|Diseases Affecting Bonsai: Cedar Rusts|
by Nina Shishkoff, Ph.D. and Andy Walsh, M.S.
On the whole, bonsai are probably the healthiest trees on Earth. In experienced hands, they are well cared for, provided with appropriate environmental conditions, and supplied with adequate amounts of macro-nutrients (NPK) and micro-nutrients. While such good horticultural practices can avoid most pests and diseases, there are some diseases that can still strike even the healthiest of bonsai.
One of these is a trio of fungal diseases, called "Cedar Rusts", that affects Junipers and members of the Rose family. These diseases can seriously damage or disfigure a bonsai [ed: the diseases rarely kill the tree]. With an understanding of disease epidemiology and some precautions these diseases can be avoided. This trio of diseases are known as Cedar-Apple Rust, Cedar-Quince Rust and Cedar-Hawthorn Rust. They are caused by different species of the fungus genus Gymnosporangium. What is peculiar about all three is that the fungus lives alternately in Junipers and then in Rose family members. The three diseases are similar so only an explanation of Cedar-Apple Rust will be given. The differences between the three types can be seen in Table 1.
Life Cycle: The fungus overwinters in swollen "galls" on juniper branches (Figure 1). In the spring, especially after a rainfall, large yellow-orange gelatinous "sporehorns" (Figure 2) emerge from these "galls" and spores (teliospores) are produced inside them. These spores will not infect junipers; they will only infect some plants in the rose family, particularly apples, crabapples, and hawthorn. When it rains, spores are spread by the splashing of raindrops. As the "sporehorns" dry, the spores are forcibly discharged into the air and can be carried by wind to nearby apple leaves, fruits, and twigs. About 30 days after apples have bloomed, the "sporehorns" have discharged all their spores and most apple tissue is no longer susceptible. Within five or six hours after landing on the leaf, the spores become attached to the surface, germinate, and penetrate the upper leaf surface. After 1 to 2 weeks, yellow spots develop on the upper leaf surface. Several weeks later, numerous pustules form on the under-surface of the leaf (Figure 4). These pustules produce a different type of spore, which can't infect rosaceous plants, but can infect junipers. These spores are carried by wind. When they contact a Juniper twig, they become firmly attached and germinate in the warm moist weather of late summer or early fall and penetrate the needle. A pea-size, greenish-brown "gall" develops which is sometimes called a "Cedar Apple" (Figure 1 again). The "gall" enlarges the following year, but does not produce "sporehorns" until the second spring. After the second spring the life cycle repeats itself. The complete disease cycle requires almost two years. An example of this alternating life cycle for Cedar-Apple Rust is depicted in Figure 5 (artwork by Dr. Nina Shishkoff).
Treatment: Once the fungus has entered your collection a vigilant eye is necessary. Keep an eye out for "galls" on Junipers and the developing yellow spots on the leaves Crabapple, Hawthorn, etc.. On Hawthorn, the twigs can become infected (Figure 6) and these will eventually die (Fig 7) When there are only a few plants involved, such as a bonsai collection, it is easy to simply remove galls from junipers and pick the infected leaves off the Crabapples and destroy them. This is easier to do with Cedar Apple Rust and Cedar Hawthorn Rust, since the "galls" are more conspicuous than with Cedar Quince Rust. The "galls" may be picked off or the infected branch pruned. This is practical if a few plants are infected and the number of galls per plant is limited. Try to remove infected leaves before pustules on the underside develop to prevent them from releasing spores. Fungicides are not recommended for treatment. But fungicides can be effective for prevention and control.
1. Grow only resistant or immune apples, crabapples, and
First, avoid the problem. When buying any tree, ask the nursery about its rust resistance. Check the types listed in Table 2. If it's listed as resistant, then it's a better bet. This should be your first step in reducing the possibility of running into these problems. This spring I found Cedar-Quince Rust on all my San Jose Juniper bonsai. None of my other Junipers, including Shimpaku and Rocky Mountain, showed any signs of it. San Jose Junipers are not resistant. If you create bonsai from trees collected from the wild or from old gardens you may run into problems. It also may a good idea to choose whether you grow Junipers or Crabapples, Hawthorns, Quinces etc. This may mean limiting your collection to one type of tree. However, by choosing only resistant to immune varieties, you may be able to enjoy the best of both worlds.
Even though your bonsai collection may be comprised of resistant varieties your backyard may be full of susceptible plants. This is especially true with older homes with older gardens. It is important to understand that the fungus must alternate between a Juniper and a Rose member. It cannot go from Juniper to Juniper or a Rose member to a Rose member. If you grow only Juniper bonsai but your yard if full of old Rose bushes you may still have problems. Or if you grow only Crabapples bonsai but your yard if full of old Eastern Red Cedars you may still have problems too. If you can keep them apart you may have an easier time controlling these diseases. If possible, destroy nearby, worthless or wild junipers infected with rust galls. This may not be practical as the spores carry great distances and your neighbor may like his ratty old Junipers.
Fungicidal spays are highly effective if applied correctly. However, apple leaves are susceptible when they are young and rapidly expanding, so protective fungicides will need to be applied frequently to cover all the tissue. Remember, if you see the "galls", "sporehorns" or the yellow spots on leaves, it is too late to apply fungicides. The fungus is already there. Remove the infected leaves and twigs then consult your local horticultural agent and find out what you should apply for the following year. Some commonly recommended fungicides include: Mancozeb (Fore, Dithane, Mancozeb); Chlorothalonil (Daconil); Triadimefon (Bayleton, Strike) and propiconazole (Banner). Fungicides such as Bayleton and Rubigan EC will protect Crabapple leaves from becoming infected. It is the user's responsibility to follow all label instructions.
Hopefully this article will help you to spot these diseases before they can do much damage. If you think you may have one of these diseases on your bonsai, or if you wish to read up more, check the Internet references listed below for further information.
(The authors would like to extend special thanks to George Laur, Publications Coordinator at the University of Missouri-Columbia for permission to use material from their website).
Moore-Landecker, Elizabeth, "Fundamentals of the Fungi", Prentice-Hall 1972.
Ohio State University:
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University of Nebraska
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University of Missouri:
West Virginia University: