Gymnosporangium sabinae Family: Pucciniaceae
Syn. Gymnosporangium fuscum
Common name: pear trellis rust; European pear rust; pear-juniper rust
Host plants: Juniperus spp. (juniper) – primary; Pyrus spp. (pear) – secondary
Life cycle: Spring – spores are wind-blown from junipers onto pear leaves
Description & Life cycle: This is a rust fungus which requires two host plants from unrelated groups – juniper and pear. In spring, wet weather triggers the production of orange jelly-like masses of spores from a spindle-shaped gall on the juniper. (These galls are typically found on the tips of the tree branches.) The wind carries these spores for distances of 4 – 6 miles (6.5 – 10 km) until they come into contact with a pear tree species. Symptoms on the pear tree first appear as orange-yellow spots on young leaves. Small branches and young fruit can also be infected. The leaf spots brighten to reddish-orange over the summer and will develop tiny black dots in the centre. These are pycnia, or fruiting structures. They play a role in the development of the spore structures which are formed later in the summer on the underside of the leaf, directly under the colour spot. In late summer, brown blister-like swellings are forming, looking quite like miniature acorns. These are the aecia*. Spores (called aeciospores) are produced in this structure and are released into the wind through the fall months to land on juniper hosts where the spores infect young branches. Fungus overwinters on the juniper host.
*Note: The aecia of pear trellis rust can produce spores for two years in a row. Aecia of apple-cedar rust only produce spores for one year.
Both plant hosts, juniper and pear, are needed for this rust fungus to complete its life cycle. However, the fungus will sometimes overwinter on pear trees in branch galls and produce new pear infections the following spring.
Special Notes: Originally native to Europe, and totally confined to this region, this rust fungus was identified in North America in the early 1930s, likely imported on ornamental junipers. The spread of this fungus was positively identified in Victoria, BC in 1961 and throughout other regions in the Pacific Northwest, including parts California, in the 1990s. Ontario reported cases in 2007; Michigan in 2009; southeastern New York State in 2011; Connecticut in 2012; Virginia in November 2014. It is also making inroads in the United Kingdom.
This fungus can cause losses in pear crops, although this appears to be truer in Canada than in Europe.
Remedial Action: Control is mainly concentrated in separating the two host plants by as much distance as possible. Recommended is minimum 100 ft (30.5 m); preferred 1000 ft (305 m). Unfortunately, these separation distances may be inadequate given the spores are carried on the wind and can travel quite a distance…up to 6 miles (10 km). If possible, prune any galls spotted on juniper species before April 1st. But, the galls can be hard to spot on juniper trees…especially when they are not producing their masses of spores. (When reproduction gets going in spring, the spindle-shaped gall produces bright orange gelatinous spores. Much easier to spot on a dark branch.) Prune galls which develop on infected pear trees and remove any infected pear leaves, fruit and small branches by the middle of August.
There are a few rust-resistant juniper species available now. These include: Juniperus horizontalis (creeping juniper), J. squamata (Himalayan juniper), and J. communis (common juniper)
Posted on October 29, 2016