Pear Trellis Rust Arrives in Ontario

Pear growers will have to learn how to manage a newly introduced disease in their orchards in the future. The disease is Pear Trellis Rust which was confirmed in several locations in Southern Ontario during 2007. Most reports of the disease were on ornamental pears growing in urban centres but at least one commercial pear orchard was confirmed to have the disease. Severely infected trees can become complete defoliated resulting in significant crop loss if the disease is not managed.

Pear Trellis rust has a complex life cycle and like many rust diseases, two alternate hosts, in this case pear and juniper, are required to perpetuate the disease from year to year. The pathogen overwinters in galls on infected twigs and branches of susceptible juniper plants. Around mid to late spring, after a wet period caused by a rain or heavy dew, the galls on the infected juniper produce tiny dark horn-like growths that become covered with an orange to orange-brown gelatinous mass. These orange horns release windborne spores that can be dispersed up to 6 km and can only infect susceptible pear leaves and fruit.

Once spores reach a susceptible pear leaf, infection takes place causing small yellow orange spots on the pear leaf. The lesions expand throughout the summer months and turn bight crimson red. By the end of August, the underside of the expanded lesion begins to swell and blister. Tiny white lantern-shaped growths protrude from the blisters which release another type of windborne spore that can only infect susceptible juniper hosts. When the spore lands on a susceptible juniper twig, infection occurs and eventually a gall is produced in which the fungus overwinters and the cycle repeats itself.

Pear Trellis Rust has been particularly severe on ornamental pear trees growing in parks and along boulevards in urban centres where susceptible juniper and ornamental pears coexist within the landscape. Ornamental pear are excellent shade trees and are one species of tree being considered to replace ash trees that were removed or killed due to emerald ash borer. With the recent introduction of pear trellis rust into Ontario, the planting of ornamental pear in urban centres where susceptible juniper already exists, may have to be re-evaluated.

It is not known where or how the disease entered Ontario. Since the pathogen produces a perennial gall on junipers, it is suspected to have entered the province on infected junipers imported from the west where the disease has been present for at least 15 to 20 years. Based on the severity and number of infected ornamental pear trees observed this past year in several urban centers within Ontario, it most likely has been here for several years.

No one know how sever this disease will become in Ontario pear orchards. There are currently no fungicides registered in Ontario to control this disease on pears or juniper, however, some of the fungicides applied to control other diseases in pear orchards may provide some control of pear trellis rust. Regardless, the best way to minimize the potential damage caused by this disease is to keep the susceptible juniper and pear at least 1 kilometre apart from each other, which for commercial pear growers may require the removal of susceptible junipers growing near their orchards.

This is Michael Celetti, Plant Pathologist, Horticulture Crops Program Lead with the Ontario Ministry of Agriculture, Food and Rural Affairs in Guelph, Ontario.

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