Sometimes similar-looking diseases can be confusing to the gardener or landscaper. A recent call reminded me of this fact. The call concerned a hawthorn tree that had blackened galls along stems and at the tips. The caller thought this was black knot. When the sample was sent to the Plant Clinic, we realized the actual disease was cedar–quince rust. Both diseases are caused by fungal pathogens. Black galls or knot on the stems are associated with both diseases.
Black knot galls (knots) are initially slight swellings on the stems. They are more visible the second year after infection, when they become hard, brittle, and coal black. These elongated, rough, black swellings may occur on twigs, branches, and sometimes the trunk. If growth of the fungus is on one side of the stem, the stem may be bent at the knot. If the knot girdles the stem, the stem beyond dies. Galls continue to spread in the branch and will be larger next year if they remain on the tree. Black knot galls may grow to a foot or longer. The disease does not typically kill a tree but causes deformed growth if left unchecked. About 24 species of Prunus are susceptible to this disease. Hosts include ornamental plums and cherries but not hawthorn. The fungus form spores on 1-year-or older knots, giving them a velvety green appearance. Removing knots controls the disease.
Cedar–quince rust was described in issues no. 1 and 9 of this newsletter. Cedar–quince rust is damaging to hawthorn because it affects fruit, stems, and petioles. The galls that form on hawthorn stems give the stem a roughened, swollen appearance that is much more obvious when the orange masses of spores are visible. By mid- to late summer, the galls are black and roughened like black knot, but still not more than twice the normal stem diameter. They do not grow as large as black knot galls. The pathogen girdles twigs, causing tip blight on hawthorn. If you look closely at old cedar–quince galls, you will see some small, cuplike fungal structures on the galls. They are white.
Images of the two diseases follow. White fruiting structures of the cedar–quince rust fungus are visible on the hawthorn galls. Black knot galls are much larger but do not have any fungal structures present.
Primary diagnostic differences between these diseases are the host, gall size, and type of fruiting structures. Black knot is found on Prunus species (ornamental or edible plums and cherries), cedar–quince rust on hawthorn, quince, and sometimes apple. Black knot galls are much larger, with green to brown sporulation on black. Fungal structures on cedar–quince rust are white.
For more information, refer to Report on Plant Disease (RPD), no. 802, “Rust Diseases of Apple, Crabapple, and Hawthorn,” or RPD, no. 809, “Black Knot of Plums and Cherries,” available on the Web at http://www.ag.uiuc.edu/~vista/horticul.htm.