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Unsightly Pear Trellis Rust

Pear Trellis Rust (Figure 1), caused by the fungus Gymnosporangium sabinae (G. fuscum), has been confirmed in several locations in Southern Ontario recently. The disease causes a yellow-orange spot that turns bight red on leaves of both ornamental and edible fruit pear trees (Figure 2). Like many rust diseases, two alternate hosts, juniper (winter host) and pear (summer host), are required to perpetuate the disease from year to year. Pear trellis rust is a regulated disease. The disease can be particularly damaging on pear, resulting in complete defoliation and crop loss if not managed.

Figure 1. Pear trellis rust lesions on ornamental pear leaf (late spring)

Figure 2. Pear Trellis Rust lesion on pear leaf appear crimson red with tiny dark pycnia in centre (early summer)

The disease overwinters in swellings or galls on infected twigs and branches of susceptible juniper plants (Figure 3). In the mid-late spring after a wet period caused by a rain or heavy dew, the galls on the juniper produce tiny dark horn-like growths that become covered with an orange to orange-brown gelatinous mass called telia. The telia release windborne basidiospores capable of infecting susceptible pear leaves. These windborne basidiospores can be dispersed up to 6 km.

Figure 3. Dormant swelling on juniper branch caused by infection with pear trellis rust fungi

Once the basidiospore reaches a susceptible pear leaf, infection takes place, causing yellow orange spots on the pear leaf. 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 called pycnia that exude a sticky sugary substance (Figure 4). These pimple-like structures contain sex spores. The sticky substance attracts insects which transfer the sex spores from one pycnia to another, resulting in fertilization (Figure 4). At the end of August the brown coloured underside of the orange lesion begins to swell and blister (Figure 5). In late summer, tiny lantern-shaped growths called aecia protrude from the blister on the underside of the pear leaf. The aecia contains spores (aeciospores) that can only infect susceptible juniper hosts (Figure 6). These aeciospores are windborne over long distances and may eventually land on a susceptible juniper host twig where infection occurs. As the fungus grows within the juniper twig or branch, a swelling or gall is produced in which the fungus overwinters. Reports indicate that the swellings or galls on juniper do not produce telia until two years after infection and continue to produce telia for several years thereafter.

Figure 4. Close up of Pear Trellis Rust lesion on pear leaf with tiny orange pimple-like pycnia in centre (mid summer)

Figure 5. Aecia cups first appear as blisters on the underside lesion on an infected pear leaf (late summer)

Figure 6. Aecia releasing the aeciospores appear as tiny lantern-like structures on the underside of infected pear leaf lesion (late summer)

There are no fungicides registered in Ontario to control this disease on pears or juniper; however, some of the fungicides applied to control other diseases will have some impact on Pear Trellis Rust. The best way to minimize this disease is to keep the alternate hosts at least 1 kilometre apart from each other. Inspect juniper plants periodically and prune out any suspicious swellings or galls. Pear growers should also insure that susceptible juniper hosts are removed within at least one or two kilometres of the orchard.