Plant Health

1. Maple bladder gall caused by Eriophyid mite. 2. Cypress twig gall caused by the midge fly. 3. Oak acorn gall caused by a gall wasp. 4. Mature horned oak gall caused by a wasp of the same name.

Plant galls are abnormal growths caused by interactions with certain insects, mites, fungi, bacteria or nematodes. Galls may occur on leaves, stems, roots, bark, buds and flowers. The multitude of organisms that cause galls makes plant diagnostics difficult. Horned oak gall is caused by a wasp, maple bladder gall is caused by a mite, cedar apple rust gall is caused by a fungus, crown gall is caused by a bacterium and root knot is caused by a nematode. Galls are ever fascinating.

Plant galls have been studied since ancient antiquity. The Greek naturalist Theophrastus (372-286 B.C.) wrote about gall-nuts of Syria. The Roman naturalist Pliny the Elder (23-79 A.D.) wrote about medical uses of gall extracts. In fact, much of our history was literally written in gall ink; a mixture of iron and gallic acid (3,4,5-trihydroxybenzoic acid) extracted from oak galls which are induced by insects. Leonardo da Vinci wrote his notes with it, J.S. Bach composed with it, Rembrandt and Van Gogh drew with it, and the Magna Carta, U.S. Bill of Rights and first drafts of the U.S. Constitution were written in gall ink.

For green industry diagnosticians the bad news is that the infinite variety of galls means you will never learn them all. The good news is — like everything else relating to horticulture and nature — you will never be bored. As Shakespeare penned: “In Nature’s infinite book of secrecy/A little I can read.” Because what caused the gall is often already departed or microscopic in size, the trick for gall identification is to match what you see with common problems and occurrences on a particular plant.

Gall hosts
It is important to note that galls caused by gall-making organisms are typically highly specific. Each gall-maker has a particular plant or set of plants it affects. For example, the horned oak gall wasp only causes its woody galls on pin oak and several other species in the red oak group. The maple bladder gall mite causes its roundish red galls on silver and red maple leaves. The fungal gall termed plum black knot only occurs on plants such as plum in the genus Prunus. In some cases, the host ranges for gall-causing organisms are more diverse, though. The crown gall bacterium causes galls on more than 600 species of plants in 90 plant families. Additionally, some gall makers, such as the spiny witch-hazel aphid and the cedar rust fungi have very specific life stories that cycle between two different types of host plants.

The overall number and variety of galls is staggering. For example, there are more than 800 insect galls on the different species of oaks alone. To name just a few: oak apple galls with their round pulpy orbs the size and color of small green apples; jumping oak gall which when the gall falls to the ground indeed jump a few centimeters due to agitation of the surprised wasp larvae inside; oak spangle galls with their pink flying-saucer like discs on oak leaves; the equally bizarre spined turban gall; two woody stem galls; the horned oak gall; and the gouty oak gall. Other plants are not quite such gall magnets, but there are literally tens of thousands of plant galls in nature’s wonderland.

Types of galls
Gall formation due to insect and mite gall-makers are stimulated by egg-laying and feeding activities by the gall-maker which result in specific reactions involving the plant’s growth hormones and growth regulating hormones produced by the insects and mites. Each case is a bit different, but the end result is that abnormal plant tissue develops which then serves as a home for the developing eggs and other stages of the gall-inciting insect or mite.

Galls caused by fungi typically are a combination of plant cells and microscopic threads of the fungus infecting the plant. Examples include the mouse gray mushroom-like flower and leaf galls on rhododendrons and their relatives to the spectacular cedar apple galls on junipers, from the warty black growths typical of black knot of plum to the Phomopsis galls on forsythia.

Cedar rusts are one of the most common fungal galls. Cedar-apple rust galls developing on junipers begin as small wart shaped greenish-brown kidney-shaped galls, eventually becoming somewhat brownish and ranging in size from pea-sized almost to malted milk ball proportions. In spring the fungus in the gall sporulates, producing spectacular bright orange gelatinous spore horns that obscure the underlying gall. Microscopic spores from these spore horns infect plants in the rose family such as apple and hawthorn in spring, which then produce spore masses that cycle spores back to the junipers in summer and fall. Fungicides preventing infection of the rosaceous host are controls used by orchardists and rarely in landscape situations.

Galls caused by bacteria are less common than insect or fungal galls but probably the most famous gall disease of all is bacterial, namely crown gall. The bacterium Agrobacterium tumefaciens enters plant roots and root crown cells through tiny wounds. It then exchanges bacterial DNA with plant cells, becoming integrated into chromosomes in the plant cell’s nucleus, and induces production of plant growth hormones that result in galls or tumors.

The galls develop on roots and crowns, and on some plants, migration of bacteria in the vascular system upward results in galls on upper stems. Crown gall is historically important not only because it is a real economic problem on many fruit crops such as apples and stone fruits, on ornamentals such as rose and euonymus, but also because the method of insertion of bacteria DNA into host plant chromosomes is exploited in genetically modified organism biotechnology.

Hawthorn pod gall caused by the midge fly.

Most galls cause little plant damage in terms of overall plant health. Maple spindle galls and maple bladder galls are common samples brought in to any garden store, botanical garden or university clinic or extension office, but they do not hurt the plants and require no controls as far as plant health is concerned. In fact they often provide the diagnostic response: “Your maple will be fine, sir.”

Some galls are true plant problems, including a few oak galls. Not the leaf galls, but two that occur on stems, horned oak gall and gouty oak gall, sometimes develop so abundantly that they interfere with vascular movement in the stem and contribute to plant decline. Bacterial crown gall is certainly a bad actor for stone fruit orchardists and for rosarians, debilitating plants, predisposing them to other pathogen infection and pest infestations and making them more susceptible to drought and winter injury. Cedar rusts once resulted in cedar eradication laws in the Carolinas. Campaigns of “Cedar or Cider” clearly illustrated the apple growers’ concerns. Root knot nematodes debilitate many commercial plantings, especially in southern states.

Gall control
As for control of gall inducers, each case is different, with removal (black knot), exclusion (carefully inspect plants), rotation, soil replacement (crown gall) and sanitation common practices. Preventive fungicides and insecticides are sometimes in order for professionals who sell or display plants, though often life cycles of the pest make it difficult to effectively target controls. It is also important to note that the importance of galls is in the eye of the beholder. A little cedar apple rust on a backyard crabapple may be a conversation piece and not be a plant killer, but for your orchardist neighbor rust lesions on fruits will surely kill sales.

If not viewed with a sense of wonder and fascination, at least insect and mite gall-makers should garner begrudging respect. So far, no human has managed to duplicate the work so handily done be a group of organisms that are often viewed with disdain. Imagine the tree secrets that would be unlocked if we could.

Jim Chatfield is assistant professor and extension specialist, Department of Horticulture & Crop Science/Department of Plant Pathology, Ohio State University. Joe Boggs is assistant professor and extension educator, Hamilton County and Department of Entomology, Ohio State University.