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Re: Leaf Spot: Gene, Culture, Location, Gardener

  • To: Multiple recipients of list <iris-l@rt66.com>
  • Subject: Re: Leaf Spot: Gene, Culture, Location, Gardener
  • From: "Jeff and Carolyn Walters" <cwalters@digitalpla.net>
  • Date: Sun, 8 Feb 1998 09:42:31 -0700 (MST)

In my experience it seems that susceptibility to leaf spot is greatest in
TBs, less in IBs (which are typically 3/4 TB by descent), less still in
SDBs (which are 1/2 TB by descent) and non-existent in MDBs and beardless
iris. There does seem to be a degree of differential resistance to leaf
spot among TBs, but if the infestation is serious enough, I think any of
them are susceptible.

I grew TBs in this dry climate for more than 15 years without any sign of
leaf spot. Then in '95 (an unusually wet Spring, but not the first such in
that time span) it suddenly showed up with about a dozen cultivars being
seriously affected. I tried the approach of trimming and removing the
affected leaves. In '96 the leaf spot spread further and I kept trimming.
Last year I could see early on that the leaf spot was going to be
overwhelming, so I started spraying. Daconil, sprayed once every three
weeks, proved 100% effective in suppressing leaf spot through the bloom
season. After bloom season it tends to become very dry here with relative
humidity of 25-30% or lower through the summer. This in itself suppresses
leaf spot. However, last summer, perhaps due to the early development of
the current El Nino, it was untypically rainy in August and early
September. The leaf spot returned with a vengeance. Again Daconil was
effective, but when I ran out of it, I followed the advice to rotate
fungicides and tried Black Leaf Rose and Ornamental Fungicide (active
ingredient: Dimethyl 4,4'-o-phenylenebis-(3-thioallophanate)). For whatever
reason, this product proved totally ineffective here, although it is
specifically labeled for iris leaf spot. 

Jeff Walters in northern Utah  (USDA Zone 4, Sunset Zone 2)

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