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Ministry of Agriculture, Food and Rural Affairs

Pear Trellis Rust

Aecia on underside of pear leaf Orange lesion on pear leaf Pear trellis rust pynia on leaf
Click to enlarge.

Beginner

This pest affects:

Pear

Scientific Name
Gymnosporangium sabinae

Identification
Summer host (pear):

  • The disease causes yellow-orange spots that turn bight red on leaves, fruit and branches.
    • In the centre of the yellow spots are tiny raised pimples that exude a sticky sugary substance and spores.
    • Spots get covered in grey, fuzzy growth in the fall.
  • Brown underside of lesion swells and blisters later in season
  • Occassionally infected fruit mummifies.

Overwintering host (juniper):

  • Swelling or gall on branches in which the fungus overwinters.
  • Tiny, dark horn-like growths (telia), covered with an orange to orange-brown gelatinous mass protrude from galls.

Often Confused With

Period of Activity
The disease overwinters on juniper. Basidiospores are released mid to late spring from juniper, following a wet period. Infection on pear progresses through summer. Aeciospores that infect juniper are released from aecia on pear from August to leaf drop in late fall.

Scouting Notes
The disease is easily detected on pear leaves with orange spots in early summer. Infected junipers will continue to grow and appear healthy until bright orange telia develop.

Threshold
There are no thresholds for treatment for pear trellis rust.

Advanced

This pest affects:

Scientific Name
Gymnosporangium sabinae

Identification
Summer host (pear):

  • The disease causes yellow-orange spots that turn bight red on leaves, fruit and branches.
    • In the centre of the yellow spots are tiny raised pimples (pycnia) that exude a sticky sugary substance and spores.
    • Spots get covered in grey, fuzzy growth in the fall.
  • Brown underside of lesion swells and blisters later in season
    • Lantern-shaped growths (aecia) with grey hair-like projections resembling trellises protrude from underside of leaves.
  • Occassionally infected fruit mummifies.

Overwintering host (juniper):

  • Swelling or gall on branches in which the fungus overwinters.
  • Tiny, dark horn-like growths (telia), covered with an orange to orange-brown gelatinous mass protrude from galls.

Often Confused With

Biology
Like many rust diseases, two alternate hosts, juniper (winter host) and pear, ornamental pear and quince (summer hosts), are required to perpetuate the disease from year to year.

The disease overwinters in swellings or galls on infected twigs and branches of susceptible juniper plants. In mid to late spring, after a wet period caused by a rain or heavy dew, the galls on the juniper produce telia, which are horn-like growths that become covered with an orange gelatinous mass. The telia release windborne basidiospores capable of being dispersed up to 6 km, infecting susceptible pear leaves.

Once the basidiospore reaches a susceptible pear leaf, infection takes place, resulting in the development of yellow-orange spots. The spots eventually enlarge and become crimson red along the margins, making them very noticeable in early summer. In the centre of the yellow spots are tiny raised pimples, (pycnia) that exude spores in a sticky sugary substance. At the end of August, the underside of the lesion begins to swell and blister. Tiny lantern-shaped growths (aecia) protrude from the blister.  These contain spores (aeciospores) that can only infect susceptible juniper hosts.

Aeciospores are windborne over long distances and may eventually land on a susceptible juniper host twig where infection occurs. As the fungus grows, a swelling or gall is produced in which the fungus overwinters. The galls on juniper start to produce two years after infection and continue to produce telia for several years thereafter.

Period of Activity
The disease overwinters on juniper. Basidiospores are released mid to late spring from juniper, following a wet period. Infection on pear progresses through summer. Aeciospores that infect juniper are released from aecia on pear from August to leaf drop in late fall.

Scouting Notes
The disease is easily detected on pear leaves with orange spots in early summer. Infected junipers will continue to grow and appear healthy until bright orange telia develop.

Threshold
There are no thresholds for treatment for pear trellis rust.

Management Notes
Remove susceptible juniper hosts within at least 1-2 km of the orchard. Where this is not possible, inspect juniper plants periodically and prune out any suspicious swellings or galls.

There are no fungicides registered in Ontario to control this disease on pears or juniper.  However, fungicides applied to control other diseases may have some impact on Pear Trellis Rust.