Issue 8, July 31, 2019

Pear Trellis Rust

Pear trellis rust (also known as European pear rust) is a fairly rare rust disease in Illinois. Other gymnosporangium rusts diseases (cedar-apple, cedar-hawthorn, and cedar-quince) are much more common, but we have confirmed a few cases of pear trellis rust at the Plant Clinic over the past few years. For more information about the common gymnosporangium rusts, please see this 2017 Home, Yard, and Garden Pest Newsletter article:

The fungal pathogen which causes pear trellis rust, Gymnosporangium sabinae, was introduced to the United States in the 1990s from Europe. As with other gymnosporangium rusts, it requires two host plants; in this case, the pathogen infects common and callery pear as the deciduous host and various juniper and eastern red cedar plants as the coniferous host.

In spring, orange, gelatinous, spindle-shaped galls appear on juniper branches. These galls produce spores which spread to the pear host via wind and rain. Symptoms begin to appear on pear trees in summer, as large, yellow/orange leaf spots develop on the leaves. As the season progresses, these spots often turn purple/brown and small fruiting structures will develop on the undersides of the leaves. Twig and branch galls may also form on the pear host.

Figure 1 Pear trellis rust in early summer displaying orange/yellow leaf spots

Figure 2 Pear trellis rust in late summer once the majority of the leaf spots have turned brown

The symptoms of pear trellis rust can be similar to those of cedar-hawthorn rust. One diagnostic difference is the shape of the fruiting structure (aecium) produced on the underside of the leaf; pear trellis rust produces a characteristic “acorn-shaped” aecium. However, these structures are quite small and can be difficult to see clearly. Samples can also be submitted to the University of Illinois Plant Clinic for identification.

Figure 3 "Acorn-shaped" aecium on the underside of the leaf

Pear trellis rust poses a threat to the health of the pear host, causing reduced growth, a thinning crown, and branch dieback following several years of repeated infection. The disease does not appear to cause damage to the juniper host. Management consists of removing unwanted juniper, pruning out galls found on junipers, planting juniper species resistant to gymnosporangium rusts, and removing infected pear leaves, fruits, and twigs in late summer. Removing infected tissue from both the juniper and the pear hosts may not be feasible, depending on the size and level of infestation. Fungicides labeled for use on pear against other gymnosporangium rusts (chlorothalonil, fenarimol, mancozeb, myclobutanil, propiconazole, pyraclostrobin, tebuconazole, thiophanate-methyl, triadimefon, and trifloxystrobin) may be effective when applied in early spring at white bud stage and continued at the labeled interval, 1 to 2 weeks past petal fall. Always read and follow label directions carefully.

Diane Plewa

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