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Falling Leaves Reveal Unsightly and Mysterious Galls

Michelle Grabowski, University of Minnesota Extension Educator

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Maple tree infected with many Phomopsis galls
M. Grabowski

As leaves are lost this fall, some gardeners are noticing unsightly lumps and bumps on the bare branches of their trees. A variety of causes, both living and non living, can result in irregular tree growth. However if round rough balls of wood are clustered on the trunk, branches, and twigs of the tree this is likely phomopsis gall. Phomopsis gall is a fungal disease caused by several species of Phomopsis. In Minnesota, Phomopsis galls are most commonly seen on Hickory (Carya spp.) and Maple (Acer spp.) trees. This disease can also infect American elm (Ulmus americana), several species of oak (Quercus spp.), Viburnum spp., Forsythia sp., rhododendrons and azaleas (Rhododendron spp.).

Phomopsis galls are spherical wood balls growing on tree trunks or branches. Galls may be as small as a pea or up to 10 inches in diameter. In most trees, the galls look like a cluster of small nodules or bumps, all pushed together into one lump like a popcorn ball. In maple trees, however, galls often start as smooth round balls and become rough with age as the bark cracks. In all cases, the galls are made of hard disorganized wood that may be difficult to cut through. Examination of young growing gall tissue under a microscope reveals strands of fungal mycelia growing within the wood, but signs of the fungal pathogen are rarely visible to the naked eye.

It is common for a tree to have many galls scattered throughout the branches. Galls may occur individually on branches or be clustered together in small groups. Often one or a few trees in an area will be heavily infected with phomopsis galls and nearby neighbors of the same species will be completely healthy and gall free. Heavily infected trees may have slower growth than their uninfected neighbors. Small twigs that are girdled by one or more galls can be killed. Typically however trees with phomopsis galls are able to continue to grow despite their unusual appearance.

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Phomopsis galls on maple
M. Grabowski

Phomopsis gall is a mysterious disease because very little is known about its lifecycle. Spore producing structures of the fungus are almost never seen on galls. As a result, it is unknown how the fungus spreads from one tree to another and under what conditions infection occurs. No one knows why some trees are highly susceptible to this disease while a neighboring tree of the same species seems to be completely disease free. As a result of this gap in knowledge, few control strategies are available to manage Phomopsis galls. In most cases gardeners are encouraged to tolerate the galls; although heavily infected branches can be pruned off and disposed of. Reducing other stresses on infected trees by mulching the soil around the tree with woodchips or other organic matter, providing water during times of drought, and avoiding accidentally wounding the tree will help the tree to continue to grow at the best rate possible.

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