Hi Dave,
I've wondered about this. In several of our orchards. All of the
epiphytes all nearly have 100% of the population strep resistance even
at the start of the season, so in theory no applications of strep would
just as bad as several. The predominance changes over the season and it
makes me wonder if one of the species that comes in later in the season
is more like to transfer the plasmid. Also, feel like there probably has
to active cankers or shoot blight to get some Ea present before the
transfer could occure. The other bacteria seem to be better epiphytes
and it's getting hard to even get Ea on shoots in Kiersten's
experiments. We do need more seasons of data though.
One theory about how strep resistance develops (and I think this is
still valid) is that the initial selection for resistance is not in
Erwinia, but rather in other bacteria that exist in the orchard
environment. These other bacterial species may then pass on the
strep-resistance genes to Erwinia because bacteria have mechanisms for
transferring useful DNA (i.e., DNA that enhances survival) from one
species to another. When strep is applied to apple and pear flowers
in springtime, the over-all bacterial populations on leaves and in
soil are still rather low because bacterial population build slowly as
plants and soil warm up. Therefore, there is less selection pressure
for resistance in the non-Erwinia species when strep is applied during
bloom as compared to after bloom. Bacterial populations in the
orchard environment increase very rapidly as temperatures rise, so
summer applications of strep impact a much larger universe of bacteria
and therefore are presumed to be more likely to trigger resistance
that can later be transferred to Erwinia.
--
Kerik D. Cox, Ph.D., Associate Professor
Plant Pathology and Plant-Microbe Biology Section
School of Integrative Plant Science
Cornell University
221 Barton Lab
NYSAES
630 West North Street
Geneva, NY 14456 USA
E-mail: kd...@cornell.edu
Faculty Office: (315) 787-2401
Fruit Pathology Lab: (315) 787-2402
FAX: (315) 787-2389
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