Re: Iris Rot

Dave writes

:Someone asked what makes certain iris rot when no others will.  If we
:investigate the hybridizer and his practices you will find that in many
:instances that there is a lot of line breeding ( iris incest if you will
:!!!).  Continued practice will alway magnify the weak spots in the genetic
:character, as with the classic biological case studies of the Jukes and the
:Kallikaks (sp?).  I may be right or I may be wrong.  COMMENTS?????

What inbreeding does is increase the frequency of homozygous recessive
genes. Because many defective genes are recessive, weakening and disease
susceptibility are often consequences of inbreeding. Of course, there
are also desirable recessive traits, and these are also enhanced by
inbreeding (one of the reasons hybridizers often make use of the practice).

My feeling (no solid evidence to back it up) is that tetraploid TB irises
are a pretty inbred group of plants, not so much because of the programs
of particular hybridizers, but because tetraploidy was introduced from
a very small number of tetraploid clones around the turn of the century,
and crossing them with older diploids was so difficult that there was
probably not a huge variety in the diploid gene pool that contributed to
our modern TBs. There have been occasional influxes from other species
(Ii. aphylla, reichenbachii, and balkana), but genes from these species
are probably nearly negligible in most modern TBs.

I think this situation does contribute to the relative weakness of modern
TBs compared with their diploid ancestors. However, I'm reluctant to
attribute an iris's growth problems to the use of line-breeding by its
hybridizer. I think most hybridizers select irises that grow well in their
own garden; inadequate testing in other climates, rather than inbreeding,
is probably the most important reason why some irises do not grow well
everywhere. I'll also reiterate my point of several months ago that
claims of "weakness" in a particular cultivar are usually supported by
only anecdotal evidence, and should be taken tentatively, at best.

Happy irising, Tom.


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Tom Tadfor Little         tlittle@lanl.gov  -or-  telp@Rt66.com
technical writer/editor   Los Alamos National Laboratory
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Telperion Productions     http://www.rt66.com/~telp/
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