Rust-infected Juniperus species (cedars and junipers) are revealing bright orange evidence of the rust fungi infecting them during moist periods this spring.  In Kentucky, we commonly see three different rust fungi on cedars and junipers, often on the same tree.  These rust diseases are cedar-apple rust, cedar-hawthorn rust, and cedar-quince rust.  All three rusts are caused by different species of the fungus Gymnosporangium, each of which must spend a phase of its life cycle as a parasite on Juniperus species such as native red cedars or ornamental junipers.  The other phase of the life of these rust fungi is spent on susceptible alternate hosts such as apple, crabapple, hawthorn, mountain ash, pear, and serviceberry.

Cedar apple rust.  The cedar-apple rust fungus (G. juniperi-virginianae) forms light brown to reddish or chocolate brown galls in the leaf axils of infected Juniperus species.  These galls are usually rounded and range from pea-sized to 2 inches in diameter.  As galls mature, the flesh becomes corky and the surface becomes pitted with circular depressions.  In spring, following rainy periods, slimy, yellow-orange tendrils, or “spore horns,” up to 2 inches long swell and protrude from these depressions (Figure 1, Figure 2).  A gall may produce many spore horns, which cause it to resemble orange-colored blossoms from a distance.  Severely rusted Juniperus can be very conspicuous.

Cedar-hawthorn rust.  Galls produced by cedar-hawthorn rust (G. globosum) are similar in appearance but are smaller and more irregular in shape and do not develop the regular arrangement of circular depressions. Spore horns, too, are shorter, generally fewer in number, and wedge- or club-shaped (Figure 3).  Because of its smaller size, cedar-hawthorn rust is less conspicuous than cedar-apple rust.

Cedar-quince rust.  Cedar-quince rust (G. clavipes) does not form rounded galls but instead forms perennial, spindle-shaped swellings on the twigs, on which a gelatinous, orange-brown mass of spores is borne in the spring (Figure 2 and Figure 4).

Disease cycle.  All three fungi have similar life cycles.  From dormant galls and swellings, orange spore “horns” or spore masses begin to appear in spring about the time that flowering crabapples are in bloom and apple buds are in the pink to early bloom stage.  These spore horns or spore masses are actually columns of fungal spores (teliospores), each of which can germinate under moist conditions to form four new spores (basidiospores).  Basidiospores are then carried to apple, crabapple, hawthorn and other susceptible trees by wind currents where they can germinate and cause infection during relatively short periods of wetness (about six or seven hours for moderate levels of infection when temperatures are in the 50s and 60s).  The springtime period of infection usually ends about 30 days after apples bloom when the fungus no longer produces basidiospores on cedars and the majority of alternate host leaf, shoot or fruit tissues have aged to the point where they are no longer susceptible.

Depending on the alternate host, spots may begin forming on the upper leaf surface or on shoot or fruit surfaces about 10 to 14 days after infection and spores (aeciospores) are produced in tiny “cups” called aecia on the infected tissues several weeks later.  Aeciospores produced in summer in the tubular aecia on diseased apple, crabapple, or hawthorn tissue are blown to cedar during the summer where they germinate during moist weather, infect, and cause small pea-sized galls to form on the twigs.  The fungus over-winters in these galls which continue to grow and enlarge during the following year. The fungus survives a second winter within the gall, then begins producing its long, orange spore horns the next spring, completing the disease cycle that began two years earlier.

Although cedar rusts can cause unsightly growths on Juniperus, they do not usually cause serious damage to these plants.  However rust diseases can cause serious fruit losses on apples and weaken and kill shoots of crabapples and hawthorns.  Infected fruits can drop prematurely or have a reduced commercial value if they remain on the tree through harvest.  Leaf infections often result in premature leaf loss, reducing the size and quality of the fruit crop, weakening the tree, and reducing bloom the following year.  Hawthorn and crabapple twigs infected by the cedar-quince fungus can become swollen and die.

Cedar rust disease management efforts are usually aimed at reducing damage to apples, crabapples, and hawthorns.

1. In the landscape and nursery, keep cedars and alternate hosts such as apple, crabapple, and hawthorn separate.  The closer these hosts grow to one another, the greater the risk of cedar rust diseases on both.
2. Grow resistant apples (crabapples, hawthorn, mountain ash) or junipers.  Resistant apple varieties were listed in Kentucky Pest News issue 999 (August 18, 2003).  Resistance may vary among localities, however, depending on the specific races of the rust species present in the area.
3. Destroy nearby wild, abandoned, or worthless apples, crabapples, hawthorns, cedars, or junipers. When practical, prune out and destroy cedar galls found on ornamental junipers and cedars.  Although apples may occasionally become infected by spores produced up to several miles away, most infections result from spores produced on infected Juniperus within a few hundred feet of the apple or hawthorn tree.
4. Follow a recommended fungicide control program. Early protection of apples, crabapples, and hawthorns (beginning at the pink-bud stage) is especially important for control, as most infections occur within the first 30 days after bloom.  For specific spray recommendations, commercial apple growers should consult UK Extension publication ID-92, “Commercial Fruit Spray Schedule.” Homeowners should consult publication ID-21, “Disease and Insect Control Program for Home Grown Fruit in Kentucky.”  Landscape managers need to consult UK Extension publication ID-88, “Woody Plant Disease Control Guide for Kentucky.” All three are available through local county Extension offices.
   
   

Figure legends:

Figure 1.  Telial “horns” emerging from a cedar-apple rust gall. 



Figure 2.  Cedar-apple rust with telial “horns” (left) and cedar-quince rust (right).
University of Georgia photo.

Figure 3.  Several cedar-hawthorn rust infections on juniper.

Figure 4.  Cedar-quince rust on juniper twig.