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Disease 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.

 

Table 1 -  Cedar Rust Differences
 

 Cedar-apple Rust

 Cedar-hawthorn Rust

 Cedar-quince Rust
Deciduous hosts  Apple/crabapple  Hawthorn, apple/crabapple,
sometimes pear, quince,
serviceberry
 Many of the genera in the
rose family. Includes
mountain ash, hawthorn,
flowering quince, and  serviceberry
 Affected deciduous plant
parts
 Mostly leaves  Mostly leaves occasionally
fruit, stems and thorns
 Mostly thorns, new twigs
and fruits. Sometimes
petioles and veins of leaves
 Deciduous Leaf spot
symptoms
 Start as greenish yellow
then yellow followed by
orange (sometimes with
black centers), there is
generally a reddish halo
between infected lesion and
healthy tissue
 Start as yellow spot
followed by orange
(sometimes with black
centers )
 Basically none except upon
close inspection of veins
and petioles that are
swollen and occasional
chlorotic spots on some
rosaceae hosts.
 Evergreen hosts  Mostly Eastern red cedar  Eastern red cedar; Rocky
Mountain, common and
prostrate junipers
 Eastern red cedar, common,
prostrate, Rocky Mountain,
and savin junipers
 Gall shape  Kidney shaped to round;
sites of telial horns
egularly arranged
 irregular-shaped; sites of
telial horns scattered
 Elongated swelling of the
twig; telia emerging from
cracks
 Telial horn appearance  Long and thin; membranous  Short and stubby  Orange gelatinous ooze
 Number of years telia are
 produced
 One year (spring following
gall development)
 May produce telia horns
for several growing seasons
or years
 One or more years (4-6 but
sometimes for 20)
 Damage to evergreen  Galls form on needles on
small twigs, foliage above
gall almost always dies
 Galls form on stems; twig
seldom dies, but is seriously
disfigured
 Small twigs often die; larger
branches continue to grow
but are seriously disfigured
 Distance between hosts
that spores can travel to
cause infection
 Usually within several
hundred feet but possible
for several miles
 Usually within several
hundred feet but possible
to 14 miles
 Not known?

 

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).



 
Cedar-apple  rust
 Figure 1

 
Cedar apple rust on San Jose
Figure 2

 
Cedar apple rust on crab apple leaf
Figure 3


Cedar apple rust - Aeciopores under leaf
 Figure 4

 

Cedar apple rust - life cycle
  Figure 5

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.


 
Hawthorn rust on new twig
Figure 6

 
Hawthorn rust on old dead twig
Figure 7

Prevention/Control:

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There are 3 main approaches to prevention and control. These are:

1. Grow only resistant or immune apples, crabapples, and junipers.
2. Keep Junipers and Rose members away from each other.
3. Apply protective fungicides.

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.


Table 2 - Cedar Rust Differences
Crabapple cultivars (Malus sp.) resistant to cedar-apple rust
• Adams
• Baccata 'Jackii1
• Beverly
• Bob White
• Candied Apple
• Dolgo
• Donald Wyman
• Floribunda
• Halliani 'Parkmanii'
• Indian Magic
• Indian Summer
• Liset
• Mary Potter
• Molten Lava
• Mt Arbor Special
• Ormistron Roy
• Pink Spires
• Prairie Fire
• Profusion
• Professor Sprenger
• Red Jade
• Red Jewel
• Robinson
• Royalty
• Sargentii
• Strawberry Parfait
• Sugar Tyme
• Tina (sargentii)
• White Angel
• Zumi 'Calocarpa1

 

Hawthorns resistant to rust

 
 • Crataegus crus-galli (Cockspur
   Thorn)
 • C. intricara
 • C. laevigata (Autumn Glory)
 • C. phaenopyrum (Washington
   Thorn)
 • C. pruinosa
 • C. viridis (Winter King)
   

 

Junipers resistant to cedar-apple rust

 
J. ashei (Ashe juniper)
J. chinensis (Chinese juniper)  
• Fortune!
 • Hetzii
 • Keteleeri
 • Pfitzeriana
 • Pfitzeriana compacta
 • Pfitzeriana glauca
 • Plumosa aurea
 • Pyramidalis
 • Sargentii
J. communis (common juniper)
• Aurea
 • Aureo-spica
 • Cracovia
 • Depressa
 • Hibernica
 • Oblonga pendula
 • Pyramidalis
 • Saxatilis
 • Saxatilis pallas
 • Suecia
 • Suecia nana
J. horizontalis (creeping juniper)
 • Admirabilis
 • Adpressa
 • Argenteus
 • Blue Rug
 • douglassi
 • Filcinus
 • Glomerata
 • Livida
 • Petraea
 • Plumosa
J. procumbens (= J. chinensis var. procumbens)
J. sabina (savin juniper)
 • Broadmoor
 • Fastigiata
 • Skandia

 

J. scopulorum (Rocky Mountain juniper)
 • Dewdrop
 • Medora
 • Moonglow
 • Pathfinder
 • Platinum
 • Silver Globe
 • Sky Rocket
 • Welchii
 • Wichita Blue
J. squamata
 • Blue Star
 • Meyeri
J. virginiana (eastern redcedar)
 • Globosa
 • Kosteri
 • Pseudocupressus
 • Pyramidalis
 • Tripartita
 • Venusta
 

 

(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).

References:

Moore-Landecker, Elizabeth, "Fundamentals of the Fungi", Prentice-Hall 1972.

Internet References:

Ohio State University:
http://ohioline.osu.edu/hyg-fact/3000/3024.html

University of Illinois:
http://www.urbanext.uiuc.edu/focus/cedarapplerust.html
http://www.urbanext.uiuc.edu/focus/cedarhawthornrust.html
http://www.urbanext.uiuc.edu/focus/cedarquincerust.html
http://www.urbanext.uiuc.edu/focus/rustdifferences.html

University of Nebraska
http://www.ianr.unl.edu/pubs/plantdisease/g1327.htm

University of Michigan:
http://www.msue.msu.edu/msue/imp/mod03/01701181.html

University of Missouri:
http://muextension.missouri.edu/xplor/agguides/pests/g07870.htm

West Virginia University:
http://www.caf.wvu.edu/kearneysville/disease_descriptions/omcar.html

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