Cedar Apple Rust.pdf

Extension

SP 277-A

Cedar-Apple Rust

Steve Bost, Professor

Entomology and Plant Pathology

Cedar-apple rust is a common disease of apple

and crabapple. The fungus that causes the disease,

Gymnosporangium juniperi-virginianae, is unusual

because it must spend a phase of its life cycle as a

parasite on Juniperus species, such as red cedar or

ornamental junipers.

Cedar-apple rust can be severe on apple.

Infections of apple fruit result in lower fruit quality

and early fruit drop. Leaf spots may cause early

defoliation, especially during dry summers. If trees

are defoliated several years in a row, they become

weakened and unthrifty. Fruit bud formation may be

reduced after one year. The disease is not as harmful

to juniper, causing galls but not severely affecting

plant vigor.

Figure 1. A “cedar-apple” gall on cedar.

These galls are light brown, reddish or chocolate

brown, round or kidney-shaped, and up to 2 inches in

diameter (Figure 1). The galls develop round pits with

small pimple-like spots in the center.

The following year, after a few warm spring

rains, the galls (sometimes called cedar-apples) swell

and produce bright orange, jelly-like tendrils from the

circular pits. These tendrils are called spore horns or

telial horns, and may be up to 2 inches long. They are

highly visible on junipers.

Symptoms and Disease Cycle

The fungus causing cedar-apple rust lives nearly

two years on juniper, the primary host. From July

to September, spores from apples fall on junipers.

During warm, moist weather, the spores germinate

and infect twigs. Small galls become apparent on

juniper the next spring and grow during the summer.

One large gall can produce billions of spores,

borne on the spore horns. These “basidiospores” can

be carried several miles on air currents. Those that

land on young apple tissue germinate and cause infec-

tion during short periods of wetness.

Apple leaves four to eight days old are the most

susceptible to infection. Fruit lesions result from in-

fections that took place before and during the blossom

stage, from tight-cluster until just after petal fall. Most

infections occur in Tennessee between early April and

early May.

One to two weeks after infection, small yellow to

orange spots appear on the upper leaf surface (Figure

2). These spots increase in size up to 1/4 inch. They

become bright yellow or orange, and often have a dark

border. Small black dots (pycnia) develop in the spots.

Similar spots develop on the fruit.

A few weeks after these pycnia develop, yellow

spots begin to form on the lower apple leaf surface

(July-August). The leaf thickens and small tubular

projections (aecia) develop. These split open and curl

backward forming tiny, cup-like structures sometimes

called “cluster cups.” The aeciospores released from

these cluster cups are wind blown to junipers, where

infection occurs. These spores cannot reinfect apples.

Control

(1) Grow resistant apple or crabapple varieties.

Apple varieties that normally show good to excellent

resistance to cedar-apple rust include Red Delicious,

McIntosh, Arkansas Black, Winesap, Mollies

Delicious, Spartan, Priscilla, Liberty and Empire.

Varieties that are usually highly susceptible include

Prima, Sir Prize, Lodi, Jonathan, Rome, Golden

Delicious and Jonafree.

(2) Destroy nearby wild, abandoned or worthless

apples, crabapples, cedars or junipers. If only a few

“cedar-apples” are present, prune and destroy them.

Although apples may occasionally become infected by

spores produced several miles away, most infections

result from spores produced on Juniperus within a few

hundred feet from the apple trees.

(3) Follow a recommended fungicide spray

schedule, beginning at the pink-bud stage and continu-

ing through the second cover spray. The cedar-apples

have usually exhausted their spores by this time.

Immunox ® is a fungicide available to homeowners

(see Extension PB1622, “Disease and Insect Control

in Home Fruit Plantings,” for timing and rates).

Captan ® , the fungicide component of many pre-mixed

home fruit sprays, is not effective against cedar-apple

rust. Commercial growers should refer to “Integrated

Orchard Management Guide for Commercial Apples

in the Southeast” for additional fungicides for control

of cedar-apple rust.

Precautionary Statement

To protect people and the environment, pesticides should be used safely.

This is everyone’s responsibility, especially the user. Read and follow label

directions carefully before you buy, mix, apply, store, or dispose of a pesticide.

According to laws regulating pesticides, they must be used only as directed

by the label. Persons who do not obey the law will be subject to penalties.

Disclaimer Statement

Pesticides recommended in this publication were registered for the prescribed

uses when printed. Pesticides registrations are continuously reviewed. Should

registration of a recommended pesticide be canceled, it would no longer be

recommended by the University of Tennessee.

Figure 2. Leaf spots of cedar-apple rust on apple.

Use of trade or brand names in this publication is for clarity and information; it

does not imply approval of the product to the exclusion of others that may be

of similar, suitable composition, nor does it guarantee or warrant the standard

of the product.

Visit the UT Extension Web site at

http://www.utextension.utk.edu/

Programs in agriculture and natural resources, 4-H youth development, family and consumer sciences, and resource development.

University of Tennessee Institute of Agriculture, U.S. Department of Agriculture and county governments cooperating. UT Extension provides

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