October 15, 2009
Pear trellis rust found in several Ontario locations
By Jennifer Llewellyn
OMAFRA nursery crops specialist, and
Michael Celetti
OMAFRA plant pathologist
Over the last few years, pear trellis rust has been found in the landscape at several locations in southern Ontario. This disease causes very striking, orange-red lesions on ornamental and fruiting pear trees by mid-summer (Figure 2, 4).
It is caused by the fungus Gymnospor-angium sabinae (G. fuscum). You may know the genus Gymnosporangium, since other species of this fungus cause cedar-apple rust, cedar-quince rust and hawthorn rust on ornamentals. Like many rust diseases, two hosts are required to perpetuate pear trellis rust from year to year. Juniper is the perennial host and pear is the summer host. The disease can be particularly damaging on pear, resulting in complete defoliation and crop loss if the disease is not managed.
The life cycles of Gymnosporangium rusts are complex, often involving alternate hosts (heteroecious) and various types of spores, so bear with us. It is important to note that pear trellis rust does not overwinter on the pear. So, if you are a nursery that grows pear trees and you find Gymnosporangium rust on the trees, it will no longer be infected once the leaves drop. Therefore, pear trees are not likely involved in the long distance spread of this disease through nursery stock. But, not so the juniper hosts. The disease over-winters in subtle swellings, or galls on infected twigs and branches of susceptible juniper species (J. sabina or Savin juniper) as seen in Figure 1. You can take our word that it is very difficult to detect these galls when they are in the dormant state. In the mid- to late-spring, after a warm, wet period, the galls on the juniper produce tiny dark horn-like growths that become covered with an orange to orange-brown gelatinous mass called telia (Figure 3). The telia release windborne spores (basidiospores) capable of infecting susceptible pear leaves. These windborne basidiospores can be dispersed up to six km. After a few days, the gelatinous telia masses fall off, leaving creamy-white scars behind on the swollen galls (Figure 2). After a week or two, the scars close and the galls shrink down to the dormant state.
Once the basidiospore reaches a susceptible pear leaf, infection takes place. The first symptoms of infection are tiny, 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 (Figure 4). In the centre of these orange-red lesions are tiny, raised pimple-like structures called pycnia (Figure 5). The pycnia exude a sticky sugary substance that contains sex spores. The sticky substance attracts insects that transfer sex spores from one pycnia to another, resulting in fertilization.
At the end of August, the undersides of the orange-red leaf lesions begin to swell with brown blisters (Figure 6). In late summer, tiny lantern-shaped growths (aecia) protrude from the blisters on the underside of the leaves (Figure 7). The aecia contain spores (aeciospores) that can only infect susceptible juniper hosts. 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, 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. Similar to other Gymnosporangium rusts, galls on juniper are perennial, they continue to produce telia for several years and become larger over time.
Thankfully, not all species of junipers are susceptible to this disease. Juniperus horizontalis, J. communis and J. squamata seem resistant to pear trellis rust and can be grown near pears without concerns for this species of rust. The fungicide, Nova, is registered in Ontario to control rust diseases on pear and juniper. However, some of the fungicides applied to control other diseases may have some impact on pear trellis rust. Fungicides should be applied to protect pear foliage when the juniper hosts are sporulating (Figure 3), similar to the timing of fungicides for other Gymnosporangium diseases. The best way to minimize this disease is to keep alternate hosts a distance of at least one kilometre. Inspect juniper plants periodically and prune-out any suspicious swellings or galls. Pear growers should also ensure that susceptible juniper hosts are removed within at least one or two kilometres from the orchard.
By the 1970s, this disease was widely distributed in Europe, Asia and North Africa. The geographic distribution of this disease in North America started out quite small. Pear trellis rust was first recorded in Canada (B.C.) in the 1960s. Prior to that it was only known to occur in one county in California. At that time, there were no other known sites of this fungus in North America, so the disease was regulated as a quarantined pest in hopes of eradication and slowing its spread. B.C. developed a certification program for pear and juniper nursery stock, based on clean stock, sanitation and a large separation (one km) between the two alternate hosts of juniper and pear. During the late 90s, pear trellis rust was detected in Washington and from there, the U.S. and Canadian governments considered de-regulating this disease. Since then, the disease has become more common in the lower mainland of B.C. and now we are seeing it in southern Ontario.
Jennifer Llewellyn may be contacted at 519-824-4120, ext. 52671, Fax: 519-767-0755, or jennifer.llewellyn@ontario.ca.
