North American Fruit Explorers mailing list at ibiblio

["dwalsh/wchase" <wchase@interchange.ubc.ca>]

Ginda,

To quote from the Compendium of Apple and Pear Diseases, ed: Jones and 
Aldwinckle,
APS Press:

CAR: The galls ("cedar apples")  on Juniperus virginiana (eastern red cedar) 
exude
horn-like telia during spring rains.  During rains the telia become swollen 
and
jellylike, afterwards drying back to dark brown threads.  Swelling and drying 
of
the telia may occur 8-10 times during the spring.

The telia contain teliospores, some of which are extruded during each 
rain-induced
swelling, until the supply is exhausted.  During a rain, teliospores 
germinate to
produce basidiospores within four hours at optimum temps (11-25°C).

Basidiospores are forcibly discharged into the air immediately after 
formation and
can be carried one kilometer on air currents.  Those that land on young apple
tissue may germinate and infect the host if a film of water is present for a
sufficiently long period.  Apple leaves four to eight days old are the most
suceptible to infection from basidiospores, and fruit are susceptible from the
tight-cluster stage until just after petal fall.  One to two weeks after
infection, orange-brown pycnia containing pycniospores appear on the upper 
sides
of the leaves or on the fruit.

One to two months after the appearance of pycnia, aecia appear on the lower
surfaces of the leaves or on the fruit.  The aecia produce aeciospores, which 
are
released during dry weather in late summer.  Those that land on young twigs of
Juniperus virginiana may germinate and infect them.  Eventually during the 
second
year after infection, mature galls are formed, thus completing the disease 
cycle.
Because the galls of cedar apple rust produce teliospores during one spring 
only,
fresh infections of eastern red cedar are required each year for infection of
apple trees.

Cultivars vary greatly in susceptibility to this disease. Resistant cultivars 
such
as Delicious, Liberty, McIntosh, Priscilla, and Tydeman's Early Worcester can
always be grown without chemical sprays, whereas susceptible cultivars such as
Golden Delicious, Jonathan, Prima, Rome Beauty, Twenty Ounce, and York 
Imperial
suffer severe damage where the fungus is present and conditions are 
favourable for
infection.

So to summarize:

You need the susceptible species of Juniper in your area (within one km)
You must have rain to swell the telia
You need more rain for the teliospores to germinate and produce basidiospores
You need wind to carry the basidiospores from the Juniper to your apple tree
The apple leaf must be young (4-8 days old) and wet to be infected
This sequence must occur every year.

The first thing to do is get to know the junipers (a good reference is Manual 
of
Trees and Shrubs by Hillier Nurseries, England ... eleven pages of junipers
listed)
Next, get rid of the Juniper virginiana on your property
If there are only a few apple leaves infected, remove and garbage those leaves
Hope for dry weather in Spring.

I'm familiar with pear trellis rust which has a similar life cycle and I find 
it
fascinating.  Try looking at the developing pycnia on the apple leaves with a
magnifying lens.

Despite watching the development of pear trellis rust for about fifteen 
years, and
searching for telia in my neighbourhood, last month was the first time I saw 
the
pear trellis rust telia on Junipers ... on scrubby junipers in the alpine 
area of
Potato Mountain in the Chilcotin area of B.C.

Derry

Derry Walsh & Bill Chase  email:  wchase@interchange.ubc.ca
Aldergrove,  B. C.,  Canada
phone/fax (604) 856-9316
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Home web page  http://www.interchange.ubc.ca/wchase/HTML
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: Having seen lots of pictures of CAR on the internet, I'm pretty sure
: that's what I have on the crab, and to a lesser extent on the Jonathan.
: It's not very surprising, as junipers and their kin are a common
: landscaping plant around here.  I went looking to see if my shrubs have
: any visible CAR galls, and didn't find any.
:
: 1)  Would they be visible at this time of year?
: 2)  Is hand removing of fruiting bodies worth the effort?
: 3)  Is arborvitae a host?
: 4)  How about blue mat juniper?
:
: I also have two unidentified junipery shrubs (neither with visible
: galls).
:
: 1)  If levels of infestation seem low, is it important to treat?  (My
: one edible apple in not a fruiting age.  Does this matter?)
: 2)  Can anyone point me to information on how to predict heavy infection
: years, or otherwise determine when it's most important to treat.
:
: Thanks,
: Ginda



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