Cedar Apple Rust

Cedar apple rust is a two-cycle fungus that lives one stage of its life on red cedar (Juniperus virginiana) and the other stage on apple or crabapple (Malus).  There is also a strain that affects hawthorn and the controls are the same as for cedar apple rust.  Although the disease can be quite unsightly on juniper, it does not usually harm the plant.  Apple, crabapple and hawthorn, however, can be quite disfigured.

SYMPTOMS & DISEASE CYCLE

The fungus is most prevalent where junipers are grown within 300 yards of apple, crabapple and hawthorn.  The infection starts when spores land on the juniper needles during the summer.  They overwinter on the needles enclosed in galls.  By the following spring, a small greenish brown swelling appears on the upper or inner leaf surface.  The juniper may be covered with gall varying from 1/16 to 2 inches across.  When the weather is wet, long yellow tongue-like growths form on these galls.  These outgrowths produce small spores that are carried by the wind to the nearby leaves of apple, crabapple and hawthorn.

By midsummer, the spores cause the leaves of infected trees to develop small yellow spots with tiny black fruiting bodies.  In addition, yellow, red or black powdery pustules appear on the undersides of the leaves.  The infection can cause premature defoliation, dwarfing and poor quality fruit.  The leaves of certain varieties are often badly damaged.  The pustules on the leaves produce another spore stage that reinfects junipers.  The complete disease cycle, therefore, usually requires almost 2 years.

CONTROL

Do not plant red cedar and apple, crabapple or hawthorn close to each other.  Since fungal spores are spread by wind, separate susceptible varieties by windblocks such as houses, walls, non-host plants or hedges to greatly decrease infection.  In addition, thoroughly rake up and dispose of infected leaves and prune the trees’ canopy to increase air circulation.  Select varieties that are resistant to cedar rust.  Avoid overhead watering.  Prune out galls on juniper.

Chemical:  For control on apple, crabapple, serviceberry and hawthorn use Mancozeb with Zinc, Immunox or Infuse.  Spray as new growth first appears (bud break) and the leaves start to open.  Repeat this process 3-4 times at 7-10 day intervals.  Carefully follow all label directions.  Chemical control in not usually recommended for juniper.