Captions: Figure 1 - Pear trellis rust in dormant state on juniper
Figure 2 - Scars after telia masses fall off juniper branches
Figure 3 - Telia on juniper
Figure 4 - Early lesion on pear leaf
Figure 5 - Pycnia seen on pear leaf
Figure 6 - Aecial cups
Figure 7 - Aecial cups spore on the underside of pear leaf
OMAFRA nursery crops specialist, and
Michael Celetti
OMAFRA plant pathologist
Over the last few years, pear trellis rust has been found in the landscape at several locations in southern Ontario. This disease causes very striking, orange-red lesions on ornamental and fruiting pear trees by mid-summer (Figure 2, 4).
It is caused by the fungus Gymnospor-angium sabinae (G. fuscum). You may know the genus Gymnosporangium, since other species of this fungus cause cedar-apple rust, cedar-quince rust and hawthorn rust on ornamentals. Like many rust diseases, two hosts are required to perpetuate pear trellis rust from year to year. Juniper is the perennial host and pear is the summer host. The disease can be particularly damaging on pear, resulting in complete defoliation and crop loss if the disease is not managed.
The life cycles of Gymnosporangium rusts are complex, often involving alternate hosts (heteroecious) and various types of spores, so bear with us. It is important to note that pear trellis rust does not overwinter on the pear. So, if you are a nursery that grows pear trees and you find Gymnosporangium rust on the trees, it will no longer be infected once the leaves drop. Therefore, pear trees are not likely involved in the long distance spread of this disease through nursery stock. But, not so the juniper hosts. The disease over-winters in subtle swellings, or galls on infected twigs and branches of susceptible juniper species (J. sabina or Savin juniper) as seen in Figure 1. You can take our word that it is very difficult to detect these galls when they are in the dormant state. In the mid- to late-spring, after a warm, wet period, the galls on the juniper produce tiny dark horn-like growths that become covered with an orange to orange-brown gelatinous mass called telia (Figure 3). The telia release windborne spores (basidiospores) capable of infecting susceptible pear leaves. These windborne basidiospores can be dispersed up to six km. After a few days, the gelatinous telia masses fall off, leaving creamy-white scars behind on the swollen galls (Figure 2). After a week or two, the scars close and the galls shrink down to the dormant state.
Once the basidiospore reaches a susceptible pear leaf, infection takes place. The first symptoms of infection are tiny, 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 (Figure 4). In the centre of these orange-red lesions are tiny, raised pimple-like structures called pycnia (Figure 5). The pycnia exude a sticky sugary substance that contains sex spores. The sticky substance attracts insects that transfer sex spores from one pycnia to another, resulting in fertilization.
At the end of August, the undersides of the orange-red leaf lesions begin to swell with brown blisters (Figure 6). In late summer, tiny lantern-shaped growths (aecia) protrude from the blisters on the underside of the leaves (Figure 7). The aecia contain spores (aeciospores) that can only infect susceptible juniper hosts. 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, 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. Similar to other Gymnosporangium rusts, galls on juniper are perennial, they continue to produce telia for several years and become larger over time.
Thankfully, not all species of junipers are susceptible to this disease. Juniperus horizontalis, J. communis and J. squamata seem resistant to pear trellis rust and can be grown near pears without concerns for this species of rust. The fungicide, Nova, is registered in Ontario to control rust diseases on pear and juniper. However, some of the fungicides applied to control other diseases may have some impact on pear trellis rust. Fungicides should be applied to protect pear foliage when the juniper hosts are sporulating (Figure 3), similar to the timing of fungicides for other Gymnosporangium diseases. The best way to minimize this disease is to keep alternate hosts a distance of at least one kilometre. Inspect juniper plants periodically and prune-out any suspicious swellings or galls. Pear growers should also ensure that susceptible juniper hosts are removed within at least one or two kilometres from the orchard.
By the 1970s, this disease was widely distributed in Europe, Asia and North Africa. The geographic distribution of this disease in North America started out quite small. Pear trellis rust was first recorded in Canada (B.C.) in the 1960s. Prior to that it was only known to occur in one county in California. At that time, there were no other known sites of this fungus in North America, so the disease was regulated as a quarantined pest in hopes of eradication and slowing its spread. B.C. developed a certification program for pear and juniper nursery stock, based on clean stock, sanitation and a large separation (one km) between the two alternate hosts of juniper and pear. During the late 90s, pear trellis rust was detected in Washington and from there, the U.S. and Canadian governments considered de-regulating this disease. Since then, the disease has become more common in the lower mainland of B.C. and now we are seeing it in southern Ontario.
Jennifer Llewellyn may be contacted at 519-824-4120, ext. 52671, Fax: 519-767-0755, or jennifer.llewellyn@ontario.ca.
Captions: Figure 1 - Pear trellis rust in dormant state on juniper
Figure 2 - Scars after telia masses fall off juniper branches
Figure 3 - Telia on juniper
Figure 4 - Early lesion on pear leaf
Figure 5 - Pycnia seen on pear leaf
Figure 6 - Aecial cups
Figure 7 - Aecial cups spore on the underside of pear leaf