Pear trellis rust (Gymnosporangium sabinae) is an interesting fungus in that it requires two entirely different plant hosts to complete its life cycle – pear and juniper. Spores produced on junipers only infect pears, and those produced on pear only infect juniper. It gets its name from the fruiting structures on the underside of the pear leaves- they look a bit like a trellis when examined under a magnifying lens. The symptoms on pear generally only effect the leaves. Starting as small bright orange spots in the spring and gradually turning into thick growths later in the summer as they release spores. It’s a relative newcomer to our area. It was found on pear trees in Bellingham in 1997. Previously known only from Southern California and Southern British Columbia, it has now spread throughout most of Western Washington and is a fairly common sight in Seattle.
Here are some non-chemical control suggestions from WSU:
-Prune out swellings or galls from junipers.
-Remove and destroy infected material from pear trees (fallen leaves, mummified fruit, heavily infected twigs, etc.) to help minimize disease spread. To help protect junipers, infected plant material must be removed from the pear trees before spores form, usually around late August in western Washington. This may not be practical on large trees.
-Plant only disease-resistant junipers in areas where this disease is a concern; cultivars of Juniperus squamata, J. horizontalis, and J. communis are resistant.
-Do not plant pears and junipers within 1,000 feet of each other. Most local transmission of this disease is by wind-blown spores.
-Complete removal of one host is the only completely effective cultural control.
-Carefully examine plants before adding them to your landscape. Many diseases are introduced on infected planting material.
This is from the ministry of Agriculture in B.C.
-Plant junipers and pears as far away from one another as possible. Consider your neighbor’s plants, as well as your own. If the two hosts are separated by at least 150 meters [~500ft], there will be minimal damage to the pears.
-If infected pear and juniper are in close proximity, consider removing one or the other. Don’t leave juniper brush piled in your yard as the fungus will sporulate on it and spread to the pears.
-To prevent spread of the disease from your pear tree to nearby junipers, pick the infected leaves before mid-August each year, if they are not too numerous. No special disposal of these leaves is required. The fungus will die out in a few days when the leaves shrivel up.
-Prune out any gall-like growths at the base of twigs on pear.
-Avoid planting ornamental pears. They are also susceptible to the disease.
-If selecting junipers for a landscape containing pears, select resistant juniper species, including Juniperus horizontalis, J. communis and J. squamata, or choose a different type of conifer.
Also from the ministry of Agriculture, B.C.:
-Symptoms on juniper are much less obvious. They can only be seen during wet weather in April and early May. At that time, orange, jelly like masses (telia) swell and enlarge on infected juniper branches. Telia release release spores which are capable of infecting nearby pear leaves which are just starting to grow. After the spores are released, the telia shrink and dry up, and infection on juniper remains dormant until the next spring. Infected junipers continue to grow and appear healthy. Some varieties may develop spindle-shaped swellings on the branches.
The fruiting structures can be seen protruding from on the underside of these infected leaves, in this photo courtesy of Jeremy Harrison-Smith. The initial infection is often bright orange, sometimes darkening to red late in the summer.
It seems the consensus is that the only sure way of eradicating the fungus is to take out any junipers near your pear. 300-500ft some sources say, To 1,000ft (the most commonly stated number). This is fairly impractical for a city neighborhood situation like most of Seattle, but the further away an infected neighboring juniper is the less likely spores from it will land on your pear. So if there’s a juniper directly next door, perhaps some discussion with the neighbor about its replacement could happen as there are resistant varieties, and (in my opinion) much more interesting plants that could replace it. In general keeping the pear tree as happy as possible will be of benefit. Putting down a wood chip (not bark) mulch under the tree will help build the soil, moderate soil temperature and retain moisture. Making sure it has sufficient water during the drought time at the end of the summers will also help. Don’t use overhead sprinklers, as the fungus needs water to infect the leaves.
-Pear (Pyrus spp.)-Trellis Rust In: Pscheidt, J.W., and Ocamb, C.M., senior editors. 2014. Pacific Northwest Plant Disease Management Handbook [online]. Corvallis, OR: Oregon State University. http://pnwhandbooks.org/plantdisease/pear-pyrus-spp-trellis-rust (accessed 4 December 2014).