Fungal Rust Disease of Pears and Many Other Trees

Rust Diseases on Pears and Others

Tom Ingram: Ask a Master Gardener

Thursday, May 14, 2020

We break from tradition today and are not going to answer a single question but rather address a topic that has been flooding our Diagnostic Center via phone and email: fungal diseases in trees and shrubs. And when I say flooding, I mean flooding.

This spring has been cool and moist for the most part and still is. These are perfect conditions for fungal diseases in plants.

There are a variety of fungal diseases with a variety of names, but in my book, the most interesting one is rust disease. There are several rust diseases: cedar-apple rust, Asian pear rust, cedar-hawthorn rust and cedar-quince rust, to name a few. These diseases are not extremely harmful but can be detrimental to the vigor of both hosts, as well as diminish the productivity of fruiting trees.

One thing that is interesting about these diseases is that they bounce back and forth between different plants with the common link being the cedar (aka juniper). This year, it seems like almost every pear tree in town (including Bradford pears) is covered with Asian pear rust.

While this explanation can seem a little like “which came first: the chicken or the egg,” let’s give it a shot.

The spots we are seeing on pear and apple tree leaves this spring are the result of fungal spores that have blown from a nearby cedar. Nearby is a relative term and typically means within a mile or so, maybe more.

The fungal spots on the apple and pear leaves mature somewhere around June or July. Once mature, they begin to release their own spores, which are then blown by the winds in hopes of finding a cedar to call home.

Asian pear rust causes fairly small rust twig cankers on the cedars, while cedar-apple rust causes small galls to be formed. Fast forward a few months and spring rains cause these cankers to release their own spores, which go out in search of another broadleaf host. Lather, rinse, repeat.

While the Asian pear rust cankers are fairly small, cedar apple rust galls can be golf ball sized. But it gets better. When spring rains soak the cedar-apple rust galls, they begin pushing out these long orange gelatinous tendrils full of spores, which then ride the wind in search of a new home. It is quite a sight.

To control these diseases, the hosts need to be separated, but in town, that is next to impossible. So we are primarily left with treatment options.

Susceptible broadleaf trees, such as apples or pears, can be treated in the spring from the point their leaves emerge through April on seven- to 10-day intervals with a fungicide that includes one of the following ingredients: copper hydroxide, chlorothalonil, myclobutanil and propiconazole (use as specified on the labels). Cedars can be treated in late June-July with the same fungicides. However, you can always physically remove the galls from the cedars when you see them.

For fruiting trees, we recommend you follow a pesticide spraying schedule that can be found in the Hot Topics section on our website, tulsamastergardeners.org.

Garden tips

·        Nutsedge weeds are emerging now. Post-emergent treatments are best applied for the first time this month. Make certain warm-season grasses have completed green-up. Nutsedge control requires specific treatment for control; standard broadleaved post emergent herbicides are not effective. Contact the OSU Tulsa County Master Gardeners for recommendations.

·        Plant summer bulbs such as cannas, dahlias, elephant ear, caladiums and gladiolus.

·        Remember, working wet soil will cause significant damage to the soil structure. Give it time to drain from recent rains, before tilling. Damage from tilling while wet may last for a very long time.