As autumn brings a chill to the air and the leaves change their colours, a woodland walk can be a pleasant way to spend a sunny afternoon. But have you ever looked closely at those leaves and twigs beneath your feet? On closer inspection you may notice some have strange spots or bumps. These abnormal outgrowths are collectively known as plant galls - fascinating but often overlooked natural phenomena. There are a variety of organisms that cause plant galls including fungi, bacteria, viruses and insects. Many hundreds of gall-causing species are found in Britain.
Plant galls are prime examples of plant symbioses (organisms living together) and parasitism (one organism living off another). A gall forms on the attachment or invasion of the gall-causing species on the host plant. It causes enlargement or proliferation of the plants normal tissues in response to either the mechanical damage or chemicals secreted by the gall-causing species. These outgrowths can then function as shelter for the causative organism by providing a microclimate suited to its needs - the right growth or breeding conditions for example. The gall may also provide a nutritional role.
Whilst there are clear benefits to the gall-causing organism, the advantages or disadvantages to the host plant are often less clear. The vast majority of galls cause little or no harm to the plant. Some fungal species, if they infest one plant too heavily, can cause the necrosis of some of its parts and weakening it or even killing the entire plant. For horticulturalists, plant galls can be a pain - spoiling the look and texture of prized flowers, fruit or leaves. But its not all bad news - a few of these plant/gall-causer interactions are some of the most important symbioses found in nature. Rhizobium species of bacteria can form galls on the roots of legume plants. They are able to fix atmospheric nitrogen (normally unavailable to plants) for the plants. In exchange they get food (sugary substances) and shelter, thus allowing such plants to grow in relatively nutrient-poor soils.
Identifying gall-causing organisms in their own right can be a difficult task. Many are microscopic, and even insect larvae can be tiny and transparent - not to mention the vast number of species found in the UK alone. One of the easiest ways to identify them is to look at the gall structure they have caused the plant to form. There are a variety of illustrated keys available to aid you. These range from simple fold-out charts of common species that can be easily slipped into a backpack whilst out on a walk, to heftier comprehensive keys. Plant galls can be found pretty much anywhere plants grow - so have a scan around your garden or local woodland and look at those weird little growths at little closer!
