permaculture

[Meg Mullett <meg@kcnet.com>]

Hi Mary,

The key to controlling scab on apple or pear is prevention.  Once you
see the scab lesions on the leaves and fruit, there's nothing really you
can do except try to prevent it spreading any more.  

Understanding how the fungus works is important.  Once you know the most
vulnerable times for the tree, you'll know when to spray.

Basically, it comes down to this:  The fungus overwinters on the leaves
and twigs beneath the tree.  When spring comes and the temperatures
warms a good rain makes all those fungus spores explode right back onto
the tree.  Its important to have the spray on the tree *before* that
happens.  It takes a certain amount of time of wetness on the leaf for
the fungus to actually germinate and attach, which is why its important
to have the spray on the tree before the rain comes.  There are several
charts out there showing how many hours of leaf wetness at different
temperatures are required for the fungus to attach.  

Once the fungal spore attaches to the leaf, it grows and creates its own
spores which then explode with the next rain and spread to more parts of
the tree.  This is sometimes called a "secondary infection".

Most spray regiments start when the first green tips emerge at budbreak,
and continue through the rainy season.  While many regimens state you
should spray weekly, you don't necessarily have to if you are attentive
to the weather.  If its going to be a dry week, the spray is not
necessary.  However, if its going to rain all week, you may need to
spray more than once.  The sprays are not systemic, and wash off pretty
easily in rain.  

So, what to do besides spray?  Prune the tree to encourage air flow and
sunlight penetration of the canopy, so the leaves dry faster after a
rain.  Clean up leaf litter in the winter.  If you don't want to rake it
all up, running the mower over it to chop it up and spreading a nice
layer of manure over the leaves so they decompose quickly and hot might
help.

Here are a couple factsheets explaining the fungus.  Pear scab works in
the same manner as apple scab, so looking up treatment and studies of
apple scab might also be useful for you:
http://www.caf.wvu.edu/kearneysville/disease_descriptions/ompearsc.html
http://www.attra.org/attra-pub/pear.html
http://www.agf.gov.bc.ca/cropprot/tfipm/applescb.htm

Hope this helps,
Meg in KS

 Message: 1
> Date: Wed, 29 Jun 2005 06:54:32 -0500
> From: Mary Dragich <mdragich@d.umn.edu>
> Subject: [permaculture] pear tree scab
> To: permaculture@lists.ibiblio.org
> Message-ID: <6.2.1.2.0.20050629065205.020a77a8@pop.d.umn.edu>
> Content-Type: text/plain; charset="us-ascii"; format=flowed
> 
> Does anyone have experience dealing with pear tree scab?  Our tree had it
> last year and this year.  We've used a product from Gardens Alive, but just
> started spraying when we noticed the scab.  We've pulled the diseased
> leaves off the tree, but there are now very few leaves left.  Information
> on what's worked for this problem would be very much appreciated.
> 
> Thanks.
> Mary Dragich
> Duluth, MN