Natter’s Notes:
Pear Trellis Rust, a new disease

Jean R. Natter, OSU Master Gardener

Recently, Pear Trellis Rust (Gymnosporangium sabinae) became the newest contributor to this hodge-podge-let’s-try-everything year. During 2016, the first case of pear trellis rust was reported in the northern section of the Willamette Valley, that on a Bartlett pear growing in Milwaukie, Clackamas County. (See “Pear Trellis Rust: First Report in Oregon” Metro MG Newsletter, January 2016;  http://extension.oregonstate.edu/mg/metro/sites/default/files/dec_2016_mg_newsletter_12116.pdf). Then, in mid-September 2017, an inquiry about a pear leaf problem in Multnomah County was submitted to Ask an Expert. [Fig 1; Fig 2] Yes, it’s another fruiting pear tree infected with trellis rust. It seems that gardeners are beginning to recognize this newcomer.

“Symptoms [of trellis rust] on pear begin as yellowish-orange leaf spots early in the season. Young fruit and twigs can also be infected. Leaf spots can become bright reddish orange during the summer. By mid-summer, tiny black dots (pycnia) appear in the center of the leaf spots.” [Fig 3] By late summer, brown, blister-like swellings form on the lower leaf surface just beneath the leaf spots. This is followed by the development of acorn-shaped structures (aecia) with open, trellis-like sides that give this disease its common name. (Fig 4) Aeciospores produced within the aecia are wind-blown to susceptible juniper hosts where they can cause infections on young shoots. These spores are released from late summer until leaf drop.” (“Pear Trellis Rust, Gymnosporangium sabinae” (http://www.ladybug.uconn.edu/FactSheets/pear-trellis-rust_6_2329861430.pdf)

Signs on affected alternate host junipers are difficult to detect. During wet weather in spring, look for swollen areas on branches which exude orange jelly-like horns.

Differentiate Trellis Rust from Pacific Coast Pear Rust

Trellis Rust is quite different from the widespread Pacific Coast Pear Rust you’ve likely seen every spring on Amelanchier (shadbush; serviceberry) and pears.

Pacific Coast Pear Rust infects both Asian and European pears.  And, as is common with rusts, it also has an alternate host. During spring, host incense cedars (Calocedrus decurrens) are recognized by the bright orange jelly-like globs on the foliage. On pears, the bright orange, powdery spores erupt on fruits, flowers, leaves, and twigs, often deforming them. [Fig 5; Fig 6] Management includes removing nearby hosts. A home-use spray is available for ornamental pears but not edible pears. (Keep current with the PNW Disease Management Handbook.)

Management strategies for trellis rust                             

Minimizing overhead irrigation might help reduce the number of infections. But you know how Oregon springs are.  It rains! Cultural management may help decrease infection rates:

1. Collect and discard infected leaves.
2. If practical, remove juniper hosts from a 1000-ft radius. (The PNW Disease Handbook states “J. communis, J. horizontalis, and J. squamata are immune or highly resistant.”)
3. No chemical remedies are available for edible pears.

Master Gardeners as First Responders

When MGs see a plant affected by a disease or insect, we’re required to verify our tentative diagnosis before we suggest a remedy. So, here’s an important project for you: Help track the spread of Pear Trellis Rust.

If you suspect pear trellis rust while volunteering at the MG Offices or elsewhere in the metro counties, request images and/or samples. Take pictures and jot down a history with at least these few facts, if known: the name and age of the pear; when the client first detected the problem; also, in which town the tree grows. Next, email the images and history to me (j.r.natter@aol.com). After I verify your tentative diagnosis, I will notify both you and the pathologist.

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Click the link below for a PDF containing the above text and all the images.

Pear Trellis Rust PDF

 

 

 

Natter’s Notes: Heat Stress

Jean R. Natter, OSU Master Gardener

September 2017

This has been an interesting year as far as plant problems go. The past winter was colder than usual; this spring was wetter than usual; this summer hotter and drier than usual; and, oh yes, we had a total solar eclipse (2017-08-21) even as I was writing this. Then, too, in spite of the plentiful rainfall this past winter and spring, established trees in forests and landscapes are dying from consecutive years of drought.

For the most part, causal agents of plant problems are abiotic, caused by naturally-occurring adverse environmental factors, also the garden’s care-takers, John and/or Jane Doe. So, when clients ask which disease afflicts their plants, we have a lot to consider. We need a detailed history of what occurred and when, including pre-plant preparations as well as follow-up maintenance.

Just how plants react to high temperatures depends upon numerous factors, among them the extent and duration of the heat; the relative humidity; wind conditions; soil moisture content; the kind of plant, its age, site, and general status before the heat hit. (Phew! That’s a lot to consider.) Sometimes leaves are only damaged superficially.  Other times, tissues die. Tissue survival is most likely when the plant is fully hydrated well before the heat hits. If heat is predicted, water the night before or early morning, between 2 and 6 AM.

One good thing about the recent heat waves, the accompanying low humidity has helped limit common leaf diseases. Well, except for powdery mildew, the fungus that creates a whitish film on the leaf surface. If that’s the case, recall that most fungicides are preventive and must be applied at the very first sign of disease, long before the leaf is snowy white.

Accurately diagnosing heat damage relies, in part, upon how well you “read” the signs and symptoms. It’s a skill that requires time to develop. (You know the old saw: Practice, practice, practice.)

Let’s take a look at how heat damage may be expressed, especially on leaves, since that’s often the only thing a client submits for diagnosis.

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Young dogwood (Cornus sp.), probably about 2 years old, in a commercial landscape. (Fig 1) Exposure to bright sunlight damaged superficial tissues, killed the chlorophyll (green), revealing the underlying anthocyanins (red pigments), resulting in a reddened sheen on only the most exposed leaves.  The somewhat shaded leaves retain excellent green color.

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Vine maple leaf with dry, brown edges, evidence of acute water shortage to the shrub. (Fig 2) Sudden heat exposure to a 19-year-old shrub damaged many leaves in a wide swath across the shrub. Affected leaves were tan and shriveled while others only had dry edges. Client wondered if the tree was at the end of its life span. The Ask an Expert response, said essentially this: It’s the recent heat. (Client image; 2017-08) Click image for larger view.

 

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Hosta, exposed to sudden and extreme heat, accompanied by low humidity. (Fig 3) The most severely damaged tissue at the right edge of the leaf, outlined by a zone of white tissue, still retains normal color. This kind of damage can develop in susceptible plants even if they’re in full shade. (J.R. Natter; 2017-06-24)

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Click link below for PDF with additional information and images:

2017-09 Heat stress_Natter’s_Notes