytoplasmas, sometimes causing the inflorescence to become leafy as is quite commonly seen in some plantains (Plantago).

BEGOŅA AGUIRRE-HUDSON