Rust Diseases on Fruit Trees and Cedars
A mop of orange gelatinous tentacles found growing on a tree might seem extraterrestrial, but it is a symptom of a disease that (spoiler alert) originated on this planet. Rust diseases, caused by various species of the fungus Gymnosporangium, can be found in Florida and infect a variety of trees, including apple, quince, pear and hawthorn.
Some closely related rust diseases include cedar-apple rust (Gymnosporangium juniperivirginianae), cedar-hawthorn rust (G. globosum) and cedar-quince rust (G. clavipes). A unique characteristic of these pathogens is their reliance on two separate hosts to complete their life stages: one deciduous and one evergreen tree.
Growth Stages and Tree Symptoms
The evergreen host is usually an eastern red cedar or juniper, which is why the common names of the diseases start with “cedar.” When the rust pathogen first infects the evergreen tree, small, roundish galls will develop on the needles and twigs. However, for cedar-quince rust, infection mostly causes flaky, branch swellings rather than round galls.
Eventually, the growths on these evergreen trees will produce bright orange jelly-like protrusions, or “gelatinous telial horns.” These slightly creepy horns usually appear during a spring rain. For cedar-apple and cedar-quince rust, galls will die soon after rain subsides, often killing the twig they were growing on. But in cedar-hawthorn rust, the twigs seldom die. The telial galls will produce spores for several years, sometimes up to 20 with cedar-quince rust; however, they only occur for one year with cedar-apple rust.
From these growths, spores are released that will then infect a deciduous host; while most infections occur within several hundred feet of the other host tree, the spores can be carried several miles. Each rust disease differs in its deciduous tree of choice. Cedar-apple rust favors apple and crabapple, while cedar-hawthorn rust infects those fruit trees as well as pear, quince and serviceberry (Amelanchier arborea). As for cedar-quince rust, it grows on many genera of plants in the rose family.
On the deciduous host, cedar-apple rust and cedar-hawthorn rust mainly infect the leaves, which will appear yellow followed by orange with black spots (spermagonia). Cedar-hawthorn rust can sometimes infect the fruit, stems and thorns. Cedar-quince rust does not usually show any leaf-spot symptoms and will infect the thorns, new twigs and fruits.
One dominant feature of the rust diseases on the deciduous host is the development of cup-like structures with tubes (aecia) that grow on the infected plant parts. Cedar-quince rust can even form spindle-shaped cankers, or dead and sunken areas of the plant.
Management
Integrated pest management strategies are recommended to prevent the spread of rust diseases and infection of your plants. Action items include removing infected branches or even removing the entire evergreen host species if infection is serious. You can also plant rust disease-resistant species and avoid planting susceptible evergreens next to susceptible deciduous trees. Preventative fungicidal sprays can also be used.
While scary to behold, the rust diseases rarely kill the whole plant. However, it may affect your fruit production, and let’s face it, the energy of your garden might be in jeopardy from just looking at it. Understanding the life processes of these diseases and taking preventative care can save you a lot of trouble down the line.
For more tips on managing rust diseases, contact your local UF/IFAS Extension office.