Cedar Rusts Threaten Apple

Thanks to this beautiful weather, cedar rust galls are developing.  As galls swell, they produce spores that threaten apple (and sometimes crabapple and hawthorn).

 Images of swelling galls were taken from cedar in Lexington on Monday March 19.  Warmer temperatures in western KY allowed even earlier gall development.

These galls indicate that rust pathogens are releasing, or are preparing to release, infective spores.  Growers should protect trees with fungicides that are proven to be effective against rusts (listed below).  Once diseases symptoms develop on apple, it is often too late for control. 

Here in Kentucky, 3 types of cedar rusts affect apple: 

1.       Cedar-apple rust produces large brown galls on cedar and other species of Juniperus.  Soon after a rain, galls produce slimy yellow or orange “horns” that are made up of infective spores (basidiospores).  These spores immediately infect apple.  Upon infection, the disease causes yellowish leaf spots with red rings (called halos).  Leaf yellowing and leaf drop follow.  Infected fruit develop large spots near the calyx end.  These fruit are often stunted and may drop prematurely. 

Gall beginning to form "horns."

"Horns" contain infective spores in a gelatinous material. 
This is an old photo, but we can expect to see this fully-developed stage soon in central KY.
Never wait until this stage to start a spray program on apple.

2.       Cedar-quince rust produces orange swellings on twigs.  Spores produced in these slimy lesions affect apple fruit, but not leaves.  Infection of blooms and young fruit occurs early in the season, but symptoms do not develop until fruit mature.  Diseased fruit are puckered and have spongy lesions at the calyx end.

Sometimes growers must look closely to recognize rusts, so scouting is important.

3.       Cedar-hawthorn rust forms galls similar to those of cedar-apple rust, only smaller.  Spores produced from short “horns” infect apple, crabapple, and hawthorn.  Leaf spots on apple appear similar to those caused by apple cedar rust, and can cause defoliation.  Fruit infection is not common.

Often, many rust types occur on the same tree.

Fungicides should be used as protectants to prevent rust pathogens from infecting.  After symptoms develop on apple, it is often too late for control. 

Commercial and homeowner fungicide recommendations are listed below.

Commercial Fungicides for  Management of Rust on Apple
Bayleton	Triadimefon
Dithane	Mancozeb
Indar	Fenbuconazole
Inspire Super	Difenoconazole
Pristine	Boscalid + pyraclostrobin
Procure	triflumizole
Rally	Myclobutanil
Sovran	Kresoxim-methyl
Topsin-M	Thiophanate-methyl
Homeowner Fungicides for  Management of Rust on Apple
Dithane	Mancozeb
Immunox Multi-purpose	Myclobutanil
Mancozeb	Mancozeb

More details for management of cedar rusts can be found in Kentucky Pest News or PPA-23 (http://www.ca.uky.edu/agc/pubs/ppa/ppa23/ppa23.pdf ).